Peptide receptor radionuclide therapy in neuroendocrine neoplasms and related tumors: from fundamentals to personalization and the newer experimental approaches

ABSTRACT

Introduction

Peptide Receptor Radionuclide Therapy (PRRT) is a type of molecular-targeted endo-radionuclide therapy in which unsealed radiolabelled somatostatin analogues are specifically directed toward somatostatin receptors (SSTR) over-expressed in Neuroendocrine Neoplasms (NEN) and related malignancies. This modality has witnessed a benchmark growth over last decade and symbolizes remarkable development in precision oncology. The tumor selectivity and excellent tolerability with minimal side effects make this one of the preferred therapeutic options in metastatic progressive NENs.

Areas covered

This review deliberates upon fundamentals of PRRT, its indications and employment as a therapeutic modality for precision oncology. A few special scenarios based on our experiences and expertise have also been mentioned. A short discussion on current advancements worldwide and future directions has also been included.

Expert opinion

SSTR-based PET and PRRT are now an established theranostic approach in management of NENs. Its spectrum has expanded in the form of Neo-Adjuvant-PRRT, Sandwich Chemo-PRRT, Duo-PRRT, Salvage-PRRT, and Intra-arterial-PRRT. Its application to other SSTR-expressing tumors includes metastatic/inoperable medullary thyroid cancer, neural crest malignancies like pheochromocytomas-paragangliomas (PPGLs), Merkel cell carcinoma, radioiodine-refractory thyroid cancer, and meningiomas. Though substantial prospective evidence is still lacking, a multidisciplinary team should adopt PRRT in appropriate indications by using dual tracer PET imaging as a gatekeeper.

KEYWORDS:

• Peptide receptor radionuclide therapy
• PRRT
• somatostatin receptor
• Neuroendocrine tumor (NET)
• PET-CT
• Radionuclides
• ¹⁷⁷Lutetium
• Theranostics

1. Introduction

Peptide Receptor Radionuclide Therapy (PRRT) is a kind of molecular radiotherapy that targets specific receptor molecules on particular cancer cells (such as NETs) with therapeutic radiopharmaceutical delivering targeted radiation dose. Being efficacious, minimally toxic and having a convenient therapy schedule, this treatment modality has evolved rapidly and integrated into the NET therapy armamentarium over the last 10–15 years. In NENs, somatostatin receptors are over-expressed in the well-differentiated set of tumors, known as NETs, which are the target molecules for PRRT. This therapy has completed a journey from ‘experimental laboratory to bedside.’ Given limited established therapies in inoperable and metastatic NETs, PRRT has become a promising option. Within last decade it became most popular radionuclide therapy after ¹³¹I in the radio-pharmacy world and worldwide its market is projected to double in the present decade [1] In January 2018, the FDA approved ¹⁷⁷Lu-DOTATATE for first time in the gastroenteropancreatic (GEP) NETs on the basis of results of prospective NETTER-1 trial. This phase III prospective randomized trial and multiple other retrospective data (523 articles and 158 reviews in last 20 years) have validated the novel approach [2] and its role is being further explored with other agents and other classes of NETs.

The principle of PRRT is a classical example of ‘THERANOSTICS’ (‘to treat what we see and see what we treat’), where the SSTR targeting PET/CT imaging plays a pivotal role in diagnosis, staging, and receptor status evaluation. The molecular PET imaging agents depicting tumor biology and their corresponding therapeutic implications can be considered as a major development in the inception of ‘Precision Medicine’ management of NENs. The present article is an updated overview of PRRT as a treatment option in the domain of metastatic NENs and other SSTR expressing tumors encompassing present-day practices, our experience and recent developments.

2. PRRT- The fundamentals

2.1. Radiopharmaceuticals

Radiopharmaceuticals are special type of radioisotope tagged biological molecules (termed as ligands), employed for diagnostic or therapeutic purposes, herein used for the purpose of therapy. The word itself can be spliced to ‘Radionuclide’ and ‘pharmaceutical agent’ where the pharmaceutical molecule (termed as ‘ligand’) which acts as a vehicle binds to the specific target (in this case, the SSTR analogs such as octreotate which target the SSTR receptors on NEN cells) and radionuclides are specific radioactive isotopes (e.g. β-emitter like ¹⁷⁷Lu or ⁹⁰Y) which deliver high energy radiation and irradiate tumor tissue to produce therapeutic effects. The concept of bifunctional chelating agent (BFCA) is also important here, these are special molecules like DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) which possess co-ordinating sites that chelate the metallic radionuclides (¹⁷⁷Lu,⁹⁰Y) and a functional group enabling the attachment to the carrier pharmaceutical agents (SSTR analogs). This assembly is illustrated in Figure 1 which is the typical structure of radiopharmaceuticals used in PRRT. After binding to SSTR receptors which is a type of G protein-coupled receptor, it gets internalized and irradiates intracellular structures including DNA molecules by its attached radioisotope to produce lethal and specific damage. Bystander and abscopal effects related to cellular damage on the adjacent tumor cells have also been postulated.

Figure 1. PRRT fundamentals: radio-pharmaceutical assembly in theranostics.

A typical radiopharmaceutical is comprised of (from left to right), (a) radionuclide (radioactive atom) of choice attached to suitable (b) chelator, (c) linker molecule which is further linked with targeting molecule commonly peptide/monoclonal antibody, localizing sequence, etc. The whole assembly gets bound to specific target receptor on cells of interest such as tumor cells.

As outlined in Table 1, there are several desirable factors considered while deciding the use of radio-isotope in the therapy e.g. (i) it should produce high energy particulate emission to produce lethal effects to the malignant cells, (ii) moderate half-life to exert adequate action, (iii) suitable tissue penetration range to irradiate tumor tissue in maximum extent but not normal surrounding tissue. Another important factor to consider in a clinical setting is the availability and cheap production method of a given radioisotope.

Table 1. Characteristics of therapeutic radionuclidesemployed in PRRT in NENs.

Therapeutic Radionuclides(References)	Corresponding Diagnostic Radionuclides	Half-Life (days)	Emissions of Therapeutic Radionuclide	Maximum Energy of Therapeutic Radionuclide(keV)	Maximum Tissue Penetration Range (mm)	Production of Therapeutic Radionuclide
^177Lu [3–5]	^68Ga	6.7	β^−,γ	497	2–3	Reactor
^90Y [6]	^68Ga	2.67	β^−	2284	11	^90Sr/^90Y generator
^67Cu [7–9]	^61Cu,^62Cu, ^64Cu	2.58	β^−, γ	562	2–3	Accelerator
^161Tb [10]	^152Tb, ^68Ga	6.89	β^−, Auger e, γ	593	3	Reactor
^225Ac [11]	^132La, ^68Ga	10.0	α and β ^−,γ	5935	0.1	Separation from ^229Th (Decay product of ^233U)
^213Bi [11,12]	^132La, ^68Ga	46mins	α and β ^−,γ	5870	0.1	^225Ac/^213Bi generator
^111In [13]	^99mTc,^111In	2.8	Auger e,γ	0.13to25.6	0.001	Cyclotron

2.2. Clinically used radioisotope molecules for PRRT

2.2.1.

1. Lutetium-177 (177Lu): This is presently the most preferred molecule due to its favorable decay characteristics and suitable production routes [3,4]. It decays to stable ¹⁷⁷Hf with emission of moderate energy β^– particles of 497 keV and maximum tissue penetration range of 2–3 mm, this makes it less toxic than other similar high energy radioisotopes(e.g.⁹⁰Y) and very suitable to use in tumors measuring~2 cm in size [14]. Another important benefit of ¹⁷⁷Lu is its additional low energy γ photon emissions (208 keV − 11% abundance and 113 keV-6.4% abundance), this makes post-therapy imaging and dosimetric applications very smooth, without stringent radiation safety-related issues.

In one of the recent reviews [5], the technicalities of ¹⁷⁷Lu production have been highlighted (produced by direct and indirect routes), along with clinical results, ease of use, long-term outcome, and comparable efficacies of using indigenous and cheap clinical-grade ¹⁷⁷Lu produced by medium to high flux reactors in India. This makes ¹⁷⁷Lu an attractive metallic candidate for therapeutic radionuclide worldwide for its overall performance. It is also the most studied molecule in literature on PRRT and has experienced several cycles over a long period [15].

2. Yttrium-90 (90Y): Next to ¹⁷⁷Lu, ⁹⁰Yis the most used and studied molecule for PRRT applications. The use of ⁹⁰Y was started even before than ¹⁷⁷Lu in patients and had proved its efficacy in the early days. It has advantages over ¹⁷⁷Lu, due to emission of a higher-energy beta particle emission (2.28 MeV) and a higher tissue penetration range of 11 mm,that makes it more useful in large-sized tumors when compared to other relatively low-energy beta emitters. Being a pure beta particle emission, it has a better radiation safety profile, and post-procedural imaging is carried out with approaches like bremsstrahlung imaging or PET-CT. It is also proved more toxic especially to kidneys than ¹⁷⁷Lu [16].

There are also issues related to the availability of ⁹⁰Y in its NCA form on a large scale for clinical use. It is obtained from the ⁹⁰Sr/⁹⁰Y radionuclide generator system. The separation of NCA ⁹⁰Y suitable for clinical utilization from ⁹⁰Sr is highly challenging due to the strict regulatory requirement of a very low permissible limit of ⁹⁰Sr in separated ⁹⁰Y. Strontium-90 in ionic form localizes in the skeleton and owing to its long half-life (28.8 y) is radiotoxic [6]. This causes restrictions on the commercial availability and widespread use of clinical-grade ⁹⁰Y.

3. Actinium-225 (225Ac): ²²⁵Ac has grown popular in recent years because of the advantage of being an α particle emitter, which is a high linear energy transfer (LET) radiation compared to the beta-emitters and has an energy of 5.93 MeV. Its role has been emphasized in patients with advanced cases which were resistant even to ¹⁷⁷Lu, wherein its efficacy and safety profile have been observed [17]. Interestingly, these radionuclides have lesser tissue penetration range and thereby saves surrounding non-target organs from being irradiated with unwanted irradiation. They also have a better radiation safety profile. With 10 days half-life and multiple emissions like β ⁻, γ rays in lower abundance it is quite suitable for use in therapeutic seating. Researchers are looking forward to observing the results and whether it will improve the outcome of PRRT in metastatic/advanced NETs. Similar to ⁹⁰Y, there are issues regarding production and availability ²²⁵Ac, as it is available in limited quantities by radiochemical separation from two ²²⁹Th sources, one located at Oak Ridge National Laboratory (ORNL), U.S.A, and the other at the Institute for Transuranium Elements in Karlsruhe, Germany. There have been also a few issues regarding the chemical stability of intermediate species of the decay chain of ²²⁵Ac with chelators which are under research (sumarrized in Table 2).

Table 2. Comparison of commonly used therapeutic radionuclides employed in PRRT in NENs.

	^177Lu-DOTATATE	^90Y-DOTATATE	^225Ac-DOTATATE
Advantages	Easy production, Widely available,Cheap, Less toxic, suitable for post-therapy imaging, Easier and well-established dosimetry	High path length (to handle larger tumors), less radiation exposure to healthcare workers or comforters	High LET (theoretically more powerful), less penetration to non-tumour cells, performs better in treatment resistant disease
Disadvantages	Lesser energy, path length and LET	More toxic owing to higher path length, post-therapy imaging is based upon bremsstrahlung and PET, complicated dosimetry, and post processing.	Expensive, availability problems, less evidence available both in terms of clinical results and toxicity, post-therapy imaging difficult, complicated dosimetry

2.3. Other investigational radioisotopes

2.3.1.

4. Terbium – 161 (161Tb): Recently,¹⁶¹Tb is being discussed as an alternate option of PRRT with suitable physical characteristics (T_1/2 = 6.89 days; Eβ^¯_av = 154 keV; Eβ^¯_max = 593 keV) and imageable low energy γ rays (peaks at 49 keV and 77 keV with 20% & 15% energy windows respectively), and forms stable coordinates with DOTA – the characteristics are quite similar to well-established ¹⁷⁷Lu [10]. In addition, they have shown co-emission of a substantially larger number of conversion and Auger electrons at a favorable energy range that can enhance the therapeutic effect.

Also, importantly the production route proposed is via the ¹⁶⁰Gd(n,γ)¹⁶¹Gd → ¹⁶¹Tb nuclear reaction, which provides no-carrier-added radio-lanthanide at high specific activities (HAS). Production and post-processing of these Gd targets are supposed to be less tedious and cheaper than HSA ¹⁷⁷Lu production. Its clinical use in PRRT for NETs is yet to be validated in routine patient settings.

5. Copper-67 (67Cu): With the improvement in understanding of the copper chelation chemistry, multiple copper isotopes are being explored for their medical use. Of them ⁶⁷Cu with its half-life of 2.58 days, Eβ−max 562 keV (100% abundance) and imageable γ photons (185 keV, 49%, and 93 keV, 16%) became an attractive option for therapeutic applications. It also suitable forms theranostic pair with ⁶⁴Cu (Positron emitter with half-life of 12.8hrs) [7]. In addition, ⁶⁷Cu can be successfully produced from natural zinc (⁶⁸Zn) with an eLINAC, and this makes copper not dependent on natural radioactive sources like ²³⁵U and nuclear reactors [18]. Promising dosimetric studies have highlighted the use of sarcophagine ligand chelator with copper compounds [8] and preclinical data of therapeutic efficacy has been reported in animal models [9]. The ongoing prospective trial of ⁶⁷Cu PRRT in patients of high-grade neuroblastoma [19]will further guide its use in clinical seating.

6. Bismuth-213 (213Bi): ²¹³Bi is an α emitter (T_1/2 = 45.6 min, E_α = 8.4 MeV, γ = 440 keV, α-particle range = 40–80 µm) and decay product of ²²⁵Ac. It also emits β⁻ particles (492 keV in 98% abundance). Just like ²²⁵Ac, this has also shown proven efficacy even in ¹⁷⁷Lu/⁹⁰Y resistant patients [12]. It forms a stable chelate complex with DTPA, NETA, and DOTA and has more stable chemistry for therapeutic uses [11]. It has the benefit of manufacturing and suitable scheduling patients in clinical seating as it can be extracted from ²²⁵Ac in form of the ²²⁵Ac-¹³¹Bi generator system.

7. Indium − 111 (¹¹¹In): Indium-111 is one of the oldest candidates which was used for PRRT in early days with ¹¹¹In-DTPA-Octerotide, it emits auger electron which has a very low therapeutic range (in nanometers), thus, the therapeutic response depends on internalization ability and closeness to the nucleus of the cell. Though it emits multiple imageable gamma rays, the dosimetric calculations are difficult and not well established.

2.4. Somatostatin analogues (SSAs)

As discussed previously the somatostatin analogues target SSTR on tumor surfaces. There are 5 subtypes of SSTRs (SSTR1 to SSTR5) present naturally in the body. SSTR-2 receptor-mediated action inhibits hormone release and also causes anti-proliferation, whereas stimulation of SSTR-2 and 3 causes apoptosis, which forms the basis of the use of (non-radio-labeled)octreotideand SSAs in the treatment of NETs. They show proven efficacy in controlling secretory symptoms and inhibiting tumor growth [20,21].

For PRRT purposes, SSTR-2 receptors are of special interest as it is the predominant receptor compared to other subtypes, and overexpressed in a spectrum of NETs and related neoplasms. Most exclusively used ligand in the present-day PRRT practice is [DOTA 0, Tyr 3]octreotate. It is an amino acid peptide compound containing tyrosine3-octreotate bonded covalently with a bifunctional chelator (DOTA or tetraxetan). Octreotate is similar to octreotide, but differs in that the C-terminal threoninol (amino alcohol) is replaced by threonine in the former. The chemical modification provides a 9-fold enhancement in the affinity of SSTR receptors, finally results in 4–5 times enhancement in the tumor uptake of the radiopharmaceutical and correspondingly enhanced radiation absorbed dose [22], makes this a preferred agent of therapeutic use. When attached with unsealed radio-isotopes mentioned above, DOTATATE selectively binds to SSTR-2a receptors after intravenous (IV) administration and gets internalized in tumor cells to produce therapeutic effects. Among all other chelator-peptide analogs (TETA, TEMA, SAR) DOTATATE is most studied and clinically used (tagged with ¹⁷⁷Lu/⁹⁰Y) worldwide. Copper-67 (⁶⁷Cu) shows stable binding with hexamine cage ligands of the bicyclo[6.6.6]icosane type, and these form complexes with copper(II) that are more stable in vivo than copper(II) complexes of DOTA. Preclinical evaluation of a sarcophagine ligand functionalized with Tyr3-octreotate, ⁶⁴Cu-CuMeCOSar-Tyr3-octreotate (⁶⁴Cu-CuSarTATE), the animal model revealed high uptake of ⁶⁴Cu- CuSarTATE in somatostatin receptor-expressing tumors at 2 h after injection and remained high 24 h after injection [23], indicating its suitability for therapeutic use.

3. Indications of PRRT and pateint management

3.1. Role of dual tracer PET-CT findings and NET-PET score: patient selection for PRRT & personalising treatment strategies in NETs

Dual tracer PET-CT i.e. molecular imaging with both SSTR targeting agents like ⁶⁸Ga-DOTATATE/NOC/TOC and metabolic imaging with ¹⁸F-FDG has emerged as an important tool for evaluation of metastatic NENs, before the treatment decision making [24]. This is being adopted by most of the centers alongside histopathology, the Ki-67 proliferation index. The proliferation index is the proportion of MIB-1-positive cells out of at least 2000 manually counted cells in areas of highest mitotic density. According to the WHO 2017 criteria [25], there are three grades of well-differentiated NENs classified as G1 (Ki-67 < 3%), G2 (Ki-67 3–20%), and G3 (Ki-67 > 20%), but there are situations where the reliability of Ki-67 index has been questioned as (i) a limited biopsy from the single lesion may not represent actual tumor biology, and (ii) there are also factors producing errors as inter-lesional heterogeneity which can be better addressed with whole-body molecular imaging with PET/CT.

Tumors with Ki-67 labeling index between 20–30% though classified as high grade pathologically, in clinical practise, on dual tracer PET imaging they show significant SSTR expression as well as variable FDG uptake, and this information has been further utilized for deciding between PRRT versus combined chemo-PRRT versus chemotherapy (discussed in further sections of chapter).

FDG-PET has also been shown to be of importance in low-grade NETs. One study by Binderup et al showed that G1 tumors with Ki-67 < 2% are in 40% of cases FDG-positive, 70% of G1/G2 tumors with Ki-67 between 2% and 15% and93% of tumors with Ki-67 > 15%. This means that even low-grade NET may be FDG-positive, which should be managed appropriately [26].

Dual tracer PET-CT evaluates relative tumor expression of SSTR and FDG uptake and metabolism covering almost all lesions in the whole body in a single go. This makes dual tracer PET a very reliable and robust tool for clinicians for assessing the dynamic tumor biology in continuum and thereby personalizing the treatment strategies [27] and also useful for predicting individual prognosis.Basu et al endeavored toestablish an approach correlating dual tracer PET-CT imaging with genomic and proteomic markers and selecting appropriate therapy options such as PRRT and multiple biomarkers based chemotherapeutic and molecular targeted therapy options [28] (Figure 2).

Figure 2. NEN assessment with correlated genomic and proteomic assessment with dual tracer PET/CT imaging for personalized management approach (Reproduced with permission from Basu et al [28].

One of the accepted and used grading systems for uniform clinical interpretation among multiple centers in this context is the NET-PET Score proposed by Chan et al [29]. The authors divided patients as per dual tracer PET-CT scan findings into solely SSTR +ve, solely FDG +ve, both SSTR and FDG +ve and both SSTR-ve and FDG – ve. NET-PET grades correlated significantly with the overall survival (p = 0.0018 in the univariate analysis) in contrast to the initial histopathological WHO grade, which did not seem to be prognostic in this cohort of patients. They also further discussed the role of the scoring system for therapeutic decision making, with patients having SSTR +ve/FDG -ve lesions, patients having FDG+ve lesions (that is less avid/equivalent SSTR positive lesions) PRRT should be considered with or without the addition of other therapies based on uptake characteristics. Though strong prospective data is still lacking to support this scoring system, most of the tertiary care centers follow a similar individualized management approach in multidisciplinary NET tumor board meetings, strongly based on dual tracer PET/CT findings and scores

Figure 3. Illustration of Spectrum of NENs based upon Dual tracer PET-CT (⁶⁸Ga-DOTATATE based SSTR PET/CT and ¹⁸F-FDG PET/CT (image display thresholds are set as per original paper [29]i.e., SUVmax of FDG PET at 7 and ⁶⁸Ga-DOTATATE PET at15). (a) Example of a patient of metastatic NEN with NET-PET Score: P1, in which lesions are SSTR +ve but FDG – ve. (b) Example of a patient with NET-PET Score: P2, in which lesions are both SSTR +ve and FDG +ve, but the number of lesions and their demonstrated uptake on SSTR PET >FDG-PET. (c) Example of a patient of metastatic NEN with NET-PET Score: P3, wherein lesions are both SSTR +ve and FDG +ve, and lesions and uptake on SSTR PET are nearly similar to FDG-PET. (d) Example of NET-PET Score: P4, in which lesions are both SSTR +ve and FDG +ve, but lesions and uptake on SSTR-PET. (e) Example of a patient of metastatic NEN with NET-PET Score: P5, in which lesions are SSTR-ve, but FDG+ve (Figure 3).

In a recent 10-year follow-up study in a cohort of 166 patients by Binderup et al elucidating the role of FDG PET in the spectrum of NENs, lesional uptake on FDG PET/CT was found to be useful for risk stratification of all NEN grades and even more robust to histologic grading. FDG-PET could differentiate G1 and G2 tumors into low- and high-risk groups. For patients receiving PRRT, FDG-negative cases showed significantly longer survival than FDG-positive cases, whereas no difference was identified for tumor grading. This cohort also showed FDG-positive patients receiving PRRT had a significantly longer median survival than patients not receiving PRRT whereas no difference was seen for FDG-negative patients [30,31].

The WHO 2017 classification introduced the novel well-differentiated neuroendocrine tumor of high grade (NET G3) and differentiated it from poorly differentiated neuroendocrine neoplasms also known as poorly differentiated neuroendocrine carcinoma (NEC) of high grade emphasized to highlight substantial biological differences within the same subgroup. The latter tumors are composed of highly atypical small or large cells expressing faint neuroendocrine differentiation markers whereas NET G3 shows adequate differentiation. The same reflects in molecular imaging as G3 NETs many times show SSTR expression and uptake on SSTR PET while PD-NECs show absent or very low uptake on SSTR-based PET [32]. Invariably dual tracer PET can play valuable adjunctive role in differentiating these two subtypes with G3-NENs, where clinical and pathological dilemma appears including intra- and inter-lesional variability. Furthermore SSTR PET-positive lesions especially in high-grade NENs (especially NET G3) can be targeted with PRRT and combined with other therapies (discussed in latter sections), while the same is not possible in SSTR-negative PD-NEN patients.

3.2. Indications of PRRT

3.2.1.

As per the published guidelines [33,34], and [35] PRRT is classically indicated in:

3.2.1.1.

1. Advanced, metastatic or inoperable, progressive NETs (traditionally grade 1 and grade 2 showing high uptake on SSTR-based imagingin the lesions)

Histologically these are well-differentiated NENs of gastroenteropancreatic origin of grades 1 or 2 (as per the WHO 2017 classification) having Ki-67/MIB- 1 labeling index up to 20%. High-grade uptake on different SSTR based ⁶⁸Ga-DOTA-TOC/TATE/NOC PET-CT or ^99mTc-HYNIC-TOC SPECT-CT is usually designated to uptake of Krenning’s score 3 (uptake more than that of physiological uptake in the liver) or 4.

2. While the usual clinical setting has been disease progression on cold somatostatin analogues (i.e. long acting octreotide or lanreotide), PRRT can be considered as first-line in patients with extensive metastatic/large-bulk disease at diagnosis [22,24]. In pancreatic NET, PRRT is an option if cytotoxic chemotherapy, everolimus, and sunitinib fail and/or are contraindicated according to current guidelines.

3. PRRT can be considered in symptomatic patients with functioning NETs, where the symptoms are not controlled by the long-acting SSAs (octreotide/lanreotide).

4. Extended Indication [5]:

Considering gratifying results, excellent tolerability, and minimal toxicity experienced in thousands of PRRT cycles in multiple centers across the world, the therapy has been further explored beyond the above-mentioned classical indications with the following extended indications that can be useful, especially in improving quality of life in a substantial fraction of patients:

(a) PRRT in patients up to Ki-67 LI of 30% who demonstrate high uptake on SSTR based imaging: This is a ‘Gray zone’ and frequently these groups of tumors demonstrate high uptake on ⁶⁸Ga-DOTATATE PET-CT and has been an area where PRRT has been advocated successfully [36], [37]. They sometimes demonstrate high uptake of FDG, where combined chemo-PRRT is now an available option with encouraging results (explained in later sections of the chapter).

(b) Beyond Gastroenteropancreatic NENs (GEP-NENs): GEP-NENs have been the major indications of PRRT and have level one evidence in the literature. There is also decent evidence worldwide for ‘beyond GEP-NEN’ applications and include:Metastatic/inoperable Bronchopulmonary and Mediastinal/Thymic NENs [38],

Metastatic/inoperable Medullary thyroid carcinoma,Metastatic Paraganglioma & Pheochromocytoma especially which are not amenable to other therapies,

Non-iodine concentrating metastasis of differentiated thyroid carcinoma (TENIS), other tumors with neuroendocrine tumor differentiation, e.g. metastatic Merkel Cell carcinoma, Meningioma and Recurrent/inoperable Phosphaturic Mesenchymal tumors, have also been discussed in the literature [39–42].

3.3. Contraindications

1. Other curative approaches (surgery) possible

2. Pregnancy

3. Severe acute concomitant illnesses.

4. Severe unmanageable psychiatric disorder where the patient is unable to comprehend and consent for PRRT

5. Breastfeeding (if not discontinued).

6. Severely compromised renal function and bone marrow function.

3.4. Specific selection criteria

• A. General health condition:

  • Karnofsky Performance Status>50 (ECOG<3)

  • Expected survival>3 months

• B. Renal function:

Especially with ⁹⁰Y-DOTATATE, age-adjusted normal renal function is essential. Patients with some compromise in renal function can be considered for ¹⁷⁷Lu-DOTATATE treatment

Contraindicated reference values are:

• Creatinine :> 1.7 mg/dl

• Glomerular filtration rate (GFR) and tubular extraction rate (TER) below 60% of mean age-adjusted normal values (eGFR values not less than<30 ml/min)

C. Bone marrow function: The patient should not have severely compromised bone marrow function (which is not uncommon in patients who are previously heavily pre-treated with cytotoxic chemotherapies)

Contra-indicated reference values are:

WBCs<3,000/μl, with absolute neutrophil count<1,000/μl, platelets :< 75000/μl for ¹⁷⁷Lu-DOTATATE, <90,000/μl for⁹⁰Y-DOTATOC, RBC<3,000,000/μl

3.5. Procedure

Pre-therapy assessment in a referred patient for PRRT:

1. Histopathology evaluation (with immuno- histochemistry) proving type and grade of NET

2. Dual tracer PET/CT (SSTR based PET-CT alone followed in few centers) ⁶⁸Ga-DOTATATE/NOC/TOC PET/CT scan (or ¹¹¹In-pentetreotide scan) and ¹⁸F-FDG PET/CT scan

3. Clinical evaluation – general condition (Karnofsky/Lansky/ECOG performance status)

4. Evaluation of any concomitant systemic and other illness, if needed specialist review

5. Evaluation of renal Function: Sr. Creatinine, eGFR calculation, some centers also evaluate renal functions with ^99mTc DTPA renogram (GFR estimation and drainage) and OR ^99mTc EC scan (ERPF estimation and drainage)

6. Evaluation of blood parameters: CBC (at least Haemoglobin concentration, WBC count, Platelet count)

7. Evaluation of liver function (liver is most commonly involved organ of metastases in GEP-NETs): Sr. Bilirubin, AST/ALT, and alkaline phosphates

If any other parameter is observed insufficient or borderline as per inclusion criteria (discussed in previous sections), measures such as dietary supplements, hematopoietic growth factors, blood transfusions (whole blood/packed cell/platelet concentrates), and specialist review (nephrologist, hematologist, hepatologist) should be planned before therapy and corrective measures should be undertaken.

Patients with renal outflow obstruction, previous myelotoxic chemotherapy/extended EBRT, and pending liver failure should be evaluated.

As per current practice protocols there is a practice to stop long-acting SSAs (e.g. inj. octreotide LAR 30 mg IM) at least 4 weeks before the PRRT date to avoid interference with a radiolabelled agent; if the patient has severely symptomatic functioning disease or carcinoid syndrome, they are switched to short-acting formulations (e.g. inj. octreotide 100mcg sc 8 hrly), continued upto 1 day before PRRT [5,43]. We must mention the other school of thought, wherein it was hypothesized indeed that the administration of LA-SSA therapy prior to PRRT may improve the PRRT anti-tumor activity in most patients, by saturating the physiological binding sites and by increasing the binding of [¹⁷⁷Lu]DOTA-peptide-SSTRs in pathological sites, just like the administration of ‘cold’ Rituximab prior to administration of radiolabelled Rituximab in patients with CD20 + B-cell lymphoma [44].

A prospective study by Aalbersberget et al suggested that the discontinuation of cold somatostatin agonists is not mandatory [45]. But most of the centers and guidelines still recommend stopping long-acting analogue, and the topic remains a matter of debate.

3.6. Facility and personnel

The facility requirements depend on the national legislation on the therapeutic use of radioactive agents. If in-patient therapy is required, the treatment should take place in an approved dedicated facility for use of specific radio-isotopes. The facility must have appropriate personnel (nuclear medicine physician, trained nursing staff, RSOs who have adequate training in handling therapy-related radioactive as well as medical emergencies), radiation safety equipment, and procedures for waste management and handling accidental contamination of the site or personnel [33].

3.7. Treatment scheduling

3.7.1. For routine metastatic therapy

¹⁷⁷Lu-DOTATATE [33]

Administered activity:5.55–7.4 GBq (150–200 mCi)

The time interval between cycles: 6–12 weeks

Number of cycles:three to five

⁹⁰Y-DOTATATE [33]

Administered activity:3.7 GBq (100 mCi)/m² body surface, typical dose~75-120mCi

The time interval between cycles: 6–12 weeks or

Number of cycles:two to four

²²⁵Ac-DOTATATE [17]

Administered activity:100 kBq/kg of body weight (2.7 μCi), typical dose~150–180 μCi.

The time interval between cycles: 8 weeks

Number of cycles:one to five (depending on toxicity – maximum cumulative dose possible: 55500 kBq)

(Scheduling of special scenarios like neo-adjuvant/salvage PRRT are discussed under specific headings)

3.8. Therapy administration

3.8.1. Patient preparation

On the therapy day patients should be reevaluated for their general condition, special precaution to be taken regarding severe active carcinoid symptoms and dehydration. The patient should be encouraged to drink plenty of fluids no usual dietary restrictions are required, allowed upto at least 2 hrs before the scheduled PRRT time, to allow gastric emptying and minimize nausea. The patient must be reassured that both nuclear medicine physician and nursing staff will be communicating daily and any essential routine medical and nursing care will be provided. This psychological preparation of the patients and their co-operation play a considerable role in the optimization of radiation exposure to the medical, paramedical, nursing staff, and comforters. All medical and radiation-related queries to be solved before administration of radioactive drugs, measures to be taken address and relieve treatment-related anxiety of the patients. Due consent to be taken as per national clinical and radioactive regulations from the patient. Patients to be instructed to take almost all of their routine medications as per schedule (anti-hypertensive drugs, anti-diabetic drugs including insulin, etc). Patients should be given hospital clothes, and handled safely as per radiation- protection protocols.

3.8.2. PRRT administration protocol

Standard procedure involves placement of two peripheral intravenous (IV) lines on both hands (using 22 G IV cannula). If single IV line is employed, then a 3-way IV cannula with stopcock is used, with saline flush after every fluid change (i.e. after anti-emetic-dexamethasone combination, amino acid and the radiopharmaceutical infusions).

3.8.3. Renal protection

Kidneys are one of the critical and dose-limiting organs in PRRT considering retention of radio-labeled peptide in kidneys. This is partly attributed to megalin-mediated cubulin dependent endocytosis process across the proximal epithelium of renal tubule. This toxicity also gets aggravated by preexisting morbidities such as diabetes mellitus and hypertension. The following considerations and approaches are employed to tackle this problem in the clinical settings:

a) Consideration on difference in the Nephrotoxicity of the Radioisotopes:

As described above, due to different energy and tissue penetration range of the radionuclides, they exert different radiation doses including the in-vivo renal dose. In a large retrospective Swiss study significant permanent renal toxicity was reported in 9% of the population using ⁹⁰Y-DOTATOC PRRT [46]; this value was significantly less with¹⁷⁷Lu-DOTATATE (around 1%) because of its lesser renal irradiation [47,48].

b) Amino acid protocols forrenal protection:

A mixture of positively charged basic amino acids (arginine and lysine) in adequately diluted formulations have been widely advocated; these positively charged amino acids competitively inhibit the proximal renal tubular reabsorption of the radio-peptide. When co-administrated with radiolabelled DOTA peptides they have shown to result a reduction in the renal absorbed dose, ~40% (range: 9% − 53%) [13] and can be further reduced by up to 39% by extending the infusion time of the amino acid solution over 10 hrs, and up to 65% by extending the protection over 2 days as per shown in studies [49]. Different institutions follow different protocols ranging from 2 to 10hrs.

Due to the non-availability of pharmacological formulations of positively charged AAs, many centers have also successfully used pharmaceutical grade mixed amino acid infusion which are widely available have also worked well in the clinical setting [5]. Centers have titrated these formulations as per concentration of positively charged AAs in them which would closely simulate the suggested dose of 50 g (25 g lysine+25 g arginine), e.g. when using the formulation inj. Hermin® in its usual 200 ml bottles, a total of 6 bottles of this are infused over 4-5hrs. with 1st bottle infused before administration of ¹⁷⁷Lu-DOTATATE.A slower single-day infusion or 2 days AA infusion is also suggested in review articles [5]when using ⁹⁰Y-DOTATATE owing to its higher toxicity. While using ²²⁵Ac, similar to guidelines 50 g single day AA infusion over 4 hours has been successfully used [17,37] (summarized in Table 3).

Table 3. The joint IAEA, EANM, and SNMMI practical guidance [33] have shown multiple following AA protocols.

Protocol Name	Cocktail	Dosage protocol
Single-day 50-g	25 g of lysine and 25 g of arginine in 2L of NS	AA cocktail infused over 4 h, starting 30–60 min before PRRT
Single-day 50 g+ Gelofusine	25 g lysine+25 g arginine diluted in 2L of normal saline and Gelofusine500mL	AA cocktail infused over 4 h, starting 30–60 min before PRRT And Gelofusine infusion as a bolus of 1 ml/kg body weight over 10 min before PRRT, infusion at 0.02 ml/kg/min over 3 h after radiopeptide infusion
Three-day 25-g	Day 1 : of 25 g of lysine diluted in 1 L NS Day 2 and 3 :12.5 g lysine solution in 500 ml of NS	Day 1 AA cocktail infused over 4 h, starting 30–60 min before PRRT Day 2 and 3 AA cocktail infused over 3 h twice a day
Three-day 50-g	Day 1: 50-g solution of lysine and arginine (25 g/25 g) diluted in 2 l NS Day 2 and 3: 12.5 g lysine diluted in 500 ml of NS	Day 1 AA infused over 4 h, starting 30–60 min before PRRT Day 2 and 3 AA cocktail infused over 3 h twice a day

Special precautions should be taken in patients with severe cardiac failure who have volume restrictions; here lower volumes of AA infusions should be formulated with a recommendation from the cardiologist. Due to the high osmolarity of the infusion, pain along the infusion line and phlebitis are not uncommon. The patient should be adequately counseled regarding the same, vasoprotective creams can also be used.Nowadays, a few AA mixtures are commercially available such as LysaKare 25 g/25 g (One 1,000 mL bag contains 25 g of L-arginine hydrochloride and 25 g of L-lysine hydrochloride) specially for PRRT.

Antiemetic protocol: One of the commonly reported side effects during PRRT infusion is nausea (~30%) and emesis (~12%) of grade I and II [48], this primarily occurs due to metabolic acidosis related to charged AA infusion. There have been different approaches have been used to address this; commonly inj. Ondansetron (owing to its central 5HT–3 antagonizing action) and inj. Dexamethasone has been used successfully in most centers. Both Inj. Ondansetron (8 mg) + Inj. Dexamethasone (8 mg)IV are administered 30 mins before administration of PRRT concurrently with AA infusion in two different infusion lines, this approach has been effective in most cases [22]. However, in a smallfraction of patients who develops uncontrolled vomiting even after the above-mentioned combination, can be managed with oral aprepitant (NK-1 receptor antagonist) with its standard 3-day oral regimen (125 mg, 80 mg, and 80 mg on days 1, 2, and 3, respectively), such patients should be followed up keenly at time of next PRRT cycles also and considered for this additional anti-emetic medication.

The following options have been suggested to adequately manage patients with functioning NENs in specific clinical settings [5]:

a. Priming with Cyproheptadine (having potent 5HT–2 antagonizing action) from 1 to 2 days before scheduled PRRT is quite an effective option for preventing serotonin syndrome esp. in patients with bulky hepatic metastatic disease; it can be further continued with no harm for 10–15 days post-PRRT. This medication is commonly available in syrup formulation and administered in OD/BD schedule as 10 mL dose.

b. Short-acting octreotide injection (100-300mcg titrated dose subcutaneously): can be administered till 1 day before PRRT and start back the next day following PRRT and continued till 10–14 days after therapy in patients with functioning disease; this mostly takes care in preventing an acute carcinoid syndrome. Later, if needed, can be successfully switched over to a suitable long-acting intramuscular formulation (inj. octreotide LAR 30 mg IM).

The patient’s vital signs (pulse, BP) should be monitored just before therapy as well as regularly post-PRRT during AA infusion. Other medical emergencies (MI, stroke) though very rarely occur after adequate pre-therapy patient selection should be promptly addressed similar to any non-radioactive therapy; there is no observed significant adverse interaction of any emergency drug with PRRT radiopharmaceuticals. The resuscitation cart and the trained emergency team must be available round the clock in such facilities.

4. Dosimetry and its value in treatment personalization in prrt

Dosimetry refers to calculation of the radiation dose absorbed by tumor tissue, normal tissues, and the whole body, and is an important physical factor that influences the response of tumors and the adverse effects to the rest of the body to radiation. Absorbed dose is the amount of radiation energy that is deposited in tissue divided by the mass of the tissue. It is determined by a combination of measurement and calculation.

For ⁹⁰Y-DOTATOC, due to lack of γ-emission by ⁹⁰Y makes direct dosimetry is difficult, Bremsstrahlung images are also difficult to quantify and require the application of complex corrections. Thus, alternative options are used in clinical practice: ¹¹¹In and 86Y simulations. PET with ⁸⁶Y-DOTATOC offers improved spatial resolution and quantitative analysis. A PET-based method promising quantitative imaging of ⁹⁰Y distribution is also being evaluated [50]. Advances in SPECT imaging such as quantitative SPECT/CT enable the absolute quantitative measure of the true radiopharmaceutical distribution providing for PRRT dosimetry in each patient [51].

In case of SSTR-based PRRT, the kidneys and the bone marrow are considered as dose-limiting organs [52]. Using dosimetric estimations one can derive a therapeutic window that causes minimum toxicity to these organs and exert the maximum possible dose to tumor tissues. A pioneer work by Sandström et al observed that kidneys are the dose-limiting organs in most cases (98.5%) while bone marrow contributed a smaller number of therapies (1.5%), considering maximum safe dose determined by ICRP to kidneys of 23 Gy and for bone marrow 2 Gy. The AA co-infusion protocols help to reduce this renal dose by around 40%. Considering these limits, the studies have claimed a cumulative dose of ¹⁷⁷Lu-DOTATATE up to 40Gbq (~1080mCi) can be administered safely as PRRT [53]. It is important to note that the maximum tolerated doses in such prescribed limits are derived from data using EBRT (which is high dose-rate radiation), while the intravenous targeted type of unsealed sources therapy used in Nuclear Medicine (including PRRT) is inherently low-dose-rate, allowing the maximum tolerable dose by these organs to higher limits. Many groups thus believe that by using exclusive PRRT dosimetric studies and deriving their maximum tolerable dose one can further increase the limits of administered dose, thereby increasing therapeutic efficacy.

Forrer et al, exclusively studied bone marrow toxicity using blood sampling and organ level dosimetric calculations [54]. The total absorbed dose to the red marrow after application of 3700 MBq (100 mCi) 177Lu-Octreotate was median 83 mGy (range: 40–466 mGy). As result, the inter-individual bone marrow dose differed significantly and the need for individual dosimetry was suggested to reduce the bone marrow toxicity and the risk of developing MDS.

Apart from kidneys and marrow, the other organs getting most of the irradiation are liver and spleen (even greater than kidneys), but considering their cellular radiation biology and regeneration potential, the prescribed maximum tolerable absorbed doses are much higher for these organs (range of 30–40 Gy). In a clinical PRRT setting, this high radiation dose is never achieved and is of less concern in dosimetric regard, except for some special cases like extensively involved hepatic parenchyma, where normal functioning liver volume is minimal, dosimetric parameters should be considered for individual decision making.

As ⁹⁰Y is a pure β emitter with higher energy than ¹⁷⁷Lu, the dose estimates for ⁹⁰Y show an obvious increased absorbed dose to all organs, particularly kidneys. This information can be utilized in clinical decision-making when treating patients with compromised renal function, where the cocktail approach can benefit than single radionuclide in lesser toxicity.(summarized in Table 2)

Deciding dose and scheduling by performing dosimetric calculation for each patient has a number of connotations. This has defiantly proven to improve efficacy using the maximum possible dose for a specific person [55].

Studies have claimed that we can increase tumor absorbed dose of ¹⁷⁷Lu-DOTATATE using individualized dosimetry to up to 43.0 ± 29.7 Gy per cycle that gives 163.4 ± 85.9 Gy for routine 4 cycle schedule [56]. Thus, through such personalized way, the cumulative maximum tumor absorbed dose can be increased upto 2.64-fold (1.48-fold on average) over a four-cycle course without undue toxicity increment. Though these approaches are promising, their superiority over routine empirical approaches especially the final therapy outcomes (OS and PFS) has not yet been proven in larger studies (summarized in Table 4). Presently, most of the centers worldwide use the empirical fixed dose approach in the routine clinical setting. Increasing the number of trained personnel performing dosimetry and simplified dosimetric protocol in a routine clinical setting is now a need considering the increasing number of PRRT given worldwide.

Table 4. Advantages and disadvantages of internal radiation dosimetric approach vis-a-vis the conventional fixed-dose regimen.

Advantages	Disadvantages
Theoretically maximum possible radiation can be delivered	Time consuming
Minimizes incidence of toxicity and adverse effects	Increases number of scans, hospital visits and admission time
Delivered radiation is well documented for further planning and long term follow up studies	Requires more number of trained personnel
Aids in developing other radionuclide therapies with similar molecules in different diseases.	Not yet proven to have additional and long term benefit in prospective trials
	Higher Cost
	Cellular and molecular level complex interactions, non-uniform radioactivity distributions are not always calculated
	A number of calculations and formulas rely on multiple assumptions and models, and thus cannot be applied totally fool-proof to the biological system

5. Adverse effects of PRRT

The National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) system, a descriptive terminology utilized in oncological parlance for Adverse Event (AE) reporting, has led down the standardized classification of adverse effects of drugs used in cancer therapy. According to this, an adverse event is defined as any unfavorable and unintended sign, symptom, or disease during or after a medical treatment/procedure. PRRT is considered to be a very well-tolerated therapy by patients, with very few and mild side effects if adequate precautions taken.

5.1. Acute effects

These are mainly related to the AA infusion used for renal protection (for details see PROCEDURE section of the chapter). Co-infusion of this AA raises a state of metabolic acidosis and causes symptoms like nausea, vomiting, abdominal discomfort, and headaches. Different centers have reported varying ranges of these effects (vomiting in 10%, significant nausea 25%) as a reporting system and threshold of the reference population. There also occurs a mild degree of electrolyte imbalance especially hyperkalemia and hypernatremia during this period [33]. Caution should be taken if there is severe nausea/vomiting precipitating a state of significant dehydration and thereby could further worsen the electrolyte imbalance. For all patients precautionary measures are taken using a combination of corticosteroid and antiemetic administration before PRRT. Some of these patients require repeat administration of this or consideration of drugs like aprepitant for symptom control. Adequate patient counseling, preparation, and hydration form the backbone of facing these effects.

Very rarely (~1%) may develop severe hormonal syndrome/carcinoid crises [57] from the active amines secreted from an irradiated tumor. Symptoms include flushing, diarrhea, sudden blood pressure changes, metabolic changes as hyper/hypoglycemia, hyperacidity, and so on depending on tumor type. As mentioned in the above section, this is usually observed in patients with poorly controlled functioning symptoms and extensive metastatic disease or liver metastases. Such patients should be identified beforehand and should be adequately primed for therapy with pharmaceutical measures like short acting octreotide, and cyproheptadine. If such adverse event occurs in the PRRT therapy ward, they should be promptly managed primarily based on the symptomatic basis with high-dose octreotide, I.V. fluid replacement, and other supportive medications such as corticosteroids and loperamide. On rare occasions, if needed patient should be shifted to ICU facility following necessary radiation protection measures [58].

5.2. Delayed adverse effects

There are multiple symptoms related to the functioning disease and a few sub-acute and delayed clinical adverse effects reported in these patients of NETencountered in the post-treatment period following PRRT. These symptoms as nausea, abdominal pain diarrhea, abdominal distension, gastritis, weight loss, fatigue, musculoskeletal pain, loss of appetite, flushing and headache that have been reported in the literature were exclusive of grade I and II (as per CTAC v4.0) and thought to be related to natural disease course and not PRRT per se. Another recent study reported a 3% incidence of risk of bowel obstruction within 3 months in patients receiving PRRT. All patients had a mesenteric or peritoneal disease and responded to high doses of corticosteroid [59].

A number of large clinical studies have reported the toxicity profile of PRRT in the literature, few prominent ones are summarized in Table 5). Kidneys are the dose-limiting organs in PRRT and active renal protection protocols are followed in the clinics, though still there are uncommon occurrences of renal compromise due to PRRT. Valkema et al reported a creatinine clearance loss of about 3.8% per year for ¹⁷⁷Lu-DOTATATE and 7.3% per year for ⁹⁰Y-DOTATOC therapy. As summarized below different studies have reported severe grade toxicity (i.e. grades III and IV) ranging from 0.6 to 9%. Many of these patients were having preexisting renal compromise due to long-standing diabetes, hypertension, or obstructive nephropathies [60].

Table 5. Grade 3 and 4 nephrotoxicity and hematotoxicity reported in the literature.

Radiopharmaceutical used	Nephrotoxicity (Grade 3 &4)	Hematotoxicity (Grade 3 & 4)					Total number of patients in study (Reference)
		Hb	Leucocytes	Platelets	MDS	Acute leukemia
^177Lu-DOTATATE	0.6%	0.2%	<1%	<1%	–	–	468 [48]
^177Lu and ^90Y DOTATATE	1.5%	6%	5%	2%	2.3%	1.1%	807 [64]
^177Lu and ^90Y DOTATATE	1%	1%	0.2%	0.2%	2%	–	1048 [65]
^90Y-DOTATATE	9%	10%	8%	1%	–	–	1109 [46]
^225Ac-DOTATATE	0%	0%	0%	0%	0%	0%	32 [17]

Another adverse effect is bone marrow toxicity owing to its low threshold dose of 2 Gy. Up to 10% for ⁹⁰Y and 1–2% for ¹⁷⁷Lu had higher grade hematological toxicities as reported in the literature. As a fraction of the patients being considered for PRRT are pre-treated with chemotherapeutic agents, these patients require a cautious pre-therapy evaluation and corrective measures. In large studies, there has been mention of rare occurrences (1–3%) of myelodysplastic syndrome (MDS) or overt acute myelogenous leukemia. Though the reported prevalence and actual occurrence of these serious effects are relatively rare it should be anticipated and assessed in each therapy sitting [61].

One must keep in mind that most of the available data in this domain has been for period lesser than 10 years. Patients heavily pre-treated with chemotherapy (myelotoxic) or external radiation, those with high tumor load, and extensive hepatic metastases may be the susceptible subgroup for these complications. The analysis by Bodei et al showed that cumulative activity, duration of PRRT, age at diagnosis, the time between diagnosis and PRRT, toxicity grades (especially hematological), and number of cycles were all important. Age over 65 years seems to be a risk factor for the development of anemia [62]. Thus one must be watchful for these high-risk patients who are heavily pre-treated with myelotoxic drugs and show greater than NCI-CTCAE grade 2 toxicity immediately after PRRT, especially significant reduction in platelet counts and delayed normalization should be identified and documented. Such patients should be followed up rigorously and personalized therapy scheduling with reduced and delayed cycles can be planned. Even after completion of therapy cycles and adequate response they should be followed up more stringently with blood studies in long term (>10 years) and if needed investigated with other studies including bone marrow. Data from a recent retrospective study including 37 patients receiving ¹⁷⁷Lu-DOTATATE showed that only 5.5% reached 2 Gy to the bone marrow and the threshold value of 23 Gy for a kidney was reached in 21% of patients receiving 4 cycles and in 37.5% in case of more than 4 cycles. However, no long-term renal dysfunction occurred with a kidney dose of 23–29 Gy, which suggested even a possible increase in kidney threshold levels [63].

Another recent analysis in approximately 102 patients receiving PRRT (177Lu or 90Y DOTATATE) four times every 10–12 weeks and their follow-up at 12 months revealed that in 20% of cases an increase in the grading of nephron- or hematotoxicity is observed and in all of these patients toxicity findings were mild or moderate only. Also, PRRT has no critical impact on further oncologic treatment options in the case of disease progression [66].

There is also a theoretical possibility of endocrine derangements due to PRRT as many endocrine cells and organs express SSTR (pituitary, thyroid, and adrenal glands and Langerhans cells). But no significant alteration of endocrine function has been reported and currently, available data is very sparse to comment upon.Similarly, though targeted alpha therapy (TAT) use has emerged and is being administered even in previously failed patients; for commenting upon toxicity profiles for TAT agents more studies with longer follow-up period are required.

6. Efficacy of prrt

Currently, multiple therapeutic options are available for the management arsenal of NETs, including surgery, liver-directed therapies (like RFA, TACE, TARE, SIRT), somatostatin analogues (SSA), radiolabelled SSTR-based therapy (PRRT), chemotherapeutic agents, molecular targeted agents, and immunotherapy. The choice of the most appropriate option depends upon multiple factors like local tumor invasion, metastatic status, functional status, histopathological subtype and grade, SSTR, and metabolic status on dual tracer PET imaging and the patient’s clinical condition. For limited early disease- only local tumor invasion surgery is the best curative option but most of the tumors remain asymptomatic and get diagnosed when it has already metastasized or progressed to inoperable stages. The locoregional cytoreductive strategies, e.g. trans-arterial chemo-embolization (TACE), trans-arterial embolization (TAE), radiofrequency ablation(RFA), and other techniques such as selective internal radiation therapy (SIRT), are helpful in palliative and symptom control. These local embolization techniques are particularly useful when treating patients with functionally active liver metastases. TACE has shown symptomatic response rates of 60–95% and the radiological response of 33–80% with response duration of 18 and 24 months [67,68]. The procedure may require more than one treatment session and interventional radiology assistance.

The majority of NETs express SSTR (~85–90%), and considering the slow-growing indolent nature of the disease, in many patients somatostatin analogues are offered as the initial monotherapy. They have been shown to control the clinical syndrome in 40–90% of patients depending upon tumor load and dosage [69]. A recent phase III prospective trial – PROMID study compared long-acting (LAR) octreotide (30 mg) with placebo arm in progressive mid-gut NETs showed a clear benefit in PFS of study arm of 143 months vs 6.0 months in the placebo arm [70]. Similar such study – CLARINET which compared Lanreotide auto gel 120 mg vs. placebo in patients with low-grade nonfunctioning NETs showed stable disease in 66% of study arm vs 49% in placebo arm while PFS at follow up period of 24 months was not reached in study arm vs 18 months in the placebo arm. These cold somatostatin analogues have further benefitted of having long-acting formulations which can be suitable used as per month formulation and have excellent tolerability and side effect profile [71].

Chemotherapy is considered appropriate for high proliferating grade 3 tumor subtypes in NET as the vast majority of NETs are slow proliferating for many years they are insensitive to these agents. Novel agents like m-TOR inhibitors have been evaluated in NETs extensively with series of prospective phase III RADIANT trials. These studied everolimusinfunctional and nonfunctional NETs, pancreatic NETs, and lung NETs vs octreotide (in RADIANT 2) or placebo (RADIANT 3 and 4) showed significant improvement in objective response and PFS rates [72,73].

The efficacy of PRRT underwent a major breakthrough following the results of NETTER-1 trial. It was the first phase III randomized controlled trial which studied Lutathera® (¹⁷⁷Lu-DOTATATE 7.4 GBq at interval of every 8 weeks and 30 mg octreotide LAR repeatable every 4 weeks) in patients with metastatic mid-gut NETs compared to 60 mg octreotide LAR. After 20 months of follow-up, 65.2% of patients in the PRRT group were progression-free, and 14 patients died; in the control group, only 10.8% of patients were progression-free, and 26 patients died. This derived a PFS of approximately 40 months versus 8.4 months for the octreotide LAR only arm, rendering the differences in PFS and death rate as significant (p < 0.001 and p = 0.004, respectively). Relevant toxicity (grade 3 or 4) in the PRRT arm was only 1% neutropenia, 2% thrombocytopenia, and 9% lymphopenia [74]. considering this significant improvement in survival rates at the cost of minimum toxicities registered in this trial ¹⁷⁷Lu-DOTATATE PRRT received FDA and European Commission approval in GEP-NETs.

Multiple large retrospective studies have demonstrated the efficacy of PRRT (summarized in Table 6), predominantly in GEP-NETs and few other NET types.Most of the studies followed common criteria for this evaluation as follows:

1. Symptomatic response

2. Biochemical response

3. Imaging response (by RECIST or PERCIST scales)

4. Final clinical outcome in the form of overall survival (OS)and Progression-free survival (PFS)

Table 6. Efficacy and various response rates for PRRT reported in literature.

Radiopharmaceutical				Imaging response (RECIST1.1) (%)
Single center retrospective studies	Tumour type	Symptomatic response (%)	Biochemical response (%)	PR +CR	SD	PFS (months)	OS (months)	Total number of patients in study (References)
^177Lu-DOTATATE	All types	90	60–70	30	60	NA (71% at 7 years)	NA (79% at 7 years)	468 [48]
^177Lu-DOTATATE	GEP	–	–	36	35	26	55	74 [76]
^177Lu-DOTATATE	GEP + bronchial	–	–	39	43	29	63	443 [77]
^90Y DOTATATE	All types	–	–	34	40	37	38	1,109 [46]
^90Y DOTATATE	GEP	86	–	23	77	19	23	60 [78]
^177Lu and ^90Y DOTATATE	All types	–	–	–	–	22	53	782 [79]
^177Lu and ^90Y DOTATATE	All types	–	–	–	–	19	51	1048 [65]
^225Ac-DOTATATE	All types	–	80	62	37	–	–	24 [17]
Meta-analysis Studies
^177Lu and ^90Y DOTATATE	All types	–	–	35	48	–	–	22 studies −1758 patients [80]
^177Lu and ^90Y DOTATATE	All types	–	–	27	52	–	–	15 studies [81]

In a retrospective analysis of 64,791 NET patients by SEER (Surveillance, Epidemiology, and End Results) program for the period 1973–2012, the median OS rate was found to 9.3 years (112 months) while for patients with the distant metastatic disease it was only 12 months [75], indicating toward low OS in metastatic NETs; when compared to various studies (Table No. 4) using PRRT, the OS survival have significantly improved after the introduction of PRRT in management.

Though all these mentioned different studies have enrolled different tumor subtypes, grades, pre-treatment received, dual PET tracer characteristics, all of which are as confounding factors for comparison, all have successfully shown the efficacy of PRRT over cold somatostatin analogues and placebo.

Though the dual tracer PET-CT is pivotal in therapeutic decision-making, only PET uptake in assessing therapy response and efficacy is insufficient, as such they are related to many micro-tumor biological factors such as receptor density, heterogeneity, different receptor subtype binding by different types of diagnostic peptides, thus not considered as the gold standard. Thus, along with PET uptake most of the studies for treatment responsehave relied upon anatomical criteria (RECIST 1.1), which adds further objectivity for such assessment.

When compared to GEP-NETs (which is the most common form of NET and more widely treated with PRRT), the data for other subtypes such as pulmonary NETs, mediastinal NETs, other uncommon forms such as CUP-NETs is less widely available at present. Some of these tumors show inferior response rates to PRRT. This may be due to their inherent tumor patho-physiological characteristics. Overall Broncho-pulmonary and Thymic NETs have poorer prognosis when compared to GEP-NETS. Yet PRRT stands as a prominent option in this class owing to its efficacy to stabilize the disease burden, suitable schedule, and toxicity profiles when compared to other therapeutic options.

A German study showed the longest survival in patients with midgut NETs (PFS 51 months, OS not reached), followed by pancreatic NETs (PFS 39 months, OS 53 months), and the shortest survival was observed in CUP-NETs (PFS 38 months, OS 47 months) [82].

Similar such smaller studies exclusively on pulmonary NETs, showed symptomatic response following PRRT in 79% patients (¹⁷⁷Lu-DOTATATE), a total of 63%-67% patients were characterized as CR/PR/SD and had median OS of 40 months [38,83]. Another study evaluating advanced or metastatic mediastinal NETs in 27 patients, who had undergone PRRT (with ¹⁷⁷Lu-DOTATATE), metabolic PET-CT response evaluation demonstrated partial metabolic response in 33.3%, stable dis­ease was seen in 18.5%, while 44.4% of the patients showed metabolic disease progress, with median PFS and OS being 36 months and 66 months respectively. The significant level of association of PFS was observed with surgical intervention, higher cumulative PRRT dose, metabolic response, smaller sized primary lesion, and low lesional FDG uptake; with respect to the OS, higher cumulative PRRT dose, low FDG uptake and longer PFS showed significant association [84]. PRRT has been usually considered at an advanced state of this subgroup of patients, commonly after chemotherapy failure could be partly responsible for its relatively inferior outcome when compared to the GEP-NETs (Figure 4a,b).

Figure 4. Favorable response to ¹⁷⁷Lu-DOTATATE PRRT. (a) Baseline ⁶⁸Ga-DOTATATE PET/CT scan (left panel MIP and fused PET/CT images) showed SSTR expressing multiple liver lesions, para-aortic nodes and lung nodules (Krennings score 4), she received 4# of PRRT (cumulative dose of 720 mCi) and showed resolution of almost all liver lesions, the other lesions also showed reduction in size in response evaluation ⁶⁸Ga-DOTATATE PET/CT scan (right panel MIP and fused PET/CT images). (b) Post-¹⁷⁷LuDOTATATE Therapy planar gamma camera images showing adequate accumulation of tracer in lesions (corroborating with pre-therapy ⁶⁸Ga-DOTATATE PET/CT) and physiological tracer accumulation sites such as kidney.

Note: A 57 years old female, case of well-differentiated NET grade II and Ki-67 index of 5%, who had progressive disease on octreotide therapy.

A recent prominent multicenter, retrospective cohort study of 508 patients with advanced G1 and G2 NET evaluating the role of upfront PRRT immediately after disease progression with SSA treatment showed improved PFS outcomes compared with upfront chemotherapy or targeted therapy longer PFS regardless of the primary tumor site, functional or nonfunctional status and grade. This makes the sequencing of PRRT even more strong [85]. Multiple groups have studied and put factors associated with a reduction in PFS and OS post PRRT patients were ascites, marked liver metastasis burden, unusual metastatic sites, age>65 years at the time of PRRT, bone metastases and peritoneal carcinomatosis [86,87].

Carcinoid Heart Disease: One of the advanced complications, generally seen in high volume functioning disease, typically observed in patients with bulky hepatic involvement. These tumors secrete a large number of vasoactive substances (mainly 5-hydroxy­tryptamine, tachykinins, and prostaglandins) that reach the right side of the heart through hepatic veins and lead to the deposition of fibrous tissue on the endocardial surfaces of the heart. This results in the pathogenesis of carcinoid heart disease. Though reported incidence is as high as 70% within patients of carcinoid syndrome, its incidence is now much reduced as a result of widespread use of synthetic somatostatin analogues (SSA). Administration of PRRTs (¹⁷⁷Lu-DOTATATE) in such patients with resistant disease, helped in substantial reduction of 5-HIAA levels and resulted in symptomatic improvement and improved health-related quality of life (from NYHA grade III at baseline to NYHA grade I after 4–6 cycles) and also enabled taking the patient for corrective valvular surgery [88] which would otherwise have shown high morbidity and mortality.

It has been proven in literature high Ki-67 index, high pre-therapy FDG SUVmax, bone marrow involvement at diagnosis, extensive skeletal involvement are markers of aggressive disease and eventually shows a poor response to both cold SSA and PRRT6 while the symptomatic response was observed in all cases using PRRT irrespective of Ki-67 index. The symptomatic control in~90% of patients and disease stabilization in~70–80% of its patients are the principal gains achieved through PRRT. The efficacy of therapy options like duo-PRRT, the combination of PRRT with other therapeutic options like chemotherapy and SSAs have been discussed in respectivesub-headings.

Two case examples, (a) duo-PRRT and (b) alpha radionuclide therapy with ²²⁵Ac-DOTATATE, are depicted in (Figures 5a,b & 6).

Figure 5. Duo-PRRT in large grade I pancreatic NET with metastatic liver disease.

A 75 years old female patient, presented with loose stools and diagnosed to have NET of pancreas. For 2 years she received octreotide but showed disease progression and abdominal discomfort. On follow-up, she was found to have large centrally necrotic pancreatic mass and small liver lesions on 68 Ga-DOTATATE and ¹⁸F-FDG PET/CT(from left to right MIP and fused PET/CT images) showed SSTR expressing pancreatic and hypodense liver lesions which are not FDG concentrating. Considering the size of the lesion he was considered with Duo PRRT protocol (¹⁷⁷Lu-DOTATATEand⁹⁰Y-DOTATATE).

Figure 6. ²⁵⁵Ac-DOTATATE PRRT in a case of metastatic NET.

A 65 years old male, case of grade II gastric NET with hepatic metastases, post 5# of ¹⁷⁷Lu-DOTATATE PRRT and 12 # of chemotherapy. The patient continued to demonstrate progressive disease on ⁶⁸Ga-DOTATATE PET/CT scan (Left panel MIP and fused PET/CT images) showing extensive SSTR expressing liver lesions, right sided images are ¹⁸F-FDG PET/CT scan (MIP and fused PET/CT images) showing few of the lesions are FDG concentrating. Middle set of images are post ²²⁵Ac-DOTATATE SPECT and SPECT/CT images wherein gamma emission spectrum of ²²⁵Ac and its daughters was used for the imaging, the image counts are related to poor imaging characteristics, though shows adequate concentration of tracer in liver lesions.

7. Discharge and follow up

Post-therapy imaging is done to evaluate tumor uptake and bio-distribution of administrated therapeutic radiopharmaceutical during PRRT. In case of ¹⁷⁷Lu-DOTATATE, planar/SPECT-CT gamma whole body scan with its imageable gamma photons(208 keV- 11% abundance and 113 keV-6.4% abundance).In patients with the abdominal disease only on baseline imaging, SPECT/CT of the abdomen only might be sufficient for imaging during the PRRT course [89].

For therapies with ⁹⁰Y-DOTATATE – imaging options like Bremsstrahlung imaging using gamma camera is usually performed. Positron imaging and Cerenkov imaging have also been used for imaging of ⁹⁰Y.In the case of ²²⁵Ac, imaging is done using γ-co-emission of ²¹³Bi (440-keV with 26% emission probability) and γ-co-emission of ²²¹Fr (218-keV with 12% emission probability).

As per guidelines, it is recommended to keep a check and record of renal and hematological parameters post-PRRT. A complete blood cell count and renal function tests (serum Creatinine) should be performed every 2–4 weeks for at least 2 months between PRRT cycles [35,43]; even higher frequency can be adopted based on the clinical requirement in some severe cases. After completion of all scheduled PRRTs, complete blood cell count, and renal and liver function tests should be performed every 8–12 weeks for the first 12 months, and thereafter twice a year if clinically indicated. Liver function tests in similar frequency are also required in a few cases. Those having blood values lower than limits in the first PRRT cycle should be noted, they should receive corrective treatment and reference whenever indicated and lower activity and/or the interval between the PRRT cycles should be extended (Figure 7).

Figure 7. PRRT in case of metastatic Medullary Thyroid Cancer.

Note: A 59 years old male, diagnosed to have Medullary Thyroid Cancer with skeletal metastases at presentation. During post-thyroidectomy follow-up, his serum Calcitonin level was raised upto 18,220 pg/ml. He was further evaluated with ⁶⁸Ga-DOATATE PET/CT scan (left sided image panel with MIP and fused PET/CT images) showeing multiple SSTR expressing (krennings score 2-3) skeletal lesions and nodal soft tissue deposits. The patient received 2# of ¹⁷⁷Lu-DOTATATE (middle panel showing post-¹⁷⁷Lu-DOTATATE therapy planar gamma camera images showing tracer concentrating lesions in multiple bones and nodal deposits). The response evaluation scans (right sided panel with MIP and fused PET/CT image) showed reduction in size and number of tracer concentrating skeletal lesions with>50% reduction in his Sr. Calcitonin value.

Assessment of renal function is vital, esp. in patients with preexisting risk factors for delayed renal toxicity like poorly controlled hypertension and diabetes mellitus, single kidney or previously documented renal insult such as nephrotoxic chemotherapy, more precise methods to assess renal function such as ^99mTc-DTPA scan (evaluates glomerular function – GFR measurement), ^99mTc MAG3 scan or ^99mTc EC (evaluates renal tubular function) scans are recommended. As these techniques are routinely performed in the nuclear medicine departments, many centersconsider them routinely at their pre-therapy examination.

8. Special scenarios and prrt

8.1. PRRT in Non-GEP-NET tumors

Multiple groups worldwide have successfully used PRRT in targeting non-NET tumors. The fundamental mechanism of targeting SSTR and the procedure almost remains the same in these extended indications also. For therapy administration, most tumors of neural crest cell origin and other tumors like radio-iodine refractory thyroid cancers have been targeted. As these types of tumors have limited treatment options available, especially in their metastatic and non-operable stages, PRRT does hold a strong potential. Few such commonly utilized indications are discussed herein:

8.1.1.

a) Medullary Carcinoma of Thyroid (MCT)

The tumors arising from parafollicular C cells of thyroid glands express SSTR and show positive results on SSTR-PET imaging in around 60% of metastatic cases where skeletal metastatic disease with higher serum calcitonin values has higher detection rates [90]. Similar to NET cases, the SSTR PET is a vital decision-making tool for PRRT.

In our experience in 43 patients who were positive on SSTR PET imaging, 26 showed a response (61%) and 17 were non-responders (39%) based upon PERCIST criteria, and 27 were responders (62%) while 16 patients were non-responders (38%) based upon RECIST 1.1 criteria. The univariate analysis showed a significant association between responses to PRRT with a size of lesions (<2 cm), and FDG uptake in lesions (SUVmax of<5). PRRT was well tolerated in all these patients without any major grade 3 or 4 toxicity. Another recently published systemic review 131 out of a population of 220 patients with metastatic MTC who were treated with PRRT revealed that, based on the biochemical response to the treatment in 145 patients,74 (51%)were responders while in 71 cases of PD were recognized. The radiologic response was evaluated among 134 patients of which 88(65%) were responders. Major side effects like grade 3 to 4 hematotoxicity and nephrotoxicity were transient and rare (1%) [91].

b) Metastatic Pheochromocytoma (PCC) and paraganglioma (PGL)

Unresectable and metastatic PCCs and PGLs show significantly compromised prognosis compared to their curable counterparts. ¹³¹I-mIBG has emerged as a powerful treatment option in this scenario but a small class of tumors which does not concentrate this tracer, cannot be targeted with it. In this class, PRRT can be considered depending upon SSTR status on PET imaging. Multiple groups have used different radionuclides for targeting them as Kolasinska-Ćwikła et alin their phase II trial used⁹⁰Y-DOTATATE in 13 advanced and nonresectable PGL patients with median OS was 65 months and 35 months of median PFS [92]. 83% of patients were responders radiologically. Another recent long-term analysis by Severi S et on 46 metastatic PCC and PGL patients treated with ¹⁷⁷Lu/⁹⁰Y DOTATATE reported 37 patients (80.4%) as responders at a median follow-up period of 73 months. Median OS in ¹⁷⁷Lu treated patients was 143 months while for ⁹⁰Y-DOTATATE patients 74 months. None of their cases showed Grade III/IV toxicity [93]. In our experience in corroboration with similar response rates with ¹⁷⁷Lu-DOTATATE PRRT, we have observed substantial and prolonged symptomatic relief from functional symptoms which were otherwise recurrent and poorly controlled on routine medications.

Few groups also used alpha radionuclide therapy (²²⁵Ac) in this spectrum. Yadav et al used ²²⁵Ac-DOTATATE and concomitant capecitabine (radiosensitizer) at 8-weekly intervals up to a cumulative activity of ~ 74 MBq in 9 PGL patients. The radiological response revealed partial response in 50%, stable disease in 37.5% with no grade III/IV toxic events noted [94]. Still, the practice will need larger and more elaborate studies to prove the exact efficacy of these powerful agents. A recent case report by Auerbach et al also described a case of progressive pheochromocytomatosis (pheochromocytoma developed as a result of seeding of tumor cells around the surgical bed due to intraoperative tumor capsule rupture and tumor cell spillage) despite surgical debulking, who underwent PRRT and this stabilized the pheochromocytomatosis progression [95].

c) Radioiodine refractory Thyroid Carcinoma and Tg elevated negative iodine scintigraphy (TENIS) (Figure 8)

Figure 8. PRRT in TENIS/Radio-iodine refractory thyroid cancer.

Note: A 75 years old male, known case of differentiated papillary thyroid cancer with nodal and lung metastases post 2 cycles of RAI therapy showed no tracer concentration on radioiodine scan but high serum thyroglobulin value. He was labeled as Radio-iodine refractory thyroid carcinoma (TENIS) at this point. Considering progressive disease he was scanned with ⁶⁸Ga-DOTATATE PET/CT (left panel with MIP and fused PET/CT images) to look for feasibility of PRRT as therapeutic option (right sided panel with MIP and fused PET/CT images) showed SSTR expressing nodal and lung lesions. He subsequently received 2# of ¹⁷⁷Lu-DOTATATE PRRT (middle panel showing post-¹⁷⁷Lu-DOTATATE Therapy planar gamma camera images showing tracer concentrating in multiple nodal and lung fields). His response evaluation scans (right panel with MIP and fused PET/CT images) showed reduction in size, uptake and number of tracer concentrating nodal and lung lesions.

These are a class of thyroid cancers that have either been not able to concentrate radioiodine or lost their ability to concentrate radioiodine over the period. This disease subgroup is a challenging condition and often continues to progress and has a guarded prognosis compared to the differentiated thyroid cancers. A fraction of these tumors may express adequate SSTR expression which can be examined in-vivo using SSTR-PET. Similar to other PRRT indications if uptake is adequate on primary as well as metastatic sites the disease can be targeted [96].

A recent review on157 such patients (who received ⁹⁰Y,¹⁷⁷Luand ¹¹¹In labeled PRRT) revealed that out of 91 patients whose radiological response rates were available, 48 (52%) were responders. The toxicity profile was similar to MCT patients [97].

In our experience with a pilot study of eight patients with a medial follow-up period of 34 months complete disappearance of symptoms was found in one patient (12.5%), whereas three patients (37.5%) showed partial improvement in symptoms after PRRT and four patients (50%) showed worsening of and appearance of new symptoms. Imaging response showed stable scan in two patients (25%) and progressive disease (PD) in six patients (75%), following a progression-free survival ranging from 7 to 16 months, when they were considered for tyrosine kinase inhibitors because of PD;thus, compared to GEP-NET, there is less percentage of obvious objective response to PRRT in patients withTENIS owing to their inherent aggressive nature. The low SSTR uptake especially in FDG concentrating progressive symptomatic tumors, is the primary reason for which this therapy is not widely employed in this group of patients. We also suggest after labeling them as TENIS, that this disease can be screened with SSTR PET and FDG PET for prognostification and future feasibility of PRRT and other therapy options. In our experience, only 15–20% only such cases show adequate uptake on SSTR PET making them amenable to therapy [98] (Figure 9).

Figure 9. Evaluation for PRRT in Metastatic Merkel Cell Carcinoma.

Note: A 65 years old male, known case of Merkel Cell Carcinoma post-resection of primary tumor from back, presented with multiple subcutaneous nodular lesions, mainly in neck region. Later, on histopathology they were confirmed for recurrence. The patient then underwent dual tracer PET (from left to right ⁶⁸Ga-DOTATATE and ¹⁸F-FDG MIP and fused PET/CT images in upper and lower panel respectively) showed SSTR expressing (Krennings score 2-3) and FDG avid multiple subcutaneous lesions.

In today’s era, TKIs have evolved as promising agents such and have become a mainstay of treatment in inoperable, progressive metastatic MCT and TENIS patients. However, the agents are not curative, and have demonstrated modest results primarily in prolonging the PFS by a few months and stabilizing the disease. Their long-term efficacy data and safety profiles are not yet available. In view of the fact that TKISs are associated with multiple dose-limiting toxicities with moderate response rates, radionuclide-based treatments like PRRT have been considered and investigated as a promising option in SSTR expressing MCTs and TENIS patients. The prospective trials comparing both therapies are lacking at the present. PRRT should not be considered in non-SSTR expressing tumors or tumors that have progressed on previous PRRT. Dual tracer PET-CT, with inputs from FDG-PET/CT plays a similar useful role in PRRT decision-making. Tumor lesions with highly FDG concentrating and minimal or absent SSTR uptake, and symptomatic, the progressive disease should thus be considered to appropriate TKI and not be considered for PRRT. As per our experience [5], few of these indications like MCT and TENIS, PRRT has been considered even though there was a lesser degree of uptake (Krenning score 2) on SSTR-based scanning, since alternative experimental therapies were either potentially toxic, only modest efficacious or expensive. Overall, this has emerged as a tolerable and efficacious treatment option in the aforementioned conditions [99].

d) Progressive or Metastatic Meningioma:

Though meningioma is usually considered benign and surgically curable, there is a subgroup which shows aggressive, recurrent, and metastatic disease. Considering the critical brain regions, these tumors become difficult to handle surgically and even with EBRT. Meningioma is known to have overexpression of SSTR2aand is many times detected incidentally on SSTR PET imaging [100]. This makes meningioma a possible target for PRRT.

In one study on 20 Meningioma patients, stable disease was noted in 50% and median OS was not reached at a median follow-up period of 20 months [101]. WHO I and II patients did better than WHO III patients, with stable disease after treatment in 100%, 57%, and 13%, respectively. SSTR expression on PET imaging was significantly lower in WHO III meningiomas compared to WHO I/II meningiomas.

A phase II trial by Marincek et al with 34 patients showed similar results with long-term stable disease in 65.6% and a mean overall survival (OS) of 8.6 years [102]. In another similar study EBRT and PRRT (¹⁷⁷Lu labeled with a mean dose of 7.2 Gy) together have been studied by Philipp et al in 10 patients with unresectable Meningioma and a long-term follow-up of 105 months, resulting in disease stabilization of 7 of the 10 patients with a median progression-free survival of 107.7 months (range, 47.2–111.4 months) vs. 26.2 months (range, 13.8–75.9 months) for the patients with progressive meningioma, with no chronic toxicity in any of patient [103].

Another group successfully tried intra-arterial administration of PRRT with a view to examine safety and increased efficacy with this approach [104]. In a case report PRRTwas successfully used in metastatic anaplastic meningioma which resulted in significant improvement in the patient’s quality of life and inhibition of tumor progression in this aggressive tumor [105]. Overall PRRT stands as a promising option where other options are surgery/EBRT are not possible or tumors refractory to previous treatments with proven PFS improvement especially in grade I and II refractory meningiomas [106].

8.2.

e) Miscellaneous SSTR expressing tumors:

As mentioned before, a wide variety of tumors express SSTR and it is theoretically feasible to target them with PRRT but no strong evidence is not yet available apart from case reports/series considering the rarity of the tumors.

8.2.1.

Merkel Cell carcinoma (MCC): is a similar type of skin tumor of Merkel cells present in the dermis. The origin and function of these cells have been thought to have a neuroendocrine origin, further SSTR expression and SSTR PET imaging in these tumors have been well established for disease mapping [107]. Few discrete cases report have successfully used PRRT in metastatic MCC where no obvious other treatment options are yet discovered apart from immunotherapy in recent years [108]. Our experience also corroborates with them, We have noted good tolerability, minimal side effects, and good disease control with radiological partial response in a few cases targeted with PRRT [39].

Currently, phase I and II trial is recruiting named ‘Targeted Therapy and Avelumab in Merkel Cell Carcinoma (GoTHAM)’ to evaluate the safety and anti-tumor activities of the novel combination of avelumab (an immunotherapeutic agent) with ¹⁷⁷Lu-DOTATATE or EBRT in patients with metastatic Merkel cell carcinoma (mMCC) [109]. This will gather strong evidence and experience to establish this practice in routine.

Another such SSTR expressing tumor is mesenchymal phosphaturic tumors which are known to secrete FGF-23 factor causing crippling paraneoplastic syndrome named tumor-induced osteomalacia (TIO), the patients usually present with skeletal symptoms only. Recently SSTR PET has emerged as an excellent and sensitive tool to find the tumor as commonly present presents with severe skeletal symptoms [110].

TIOs are handled surgically, but sometimes they recur or progress to metastatic forms and become inoperable. The other therapy options are not well established, importantly the patients suffer from skeletal pathology and become bedridden in a short course. Here PRRT can be successfully employed to address the systemic disease and the paraneoplastic syndrome. This therapy option has currently been studied in very limited centers, case by Nair A where there was persistent hypophosphatemia even after the amputation of limb and transient response to octreotide, the patient was administered four cycles of peptide receptor radionuclide therapy using ¹⁷⁷Lu, which showed moderate improvement of serum phosphorus levels [111]. Our experience in a couple of such cases conveyed similar results. One case with recurrent skull-based tumor non-operable and symptomatic disease targeted twice with ¹⁷⁷Lu-DOTATATE therapy owing to high-grade SSTR expression on PET [112]. The patient showed excellent symptomatic and partial biochemical response, followed by stable disease for several years; hence, PRRT can be considered quite efficacious than currently available medical therapy.

8.3. Neo-adjuvant PRRT

Surgery is the best curative option in patients with operable NETs, but many patients present quite late when the tumor is too advanced, and its total excision is generally considered impossible due to infiltration of other tissues and/or important blood vessels. Neo-adjuvant therapy is an intervention to decrease tumor size, changing it from inoperable to operable state and therefore enabling surgical intervention, has been commonly practiced in different malignancies using interventions like chemotherapy and radiation therapy. A similar approach has been thought of in the arena of NETs but systemic chemotherapy here only showed modest benefit in down-sizing the tumor. Other therapies such as SSAs or interferon-alpha can decrease symptoms of the disease and lengthen the time to tumor progression but don’t show in objective tumor response in most cases. When these SSAs attached with radionuclide, PRRT which carry isotopes to specific tumor sites and give tumoricidal effects eventually downsizing the tumor mass has been investigated as one of the promising options for neoadjuvant therapy in NETs.

Though no level one evidence is available in the literature to prove its actual efficacy, but multiple single-center studies and case reports that have used both ⁹⁰Y-DOTATATE and ¹⁷⁷Lu-DOTATATE in neo-adjuvant settings have registered modest success. The study by Sowa-Staszczak et al,on large 6 inoperable GEP NETs patients treated with ⁹⁰Y-DOTATATE (7.4 GBq/m² of ⁹⁰Y-DOTATATE in 4–5 cycles every 6–9 weeks). Disease stabilization was observed in four patients while 2 showed partial response [113]. In another Dutch study with 29 patients (having a borderline or unresectable pancreatic tumor or oligo-metastatic disease≤3 liver metastases) post-¹⁷⁷Lu-DOTATATE neo-adjuvant therapy cycles, the successful surgery was performed in 9 patients and significant improvement of PFS was noted in all treated patients [114]. A similar larger dataset by Parghane et al, with 57 patients also showed a similar response where 26% of patients became surgically operable and disease-free. They have suggested higher-end dose (ie, 200 mCi of ¹⁷⁷Lu-DOTATATE) should be administered, with a short time interval (8 weeks between 2 cycles) when compared to routine palliative protocol with higher intervals [115].

8.4. Sandwich/chemo-PRRT and PRCRT

In the selected subgroup of patients where dual tracer PET shows uptake in both SSTR and FDG-PET/CT metabolic imaging, combining chemotherapy with PRRT was thought to be rational approach and benefits more from rather than prescribing chemotherapy or PRRT alone. The dual tracer PET with NET-PET score divides the spectrum of NETs as follows:

1. Low Ki-67 index, which is usually positive on SSTR imaging with low/absent FDG uptake: which are suitably treated with SSA and PRRT.

2. Tumors with very high Ki-67 index, which are usually negative on SSTR-based imaging and shows high uptake on FDG PET: are treated with chemotherapy (CAPTEM if Ki-67 < 55% or platinum-based chemotherapy if Ki-67 > 55%)

3. An intermediate gray zone exists between the aforementioned two groups with the tumor demonstrating both high ⁶⁸Ga-DOTATATE and ¹⁸F-FDG uptake on dual tracer PET/CT: This is the target group to consider for this novel approach [116].

Presence of inter-organ heterogeneity on dual tracer PET in the same individual that is few lesions are SSTR expressing while other few are non SSTR expressing and FDG avid can also be treated well using this approach, the 68 Ga-DOTATATE avid lesions to be targeted by PRRT while FDG avid lesions theoretically would be more amenable to chemotherapy.

This approach is also backed by the fact that minimally cytotoxic doses of chemotherapy may induce a radiosensitizing effect in tumor cells, which eventually increases the tumor response after PRRT [117]. This effect can be explained by increased DNA damage and inhibition of DNA repair, inhibition of cell proliferation, tumor cell re-oxygenation, apoptosis, or synchronization of the cell cycle. Commonly used such agents are capecitabine, temozolomide, and 5-fluorouracil. Multiple studies have highlighted benefit and proven safety of this approach. A phase II study by Claringbold et al. studied toxicity and outcome of combined capecitabine and PRRT (¹⁷⁷Lu-Octreotate) in 33 patients – resulted in progression of only 6% of patients while the majority of patients showed stable disease (70%) therapy was well tolerated and resulted in tumor control and stabilization of disease in 94% of these patients [118]. Another such study by Parghane et al on 38 patients positive on dual tracer PET scans, they received at least 2 cycles of each PRRT and chemotherapy (between two cycles of ¹⁷⁷Lu-DOTATATE, two cycles of oral capecitabine and temozolomide (CAPTEM) were sandwiched) 45% patients showed anatomical imaging response with a disease control rate of 84%. An estimated PFS rate of 72.5% and OS rate of 80.4% were found at 36 months. None of the patients developed grade 4 hematotoxicity and nephrotoxicity of any grade [116]. In another retrospective study, Ballal et al evaluated the clinical efficacy of combined ¹⁷⁷Lu-DOTATATE with CAP over PRRT alone in advanced NETs. They suggested that combined ¹⁷⁷Lu-DOTATATE and CAP were highly effective in delaying time to progression and also improving OS in metastatic NETs [119] (Figure 10).

Figure 10. Sandwich Chemo-PRRT in case of grade II metastatic NET.

Note: A 65 years old male, presented with chronic abdominal pain and multiple liver lesions on USG. On liver biopsy he was diagnosed to have II grade NET. His baseline ⁶⁸Ga-DOTATATE PET/CT and ¹⁸F-FDG PET/CT images (left sided upper and lower panel MIP and used PET/CT images respectively) showed highly SSTR expressing (Krennings score 3-4) liver lesions of which few of them demonstrated FDG concentration. The patient received 2 cycles of ¹⁷⁷Lu-DOTATATE PRRT (post-therapy planar gamma camera scans in middle panel) and 4# of chemotherapy (CAP-TEM). His response evaluation (right sided panel) PET/CT scans showed reduction in number and size of SSTR expressing lesions and also significant reduction in FDG concentrating lesions.

One has to keep in mind that high incidence of toxicity may be seen in the combined therapy approach as compared to monotherapy with a higher prevalence of nausea (Grade 2–33%), thrombocytopenia (Grade 3–10%), and anemia (10%) [120]. This toxicity can be decreased by setting proper protocol spacing of both therapies, suggested as- PRRT followed by 2 cycles of CAPTEM followed by PRRT. CAPTEM regimen comprising of oral capecitabine (CAP), 750 mg/m² twice daily for 14 days (D1-D14) and oral temozolomide (TEM) 200 mg/m² once daily for 5 days (D10-D14) followed by two weeks rest period and another CAPTEM cycle given for total 28 days is followed by next cycle of PRRT at around 3 months [116].

In our initial experience of 38 patients, who had both SSTR and FDG positive aggressive disease, 73% of patients showed a symptomatic response, 39% showed biochemical, while 45% had anatomical imaging response with DCR of 84% based upon the RECIST 1.1 criteria. An estimated PFS rate of 72.5% and OS rate of 80.4% were found at 36 months. All of them tolerated this schedule well with none developing grade 4 hematotoxicity and nephrotoxicity of any grade, while grade 3 hematotoxicity was observed in 4–5% of patients. Figure 11 depicts the Sandwich Chemo-PRRT Protocol flowchart used in our setting. [37]

Figure 11. Sandwich Chemo-PRRT protocol flowchart used in our setting (Reproduced with permission from Basu et al [37].

In the ongoing prospective PReCedeNT trial from India which is recruiting patients progressive low grade NETs and G2/G3 NETs with disease positive on both FDG and SSTR PET/CT scans, patients being divided randomly into 2 arms as PRRT (¹⁷⁷Lu-DOTATATE) vs Chemo(CAPTEM)-PRRT will put strong evidence for rationalizing this practice in multidisciplinary treatment protocols and guidelines.

PRCRT-Peptide receptor chemoradionuclide therapy is usually done with concurrent 5FU chemotherapy usually done using ¹⁷⁷Lu-octreotate with concomitant 5-FU radiosensitizing infusional chemotherapy has been suggested by the Australian group. The dosage of 5-FU in PRCRT in this regimen has been (200 mg/m²/24 h), starting 4 days before the day of PRRT administration, and continued for 3 weeks in total and early discontinuation in the event of hand-foot syndrome or other acute toxicity. Long progression-free survival of 48 months in a cohort of 52 patients with clinical benefit in the imaging (objective response or stable disease) showed about 70% of patients; the biochemical response was observed in about 60% of patients [121].

Everolimus – an oral drug that inhibits the mammalian target of rapamycin (mTOR) has proven its efficacy in NETs [72,73], this in combination with PRRT (¹⁷⁷Lu-DOTATATE) has also been studiedrecently [122]. In this study of 16 patients were treated with four cycles of 7.8 GBq¹⁷⁷Lu-Octreotate every 8 weeks. Successive cohorts of three patients were treated with escalating doses of everolimus: 5, 7.5, and 10 mg daily for 24 weeks, the maximum tolerated dose of everolimus, in combination with ¹⁷⁷Lu-Octreotate was 7.5 mg daily as higher doses than doses were associated with high hematological and nephrotoxicity. Future larger studies are needed to prove its efficacy and its introduction in a clinical setting.

8.5. PRRT in combination with cold SSAs

The results of NETTER-1 trial where arm with PRRT with long-acting octreotide has proven its benefit, cold SSAs have proven its anti-tumor and disease stabilizing action – it has been routinely employed nowadays to interrogate both PRRT and cold SSAs for added gain from both modalities. A study by Yordanova et al has evaluated the survival and response rate of patients with GEP-NET treated with cold SSA as a combination therapy with PRRT and/or as maintenance therapy after PRRT. In this analysis, a significantly improved PFS and OS (91 months vs. 47 months) and lower death-event rates (26% vs. 63%) could be achieved with the combined treatment compared to PRRT alone [123]. This proves cold SSA as a combined therapy and/or maintenance treatment may play a significant role in tumor control in patients who underwent PRRT. Precaution to be taken in timely scheduling and interval stoppage of long acting SSA analogues in between PRRT cycles.

8.6. Duo-PRRT and Tandem PRRT

⁹⁰Y owing to its higher β⁻particle energy (Eβ⁻max of ⁹⁰Y is 2.28 MeV and mean energy of 0.94 MeV compared to that of ¹⁷⁷Lu Eβ⁻max is 497 keV)and tissue penetration length(⁹⁰Y have maximum soft tissue range 11 mm vs 2.5 mm for ¹⁷⁷Lu) proves more useful in bulky tumors when compared with ¹⁷⁷Lu, but this also has been proposed to contribute to higher renal toxicity from ⁹⁰Y. So the approach of combining both these agents as ⁹⁰Y is more suitable for larger lesions (especially>5 cm), while ¹⁷⁷Lu is most useful in eradicating smaller lesions, and at the same time balancing their toxicities is being studied in a handful of centers [119]. This approach of combination can be undertaken in two ways in clinical PRRT scheduling:

Duo-PRRT: ¹⁷⁷Lu-DOTATATE sequenced with ⁹⁰Y-DOTATATE e.g. baseline routine 2cycles of ¹⁷⁷LuDOTATATE at 10-12 weeks of interval – response assessment PET/CT evaluation – followed by 1 cycle of ⁹⁰Y-DOTATATE (4.4–4.48Gbq) – followed by 2 cycles of ¹⁷⁷Lu-DOTATATE and ⁹⁰Y-DOTATATE at 10–12 weeks interval in a similar manner [124,125].

Tandem PRRT: the combination of both ⁹⁰Y-DOTATATE and ¹⁷⁷Lu-DOTATATE (preferably in 1:1 proportion) as cocktail radio- pharmaceutical infusion simultaneously [126].

A higher response rate has been observed for both types of approaches when compared with ¹⁷⁷Lu/⁹⁰Y monotherapy in bulky and progressive tumors [107,124]. Study by Kong et al on 26 patients with bulky(>4 cm) and progressive disease − 79% of patients showed PR and in rest, 21% disease was stabilized on dual PET/CT scans. Median PFS was calculated to 33 months while OS was not achieved at 35 months which was superior to other options like sunitinib and everolimus. Overall, the therapy was well tolerated and produced lesser toxicity than ⁹⁰Y alone.

Similarly, the Polish multi-center trialwhich treated 103 NET patients with 4 cycles of 3.7 GBq of intravenous infusions of a mix of ⁹⁰Y/¹⁷⁷Lu-DOTATATE (1:1) at 6–12 weeks intervals, 78% of patients resulted in SD and 17% showed PR as per RECIST1.1. They showed PFS of 30 months and OS of 89 months [126]. The results were favorable when compared with ¹⁷⁷Lu-DOTATATE used with inj. Octreotide. Still, more evaluation using randomized controlled trials and dosimetric calculations is awaitedfor this approach (Figure 12).

Figure 12. Mediastinal NET on Chemo-PRRT. (a) the baseline (left sided panel) dual tracer PET imaging (⁶⁸Ga-DOTATATE PET/CT on top and ¹⁸F-FDG PET/CT in the lower panel) showed SSTR expressing (Krennings score 3) and FDG avid mediastinal mass, nodes, pleural deposits and duodenal lesions . He received 3# of 177LuDOTATATE PRRT (cumulative dose: 559 mCi), and 1 cycle of ⁹⁰Y-DOTATATE with dose of 102 mCi and CAPTEM of 8 cycles. He got symptomatically better, breathlessness got reduced. His Sr. chromogranin a was reduced to 520 ng/ml from 1440 ng/ml. His post therapy response evaluation dual tracer PET (right sided panel) showed some reduction in size and number of SSTR expressing and FDG avid lesions, rest of lesions showed stable disease. (b) the upper panel shows post-¹⁷⁷Lu-DOTATATE therapy planar gamma camera images, while bottom panel is showing 90Y-Bremsstrahlung planar gamma camera and SPECT/CT fused and corresponding plain CT images.

A 43 years old male presented with chest discomfort and breathlessness, the CT guided biopsy from mediastinal mass turned out to be Grade II NET with Ki-67 index of 10-12%.

8.7. Salvage PRRT/Re-challenge PRRT

Similar to other oncology settings, despite improved PFS and OS achieved by novel therapies like PRRT and molecular targeted therapies, many of the NET patients do not show complete response to therapy and may progress in due course. In this scenario where other options are not acceptable, PRRT has been thought of as a suitable option for disease control and symptomatic relief in this palliative setting. PRRT has definably many advantages compared to other therapies like excellent tolerability, suitable scheduling, long lasting effects, and lesser toxicities (especially haematotoxity). Several large oncology centers have considered this option forward in their multidisciplinary boards. The study by Van Essen et al reported on 33 such patients who progressed after initial stabilization of disease with PRRT, and received 2 additional cycles of ¹⁷⁷Lu-DOTATATE, though the effects were less favorable than for the initial therapy but the demonstration of antitumor effects with a progression-free survival of 17 months in such advanced stage of disease was gratifying [127].

Another similar study where 26 patients who relapsed after initial ⁹⁰Y-DOTATATE therapy received 4–5 cycles of ¹⁷⁷Lu-DOTATATE as re-challenge PRRT (median total activity of ¹⁷⁷Lu-DOTATATE PRRT of 16.5 GBq), disease control rate was observed in 85% of patients and median PFS of 22 months which was comparable even to primary ⁹⁰Y-DOTATATE therapy [128].

In our experience in a group of 22 patients, the median PFS after salvage PRRT was 17 months. The median OS was not attained after the initial course PRRT and salvage PRRT while the Estimated OS rate was 82% at 18 months after salvage therapy. None of the patients developed Grade 3/4 hematotoxicity, nephrotoxicity, hepatotoxicity, or AML/MDS after salvage PRRT at a median follow-up of 12 months respectively [129].

With much such evidence, salvage/re-challenge PRRT is considered a safe and effective approach. The centers have their different patient selection criteria and protocols but commonly 1–2 cycles of ¹⁷⁷Lu-DOTATATE are being tried by most centers followed by re-evaluation. Nuclear medicine physicians should be more watchful for patient’s clinical condition, lab parameters, analgesic, and other medication and PRRT should be administered only when clinical benefits out-weigh the risk associated with this progressed disease and patients should be well counseled on possible outcomes.

8.8. Intra-arterial PRRT

Liver being the most common site of metastasis in patients of NET, multiple liver directed therapies have been tried in the metastatic hepatic NETs. TACE and its modified form using radio-nuclides i.e. trans-arterial radio-embolization (TARE) which uses IR procedures and selectively administers radioactive isotope in a hepatic artery feeding the tumor has been commonly used in the treatment of HCC. Similar molecules have also been tried in patients with NET with liver-only metastasis. Intra-arterial administration of PRRT has been thought to give more efficacy than routine IV PRRT and TARE with other agents. Owing to its selective nature larger tumor dose can be achieved in less nephrotoxic doses. A German studyassessed the effect of such intra-arterial PRRT (⁹⁰Y/¹⁷⁷Lu Octreotate) in 15 liver dominant NET patients, complete response was observed in one (7%) patient, and partial response was achieved in eight (53%) patients. About 40% of patients demonstrated stable disease. The median PFS was not reached at 20 months. Due to its highly selective nature, no acute side effects or impairment of the renal function occurred in this study [14]. Another recent study studied PRRT in 4 surgery and radiotherapy refractory meningioma using intra-arterial (external carotid artery) ¹⁷⁷Lu-HA DOTATATE as salvage therapy, showed significantly increased tracer uptake compared to IV PRRT and also highlighted its safety with no angiography related or unexpected adverse events occurred in the small group. The response rate of the intra-arterial therapy was found comparable with responses after chemotherapy and intravenous PRRT [130]. Complex dosimetry procedures, costly and less commonly available IR support, patient scheduling issues are major limiting factors for using this therapy in routine clinical settings compared to IV PRRT. But this option can be offered to a patient who would greatly benefit from PRRT but renal parameters are not allowing PRRT administration.

8.9. PRRT in high-grade NET (G3-NET)

As mentioned in the earlier section, the dual tracer PET imaging approach is now considered an important disease assessment parameter in addition to histopathology/Ki-67 index in the NEN spectrum. It is not infrequent to encounter Krenning 3 and 4 grades of SSTR expression on SSTR based imaging in histologically G3NENs. A number of groups worldwide have evaluated the utility of PRRT in this subgroup. In one of the largest cohorts of 149 patients by Carlsen et al, NEN G3 population separated into three categories: NET G3, low-NEC (Ki-67 21–55%), and high-NEC (Ki-67 > 55%). The response rate for NET G3 and low NEC were similar (42–43%), but immediate disease progression was more frequent in low NEC than NET G3 (26 vs 7%). PFS was 19 months in NET G3, 11 months for low-NEC, and 4 months for high-NEC, whereas overall survival was 44 months for NET G3, 22 months for low NEC, and 9 months for high-NEC. As per this study, the progressive NET G3 can be targeted with PRRT and demonstrated promising response rates, disease control rates, PFS, and OS as well as toxicity in patients [131].

As per the review by Sorbye et al high-grade uptake on SSTR PET enables the potential for a theranostic approach by the PRRT and indicates a good prognostic factor for GEP-NEN G3 patients, reflecting a more well-differentiated neoplasm with less aggressive tumor growth. As per their comparison with other GEP-NEN G3 studies of different therapeutic modalities, the PFS and OS seem impressive as second- and third-line therapy, especially for the NET G3 and low-NEC groups. Importantly, the favorableresponses (RR, DCR, and PFS) from PRRT, despite the high number of patients with documented progressive disease, indicate that PRRT is an active treatment for these patients. Platinum-based chemotherapy as first-line for metastatic NET G3 has resulted in RR of 0–24%, PFS 3–5 months, and OS of 30 months, while PRRT given as mainly second or third line in NET-G3 showed RR 42%, PFS 19 months and OS 44 months – indicating a very important and potential benefit of PRRT in G3-NET patients. PRRT is one of the best tolerable therapies in this subgroup also with no reported high-grade toxicity [132].

Thus, an important question in PRRT decision making particularly for the G3 NEN is which patients to be offered therapy in this subgroup. From the available literature and theory, PRRT should be considered for all GEP NEN G3 cases with well-differentiated tumors (i.e. G3 NET) and may also be considered for other G3 NEN cases with low Ki-67 as long as the lesions demonstrate high uptake on SSTR-based PET-CT.

It is imperative that, in patients with low grade uptake on SSTR PET (less than Krenning score 2), the PRRT should be strictly avoided. Overall, the predominantly retrospective data available at present is not sufficient enough and upcoming prospective trials including NETTER-2 will help define the role and optimal scheduling of PRRT to improve outcomes of patients with WD-G3 NET [133]. This role is yet to be further extended using combinatorial treatments including chemotherapies and other targeted therapies (discussed in detail in the section, Novel Approaches to improve the efficacy of PRRT).

8.10. PRRT in children

NETs, although rare, do occur in children as young as 4 to 6 years and are often diagnosed late because of the lack of recognition of the signs and symptoms. As per the SEER database, the most common NET sites in the 0- to 29- year age group were lung, breast, and appendix with incidence rates of 0.6 per million, 0.6 per million, and 0.5 per million respectively [134]. Mostly they are sporadic but also occur as part of familial syndromes like MEN, VHL syndrome, NF-1, or tuberous sclerosis etc. As most of the studies using PRRT excluded children less than 18 years, the data is sparse compared to adult groups. In other tumor types such as neuroblastoma PRRT has been extensively studied and proven efficacious and safe in the pediatric age group.

As per phase I trial on⁹⁰Y-DOTATATE in 17 patients of age 2-25 years old with SSTR expressing different types of progressive NETs, administered in 3 cycles at 6-week intervals, co-administered with an amino acid infusion for renal protection. There were no dose-limiting toxicities and no individual dose reductions needed because of renal or hematologicaltoxicity [135]. The most frequent toxicity was reversible nausea in 70%, even in the presence of anti-emetics. 3 subjects had a partial response (PR), 5 had MRS, 5 had stable disease, 2 had PD. Dosimetry performed on subjects in the 1.85 GBq/m²/cycle cohort demonstrated an average 2.24 mGy/MBq dose to kidneys, similar to the dosimetry estimates in adults. This trial in hand with few other smaller studies concludes that PRRT is safe in children and young adults when given with renal protection, dosing should be done as per children body surface area and dosimetric calculations not exceeding a renal absorbed dose of 23 Gy (for ⁹⁰YDOTATATAE is recommended at 1.85 GBq/m2 in all children and at less than 1.73 GBq/m2 in young adults) [136]. Another similar phase I/II clinical trial using ¹⁷⁷Lu-DOATATE is expected to release results shortly which will further clarify dosimetric considerations in this age group of patients [137].

In summary, with properly selected and individually tailored settings, PRRT can be safely administered in children with progressive and metastatic NETs similar to adult.

9. Future perspectives

Nuclear medicine, and the domain of theranostics, is one of the most dynamically changing fields with multiple new targeted approaches, radionuclides and targeting molecules (like peptides, antibody fragments) that are getting introduced frequently. Such ongoing developments are described briefly below:

9.1. Newer radionuclides and radiopharmaceuticals

a) Multiple new radionuclide molecules which are more powerful and easily produced than routine ⁹⁰Y/¹⁷⁷Lu are being studied for their therapeutic potentialse.g.²¹³Bi,⁶⁷Cu,¹¹¹In,¹⁶¹Tb. Recently a phase I trial which treated 33 treatment-naive patients, using ²¹²Pb-DOTAMTATE [138]. ²¹²Pb has a half-life of 10.6hrs and emits imageable gamma photons 238 and 300 KeV. Another benefit of ²¹²Pb is ²²⁴Ra(half-life: 3.6 days) based generator systems which can selectively produce clinical-grade ²¹²Pb, Pb shows suitable radiochemistry and forms a stable complex with DOTA and TCMC chelators [139]. As per primary results from this study 83.3% of patients who could complete all 4 cycles showed objective response as per RECIST1.1, no grade 4 and five grade 3 adverse effects were reported (back pain, dysarthria, dyspnea, acute kidney injury). All patients showed improvement in the quality of life, reduction of pain, shortness of breath in the majority of subjects, with an increase of energy. Multiple other α-emitting agents like²¹¹At in novel delivery systems like polymer nanoparticles are also being studied at pre-clinical level waiting to be translated into clinical studies.

b) Auger electrons with minimum tissue penetration range with a potent cytotoxic effect have been tried in PRRT using ¹¹¹In but the use lagged when powerful agents like ⁹⁰Y got introduced. The major problem with auger electron therapy is the requirement of delivery system/internalization so radionuclide comes closest to the nucleus and problems in dosimetric estimations. But the approach is again getting into interest with the advent of newer delivery systems, cheap production methods, and computerized dosimetric models.

c) SSTR antagonists: Though most of current practice of PRRT is based on SSTR agonist (which shows excellent uptake characteristics with the internalization of molecules), recent developments in SSTR antagonists have demonstrated to have superior uptake and lower dissociation rates. In 1996, Bass et al found that the inversion of chirality at positions 1 and 2 of the octapeptide octreotide family, converted an agonist into a potent antagonist.Ginj et al found a significantly higher tumor uptake in mice injected with the somatostatin antagonist ¹¹¹In-DOTA-sst3-ODN-8 and ¹¹¹In-DOTA-sst2-ANT compared to agonists [140]. Dalm et al did a direct comparison in mice of antagonist and agonist showed not only a higher uptake but also a longer survival and better tumor control in mice treated with the antagonist ¹⁷⁷Lu-DOTA-JR11 [141]. Nicolas et al, in their phase I/II trial examining the diagnostic value of the antagonist ⁶⁸Ga-NODAGA-JR11, established safety, pharmacokinetics, dosimetry studies, and efficacy of the antagonist molecule [142]. In patients, the first pilot study demonstrated promising results with a 1.7–10.6 times higher tumor dose with the antagonist ¹⁷⁷Lu-DOTA-JR11 compared to ¹⁷⁷Lu-DOTATATE [143]. Based on this data,¹⁷⁷Lu‐DOTA‐JR11 is currently under evaluation in an international multi-center open‐label phase 1/2 study, involving 40 SSTR +ve NET patients. The main goal of this study is to investigate the safety and tolerability of ¹⁷⁷Lu-DOTA-JR11, and secondary goals are assessment of its biodistribution, dosimetry, and preliminary efficacy. In this study, ¹⁷⁷Lu-DOTA- JR11 is planned to be administered in 2 cycles (1850 MBq) at intervals of 8 weeks. Further evaluation of the diagnostic – therapeutic pair ⁶⁸Ga- NODAGA-JR11–¹⁷⁷Lu-DOTA-JR11 is warranted in larger-scale multicentre clinical trials as a theranostic approach in patients with GEP NETs and other NETs [144].

Similarly, antagonist molecule LM3 has been successfully labeled with ⁶⁴Cu (⁶⁴Cu-CB-TE2A-LM3) which showed almost no washout from the tumor up to 24 h after injection in pre-clinical study which is a promising finding for theranostic approach with sister therapeutic radionuclide⁶⁷Cu.Satoreotide tetraxetan i.e. ¹⁷⁷Lu-OPS201 another novel SSTR antagonist that is being studied in another open label phase I/II trial on metastatic SSTR expressing lung cancer patients. The patients were to receive up to 2 i.v. administrations of ¹⁷⁷Lu-satoreotide tetraxetan, with a range of cumulative dose of 9–12.9 GBqs of¹⁷⁷Lu-satoreotide tetraxetan, fractionated into 2 administrations 6 weeks apart [145].

d) Currently ongoing phase III prospective trial (COMPLETE) using ¹⁷⁷Lu-Edotreotide aka DOTATOC, similar octreotide analogue with different SSTR subtype binding affinities. This trial is randomized patients open-label, study to evaluate the efficacy and safety of n.c.a. ¹⁷⁷Lu-Edotreotide PRRT compared to mTOR inhibitor everolimus in patients with inoperable, progressive, somatostatin receptor-positive (SSTR+) GEP-NETs. The study is currently recruiting patients in 14 countries. As part of the study, 300 patients with progressive SSTR+ Grade 1 and 2 GEP-NETs are being randomized, of which 200 receive up to 4 cycles of n.c.a. ¹⁷⁷Lu-Edotreotide (7.5 GBq/cycle) every 3 months or until the diagnosis of progression, while 100 patients receive 10 mg everolimus daily for 24 months, or until the diagnosis of progression. The overall study duration per patient will be 30 months. The study will provide data for using the peptide DOTATOC, prolonged progression-free survival (PFS) in patients in the n.c.a. ¹⁷⁷Lu-Edotreotide arm vs. Everolimus – which will make PRRT evidence stronger and also will provide insights for safety, objective response rates, and overall survival after 5 years follow-up [146]. Another similar study which is currently ongoing that has planned to recruit 200 patients and randomize 1:1 between 6 cycles Lu-177-edotreotide vs standard of care from CAPTEM, everolimus or FOLFOX therapy. Patients will be followed up for PFS and OS [147,148].

will be folloup for PFS and OS

9.2. Novel approaches to improve efficacy of PRRT

[A] Improving tumor perfusion:

With differentiation status from grade I to grade III NENs, the profile of blood vessels changes from the highly organized network to un-branched and plump vessels [149]. This triggers tumor hypoxia state and develops drug resistance. Improved delivery of ¹⁷⁷Lu-octreotate via better tumor perfusion can increase the radiopharmaceutical uptake and thus the absorbed dose and efficacy of the treatment for the same administered radioactivity. Multiple chemotherapeutic agents and targeted therapies like sunitinib have been tried to increase the efficacy and radiosensitizer effect for EBRT. The use of chemotherapeutic or anti-angiogenic drugs like VEGF inhibitors to improve the delivery of PRRT and oxygen to NETs is being thought of as a promising strategy for potentiating PRRT that could be rapidly translated into clinic settings with these already approved drugs.

[B] Improving SSTR expression and radiosensitization:

As SSTR expression on the tumor cells is the most primary determining factor for PRRT, increasing its expression would be more effective at equal or lower administered activities of radiopharmaceuticals, with the limited additional risk of toxicity. Following are various approaches that have been discussed in the literature that will help to improve SSTR expression.

1. Chemotherapeutic agents: As discussed in previous sections on chemo-PRRT, therapy with Capecitabine, Temozolomide, Gemcitabine has shown to increase uptake and sensitivity of PRRT (studied primarilywith¹⁷⁷Lu-DOTATATE therapy).

2. Targeted therapies: Multiple targeted therapies especially mTOR inhibitors like everolimus, PARP inhibitors, NAMPT inhibitors have been studied in pre-clinical studies. With their anti-tumor mechanisms, these drugs also act as radio-sensitizers owing to their actions on various DNA-related repair actions and cellular growth pathways. These have been thought of as potential candidates to synergize the effects of PRRT and eventually enhance the therapeutic effects. Except for everolimus none of these agents have yet been introduced in the clinical setting for their routine use. HSP-90 inhibition pathway is similar path evaluated in NETs; in a pre-clinical study, synergy of Hsp90 inhibition with radiation was confirmed in NET tissues and inferred further can be evaluated in combination with PRRT [150].

3. Epigenetic modifiers: Multiple pre-clinical studies have shown a modification of SSTR expression using Epigenetic drugs like histone deacetylase inhibitors (proven to increase SSTR expression up to 6 times), valproic acid, etc.m [151,152]. Thus the combination of these drugs with PRRT is thought as a promising approach to improve PRRT. But most of the research on these drugs is still in pre-clinical status; especially toxicity parameters are yet to be established to translate it well in the clinical setting

4. Micro-RNAs- Discovery of micro-RNAs in the late 90’sopened multiple avenues in molecular biology in controlling gene expressions. This practice has been widely getting in the field of oncology. Such a study on SSTR in GH secreting tumor has studied down/up regulation of these SSTR receptors using miR-185 [153]. This has opened the possibility of micro-RNA associated SSTR up regulation in NETs which is thought to improve the efficacy of PRRT.

5. External Radiation - Radiation has been shown to up-regulate SSTR2 and increase uptake radiolabelled SSTR agonists in multiple pre-clinical studies [154,155]. The molecular mechanisms of this SSTR2 regulation by radiation are still unclear multiple governing factors are associated with this as receptor recycling and proliferation rates. This approach needs further clinical evaluation for validation and incorporation in the clinical protocol as combined therapy

6. Immunotherapy –In the last decade, immunotherapy has been employed as a major breakthrough modality in multiple malignancies including NETs. Besides its anti-tumor immune modulations, they have been shown to increase the killing potential of EBRT [156]. Thus, it can be foreseen, that PRRT in combination with immunotherapy mayhave promising potentialin selected patients including the class of poorly differentiated aggressive tumors, compared to immunotherapy as monotherapy.

The ongoing multi-center, randomized, open-label NETTER II trial which is evaluating efficacy and safety of Lutathera® (¹⁷⁷Lu-DOTATATE) in combination with long-acting octreotide (30 mg), given as first-line treatment in comparison to high-dose, long-acting octreotide (60 mg) alone in patients with high proliferation rate tumors (G2 and G3). In this study, PRRT is administered every eight weeks for a total of four doses, followed by three years of post-treatment monitoring. This trial after completion is expected to provide important inputs for standardizing PRRT practice in the spectrum of NET therapy management and guidelines [157].

Currently, there are a number of ongoing studies looking the efficacy of PRRT compared with other treatment options and to optimize treatment through combination therapy, different dosing strategies, or use of different radionuclides [158]. Multiple other therapies and approaches are now sequenced or combined along with PRRT in the management of NENs. In most NEN patients who receive PRRT and also other therapies, especially in younger patients and who have longer survival, there is a need for careful review of immediate and longer-range goals of treatment. This personalized decision-making, while planning and sequencing different therapies and placing PRRT appropriately, is a need of the hour and must be defined with long-term multi-center studies follow up as well as prospective comparative studies.

10. Expert opinion

In the last decade, PRRT made its successful way from experimental medicine to the clinical settings and therapy protocols of metastatic, progressive neuroendocrine tumors worldwide. But this role is further refining with the availability of more powerful radiopharmaceutical agents, advanced dosimetric techniques, and improving evidence in new avenues like neo-adjuvant approach before surgery, as combination therapies with multiple available chemotherapies and novel molecular targeted therapies.

This emerging evidence worldwide on PRRT as described in the above sections has established this novel targeted approach of PRRT to FDA approval and initial mention in a few major guidelines. NCCN guidelines have included the use of PRRT for lung NETs, thymic tumors, pheochromocytomas, paragangliomas, and NETs of unknown primary as well. Still, the paucity of evidence in prospective RCTs is lacking in defined subgroups and clinical algorithms.

We have been utilizing PRRT at our tertiary care center on varied indications. In our view, PRRT is a powerful precision medicine tool and is still being underutilized worldwide in multiple subgroups. This underutilization reflects both paucities of availability of nuclear medicine expertise and resources in a few places and also clinical dilemmas regarding when and whether to go for this unsealed radionuclide therapy approach due to lack of its strong position in yet to be updated major clinical guidelines. We strongly support its adequate utilization in a conjunction with other approaches whenever feasible, including chemo-PRRT in tumors with aggressive biology.

Dual tracer PET-CT (FDG and SSTR PET/CT) with scoring systems like NET-PET scoring has become standard practice in this disease. This imaging is highly sensitive, specific than other available modalities, and forms the basis of appropriate clinical decision making and theranostic approach. The dual tracer PET provides robust in vivo receptor and biology status in the whole body for the same receptors to be targeted with therapeutic radiopharmaceuticals – a perfect example of precision medicine. Additionally, personal dosimetry approaches being studied in a few centers, if progresses to become routine and user-friendly, will make PRRT a more efficacious, safe and individualized, and mighty practice of precision medicine [159].

Figures 13 and 14 are flow charts of proposed management protocol in NEN and non- GEP-NEN SSTR expressing tumors using dual tracer PET demonstrating potential areas of adoption of PRRT.

Figure 13. Simplified flow-chart demonstrating potential areas where PRRT may be employed in the management of NENs.

Figure 14. Flow chart depicting the applicability of PRRT in management of metastatic/inoperable non- GEP-NEN tumors.

A few pertinent points regarding day-to-day PRRT practice are enlisted below:

1. In scenarios like NETs presenting with much progressed and extensive metastatic disease with highly SSTR expressing lesions, planning PRRT upfront rather than a trial of SSAs should be considered considering the superior tumor-cell killing power of PRRT rather than somatostatin analogs like octreotide (Cold SSAs).

2. In the setting of aggressive/high-grade tumors which continues to demonstrate adequate SSTR expression, approaches like chemo-PRRT are extremely useful. This approach can also be of use in cases with inter-organ tumor heterogeneity and address issues like radio-sensitizing property of a few chemotherapeutic agents providing added benefits. However, further larger prospective studies with different agents must be sought for sequencing and establishing this appealing approach.

3. PRRT is not only limited to GEP-NETs and it can impart its therapeutic effects wherever there is adequate SSTR expression. SSTR PET imaging serves as the tool for finding the adequacy of this SSTR expression and clinical decision making. Most of these current indications like inoperable, metastatic, progressive MCTs, radioiodine refractory thyroid cancers, non-mIBG-concentrating PCCs-PCLs, meningioma and other miscellaneous tumors, can be considered in a clinically difficult situation. Not all of these tumors will be adequately SSTR expressing and hence PRRT should not be used as blanket treatment here. All these tumors must be evaluated by gatekeeper SSTR PET evaluation before any decision making and only if they show adequate SSTR uptake (routinely followed as more than equal to the liver), PRRT can be tried.

4. Compared to most other available therapeutic options (like chemotherapy/molecular targeted therapy) both in NEN and non-NENs, PRRT is better tolerated, has a convenient dosing schedule, and safer will the minimal incidence of severe side effects – this has made the PRRT popular amongst oncologists and the patients.

Emphasizing the unmet need for trained personnel, facilities, and resources worldwide to meet the ever-increasing need for PRRT and such targeted radionuclide therapies, is important to enhance the patient care and this would need adequate sensitization of multidisciplinary specialists, caregivers, guidelines, and policymakers handling different sides of patient care about this greatly promising targeted treatment option.

Article highlights

• Peptide Receptor Radionuclide Therapy (PRRT) is a type of endoradionuclide therapy that selectively targets SSTR receptors on various Neuroendocrine and related tumors (e.g. Medullary Thyroid Cancer, Radioiodine non-concentrating or refractory thyroid cancer, Paraganglioma, Pheochromocytomas, Meningiomas, etc.).

• The patients are screened using dual tracer PET/CT, i.e. SSTR-based PET-CT and FDG-PET/CT, and the therapy is suitably administered in advanced, metastatic or inoperable, progressive NETs (traditionally grade 1 and grade 2 showing high uptake in the lesions on SSTR-based imaging).

• PRRT with ¹⁷⁷Lu-DOTATATE has demonstrated efficacy in terms of stabilizing disease and increase in survival rates in multiple studies worldwide and has made its way from bench to bedside. The tumor selectivity of this therapy makes it very well tolerated and safe compared to other systemic options like chemotherapy.

• ¹⁷⁷Lu and ⁹⁰Y are the commonly used therapeutic radioisotopes for the PRRT, while multiple new options including ²²⁵Ac (alpha) radionuclide therapy as well as different types of peptides like SSTR antagonists are currently on the horizon.

• Multiple newer indications, approaches, and combinations with other therapies are being successfully evaluated such as the combination of chemotherapy and PRRT, the combination of two different radionuclides (such as duo – PRRT and tandem PRRT), intra-arterial PRRT, neoadjuvant PRRT, re-challenge/salvage PRRT, etc.

Declaration of interests

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.