Photodynamic treatment modulates various GTPase and cellular signalling pathways in Tauopathy

ABSTRACT

The application of photo-excited dyes for treatment is known as photodynamic therapy (PDT). PDT is known to target GTPase proteins in cells, which are the key proteins of diverse signalling cascades which ultimately modulate cell proliferation and death. Cytoskeletal proteins play critical roles in maintaining cell integrity and cell division. Whereas, it was also observed that in neuronal cells PDT modulated actin and tubulin resulting in increased neurite growth and filopodia. Recent studies supported the role of PDT in dissolving the extracellular amyloid beta aggregates and intracellular Tau aggregates, which indicated the potential role of PDT in neurodegeneration. The advancement in the field of PDT led to its clinical approval in treatment of cancers, brain tumour, and dermatological acne. Although several question need to be answered for application of PDT in neuronal cells, but the primary studies gave a hint that it can emerge as potential therapy in neural cells.

KEYWORDS:

• Photodynamic therapy
• photosensitizers
• GTPase
• neurodegeneration
• clinical application

Overview of photodynamic therapy: the principle and components of PDT

Photodynamic therapy (PDT) involves the use of photo-excited molecules against target cells or tissues. The main components of PDT are the photosensitizers and light source [1]. Photosensitizers are molecules that get excited on irradiation of specific wavelength of light leading to the generation of singlet oxygen species [2]. Several molecules have the potency of photo-excitation but all photo-excited molecules cannot be chosen for treatment. (i) The ideal photosensitizers should have a characteristic high quantum yield, and should be able to generate singlet oxygen. (ii) The localization of photosensitizer plays a crucial role in its selection, which should specifically localize to the target tissue. (iii) Photosensitizers should not produce dark toxicity to cells and upon treatment, should be released out of cell rapidly. (iv) The photosensitizers should be cost effective, easily available for the treatment of patients [3]. Various classes of chemicals have been reported to have the property of photo-excitation. The compounds containing tetrapyrrole ring such as heam, chlorophyll and bacteriochlorophyll are widely present in the nature. Derivative of these compounds such as Photofrin, Chlorin(e6), talaporfin sodium, LUZ11, Chloroaluminium sulphonated phthalocyanine (CASP), Silicon phthalocyanine (PC4) are the examples of commonly known photosensitizers [4–6]. Apart from natural compounds, synthetic compounds are reported to have photo-excitation properties. The phenothiazine dye methylene blue, toluidine blue, xanthene dye rose bengal, DIMPy-BODIPY, Zinc(II)-dipicolylamine di-iodo-BODIPY, transition metal complex iridium, rhodium, ruthenium are well-known photosensitizers [7–10]. The mechanism of PDT involves two steps. In the first step, the photosensitizer absorbs the energy and in the second step, the absorbed energy is transferred for the generation of singlet oxygen species. The excited state is short lived and it follows the intersystem crossing after which the triplet oxygen either follows type 1 or type 2 reaction. In type 1 reaction, the triplet oxygen species form free oxygen radical and in type 2 reaction, it directly gets converted into singlet oxygen species (Figure 1) [11].

Figure 1. The principle and component of PDT. PDT involves the excitation of photosensitizer (PS) after exposure of specific wavelength of light. In the excited PS either follows type 1 and type 2 mechanism after intersystem crossing. The excited PS generated triplet and singlet oxygen species return to ground state. The singlet oxygen species act on target tissues to produce the effect. The basic components of PDT involve the photosensitive dye (photosensitizer), the light source to irradiate the photosensitizer and dark chamber to avoid exposure of undesirable light.

PDT induces the cell death via activation of various pathways

Apoptosis is the programmed cell death, which includes series of events like nuclear fragmentation, shrinking of cell, and change in their morphology [12]. Generally apoptosis follows two pathways, intrinsic and extrinsic pathways, which include the formation of the death-inducing signalling cascades assisted by Caspases and other signalling molecules, e.g. BcL-2 [13]. On other hand, necrosis is the traumatic cell death induced due to cell injury. The exposure of cells to PDT induces the cell death by initiating apoptosis, necrosis, and autophagy (Figure 2). PDT treatment induces the release of cytochrome C from mitochondria, which leads to triggering of Caspase-9-mediated apoptotic response. Bcl-2 is a known anti-apoptotic protein and it is observed that PDT inhibits Bcl-2 activity and initiates cell apoptosis. Sinoporphyrin sodium-mediated photodynamic therapy was studied to induce apoptosis and the levels of cleaved caspase-3 and Bax protein were higher in PDT-treated cells [14]. Phenalenone mediated photodynamic therapy induced the apoptosis via activation of caspase-8 and p38‐MAPK (mitogen-activated protein kinase). Similarly, the oxidative stress generated by indocyanine-mediated PDT also induced the apoptosis [15]. Berberine is a photosensitizer, which is known to cause apoptosis caspase-3 activation [16]. The phosphindole oxide containing photosensitizers were observed to induce the endoplasmic stress in cells leading to initiation of apoptosis. The activation of tumour necrosis factor after PDT treatment led to induction of necrosis [17]. Further, RIP1 binds with RIP3 leading to formation of necrosome after the PDT treatment [18]. As PDT damages Bcl-2, it also helps to stimulate the protein such as Beclin1, Atg5 and Atg7 which ultimately leads to autophagy response in cells [19].

Figure 2. Induction of cell death after PDT. PDT is known to induce the cell death. PDT induces cell death via initiating apoptosis, necrosis and autophagy pathways. PDT upregulated the cytochrome C release from mitochondria, which leads to caspase-9-mediated apoptosis of cell. Similarly the ROS produces after PDT induces necrosis in the cells. PDT also causes inhibition of anti-apoptotic protein Bcl-2, which facilitates the apoptosis.

The effect of photodynamic therapy on GTPase

The proteins belonging to GTPase family have property to hydrolyse GTP (Guanosine triphosphate) to GDP (Guanosine diphosphate) [20]. GTPases are known to be involved in several cellular mechanisms such as signal transduction, differentiation, translocation, division, nuclear transport, etc. [20]. Thus, these small GTPases have crucial role in cell division, embryogenesis, cytoskeleton dynamics, neurotic development and cell proliferation [21]. The general structure of GTPase has three subunits alpha, beta, and gamma. The alpha subunit has the GTPase activity while the dimer of beta-gamma subunits has several distinct roles [22]. The binding of activator molecules leads to dissociation of GDP from alpha subunit with the help of GEFs (Guanine nucleotide exchange factor) and facilitates GTP binding. Whereas GAPs (GTPase-activating proteins) lead to release of GTP resulting in signal termination [23]. The modulation of GTPase signalling has been associated with various pathological conditions. Rho GTPase has been reported to be associated with several pathologies such as inflammation and cancer [24,25]. Rho is known to be involved in cell apoptosis, whereas the activation of Rac pathway elevated the cell survival. Rac GTPase activates the PAK kinase, which ultimately generates the survival signal via MAPK [25,26]. On contrary, Rho GTPase stimulates the ROCK pathways, which facilitates caspase-3-mediated apoptosis [27].

GTPase plays a key role in Cytoskeleton network

Rho GTPase is known to promote actin modulation in neurons. The Rho-mediated activation of LIMK induces the downregulation of cofilin-1, which results in polymerization of G-actin to F-actin [28]. The impairment of Rho GTPase leads to generation of several neurodegenerative disorders [29]. The studies on brain from AD patients suggested that Rac and PAK levels were depleted as compared to normal brain [30]. Similarly the aberrant increase of Rho GTPases were observed in the neuroblastoma cells, which leads to neurite retraction [31]. Gene silencing studies of topo IIβ resulted in downregulation of CDC42, Rac 1and upregulation of Rho A, which leads to Parkinson’s disease-like symptoms [30]. Similarly RhoA/ROCK, Rac1, and Cdc42 signalling cascades have been reported to be involved in various aspects of AD . Extracellular treatment of Aβ aggregates leads to localization of Rho A, which upregulated the phosphorylation of collapsin response mediator protein-2 (CRMP-2) in SH-SY5Y cells [32]. In myeloid precursor protein transgenic mice (J20) it was observed that the activity of caspase-2 downregulated due to the presence of amyloid aggregates. Rho GTPase plays key role in dendrites formation, the activity of Rho GTPase is modulated by Caspase-2. The studies suggested that the downregulation of caspase leads to reduced dendrites [33]. The studies carried on AD patients suggested that Rac and Cdc activity were downregulated whereas Rho was found to be co-localized with phosphorylated Tau [34]. Ran GTPase is the class which is known to be involved in the nucleocytoplasmic transport. It was observed that presence of Tau aggregates downregulated the expression of Ran leading to impaired nuclear transport [35]. Other neurodegenerative diseases such as Parkinson’s disease (PD), Huntington’s Disease (HD), are reported to be associated with deregulation of Rho and Rac pathways. The Rac activity was reduced in HD fibroblast cells, whereas the modulation of Rac activity was restored after lowering of hungtin in cells. In HD140Q/140Q primary neurons, the Rac activity was observed to be higher than wild type, which causes the upregulation of NOX (NADPH oxidase). The upregulation of NOX increased the production of reactive oxygen species [24]. The enhanced PAK expression enhanced the Htt aggregation in-vitro [36]. In PD, Rac plays a crucial role and rotenone is known to induce the PD like symptoms. The neurons exposed to rotenone were observed to have increased and decreased levels of Rho and Rac respectively [37]. LRRK2 (Leucine-rich repeat serine/threonine-protein kinase 2) phosphorylates several signalling protein and thus is known to be involved in processes like neuronal plasticity, autophagy, and vesicle trafficking, etc. The LRRK2 mutant SH-SY5Y cells observed to have retracted neurites. When Rac1 was overexpressed in cells the neurite retraction was rescued [38].

The involvement of deregulated of GTPase machinery in cancer

Cancer is another disease which has an extensive involvement of small GTPase as it has a major function in cell differentiation and migration [39]. The Rac GTPase and Rab35 are the common GTPases known to be involved in cell migration and proliferation [29,40]. Recent studies have demonstrated that the proliferation of cancerous cells is closely associated with the deregulation of Rab35. Hence, these small GTPases have been widely studied in various aspects of therapeutic strategies [41]. The recent studies have indicated the effect of PDT on various GTPase pathways (Figure 3). Talaporfin sodium is an improved photosensitizer belonging to second generation, which activated the Rho/ROCK signalling pathways resulting in blockage of tumour vessels [42]. The treatment of pancreatic cancer cells with alkyl modified cationic porphyrins downregulates the expression of KRAS and NRAS (Neuroblastoma RAS viral oncogene homolog) [43]. Ras/MEK is an important signalling cascade involved in cell proliferation and division. The inhibition of MEK improved the potency of PDT in cancerous cells. The HeLa cells exposed to 5-aminolevulinic acid photodynamic therapy (ALA-PDT) was observed to have modulated Ras/Raf/MEK/ERK pathways, which ultimately reduced the papillomavirus load via induced autophagy [19]. Similarly, the combinatorial PDT treatment of natural isothiocyanate ‘sulforaphene’ and photofrin reduced the cell proliferation of FRO (human anaplastic thyroid carcinoma cells) cells by modulation of Ras/Raf/MEK/ERK pathways [44]. The potency of photo-active radachlorin was tested on HEC-1-A endometrial adenocarcinoma cell lines, and the results of these studies suggested that PDT showed efficiency against cancerous cells by downregulation of signalling molecules including VEGFR2 (Vascular endothelial growth factor receptor 2), EGFR (Epidermal growth factor receptor) Ras homolog gene family/ member A (RhoA) [45]. The effect of PDT has been reported to be associated with the inhibition of GTP-GDP exchange. Various studies carried on GTPase suggested that PDT modulated the exchange of GTP-GDP. Pheophorbide A (PhA) and delta-aminolevulinic acid-mediated PDT are known to hamper the GTP-GDP exchange in cancerous cells [46]. The studies have been done in context of the role of PDT in modulating GAP (GTPase-activating proteins). It was observed that in the presence of TAT-RasGAP (317–326) the effect of mTHPC-mediated photodynamic therapy (PDT) was increased [47]. Rac1/PAK1/LIMK1/cofilin signalling plays a crucial role in regulation of the cell migration by effecting actin dynamics. Chlorin e6 (Ce6-PDT)-mediated PDT downregulated the Rac1/PAK1/LIMK1/cofilin signalling, which retarded the migration of colon cancer SW620 cells [48]. Similarly Ce6-PDT was observed to destroy actin network of colon cancer SW480 cells which could be a consequence of downregulation of Rho GTPase pathway [49]. These studies indicated that proteins belonging to GTPase family are behaving as a preliminary target for PDT.

Figure 3. The modulation of key GTPase after PDT. PDT effects various signalling pathways in cell. The Rho and Ras GTPase are among the key GTPase protein which are known to involve in several cellular pathways. PDT is known to modulate the exchange of GTP and GDP which caused alternation in Rho GTPase function leading to downregulation several cellular pathways such as cytoskeleton modulation. Similarly PDT downregulated the Ras GTPase pathways leading to inhibition of cell division and proliferation. Apart from Rho and Ras, PDT also effects several other GTPase signalling cascades which results in modulation of various cellular functions.

Photodynamic therapy in neurodegenera

Neurodegenerative disorders are characterized by progressive loss of neuron, memory deprivation, language dysfunction, impaired problem solving and thinking skills, motor nerves dysfunction. Parkinson’s disease, Huntington’s disease, Creutzfeldt – Jakob disease, Fronto-temporal dementia, and Alzheimer’s disease are examples of neurodegenerative disorders [50]. Neurodegenerative diseases have been reported to be associated with protein misfolding, oligomerization and accumulation of protein aggregates [51]. Aggregates of α-synuclein lead to generation of lewy bodies, which are considered as a cause for the progression of Parkinson’s disease. Several studies suggest that mutation in α-synuclein locus leads to generation of familial Parkinson’s disease [52,53]. Similarly, the misfolded prion protein leads to the generation of sporadic Prpsc (scrapie isoform of the prion protein), resulting in generation of Creutzfeldt-Jakob disease. The tri-nucleotide disorder such as Huntington’s disease involves the aggregation of Huntingtin protein [53]. Intracellular Tau aggregates and extracellular senile plaques are known to be hallmarks of Alzheimer’s disease. Alzheimer’s disease is the neurodegenerative disease characterized by progressive memory loss and behavioural impairments [54]. PDT is widely used for the treatment of microbial infections, carcinomas, and dermatologic lesions (Figure 4). Several queries have been raised for the application of PDT on neuronal tissues. In recent years, advanced studies and techniques led to the hope that PDT cold be used against neurodegenerative diseases (NDs). Several photosensitizers were tested for their efficiency against these pathological protein aggregates of NDs. Fullerene–sugar hybrid was designed and tested against Aβ aggregates of Alzheimer’s disease. Fullerene–sugar photo activated by 365 nm UV light potentially inhibited the aggregation Aβ peptides [55]. Similarly, Polyoxometalate irradiated with 365 nm UV light degraded the Aβ fibrils [56]. 3-(4′-trifluoromethylphenyl)-5-(4′-methoxyphenyl)-1,2,4-oxadiazole is another photosensitizer, which is reported to degrade Aβ fibrils and reduced the Aβ-mediated cytotoxicity in neuroblastoma cells [57]. Moreover, the photosensitizers having excitation in visible region are reported to have the potency against Aβ aggregates [58,59]. Tetra (4-sulfonatophenyl) porphyrin (TPPS), which is excited by blue light and Thioflavin T, are the compounds which are used against Aβ aggregation. The dyes such as Rose Bengal and methylene blue attenuated the Aβ aggregation upon irradiation with particular wavelength of light [60,61]. Additionally, the metal-based compounds such as Bismuth Vanadate and Zn phthalocyanine disaggregated Aβ aggregates efficiently. In recent studies, compounds having excitation in IR range are reported for their efficiency against Aβ. RB/UCNP@ROS and βNaYF4:Yb/Er@SiO2@RB are the compounds when irradiated with 980 nm of light lead to dissolution of Aβ aggregates [55]. Tau aggregation, which is a hallmark of several neurodegenerative diseases such as AD, progressive supranuclear palsy, corticobasal syndrome, pick’s disease, chronic traumatic encephalopathy, etc. [62–64]. In some reported studies it was suggested that the photo-excited dyes could have therapeutic potential against Tau aggregation. The photo-active phenothiazine dye, Toluidine Blue (TB), induces the disaggregation of Tau fibrils. TB was found to attenuate the aggregation of soluble Tau in-vitro. The in-vivo studies on Drosophilamodel suggested that TB and PE-TB treatment improved the cognation, learning and survival of Drosophila [62]. Similarly, the effect of xanthene dye Rose Bengal (RB) was studied for its potency against Tau aggregation. The in-vitro studies suggested that RB efficiently attenuated the aggregation of Tau, whereas the treatment of photo-excited RB disrupted the mature Tau aggregates (Figure 5). The exposure to RB and PE-RB improved the cognation and survival of UAS Tau E14 transgenic Drosophila [65]. Apart from these studies, dyes which have the potential for photo-activation have been reported to attenuate Tau and Aβ aggregation. Erythrosine B (ER) is a xanthene dye, which is used as food colouring agent. The dye has high lipid solubility and hence, can cross the blood-brain barrier. It was observed that ER modulated the aggregation of Aβ and helped in reducing Aβ-mediated toxicity [66]. Curcumin is the polyphenol isolated from Curcuma longa. Curcumin is known to attenuate the in-vitro aggregation of Aβ [67]. Although the studies are in preliminary phase but the data obtained from all the studies suggested that photodynamic therapy could emerge as a new strategy against neurodegenerative diseases.

Figure 4. PDT modulates various cytoskeleton elements. The cytoskeleton is one of the primary targets of PDT. In cancerous cells PDT targets the cytoskeleton components such as actin, tubulin, and integrin the destruction of these elements ultimately leads to induction of cell death. In neuronal tissues PDT observed to modulate the cytoskeleton. The increased neurite extensions after PDT evidenced the effect of PDT on tubulin whereas cell exposed to PDT had increased actin-rich structures.

Figure 5. PDT attenuates the Alzheimer’s disease-related amyloid aggregates. Extracellular amyloid beta and intracellular Tau protein aggregates are known to be closely associated with Alzheimer’s disease. Several photo-excited molecules such as methylene blue, rose Bengal etc dissolve the mature amyloid beta aggregates and rescues the cells from toxicity. Similarly, photo-excited toluidine blue and rose Bengal improves the viability of Tau treated cells and disaggregated the matured Ta fibrils in-vitro.

Photodynamic therapy in neuroglial activation

Microglial cells are specialized type of macrophages, which have protective functions in nervous system. The microglia cells perform continuous surveillance of neuronal tissues and provide the first line of defence against invading pathogens. In resting state, microglia possesses ramified or branched morphology whereas, after activation microglia acquire an amoeboid morphology [68–70]. Actin modulation plays a crucial role in migration of microglia. The actin-rich structures such as fan like lamellipodia and figure like filopodia assist the microglia in migration [71]. The microglia cells deposit near the site of injury and release the pro-inflammatory cytokines such as TNF-γ, IL-1β, TNF-α, etc which results in inflammation [72]. Microglia cells are known to be involved in homeostasis of neuronal tissues. The microglial specific receptors such as CX3CR1, CD11b, Iba1, and F4/80, P2ry12 and TREM2 are known to be involved in sensing, autophagy, inflammation, and neurodegeneration [73,74]. Microglia are known to clear the Aβ plaques from neurons and deregulation of microglial machinery leads to elevated amyloid beta deposits and Tau hyperphosphorylation [75]. Hence, it was observed that microglia has a wide functional aspect which could be further studied. In 1980s the photofrin-mediated PDT was first tested against glioma [76]. The oxidative stress generated by PDT led to apoptotic responses in neurons and glial cells. The studies carried on using THPC-PDT on rat neurons and satellite glia, compared with human adenocarcinoma cells (MCF-7) suggested that MCF-7s and satellite glia were more sensitive to PDT than neurons. The ALA-PpIX-mediated PDT improved the survival and reduced the inflammation in glioblastoma rat cells [77]. The PDT leads to generation of nitric oxide and various studies have evidenced that the NO generated after PDT leads to death of glial cells [78]. The study carried on crayfish suggested that the radachlorin-mediated PDT induced the autophagy response in sensory neurons of glial envelope. Another study on crayfish model indicated that PDT induces the nitric oxide stress in glial cells, which ultimately causes death of cells [79]. The glial death after PDT was studied to be associated with several signalling cascades such as phospholipase C/Ca2+, Ca2+/calmodulin/CaMKII, Ca2+/PKC, Akt/mTOR, MEK/p38, and protein kinase G etc. It was observed that alumophthalocyanine-mediated PDT reduced the release of natural neuroglial mediator N-acetylaspartylglutamate resulting in generation of apoptosis [80]. The treatment of phospholipid-conjugated indocyanine green (LP-iDOPE) in rat 9 L glioblastoma model enhanced the immune response by accumulation of CD8 + T-cells and HSP70, which helped in improving survival [81]. It was observed that the Photofrin-mediated treatment induced transient proliferation of microglia, which indicated the modulation on glioma microenvironment [82]. The evidence generated by various studies indicated that PDT effects glial cells significantly.

Photodynamic therapy in clinical use

The recent research advancements in PDT led to its clinical implication against several diseases. The PDT has been found to be effective against pre-cancerous lesions, non-myeloma skin cancers, skin infections, ache, and microbial infections (Table 1). The first clinical approval of PDT was reported in year 1993 which includes the application of Hematoporphyrin-derived photosensitizers [83]. Photofrin is one of the early photosensitizer that was used clinically for treatment of carcinomas. Photofrin was used in the treatment of head and neck tumours. Subsequently, UK, USA, and Canada approved porfimer sodium for the treatment of various carcinomas like lung and oesophageal cancer [84,85]. After 2000, various new photosensitizers have been approved for the clinical use such as Temoporfin and Talaporfin, which have been used for treatment of lung and oesophageal cancers [86,87]. Another class of photosensitizer, Motexafin Lutetium (MLu), was found to be effective on patient suffering from prostate cancer [88]. After the year 2010 the advancements in field of PDT led to approval of some more photosensitizers such as Hexaminolevulinate and Padeliporfin in USA, Sweden and Mexico for the treatment of prostate and oesophageal cancer.Verteporfin is another clinically approved photosensitizer which is used in the treatment of age related macular degeneration (AMD). In recent years, PDT has been used as a combinatorial therapy for treatment of cancer [89]. This combinatorial strategy helped to overcome several side effects of chemotherapy and radiotherapy. The application of platinum (IV) complex-based prodrug monomer (PPM) and 2-methacryloyloxy ethyl phosphorylcholine (MPC) observed to facilitate the drug delivery and helped in overcoming the problem of multidrug resistance in tumour [90]. Similarly, the combination of doxorubicin and chlorin e6 loaded (Dox@MSNs-Ce6) on silica nanoparticles efficiently entered A549 lung cancer cells and increased the potency of the drug [91]. In 2000, USA approved the use of ALA-PDT for the treatment of dermatological conditions. In 2003, ALA-PDT got the approval for the treatment of Acne [92]. Addition to ALA-PDT, MLA based PDT were also approved for clinical use in the treatment of dermatological problems such as actinic keratosis (AK) [93]. Additionally, PDT has also been used for the treatment of microbial infections such as streptococci and Lactobacillus (Figure 6). Hence, PDT has emerged as a potent therapeutic approach in several fields and the recent researches on PDT provides a strong hint that PDT application would be increased in all other clinical areas as well.

Figure 6. The clinical application of PDT against various disease. Several advancements have been made in field of PDT which facilitated its clinical application. The involvement of new generation of photosensitizer and combinatorial therapies have supported the use of PDT against several diseases which are showing promising results.

Table

S.No.	Name of photosensitizer	Target	Year of clinical approval
1.	Photofrin	Head and neck carcinoma and lung cancer	1993 in Canada, 1995 in USA, 1998 in UK
2.	Temoporfin	Oesophageal cancer	2001 in china
3.	Talaporfin	Lung cancer	2003 in Japan
4.	Chlorin e6	Ladder cancer	2006 in Russia
5.	ALA	Brain tumour	2017
6.	Padeliporfin	Prostate cancer	2018 in Mexico

PDT has advantages over classical therapies

PDT is emerging therapy in several areas. Various photosensitizers have been approved for clinical use and some of them are showing promising leads in pre-clinical trials. The advantages of PDT which gives it edge over other therapies include-

• PDT has a limited long-term side effects and the degree of side-effects is certainly low as compared to other techniques such as chemotherapy.

• Low degree of invasiveness- PDT treatment requires very less or no invasive surgery. In several cases, the topical irradiation showed the effect while surgery is required only for internal tissues present under the skin.

• PDT is cost effective and affordable therapy as compared to other techniques like chemo and radio therapy. The photosensitizer and irradiation are easy to setup thus the cost of treatment reduces to appreciable extent.

• Post PDT, the treated area recovered rapidly leaving very little scars.

• The therapeutic strategies such as chemotherapy or radiotherapy require multiple rounds of treatment while in PDT single exposure causes the desired effect on target tissues.

The challenges of PDT

• PDT can only be applied to cells which are present on surface since the irradiation of cells plays a challenging role for deep tissues.

• Another key drawback is that the PDT can only be applied to localized tissues. The metastatic tissues cannot be treated with PDT efficiently.

• The dark toxicity is one of the major concern, as the molecules applied for PDT are very sensitive to light thus it is very important for a patient not to expose to light after the treatment.

• The most important question arises when we wish to apply PDT in neuronal cells. The choice of photosensitizer which can cross blood brain barrier and the delivery of light to neuronal tissues are two major challenges arises in case of PDT.

• Another challenge is in choosing the photosensitizer which will not accumulate in body for long time. If the body will not be able excrete these photosensitizers they may lead to cytotoxicity.

Consent for publication

All authors consent to the publication.

Author contributions

TD and SC collected and reviewed the literature and wrote the manuscript. SC conceived the idea for the project, resource provided, supervised and wrote the manuscript. Both the authors read and approved the final manuscript.

Acknowledgments

Tushar Dubey acknowledges the fellowship from University Grant Commission (UGC), India. We thank Chinnathambi’s lab people for their fruitful discussion on manuscript.

Disclosure statement

The authors declare that they have no competing interests.

Additional information

Funding

This project is supported in part by grant from in-house CSIR-National Chemical Laboratory grant MLP101726.