The use of venetoclax in combination with hypomethylating agents (HMAs) has changed the paradigm for the treatment of acute myeloid leukemia (AML) in older patients and those unfit for intensive chemotherapy—a population that historically had extremely limited therapeutic options.
Before the approval of the venetoclax-HMAs combination, treatment options typically included HMAs (azacitidine or decitabine), low-dose cytarabine, or supportive care. Venetoclax was first explored in AML as a monotherapy in relapsed or refractory disease, and the combination of venetoclax plus standard-of-care HMAs was evaluated to potentially improve upon its efficacy. These studies showed a significantly higher response rate with venetoclax plus HMA compared with venetoclax monotherapy and, based on these data, the FDA approved the combination for the treatment of AML in patients older than 75 or with comorbidities precluding intensive chemotherapy.
While venetoclax-based therapies in AML have been successful, many questions remain. This article reviews the proposed mechanisms of action of venetoclax in AML, predictors of response to venetoclax-based therapies, disease features in patients who relapse after venetoclax plus HMA therapy, and the potential future role of venetoclax in AML.
Mechanisms of Action of Venetoclax Plus HMAs
Venetoclax inhibits BCL2 activity by displacing protein interactions at the BH3 domain of BCL2, leading to increased apoptosis. This activity has been leveraged in other non-myeloid malignancies, including multiple myeloma, acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL), and initial studies have suggested that targeting of the canonical antiapoptotic function of BCL2 contributes to the mechanism of venetoclax in AML.
The BCL2 family of proteins are classified into 4 main groups: suppressors, activators, effectors, and sensitizers. By disrupting the BH3 domain, venetoclax releases the activator and effector proteins BIM, BAX, and BAK to execute their proapoptotic function. These proteins oligomerize and cause mitochondrial outer membrane permeabilization (MOMP) leading to cytochrome C release and apoptosis.
One proposed mechanism of venetoclax plus azacitidine activity in AML is through decreased amino acid transport to fuel the tricarboxylic acid (TCA) cycle, leading to decreased OXHPOS and eventual leukemia stem cell (LSC) death. This LSC-directed mechanism may be responsible for the durable responses seen with venetoclax + azacitidine. (Figure 1 illustrates the proposed mechanism of action of venetoclax plus HMAs.)
In both cell lines and primary human AML cells, venetoclax and HMAs have been shown to induce reactive oxygen species (ROS). In response to increased ROS levels, Nrf2 is activated to upregulate the expression of genes involved in neutralization of ROS as a protective mechanism. In AML cells, HMA activates Nrf2, most likely as a response to increased ROS. However, venetoclax treatment abrogated activation of Nrf2, resulting in cell death in both AML cell lines and primary human samples, suggesting that modulation of ROS is another mechanism of venetoclax plus HMA–mediated AML cell killing.
In some contexts, venetoclax plus HMA has been shown to modulate BH3 priming and subsequent apoptosis in AML. Additional studies have suggested venetoclax plus HMA therapy has a role in modulating T-cell–mediated killing of AML cells. Preclinical studies have suggested that there is a unique synergy between venetoclax and HMAs that results in superior outcomes for patients compared to monotherapy with either agent.
Predictors of Response to Venetoclax Plus HMAs
Initial response rates to venetoclax plus HMA range between 60 and 70%, meaning that upfront resistance occurs in 30 to 40% of patients. It is crucial to determine factors that predict sensitivity to this regimen at baseline, so that future studies can target these deficiencies by adding to venetoclax plus HMA backbone therapies or developing non–venetoclax-containing regimens.
Response prediction in AML has been largely defined by studies involving predominately younger patients receiving intensive chemotherapy regimens. It should therefore not be surprising that those factors are not necessarily predictive in older patients receiving a therapy with a different mechanism of action.
Initial analyses have suggested several gene mutations are associated with higher response rates with venetoclax plus HMA, including IDH1, IDH2, NPM1, and spliceosome mutations. Specifically, patients with these mutations had complete remission (CR)/CR with incomplete hematpoietic recovery (CRi) rates better than those observed with the phase III study benchmark of 66.4%.
Conversely, gene mutations associated with upfront resistance to venetoclax plus HMA have been postulated based on mechanisms of action, including TP53 and RAS pathway mutations, but these were either not evaluated or not significant predictors of resistance in phase III studies.
Interestingly, initial studies showed protein expression of BCL2 in primary AML samples did not correlate with venetoclax sensitivity. Rather, dependence on certain apoptotic family members (BCL2 or MCL1) correlated with venetoclax sensitivity.
Finally, patients with AML from an antecedent myeloproliferative neoplasm have been reported in case series to have suboptimal response rates; such patients were excluded from the phase III venetoclax plus azacitidine study, and more work is required to understand whether this observation persists with more robust clinical data.
Relapse Following Venetoclax Plus HMAs
While most patients respond to venetoclax plus HMA, most responders ultimately progress. Identifying the mechanisms behind relapse are crucial to improving outcomes for these patients.
Monocytic differentiation is one of the strongest predictors of refractory disease but, intriguingly, monocytic disease features have also been shown to be enriched in patients who relapsed on therapy, suggesting that this factor predicts poor response to and relapse after venetoclax.
Further analysis of monocytic AML cells derived from primary human patient samples revealed they have decreased expression of BCL2 and increased expression of MCL1. Inhibition of MCL1 by siRNA or an MCL1 inhibitor led to decreased OXPHOS activity, viability, colony formation, and engraftment in the LSC population. Other studies have also supported targeting MCL1 to circumvent venetoclax resistance in AML cell lines and patient samples.
In addition to direct targeting of MCL1, multiple studies have shown MCL1 can be targeted by inhibiting upstream pathways including MAPK signaling and XPO1. Taken together, these data suggest venetoclax resistance can be overcome by MCL1 inhibition. Other mutations have been shown to confer resistance to venetoclax plus HMA in AML, such as inactivation of TP53 and RAS mutations.
As venetoclax plus HMA–mediated changes in mitochondrial biology are crucial to the mechanism of action of this therapy in AML, it is plausible that resistance is mediated by changes in these pathways. Specifically, a CRISPR screen in venetoclax-resistant AML cell lines showed targeting mitochondrial translation was a potential strategy to overcome resistance.
Treatment of venetoclax-resistant cells with either tedezolid or doxycycline, antibiotics that inhibit mitochondrial protein synthesis, led to resensitization to venetoclax. In addition to FAO-mediated resistance, nicotinamide metabolism was also shown to modulate venetoclax plus HMA resistance in primary AML specimens, and genetic and pharmacologic inhibition of the nicotinamide metabolism pathway led to resensitization to venetoclax plus HMA. These all represent potential ways to overcome resistance to venetoclax-based regimens and prevent or delay relapsed disease.
Venetoclax-Based Regimens in the Relapsed or Refractory Setting
Most clinical trials evaluating venetoclax have focused on older, previously untreated, newly diagnosed patients. However, venetoclax-based regimens have clinical activity in the relapsed or refractory setting as well (Table 1), albeit more modest than what has been observed in treatment-naïve patients.
Therefore, understanding how this regimen can be modified to improve outcomes for these patients is crucial. Pharmacodynamic studies have suggested longer exposure to decitabine with venetoclax may lead to superior outcomes in patients with high-risk AML. For example, in a trial of 55 patients treated with decitabine for 10 days in conjunction with venetoclax, the overall response rate was 62% and patients who responded and were able to undergo successful stem cell transplant had a median overall survival of 22 months.
Furthermore, a retrospective analysis of clinical and molecular characteristics of 86 patients with relapsed or refractory AML revealed certain mutations conferred sensitivity or resistance to venetoclax plus HMA. Similar to treatment-naïve patients, mutations in NPM1 correlated with higher response rates, while mutations in TP53, RAS, and SF3B1 conferred resistance. More work is necessary to identify which relapsed or refractory patients may maximally benefit from venetoclax-based therapies.
The Future of Venetoclax-Based Therapies in AML
Currently, there are more than 80 clinical trials recruiting AML patients for venetoclax-based therapies. One potential strategy is to add additional targeted therapies to venetoclax-based regimens, including those designed to inhibit IDH, TP53, and FLT3 mutations.
Other agents being combined with venetoclax target various pathways involved in proliferation, cell survival, immune regulation, and metabolism. Before the field widely adopts the addition of therapies to the venetoclax plus HMA backbone, the ability to predict outcomes with the current regimen and make judicious and conservative changes at the individual patient level is necessary. For example, a significant population of patients has very durable responses to venetoclax plus HMA alone; these patients should be identified at baseline, as they would minimally benefit from additional up-front therapies, and any additional toxicity contributions would be unnecessary and unfortunate.
Conversely, patients likely to be refractory to venetoclax plus HMA should similarly be identified at diagnosis, and these patients should be candidates to receive additional therapies to overcome resistance, or novel non venetoclax-containing regimens.
Patients who achieve remission but are at risk of relapse, who can perhaps be identified by a metric such as measurable residual disease at some accepted post-remission timepoint, should have additional therapies introduced to prevent disease recurrence.
Finally, expanding the use of venetoclax plus HMA beyond the current labeled indication for some newly diagnosed patients may also occur in the future. Venetoclax-based therapies might also have a role in younger patients with disease features that predict adverse outcomes with standard chemotherapy. This approach is being explored in clinical trials, both single-arm and randomized designs (NCT03573024, NCT04801797), and are awaited with anticipation.
The development of venetoclax-based therapies has significantly improved outcomes in patients who have historically had poor outcomes. Continuing to understand how venetoclax modulates AML biology will be crucial in developing new therapies and to maximize success for patients.
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