Venetoclax in acute myeloid leukemia

KEYWORDS:

• Venetoclax
• AML
• therapy

1. Introduction

The cellular pathway of apoptosis is tightly regulated by a balance between pro-apoptotic and anti-apoptotic proteins [1]. One of the major regulators of the intrinsic pathway is the B-cell lymphoma 2 (BCL-2) protein, which was first identified through chromosomal mapping in follicular lymphoma. Contrary to other known oncoproteins facilitating cell growth, BCL-2 promotes oncogenesis through resistance to cell death. In vitro studies have shown that (BCL-2) overexpression is implicated in the survival of acute myeloid leukemia (AML) cells and their resistance to treatment [1], suggesting its potential targeting for antineoplastic therapy.

Venetoclax (formerly ABT-199) is a first-in-class, highly selective inhibitor for BCL-2, with a lower affinity for BCL-W and BCL-XL, a molecule crucial for platelet survival [1].

The introduction of venetoclax has changed the treatment paradigm of chronic lymphocytic leukemia (CLL) and, more recently, non-Hodgkin lymphoma (NHL), multiple myeloma (MM), and AML.

2. Venetoclax in monotherapy

In 2016, Konopleva et al. first explored the safety and efficacy of venetoclax monotherapy in relapsed refractory (R/R) AML. In this phase 2 trial, the investigators tested 800 mg/day oral venetoclax in 32 patients, obtaining an objective response rate of 19%, with 6% achieving a complete response (CR) and 13% achieving a CR with incomplete blood count recovery (CRi), often obtained during the first 4 weeks. Interestingly, among the 12 patients with IDH1/2 mutations, 4 (33%) achieved CR/CRi. Fourteen patients escalated to 1,200 mg daily venetoclax, but this dosage failed to demonstrate any additional activity. Treatment was generally well tolerated. The most common toxicities included nausea, diarrhea, and vomiting, while the most common grade >3 adverse events (AEs) were febrile neutropenia and hypokalaemia [2].

3. Combination strategies

3.1. Venetoclax in association with hypomethylating agents

Nowadays, the most widely used lower-intensity regimens of AML patients, ineligible for intensive chemotherapy, consist of DNA hypomethylating agents (HMAs), i.e. azacytidine or decitabine.

Regrettably, HMAs monotherapy yields low response rates (10–50%), with a median overall survival (OS) of less than 1 year [3], underlying the critical need to develop targeted therapies capable of improving these results.

In a large multicenter phase 1b dose-escalation and expansion study, newly diagnosed AML patients older than 65 years and ineligible for intensive chemotherapy received oral venetoclax in combination with either decitabine or azacytidine. With a median study time of 8.9 months, 67% of patients in the overall venetoclax dose cohorts achieved complete remission (CR). The median OS was 17.5 months for the whole patient population. However, it has yet to be reached for the 400-mg venetoclax cohort [4].

These encouraging data were confirmed in the randomized, placebo-controlled, phase 3 VIALE-A trial, evaluating the combination of azacytidine (5-AZA) plus venetoclax in older patients with treatment naïve AML, ineligible for intensive therapy. The combination showed an improvement in median OS (14.7 months vs. 9.6 months), more rapid and durable responses, and increased incidence of transfusion independence across different risk and molecular subgroups, including patients with secondary AML or TP53 mutation [5].

Another phase 2 trial enrolled elderly patients with treatment naïve AML not eligible for intensive chemotherapy, secondary or R/R AML to receive venetoclax with 10-day decitabine (VEN-DEC10), showing a manageable safety profile and high activity in both newly diagnosed AML and molecularly defined subsets of relapsed or refractory AML [6].

Recently, the same group compared VEN-DEC10 with intensive chemotherapy (IC) for fit and unfit patients; in both groups, VEN-DEC10 showed significantly higher CR/Cri incidence, lower 30-day mortality, and longer OS [7].

Recent data suggest that among different biological subtypes of AML, IDH1/2 mutated ones have the most significant benefit from this combination (ORR 72 vs. 60%, mOS 24.5 vs. 12.3) [8], while monocytic AML seems to be more resistant to this combination because of a lower expression of BCL2 [9].

The FDA and EMA approved the combination of venetoclax and HMAs as frontline treatment in AML patients who are ineligible for intensive chemotherapy.

3.2. Venetoclax plus low dose cytarabine

Preclinical studies showed that cytarabine could synergize with venetoclax in overcoming the resistance mechanism to the BCL-2 inhibitor [10]. This synergism was confirmed in a phase Ib/II trial where elderly treatment naïve AML patients received venetoclax 600 mg/day orally in 28-day cycles, with low-dose cytarabine (LDAC, 20 mg/m2 per day) administered subcutaneously on days 1–10. This study showed a manageable safety profile, producing rapid and durable remissions [11].

These data were confirmed in a phase III study in which 211 adult patients with newly diagnosed AML ineligible for intensive chemotherapy were randomized to receive a 2:1 ratio venetoclax (143 cases) or placebo (68 cases) in 28-day cycles plus LDAC on days 1–10. At the preplanned analysis, 40% (57/143) and 31% (21/68) of patients remained alive in the venetoclax and placebo arms, respectively, with a median OS of 7.2 vs. 4.1 months. Unplanned analysis with an additional 6-month follow-up demonstrated a median OS of 8.4 months for the venetoclax arm. CR plus CRi rates were 48% and 13% for the venetoclax and placebo arms, respectively. The most common grade ≥3 adverse events (venetoclax vs. LDAC alone) were febrile neutropenia (32% vs. 29%), neutropenia (47% vs. 16%), and thrombocytopenia (45% vs. 37%). Nowadays, the combination of venetoclax and LDAC is approved by the FDA as frontline treatment in AML patients who are ineligible for intensive chemotherapy. In Europe, this combination has yet to be approved [12].

3.3. Venetoclax plus intensive chemotherapy

In 2021, Di Nardo and colleagues reported the first experience of the association of venetoclax and intensive chemotherapy in the treatment of AML. In this Ib/II phase trial, venetoclax was combined with fludarabine, cytarabine, granulocyte colony-stimulating factor, and idarubicin (FLAG-Ida).

Phase Ib enrolled patients with R/R-AML using a 3 + 3 dose escalation and de-escalation algorithm to identify the maximum tolerated dose and dose-limiting toxicities. The phase II portion enrolled patients into two arms: treatment naïve (phase IIa) and R/R AML patients (phase IIb). Concerning toxicity, the most common grade ≥3 AEs included febrile neutropenia (50%), bacteremia (35%), pneumonia (28%), and sepsis (12%). Concerning the efficacy, the ORR was 75% in the Ib phase, 97% in the IIa phase, and 70% in the IIb phase. The CR rate was 75%, 90%, and 61%, respectively.

After a median follow-up of 12 months, the median OS was not reached in both phase II cohorts.

When venetoclax was combined with the FLAG-Ida regimen, 69% of newly diagnosed (ND) and 46% of R/R transitioned to allo-SCT. A 3-month landmark analysis in R/R-AML demonstrated improved OS with consolidative allo-SCT. After a median post-SCT follow-up of 9 months, 1-year survival rates were, respectively, 94% in ND and 78% in R/R settings; the death rate was 3% at both 30- and 60-day post-SCT [13].

A recent follow-up analysis, with a median follow-up of 20 months, showed that median EFS and OS were not reached. The estimated 24-month EFS and OS were 64% and 76%, respectively. The most common adverse events (AEs) were febrile neutropenia, pneumonia, bacteremia, and skin/soft tissue infections [14].

In another study, the addition of venetoclax to cladribine (CLAD)/LDAC alternating with 5-AZA demonstrated an effective regimen among older or unfit patients with newly diagnosed AML [15].

In a Chinese phase II study, the addition of venetoclax to 3 + 7 showed a CR rate of 91% after one cycle [16].

Combining venetoclax and CLIA (cladribine, high-dose cytarabine plus idarubicine) resulted in a safe and active strategy in patients with newly diagnosed AML or high-risk myelodysplastic syndrome, with 94% of composite CR [17].

Recently, Garcia et al. published the first report on venetoclax use in the setting of reduced-intensity conditioning chemotherapy allogeneic stem cell transplantation for AML patients. The study aimed to investigate the recommended phase 2 dose of venetoclax when added to fludarabine/busulphan in adult patients with high-risk AML, MDS, and myeloproliferative disorders undergoing a transplant. Neutrophil and platelet recovery and acute or chronic graft-versus-host-disease rates overlapped those of fludarabine/busulphan. No venetoclax dose-limiting toxicities were observed. The authors recommend venetoclax 400 mg daily for seven doses [18,19].

The role of venetoclax, either as the backbone approach preceding SCT or incorporated in the conditioning regimen or post-transplant therapy, needs further investigation and longer follow-up.

3.4. Venetoclax plus other biological therapy

Targeting BET family proteins could prevent AML cell proliferation inducing apoptosis by a double mechanism of action, i.e. BCL‐XL and potentially BCL‐2 downregulation and the BIM/PUMA upregulation [18]. Therefore, in a phase 1 study, the pan-BET inhibitor mivebresib has been used in R/R AML patients as monotherapy or in combination with venetoclax. Severe AEs occurred in 88% who received venetoclax in combination with mivebresib, with a higher efficacy than mivebresib monotherapy. The most common mivebresib-related treatment-emergent adverse events were dysgeusia, decreased appetite and diarrhea in the MIV-mono group, and decreased appetite, vomiting, and nausea in the MIV-Venetoclax group. Serious adverse events occurred in 14 patients (74%) who received MIV-mono and in 22 patients (88%) who received MIV-Venetoclax [20].

Desikan and colleagues conducted a single-center, open-label, phase 2 trial testing the combination of 5-AZA, venetoclax, and trametinib in patients with R/R AML harboring a RAS pathway-activating mutation showing unsatisfying results among 16 patients enrolled, only four patients responded. Two of the three patients who had not previously received HMA plus venetoclax responded; in contrast, only 2 of the 13 patients who had previously received HMA plus venetoclax responded. The median OS was 2.4 months, and the 6-month OS rate was 31%. All patients experienced at least one adverse event of any grade, and a grade 3–4 adverse event occurred in 14 of the 16 treated patients. Non-hematological grade 3–4 events included: pneumonia, febrile neutropenia, mucositis, heart failure, sepsis, and diarrhea [21].

Recently, venetoclax was combined with FLT3 inhibitors (FLT3I) in a phase 2 trial that enrolled 25 patients with R/R or treatment naïve AML, who received venetoclax, decitabine, and an FLT3I (midostaurin, gilteritinib, or sorafenib). The CR and 60-day mortality rates were 92% vs. 62% and 0% vs. 7% in treatment naïve and R/R, respectively. The most frequent grade 3/4 AEs were febrile neutropenia, infections with grade 3/4 neutropenia or average absolute neutrophil count, and tumor lysis syndrome [22]. This combination was further explored in an Ib/II phase study that tested the combination of venetoclax and gilterinib in patients with FLT3 wild-type and FLT3mut (escalation) or FLT3mut (expansion) R/R AML. This study demonstrates high CR and FLT3 molecular response rates, regardless of prior FLT3 inhibitor exposure. Dose interruptions were needed to mitigate myelosuppression [23].

Venetoclax was recently combined with the TRAIL receptor agonistic fusion protein, eftozanermin alfa in R/R AML, showing an overall response rate of 30% and 67% in patients with AML positive for death receptors DR4/DR5 expression [24].

The combination of venetoclax and idasanutlin (an MDM 2 antagonist) showed manageable safety in a phase 1b study and encouraging efficacy in unfit R/R AML patients. Common adverse events (occurring in ≥40% of patients) included diarrhea, nausea, vomiting, hypokalemia, and febrile neutropenia [25].

The most significant studies exploring venetoclax use in AML are summarized in Table 1.

Table 1. Most important studies exploring venetoclax in AML as single agent or in combination.

Authors	Disease status	Number of patients	Schedule	ORR (%)	CR (%)	Phase study	OS (months)
Phase I studies
Daver N [23]	R/R	61	Oral VEN 400 mg/day and GIL 80 or 120 mg/day in 28 days-cycles		18	I	10
Phase II studies
Konopleva M [2]	R/R	32	Oral VEN 800 mg/day	19	6	II	4.7
Di Nardo CD [6]	TN ineligible for IC, R/R	168	Induction: iv DEC 20 mg/mq on day 1–10 and oral VEN 400 mg/day Consolidation; in DEC 20 mg/mq on days 1–5 and oral VEN 400 mg/day + oral VEN 400 mg/day in 28-days cycles	74	61	II	18.1
Di Nardo CD [13]	IB: R/R IIA: TN IIB: R/R	IB: 16 IIA: 29 IIB: 23	IB: oral VEN 200 mg on days 1–21, 200 mg on day 1–14, 400 mg on day 1–14 in 28-days cycles IIA: iv FLU (30 mg/mq) and ARA-C (1.5–2 g/mq) on days 2–6, IDA (8 mg/mq) on day 4–6, filgrastim (5 mcg/kg) o day 1–7 and oral VEN 400 mgd/day on days 1–14 in 28-days cycles IIB iv FLU (30 mg/mq) and ARA-C (1.5–2 g/mq) on days 2–6, IDA (6 mg/mq) on day 4–6, filgrastim (5 mcg/kg) on day 1–7 and oral VEN 400 mg/day on days 1–14 IIA and IIB: Consolidation: FLU and ARA-C on days 2–4, filgrastim on days 1–5 and oral VEN 400 mg/day on days 1–7. idarubicin was permitted	IB: 75 IIA: 97 IIB: 70	IB: 38IIA: 69 IIB: 48	I–II	IB: 4.9 IIA: not reached IIB: 6
Kadia TM [15]	TN ineligible for IC	48	Induction: iv CLA 5 mg/mq on days 1–5, sc LDAC 20 mg/mq on days 1–10 and oral VEN 400 mg/day on day 1–21 Consolidation/maintenance: 2 cycles of iv CLA 5 mg/mq on day 1–3 and sc LDAC 20 mg/mq BID on days 1–10 alternating with 2 cycles of AZA 75 mg/mq on days 1–7, for up to 18 cycles and oral VEN 400 mg/day on day 1–21 in 28 day-cycles	-	77	II	Not reached
Wang H [16]	TN	33	Induction: iv daunorubicin (60 mg/mq) on days 1–3, iv ARA-C (100 mg/mq) on days 1–7 and oral VEN (100 mg on day 4, 200 mg on day 5, and 400 mg on days 6–11).	-	91	II	1y OS 97%
Kadia TM [17]	TN AML, HR-MDS, MPAL	50	Induction: iv CLA (5 mg/mq) and iv ARA-C (1,5 g/mq for <60 years, 1 g/mq ≥60 years) on days 1–5, iv IDA (10 mg/mq) on days 1–3 and oral VEN 400 mg/day on days 2–8 Consolidation: iv CLA (5 mg/mq) and iv ARA-C (1,5 g/mq <60 years, 1 g/mq ≥60 years) on days 1–3, iv IDA (10 mg/mq) on days 1–2 and oral VEN 400 mg/day on days 2–8	94	84	II	Not reached
Desikan SP [21]	R/R	16	Induction; sc or iv AZA 75 mg/mq on days 1–7, oral VEN on days 1–28 (100 mg on day 1, 200 mg on day 2, 400 mg orally on days 3–28), and oral trametinib 2 mg on days 1–28. Consolidation: sc or iv AZA 75 mg/mq on days 1–7, oral VEN on days 1–21, and oral trametinib 2 mg on days 1–28.	25	6	II	2,4
Phase III studies
Di Nardo CD [5]	TN ineligible for IC	431	Sc or iv AZA 75 mg/mq/day on days 1–7 and oral VEN 400 mg/day or placebo in 28-days cycle	66.4 vs 28.3	36.7 vs 17.9	III	14.1 vs 9.6
Wei HA [12]	TN ineligible for IC	211	Sc LDAC 20 mg/mq on days 1–10 and oral venetoclax 600 mg/day from day 4 in 28-days cycles or placebo	48 vs 13	27 vs 7	III	8.4 vs 4.1

Legend: ARA-C: cytarabine; AZA: azacitidine; CLA: cladribine; DEC: decitabine; FLU: fludarabine, GIL: gilteritinib; HR-MDS: high-risk myelodysplastic syndromes; IC: intensive chemotherapy; IDA: idarubicine; LDAC: low dose cytarabine; MPAL: mixed phenotype acute leukemia; R/R: relapsed or refractory; TN: treatment naïve; VEN: venetoclax.

4. Resistance to venetoclax

Unfortunately, resistance to venetoclax has been reported. Mutation in TP53 seems to be the major factor influencing response to venetoclax [26,27]. Consistently, clinical studies identified lower response and survival rates in patients with AML and TP53 alterations [5,12].

Mutation of BAX was found in 13.6% of a cohort of 44 venetoclax-resistant AML patient samples, while, contrary to CLL, in AML patients resistant to venetoclax, no mutations in BCL2 genes have been identified [28,29].

Moreover, a low expression of other key proteins involved in apoptotic regulation, such as BCL-XL or MCL-1, seems to correlate with the drug efficacy negatively; it has been hypnotized that the higher BCL-2/MCL-1 gene expression ratio in monocytic blasts could explain the resistance of these leukemic cells to venetoclax [9,30].

Recent studies suggested that mutations in genes involved in the mitochondrial molecules involved in the respiratory chain, ribosomal proteins, and heme synthesis could be responsible for venetoclax sensitivity [31–33].

An elevated nicotinamide metabolism, which activates both amino acid metabolism and fatty acid oxidation, was shown in relapsed stem cells’ resistance to 5-AZA/venetoclax suggesting that inhibition of nicotinamide phosphoribosyltransferase (NAMPT) could be a possible option to overcome venetoclax resistance [34].

Last year, Brett and colleagues reported that resistance to 5-AZA/venetoclax occurs via upregulation of fatty acid oxidation (FAO), either due to RAS pathway mutations or as a compensatory adaptation in relapsed disease, suggesting that targeting FAO could increase the sensitivity to venetoclax [35].

Moreover, preclinical models showed that THZ-P1–2, a pan-inhibitor of phosphatidylinositol-5-phosphate 4-kinase type 2 protein (PIP4K2s), induced cell differentiation and showed synergistic effects with venetoclax [36,37]

5. Expert opinion

Albeit a noteworthy improvement in outcome, AML patients disappointingly relapse or become resistant frequently, especially in the elderly age. Thus, new approaches are needed, as standard treatment has yet to be available for those patients. The landscape of AML treatment is constantly changing with the growing development of targeted therapy in frontline and relapsed refractory patients. In recent years, there has been an increasing shift toward combination strategies involving biological and targeted therapies. During the last years, preclinical studies have made it possible to better understand the biology of acute myeloid leukemia cells, thus allowing the development of targeted biological therapies that act on the altered intracellular signal pathways. Combination strategies involving biological molecules with different targets may have benefits compared to monotherapies, suggesting a synergism between different molecules and greater possibility of overcoming drug resistance. In this complex scenery, venetoclax-based combination strategies play a crucial role in R/R and treatment-naïve AML. Combined with 5-azacitidine, venetoclax doubled the 5-azacitidine response rate and significantly improved clinical outcomes in older adults with newly diagnosed AML. A similar scenario was seen with the combination of venetoclax/LDAC. The success of venetoclax combination strategies in unfit patients suggests that this drug’s potential role must be further explored in combination strategies for young and fit patients.

Further studies should investigate additional combination strategies and more appropriate venetoclax scheduling, as well as in different molecular settings, such as FLIT3 wild-type AML, in which gilteritinib combined with venetoclax leads to MCL-1 proteasomal degradation, thus overcoming venetoclax resistance [23].

The problem that is now arising is to understand which patients are more likely to benefit from combination strategies with venetoclax, and what are the best combination strategies to avoid the occurrence of venetoclax resistance.

Declaration of interest

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.

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Funding

This paper was not funded.