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Proteins of the bromodomain and extra-terminal (BET) domain family are epigenetic readers that bind acetylated histones through their bromodomains to regulate gene transcription. Dual-bromodomain BET inhibitors (DbBi) that bind with similar affinities to the first (BD1) and second (BD2) bromodomains of BRD2, BRD3, BRD4 and BRDt have displayed modest clinical activity in monotherapy cancer trials. A reduced number of thrombocytes in the blood (thrombocytopenia) as well as symptoms of gastrointestinal toxicity are dose-limiting adverse events for some types of DbBi1–5. Given that similar haematological and gastrointestinal defects were observed after genetic silencing of Brd4 in mice6, the platelet and gastrointestinal toxicities may represent on-target activities associated with BET inhibition. The two individual bromodomains in BET family proteins may have distinct functions7–9 and different cellular phenotypes after pharmacological inhibition of one or both bromodomains have been reported10,11, suggesting that selectively targeting one of the bromodomains may result in a different efficacy and tolerability profile compared with DbBi. Available compounds that are selective to individual domains lack sufficient potency and the pharmacokinetics properties that are required for in vivo efficacy and tolerability assessment10–13. Here we carried out a medicinal chemistry campaign that led to the discovery of ABBV-744, a highly potent and selective inhibitor of the BD2 domain of BET family proteins with drug-like properties. In contrast to the broad range of cell growth inhibition induced by DbBi, the antiproliferative activity of ABBV-744 was largely, but not exclusively, restricted to cell lines of acute myeloid leukaemia and prostate cancer that expressed the full-length androgen receptor (AR). ABBV-744 retained robust activity in prostate cancer xenografts, and showed fewer platelet and gastrointestinal toxicities than the DbBi ABBV-07514. Analyses of RNA expression and chromatin immunoprecipitation followed by sequencing revealed that ABBV-744 displaced BRD4 from AR-containing super-enhancers and inhibited AR-dependent transcription, with less impact on global transcription compared with ABBV-075. These results underscore the potential value of selectively targeting the BD2 domain of BET family proteins for cancer therapy. ABBV-744, a selective inhibitor of the BD2 domains of BET family proteins, is effective against prostate cancer in mouse xenograft models, with lower toxicities than the dual-bromodomain BET inhibitor ABBV-075.

Dual bromodomain BET inhibitors that bind with similar affinities to the first and second bromodomains across BRD2, BRD3, BRD4, and BRDT have displayed modest activity as monotherapy in clinical trials. Thrombocytopenia, closely followed by symptoms characteristic of gastrointestinal toxicity, have presented as dose-limiting adverse events that may have prevented escalation to higher dose levels required for more robust efficacy. ABBV-744 is a highly selective inhibitor for the second bromodomain of the four BET family proteins. In contrast to the broad antiproliferative activities observed with dual bromodomain BET inhibitors, ABBV-744 displayed significant antiproliferative activities largely although not exclusively in cancer cell lines derived from acute myeloid leukemia and androgen receptor positive prostate cancer. Studies in acute myeloid leukemia xenograft models demonstrated antitumor efficacy for ABBV-744 that was comparable with the pan-BET inhibitor ABBV-075 but with an improved therapeutic index. Enhanced antitumor efficacy was also observed with the combination of ABBV-744 and the BCL-2 inhibitor, venetoclax compared with monotherapies of either agent alone. These results collectively support the clinical evaluation of ABBV-744 in AML (Clinical Trials.gov identifier: NCT03360006).

Statins have shown promise as anticancer agents in experimental and epidemiologic research. However, any benefit that they provide is likely context-dependent, for example, applicable only to certain cancers or in combination with specific anticancer drugs. We report that inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) using statins enhances the proapoptotic activity of the B cell lymphoma-2 (BCL2) inhibitor venetoclax (ABT-199) in primary leukemia and lymphoma cells but not in normal human peripheral blood mononuclear cells. By blocking mevalonate production, HMGCR inhibition suppressed protein geranylgeranylation, resulting in up-regulation of proapoptotic protein p53 up-regulated modulator of apoptosis (PUMA). In support of these findings, dynamic BH3 profiling confirmed that statins primed cells for apoptosis. Furthermore, in retrospective analyses of three clinical studies of chronic lymphocytic leukemia, background statin use was associated with enhanced response to venetoclax, as demonstrated by more frequent complete responses. Together, this work provides mechanistic justification and clinical evidence to warrant prospective clinical investigation of this combination in hematologic malignancies.

Ten percent to 15% of all lung cancers are small-cell lung cancer (SCLC). SCLC usually grows and metastasizes before it is diagnosed and relapses rapidly upon treatment. Unfortunately, no new targeted agent has been approved in the past 30 years for patients with SCLC. The BET (bromodomain and extraterminal) proteins bind acetylated histones and recruit protein complexes to promote transcription initiation and elongation. BET proteins have been shown to regulate expression of key genes in oncogenesis, such as MYC, CCND2, and BCL2L1. Here, we demonstrate that approximately 50% of SCLC cell lines are exquisitely sensitive to growth inhibition by the BET inhibitor, ABBV-075. The majority of these SCLC cell lines underwent apoptosis in response to ABBV-075 treatment via induction of caspase-3/7 activity. ABBV-075 enhanced the expression of proapoptotic protein BIM and downregulated antiapoptotic proteins BCL2 and BCLxl to a lesser extent. Furthermore, BET inhibition increased BCL2–BIM complex, thus priming the cells for apoptosis. Indeed, strong synergy was observed both in vitro and in vivo when cotreating the cells with BET inhibitor and the BH3-mimetic, BCL2 inhibitor venetoclax (ABT-199). ABBV-075 interaction with venetoclax positively correlated with BCL2 expression. Taken together, our studies provide a rationale for treating SCLC with BET and BCL2 inhibitors in tumors with high BCL2 protein expression. Mol Cancer Ther; 16(8); 1511–20. ©2017 AACR.

Many small-molecule inhibitors that target both bromodomains of the BET family proteins (pan BET inhibitors) are undergoing studies in clinical trials. Emerging data are beginning to suggest that clinical responses to these pan BET inhibitors in subsets of hematologic malignancies may be modest and short lived, perhaps due, at least in part, to tolerability issues that limit dosing levels. We hypothesized that selective inhibition of four of the eight bromodomains in BET family proteins might retain the anticancer activities in certain tumor subsets while alleviating some of the tolerability liabilities of pan BET inhibitors, thus possibly providing better therapeutic benefits. ABBV-744 is a highly selective inhibitor for the second bromodomain (BDII) of the four BET family proteins, exhibiting greater than 300-fold more potent binding affinity to the BDII bromodomain of BRD4 relative to the first bromodomain (BDI) of BRD4. In contrast to the broad antiproliferative activities observed with pan BET inhibitors, ABBV-744 only displayed significant antiproliferative activities in a limited number of cancer cell lines, including AML and androgen receptor (AR)-positive prostate cancer. Studies in AML xenograft models demonstrated antitumor efficacy for ABBV-744 that was comparable to the pan-BET inhibitor ABBV-075 but with improved tolerability. Taken together, these results suggest that ABBV-744 could be a promising second-generation BET inhibitor for AML therapy. Affiliation: Oncology Discovery, AbbVie Inc., 1 North Waukegan Rd., North Chicago, IL 60064. Disclosures: All authors are employees of AbbVie. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication. Citation Format: Xiaoyu Lin, Xiaoli Huang, Richard Bellin, Emily Faivre, Paul Hessler, Lloyd Lam, Mai Ha Bui, Denise Wilcox, Tamar Uziel, Debra C. Ferguson, Terrance J. Magoc, Daniel H. Albert, Keith F. McDaniel, Warren Kati, Yu Shen. ABBV-744, a first-in-class and highly selective inhibitor of the second bromodomain of BET family proteins, displays robust activities in preclinical models of acute myelogenous leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 800.

Acute Myeloid Leukemia (AML) is a heterogeneous disease where many distinct functional mutations have been identified. While mutations in FLT3, NRAS, NPM1, and IDH genes have been well characterized in de novo AML, a subset of AML associated with leukemic transformation of chronic myeloproliferative neoplasms (MPNs) are enriched in mutated JAK2 and PTPN11. MPNs are a set of disorders characterized by the chronic and abnormal overproduction of blood cells which can ultimately progress to AML. Targeted therapy against mutated JAK2 and PTPN11 with small molecule inhibitors against the JAK-STAT and RAS-ERK pathways, respectively, are being actively investigated in AML. However, acquired resistance and non-durable responses are already being demonstrated in monotherapy treatment. Thus, AML that has arisen from leukemic transformation of MPNs remains an area of unmet medical need. This need is underscored by the fact that these patients suffer from poor outcomes and low response rates to standard chemotherapy. Together, these results indicate the need for rational combinations in this population. In this study, we sought to better define the dependencies of AML cells on BCL-2 family members by screening AML cell lines with selective small molecule inhibitors to the BCL-2 family members. The screen revealed that the majority of AML cell lines depend on BCL-2, or BCL-2 together with MCL1 for survival. Intriguingly, a subset of AML cell lines is exquisitely sensitive to BCL-xL inhibition. Treating these cell lines with BCL-xL-selective inhibitor or with navitoclax, a dual BCL2/BCL-xL inhibitor induces apoptosis. These BCL-xL inhibitor-sensitive cell lines express high levels of BCL-xL, but not BCL-2 and MCL1. Genomic analysis of BCL-xL inhibitor-sensitive AML cell lines revealed that this subset is also enriched in JAK2V16F and PTPN11 mutations. We further demonstrate positive combination effect between BCL-xL inhibitor or navitoclax with JAK inhibitor (ruxolitinb) and two MEK inhibitors (trametinib and AZD6244) in cell lines with JAK2V16F and PTPN11 mutations, respectively. Together, these results demonstrate the potential utility of a BCL-xL inhibition as a combination partner in JAK2 and PTPN11 mutated cancers. Importantly, JAK2 and PTPN11 mutations in addition to expression of BCL-xL, BCL-2, and MCL1 could also be further tested as potential biomarkers for the utility of BCL-xL inhibition based treatments for patients with these mutations. Citation Format: Richard J. Bellin, Valerie Hilgenberg, Relja Popovic, Tamar Uziel, Lloyd T. Lam. JAK2 and PTPN11 mutations as potential biomarkers for BCL-xL inhibition as monotherapy and in combination therapy for acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3812. doi:10.1158/1538-7445.AM2017-3812