Senolytic-Mediated Elimination of Head and Neck Tumor Cells Induced Into Senescence by Cisplatin.

Therapeutic outcomes achieved in head and neck squamous cell carcinoma (HNSCC) patients by concurrent cisplatin-based chemoradiotherapy initially reflect both tumor regression and tumor stasis. However, local and distant metastasis and disease relapse are common in HNSCC patients. In the current work, we demonstrate that cisplatin treatment induces senescence in both p53 wild-type HN30 and p53 mutant HN12 head and neck cancer models. We also show that tumor cells can escape from senescence both in vitro and in vivo. We further establish the effectiveness of the senolytic, ABT-263 (Navitoclax), in elimination of senescent tumor cells after cisplatin treatment. Navitoclax increased apoptosis by 3.3-fold (P ≤ 0.05) at day 7 compared with monotherapy by cisplatin. Additionally, we show that ABT-263 interferes with the interaction between B-cell lymphoma-x large (BCL-XL) and BAX, anti- and pro-apoptotic proteins, respectively, followed by BAX activation, suggesting that ABT-263-induced apoptotic cell death is mediated through BAX. Our in vivo studies also confirm senescence induction in tumor cells by cisplatin, and the promotion of apoptosis coupled with a significant delay of tumor growth after sequential treatment with ABT-263. Sequential treatment with cisplatin followed by ABT-263 extended the humane endpoint to ∼130 days compared with cisplatin alone, where mice survived ∼75 days. These results support the premise that senolytic agents could be used to eliminate residual senescent tumor cells after chemotherapy and thereby potentially delay disease recurrence in head and neck cancer patients. SIGNIFICANCE STATEMENT: Disease recurrence is the most common cause of death in head and neck cancer patients. B-cell lymphoma-x large inhibitors such as ABT-263 (Navitoclax) have the capacity to be used in combination with cisplatin in head and neck cancer patients to eliminate senescent cells and possibly prevent disease relapse.

Coamplification of miR-4728 protects HER2-amplified breast cancers from targeted therapy.

HER2 (ERBB2) amplification is a driving oncogenic event in breast cancer. Clinical trials have consistently shown the benefit of HER2 inhibitors (HER2i) in treating patients with both local and advanced HER2+ breast cancer. Despite this benefit, their efficacy as single agents is limited, unlike the robust responses to other receptor tyrosine kinase inhibitors like EGFR inhibitors in EGFR-mutant lung cancer. Interestingly, the lack of HER2i efficacy occurs despite sufficient intracellular signaling shutdown following HER2i treatment. Exploring possible intrinsic causes for this lack of response, we uncovered remarkably depressed levels of NOXA, an endogenous inhibitor of the antiapoptotic MCL-1, in HER2-amplified breast cancer. Upon investigation of the mechanism leading to low NOXA, we identified a micro-RNA encoded in an intron of HER2, termed miR-4728, that targets the mRNA of the Estrogen Receptor α (ESR1). Reduced ESR1 expression in turn prevents ERα-mediated transcription of NOXA, mitigating apoptosis following treatment with the HER2i lapatinib. Importantly, resistance can be overcome with pharmacological inhibition of MCL-1. More generally, while many cancers like EGFR-mutant lung cancer are driven by activated kinases that when drugged lead to robust monotherapeutic responses, we demonstrate that the efficacy of targeted therapies directed against oncogenes active through focal amplification may be mitigated by coamplified genes.

Navitoclax enhances the effectiveness of EGFR-targeted antibody-drug conjugates in PDX models of EGFR-expressing triple-negative breast cancer.

BACKGROUND: Targeted therapies for triple-negative breast cancer (TNBC) are limited; however, the epidermal growth factor receptor (EGFR) represents a potential target, as the majority of TNBC express EGFR. The purpose of these studies was to evaluate the effectiveness of two EGFR-targeted antibody-drug conjugates (ADC: ABT-414; ABBV-321) in combination with navitoclax, an antagonist of the anti-apoptotic BCL-2 and BCL-XL proteins, in order to assess the translational relevance of these combinations for TNBC. METHODS: The pre-clinical efficacy of combined treatments was evaluated in multiple patient-derived xenograft (PDX) models of TNBC. Microscopy-based dynamic BH3 profiling (DBP) was used to assess mitochondrial apoptotic signaling induced by navitoclax and/or ADC treatments, and the expression of EGFR and BCL-2/XL was analyzed in 46 triple-negative patient tumors. RESULTS: Treatment with navitoclax plus ABT-414 caused a significant reduction in tumor growth in five of seven PDXs and significant tumor regression in the highest EGFR-expressing PDX. Navitoclax plus ABBV-321, an EGFR-targeted ADC that displays more effective wild-type EGFR-targeting, elicited more significant tumor growth inhibition and regressions in the two highest EGFR-expressing models evaluated. The level of mitochondrial apoptotic signaling induced by single or combined drug treatments, as measured by DBP, correlated with the treatment responses observed in vivo. Lastly, the majority of triple-negative patient tumors were found to express EGFR and co-express BCL-XL and/or BCL-2. CONCLUSIONS: The dramatic tumor regressions achieved using combined agents in pre-clinical TNBC models underscore the abilities of BCL-2/XL antagonists to enhance the effectiveness of EGFR-targeted ADCs and highlight the clinical potential for usage of such targeted ADCs to alleviate toxicities associated with combinations of BCL-2/XL inhibitors and systemic chemotherapies.

Concomitant targeting of BCL2 with venetoclax and MAPK signaling with cobimetinib in acute myeloid leukemia models.

The pathogenesis of acute myeloid leukemia (AML) involves serial acquisition of mutations controlling several cellular processes, requiring combination therapies affecting key downstream survival nodes in order to treat the disease effectively. The BCL2 selective inhibitor venetoclax has potent anti-leukemia efficacy; however, resistance can occur due to its inability to inhibit MCL1, which is stabilized by the MAPK pathway. In this study, we aimed to determine the anti-leukemia efficacy of concomitant targeting of the BCL2 and MAPK pathways by venetoclax and the MEK1/2 inhibitor cobimetinib, respectively. The combination demonstrated synergy in seven of 11 AML cell lines, including those resistant to single agents, and showed growth-inhibitory activity in over 60% of primary samples from patients with diverse genetic alterations. The combination markedly impaired leukemia progenitor functions, while maintaining normal progenitors. Mass cytometry data revealed that BCL2 protein is enriched in leukemia stem/progenitor cells, primarily in venetoclax-sensitive samples, and that cobimetinib suppressed cytokine-induced pERK and pS6 signaling pathways. Through proteomic profiling studies, we identified several pathways inhibited downstream of MAPK that contribute to the synergy of the combination. In OCI-AML3 cells, the combination downregulated MCL1 protein levels and disrupted both BCL2:BIM and MCL1:BIM complexes, releasing BIM to induce cell death. RNA sequencing identified several enriched pathways, including MYC, mTORC1, and p53 in cells sensitive to the drug combination. In vivo, the venetoclax-cobimetinib combination reduced leukemia burden in xenograft models using genetically engineered OCI-AML3 and MOLM13 cells. Our data thus provide a rationale for combinatorial blockade of MEK and BCL2 pathways in AML.

Promising efficacy and acceptable safety of venetoclax plus bortezomib and dexamethasone in relapsed/refractory MM.

The antiapoptotic proteins BCL-2 and myeloid cell leukemia sequence 1 (MCL-1) promote multiple myeloma (MM) cell survival. Venetoclax is a selective, orally bioavailable small-molecule BCL-2 inhibitor; bortezomib can indirectly inhibit MCL-1. In preclinical studies, venetoclax enhanced bortezomib activity, suggesting that cotargeting of BCL-2 and MCL-1 could be an effective treatment strategy in myeloma. This phase 1b trial studied patients with relapsed/refractory MM receiving daily venetoclax (50-1200 mg per designated dose cohort; 800 mg in safety expansion) in combination with bortezomib and dexamethasone. A total of 66 patients were enrolled (54 in the dose-escalation cohorts and 12 in the safety expansion). Patients had received a median of 3 prior therapies (range, 1-13); 26 (39%) were refractory to prior bortezomib and 35 (53%) to lenalidomide; 39 (59%) had prior stem cell transplant. The combination was generally well tolerated, and common adverse events included mild gastrointestinal toxicities (diarrhea [46%], constipation [41%], and nausea [38%]) and grade 3/4 cytopenias (thrombocytopenia [29%] and anemia [15%]). The overall response rate (ORR) was 67% (44/66); 42% achieved very good partial response or better (≥VGPR). Median time to progression and duration of response were 9.5 and 9.7 months, respectively. ORR of 97% and ≥VGPR 73% were seen in patients not refractory to bortezomib who had 1 to 3 prior therapies. Patients with high BCL2 expression had a higher ORR (94% [17/18]) than patients with low BCL2 expression (59% [16/27]). This novel combination of venetoclax with bortezomib and dexamethasone has an acceptable safety profile and promising efficacy in patients with relapsed/refractory MM. This trial was registered at www.clinicaltrials.gov as #NCT01794507.

Efficacy of venetoclax as targeted therapy for relapsed/refractory t(11;14) multiple myeloma.

Venetoclax is a selective, orally bioavailable BCL-2 inhibitor that induces cell death in multiple myeloma (MM) cells, particularly in those harboring t(11;14), which express high levels of BCL-2 relative to BCL-XL and MCL-1. In this phase 1 study, patients with relapsed/refractory MM received venetoclax monotherapy. After a 2-week lead-in with weekly dose escalation, daily venetoclax was given at 300, 600, 900, or 1200 mg in dose-escalation cohorts and 1200 mg in the safety expansion. Dexamethasone could be added on progression during treatment. Sixty-six patients were enrolled (30, dose-escalation cohorts; 36, safety expansion). Patients received a median of 5 prior therapies (range, 1-15); 61% were bortezomib and lenalidomide double refractory, and 46% had t(11;14). Venetoclax was generally well tolerated. Most common adverse events included mild gastrointestinal symptoms (nausea [47%], diarrhea [36%], vomiting [21%]). Cytopenias were the most common grade 3/4 events, with thrombocytopenia (32%), neutropenia (27%), anemia (23%), and leukopenia (23%) reported. The overall response rate (ORR) was 21% (14/66), and 15% achieved very good partial response or better (≥VGPR). Most responses (12/14 [86%]) were reported in patients with t(11;14). In this group, ORR was 40%, with 27% of patients achieving ≥VGPR. Biomarker analysis confirmed that response to venetoclax correlated with higher BCL2:BCL2L1 and BCL2:MCL1 mRNA expression ratios. Venetoclax monotherapy at a daily dose up to 1200 mg has an acceptable safety profile and evidence of single-agent antimyeloma activity in patients with relapsed/refractory MM, predominantly in patients with t(11;14) abnormality and those with a favorable BCL2 family profile. Registered at www.clinicaltrials.gov: #NCT01794520.

Flavopiridol enhances ABT-199 sensitivity in unfavourable-risk multiple myeloma cells in vitro and in vivo.

BACKGROUND: The BCL-2-specific BH3-mimetic ABT-199 (venetoclax) has been reported to be principally active against favourable-risk multiple myeloma (MM) cells, prompting efforts to extend its activity to include more resistant, higher-risk MM subsets. METHODS: Effects of the CDK9 inhibitor flavopiridol (FP; alvocidib) on responses to ABT-199 were examined in MM cells. Cell death and protein expression were evaluated by western blot and immunofluorescence. Xenograft models were used to study combination effects in vivo. RESULTS: FP synergistically increased ABT-199 lethality in both ABT-199-sensitive and insensitive MM cells. FP blocked CDK9 activation/positive transcription elongation factor B phosphorylation, downregulated MCL-1, increased BCL-2/MCL-1 ratios, and upregulated BIM. MCL-1 ectopic expression or knockdown in MM cells significantly diminished or increased ABT-199 sensitivity, respectively. CDK9 knockdown triggered MCL-1 downregulation and increased ABT-199 activity, whereas BIM knockdown significantly reduced FP/ABT-199 lethality. FP also enhanced ABT-199 lethality in unfavourable prognosis primary MM cells. HS-5 cell co-culture failed to protect MM cells from the FP/ABT-199 regimen, suggesting circumvention of microenvironmental signals. Finally, FP/ABT-199 significantly increased survival in systemic xenograft and immune-competent MM models while exhibiting minimal toxicity. CONCLUSIONS: These findings argue that CDK9 inhibitors, for example, FP may increase the antimyeloma activity of ABT-199, including in unfavourable-risk MM minimally responsive to ABT-199 alone.

Potential mechanisms of resistance to venetoclax and strategies to circumvent it.

BACKGROUND: Venetoclax (ABT-199), a first-in-class orally bioavailable BCL-2-selective inhibitor, was recently approved by the FDA for use in patients with 17p-deleted chronic lymphocytic leukemia who have received prior therapy. It is also being evaluated in numerous clinical trials for treating patients with various hematologic malignancies. As with any targeted cancer therapy, it is critically important to identify potential mechanisms of resistance, both for patient stratification and developing strategies to overcome resistance, either before it develops or as it emerges. METHODS: In order to gain a more comprehensive insight into the nature of venetoclax resistance mechanisms, we evaluated the changes in the BCL-2 family members at the genetic and expression levels in seven different venetoclax-resistant derived leukemia and lymphoma cell lines. RESULTS: Gene and protein expression analyses identified a number of different alterations in the expression of pro- and anti-apoptotic BCL-2 family members. In the resistant derived cells, an increase in either or both the anti-apoptotic proteins BCL-XL or MCL-1, which are not targeted by venetoclax was observed, and either concomitant or exclusive with a decrease in one or more pro-apoptotic proteins. In addition, mutational analysis also revealed a mutation in the BH3 binding groove (F104L) that could potentially interfere with venetoclax-binding. Not all changes may be causally related to venetoclax resistance and may only be an epiphenomenon. For resistant cell lines showing elevations in BCL-XL or MCL-1, strong synergistic cell killing was observed when venetoclax was combined with either BCL-XL- or MCL-1-selective inhibitors, respectively. This highlights the importance of BCL-XL- and MCL-1 as causally contributing to venetoclax resistance. CONCLUSIONS: Overall our study identified numerous changes in multiple resistant lines; the changes were neither mutually exclusive nor universal across the cell lines tested, thus exemplifying the complexity and heterogeneity of potential resistance mechanisms. Identifying and evaluating their contribution has important implications for both patient selection and the rational development of strategies to overcome resistance.

Genomic analysis and selective small molecule inhibition identifies BCL-X(L) as a critical survival factor in a subset of colorectal cancer.

BACKGROUND: Defects in programmed cell death, or apoptosis, are a hallmark of cancer. The anti-apoptotic B-cell lymphoma 2 (BCL-2) family proteins, including BCL-2, BCL-X(L), and MCL-1 have been characterized as key survival factors in multiple cancer types. Because cancer types with BCL2 and MCL1 amplification are more prone to inhibition of their respectively encoded proteins, we hypothesized that cancers with a significant frequency of BCL2L1 amplification would have greater dependency on BCL-X(L) for survival. METHODS: To identify tumor subtypes that have significant frequency of BCL2L1 amplification, we performed data mining using The Cancer Genome Atlas (TCGA) database. We then assessed the dependency on BCL-X(L) in a panel of cell lines using a selective and potent BCL-X(L) inhibitor, A-1155463, and BCL2L1 siRNA. Mechanistic studies on the role of BCL-X(L) were further undertaken via a variety of genetic manipulations. RESULTS: We identified colorectal cancer as having the highest frequency of BCL2L1 amplification across all tumor types examined. Colorectal cancer cell lines with BCL2L1 copy number >3 were more sensitive to A-1155463. Consistently, cell lines with high expression of BCL-XL and NOXA, a pro-apoptotic protein that antagonizes MCL-1 activity were sensitive to A-1155463. Silencing the expression of BCL-X(L) via siRNA killed the cell lines that were sensitive to A-1155463 while having little effect on lines that were resistant. Furthermore, silencing the expression of MCL-1 in resistant cell lines conferred sensitivity to A-1155463, whereas silencing NOXA abrogated sensitivity. CONCLUSIONS: This work demonstrates the utility of characterizing frequent genomic alterations to identify cancer survival genes. In addition, these studies demonstrate the utility of the highly potent and selective compound A-1155463 for investigating the role of BCL-X(L) in mediating the survival of specific tumor types, and indicate that BCL-X(L) inhibition could be an effective treatment for colorectal tumors with high BCL-X(L) and NOXA expression.

Antihelminthic benzimidazoles potentiate navitoclax (ABT-263) activity by inducing Noxa-dependent apoptosis in non-small cell lung cancer (NSCLC) cell lines.

BACKGROUND: Evasion of apoptosis is a hallmark of cancer cells. One mechanism to deregulate the apoptotic pathway is by upregulation of the anti-apoptotic Bcl-2 family members. Navitoclax (ABT-263) is a Bcl-2/Bcl-xL inhibitor that restores the ability of cancer cells to undergo apoptosis. METHODS: In this study we performed a high-throughput screen with 640 FDA-approved drugs to identify potential therapeutic combinations with navitoclax in a non-small cell lung cancer (NSCLC) cell line. RESULTS: Other than a panel of cancer compounds such as doxorubicin, camptothecin, and docetaxel, four antihelminthic compounds (benzimidazoles) potentiated navitoclax activity. Treatment with benzimidazoles led to induction of the pro-apoptotic protein Noxa at the mRNA and protein level. Noxa binds and antagonizes antiapoptotic protein Mcl-1. siRNA-mediated knock-down of Noxa completely rescued benzimidazole-potentiated navitoclax activity. In addition, inhibiting caspase 3 and 9 partially rescued benzimidazole-potentiated navitoclax activity. CONCLUSIONS: We have identified compounds and mechanisms which potentiate navitoclax activity in lung cancer cell lines. Further validation of the benzimidazole-potentiated navitoclax effect in vivo is required to evaluate the potential for translating this observation into clinical benefit.

Statin-induced Mitochondrial Priming Sensitizes Multiple Myeloma Cells to BCL2 and MCL-1 Inhibitors.

The BCL2 inhibitor venetoclax promotes apoptosis in blood cancer cells and is approved for treatment of chronic lymphocytic leukemia and acute myeloid leukemia. However, multiple myeloma cells are frequently more dependent on MCL-1 for survival, conferring resistance to venetoclax. Here we report that mevalonate pathway inhibition with statins can overcome resistance to venetoclax in multiple myeloma cell lines and primary cells. In addition, statins sensitize to apoptosis induced by MCL-1 inhibitor, S63845. In retrospective analysis of venetoclax clinical studies in multiple myeloma, background statin use was associated with a significantly enhanced rate of stringent complete response and absence of progressive disease. Statins sensitize multiple myeloma cells to venetoclax by upregulating two proapoptotic proteins: PUMA via a p53-independent mechanism and NOXA via the integrated stress response. These findings provide rationale for prospective testing of statins with venetoclax regimens in multiple myeloma. SIGNIFICANCE: BH3 mimetics including venetoclax hold promise for treatment of multiple myeloma but rational combinations are needed to broaden efficacy. This study presents mechanistic and clinical data to support addition of pitavastatin to venetoclax regimens in myeloma. The results open a new avenue for repurposing statins in blood cancer.

Ubiquitin-specific cysteine protease 2a (USP2a) regulates the stability of Aurora-A.

The ubiquitin/proteasome pathway plays critical roles in virtually all aspects of cell biology. Enzymes of the ubiquitin pathway add (ligases) or remove (deubiquitinases) ubiquitin tags to or from their target proteins in a selective fashion. USP2a is a member of a subfamily of deubiquitinases, called ubiquitin-specific cysteine proteases (USPs). Although USP2a has been reported to be a bona fide oncogene that regulates the stability of MDM2, MDMX, and FAS, it is likely that there are other unidentified substrates for USP2a. In this study, we show that USP2a mediates mitotic progression by regulating the stability of Aurora-A. Through cell-based screening of a USP siRNA library, we discovered that knockdown of USP2a reduced the protein levels of Aurora-A. USP2a interacts with Aurora-A directly in vitro and in vivo. In addition, Aurora-A is a substrate for USP2a in vitro and in vivo. Our study provides a novel mechanism for the role of USP2a in mediating the stability of Aurora-A.

Aminopyrimidinone cdc7 kinase inhibitors.

We have investigated the SAR of a series of pyrimidinone-containing Cdc7 kinase inhibitors. A wide range of amine substitutions give potent compounds with activities (K(i)) less than 1nM. Kinase selectivity is reasonable and cytotoxicity corresponds to inhibition of MCM2 phosphorylation.

Stem cell architecture drives myelodysplastic syndrome progression and predicts response to venetoclax-based therapy.

Myelodysplastic syndromes (MDS) are heterogeneous neoplastic disorders of hematopoietic stem cells (HSCs). The current standard of care for patients with MDS is hypomethylating agent (HMA)-based therapy; however, almost 50% of MDS patients fail HMA therapy and progress to acute myeloid leukemia, facing a dismal prognosis due to lack of approved second-line treatment options. As cancer stem cells are the seeds of disease progression, we investigated the biological properties of the MDS HSCs that drive disease evolution, seeking to uncover vulnerabilities that could be therapeutically exploited. Through integrative molecular profiling of HSCs and progenitor cells in large patient cohorts, we found that MDS HSCs in two distinct differentiation states are maintained throughout the clinical course of the disease, and expand at progression, depending on recurrent activation of the anti-apoptotic regulator BCL-2 or nuclear factor-kappa B-mediated survival pathways. Pharmacologically inhibiting these pathways depleted MDS HSCs and reduced tumor burden in experimental systems. Further, patients with MDS who progressed after failure to frontline HMA therapy and whose HSCs upregulated BCL-2 achieved improved clinical responses to venetoclax-based therapy in the clinical setting. Overall, our study uncovers that HSC architectures in MDS are potential predictive biomarkers to guide second-line treatments after HMA failure. These findings warrant further investigation of HSC-specific survival pathways to identify new therapeutic targets of clinical potential in MDS.

Activation of RAS/MAPK pathway confers MCL-1 mediated acquired resistance to BCL-2 inhibitor venetoclax in acute myeloid leukemia.

Despite high initial response rates, acute myeloid leukemia (AML) treated with the BCL-2-selective inhibitor venetoclax (VEN) alone or in combinations commonly acquires resistance. We performed gene/protein expression, metabolomic and methylation analyses of isogenic AML cell lines sensitive or resistant to VEN, and identified the activation of RAS/MAPK pathway, leading to increased stability and higher levels of MCL-1 protein, as a major acquired mechanism of VEN resistance. MCL-1 sustained survival and maintained mitochondrial respiration in VEN-RE cells, which had impaired electron transport chain (ETC) complex II activity, and MCL-1 silencing or pharmacologic inhibition restored VEN sensitivity. In support of the importance of RAS/MAPK activation, we found by single-cell DNA sequencing rapid clonal selection of RAS-mutated clones in AML patients treated with VEN-containing regimens. In summary, these findings establish RAS/MAPK/MCL-1 and mitochondrial fitness as key survival mechanisms of VEN-RE AML and provide the rationale for combinatorial strategies effectively targeting these pathways.

Combined ibrutinib and venetoclax treatment vs single agents in the TCL1 mouse model of chronic lymphocytic leukemia.

The covalent inhibitor of Bruton's tyrosine kinase ibrutinib and the specific Bcl-2 inhibitor venetoclax are both highly efficacious single-agent drugs in the treatment of chronic lymphocytic leukemia (CLL). Based on their complementary modes of action, ibrutinib and venetoclax are hypothesized to act in a synergistic fashion. Currently, it is unclear whether combined treatment is indeed superior to continuous single-agent treatment and what mechanisms underlie the resistance to combination treatment. In addition, the effects of such treatment on the skewed T-cell compartment characteristic of CLL are as yet unknown. In the murine Eµ-TCL1 adoptive transfer model resembling aggressive CLL, we found that combined treatment resulted in the deepest responses, with the longest duration related to a combination of decreased proliferation and increased induction of apoptosis. In addition, alterations in T-cell subsets were most prominent after combination treatment, with increased naive cells and reduced effector memory cells. Remarkably, effects of single agents but also combination treatment were eventually interrupted by relapse, and we found downregulation of BIM expression as a plausible cause of acquired drug resistance. Nevertheless, in this murine model, the combination of venetoclax and ibrutinib has increased efficacy over single agents, accompanied by a restoration of the T-cell compartment.

Venetoclax imparts distinct cell death sensitivity and adaptivity patterns in T cells.

BH3 mimetics are increasingly used as anti-cancer therapeutics either alone or in conjunction with other chemotherapies. However, mounting evidence has also demonstrated that BH3 mimetics modulate varied amounts of apoptotic signaling in healthy immune populations. In order to maximize their clinical potential, it will be essential to understand how BH3 mimetics affect discrete immune populations and to determine how BH3 mimetic pressure causes immune system adaptation. Here we focus on the BCL-2 specific inhibitor venetoclax (ABT-199) and its effects following short-term and long-term BCL-2 blockade on T cell subsets. Seven day "short-term" ex vivo and in vivo BCL-2 inhibition led to divergent cell death sensitivity patterns in CD8+ T cells, CD4+ T cells, and Tregs resulting in shifting of global T cell populations towards a more memory T cell state with increased expression of BCL-2, BCL-XL, and MCL-1. However, twenty-eight day "long-term" BCL-2 blockade following T cell-depleted bone marrow transplantation did not lead to changes in the global T cell landscape. Despite the lack of changes in T cell proportions, animals treated with venetoclax developed CD8+ and CD4+ T cells with high levels of BCL-2 and were more resistant to apoptotic stimuli following expansion post-transplant. Further, we demonstrate through RNA profiling that T cells adapt while under BCL-2 blockade post-transplant and develop a more activated genotype. Taken together, these data emphasize the importance of evaluating how BH3 mimetics affect the immune system in different treatment modalities and disease contexts and suggest that venetoclax should be further explored as an immunomodulatory compound.