Proteins in the B cell CLL/lymphoma 2 (BCL-2) family are key regulators of the apoptotic process. This family comprises proapoptotic and prosurvival proteins, and shifting the balance toward the latter is an established mechanism whereby cancer cells evade apoptosis. The therapeutic potential of directly inhibiting prosurvival proteins was unveiled with the development of navitoclax, a selective inhibitor of both BCL-2 and BCL-2-like 1 (BCL-X(L)), which has shown clinical efficacy in some BCL-2-dependent hematological cancers. However, concomitant on-target thrombocytopenia caused by BCL-X(L) inhibition limits the efficacy achievable with this agent. Here we report the re-engineering of navitoclax to create a highly potent, orally bioavailable and BCL-2-selective inhibitor, ABT-199. This compound inhibits the growth of BCL-2-dependent tumors in vivo and spares human platelets. A single dose of ABT-199 in three patients with refractory chronic lymphocytic leukemia resulted in tumor lysis within 24 h. These data indicate that selective pharmacological inhibition of BCL-2 shows promise for the treatment of BCL-2-dependent hematological cancers.

Ubiquitination of receptor protein-tyrosine kinases (RPTKs) terminates signaling by marking active receptors for degradation. c-Cbl, an adapter protein for RPTKs, positively regulates RPTK ubiquitination in a manner dependent on its variant SRC homology 2 (SH2) and RING finger domains. Ubiquitin-protein ligases (or E3s) are the components of ubiquitination pathways that recognize target substrates and promote their ligation to ubiquitin. The c-Cbl protein acted as an E3 that can recognize tyrosine-phosphorylated substrates, such as the activated platelet-derived growth factor receptor, through its SH2 domain and that recruits and allosterically activates an E2 ubiquitin-conjugating enzyme through its RING domain. These results reveal an SH2-containing protein that functions as a ubiquitin-protein ligase and thus provide a distinct mechanism for substrate targeting in the ubiquitin system.

We present a phase 2, single-arm study evaluating 800 mg daily venetoclax, a highly selective, oral small-molecule B-cell leukemia/lymphoma-2 (BCL-2) inhibitor in patients with high-risk relapsed/refractory acute myelogenous leukemia (AML) or unfit for intensive chemotherapy. Responses were evaluated following revised International Working Group (IWG) criteria. The overall response rate was 19%; an additional 19% of patients demonstrated anti-leukemic activity not meeting IWG criteria (partial bone marrow response and incomplete hematologic recovery). Twelve (38%) patients had isocitrate dehydrogenase 1/2 mutations, of whom 4 (33%) achieved complete response/complete response with incomplete blood count recovery. Six (19%) patients had BCL-2–sensitive protein index at screening, which correlated with time on study. BH3 profiling was consistent with on-target BCL-2 inhibition and identified potential resistance mechanisms. Common adverse events included nausea, diarrhea and vomiting (all grades), and febrile neutropenia and hypokalemia (grade 3/4). Venetoclax demonstrated activity and acceptable tolerability in patients with AML and adverse features.

B-cell leukemia/lymphoma 2 (BCL-2) prevents commitment to programmed cell death at the mitochondrion. It remains a challenge to identify those tumors that are best treated by inhibition of BCL-2. Here we demonstrate that acute myeloid leukemia (AML) cell lines, primary patient samples, and murine primary xenografts are very sensitive to treatment with the selective BCL-2 antagonist ABT-199. In primary patient cells, the median IC50 was approximately 10 nM, and cell death occurred within 2 h. Our ex vivo sensitivity results compare favorably with those observed for chronic lymphocytic leukemia (CLL), a disease for which ABT-199 has demonstrated consistent activity in clinical trials. Moreover, mitochondrial studies using BH3 profiling demonstrate activity at the mitochondrion that correlates well with cytotoxicity, supporting an on target mitochondrial mechanism of action. Our protein and BH3 profiling studies provide promising tools that can be tested as predictive biomarkers in any clinical trial of ABT-199 in AML.

Members of the B cell lymphoma 2 (BCL-2) gene family have a central role in regulating programmed cell death by controlling pro-apoptotic and anti-apoptotic intracellular signals. In cancer, apoptosis evasion through dysregulation of specific BCL-2 family genes is a recurring event; accordingly, selective inhibition of specific anti-apoptotic BCL-2 family proteins represents an exciting therapeutic opportunity. A combination of nuclear magnetic resonance (NMR)-based screening and structure-based drug design has yielded the first bona fide BCL-2 homology 3 (BH3) mimetics, including the BCL-2 and BCL-X dual antagonist navitoclax, which is the first BCL-2 family inhibitor to show efficacy in patients with cancer. Clinical experience with navitoclax prompted the generation of the highly selective BCL-2 inhibitor venetoclax, which is now approved in the United States for the treatment of patients with chronic lymphocytic leukaemia with 17p deletion who have received at least one prior therapy. Recent advances have also been made in the development of potent and selective inhibitors of BCL-X and myeloid cell leukaemia 1 (MCL1), which are additional BCL-2 family members with established anti-apoptotic roles in cancer. Here we review the latest progress in direct and selective targeting of BCL-2 family proteins for cancer therapy.

The BCL-2/BCL-XL/BCL-W inhibitor ABT-263 (navitoclax) has shown promising clinical activity in lymphoid malignancies such as chronic lymphocytic leukemia. However, its efficacy in these settings is limited by thrombocytopenia caused by BCL-XL inhibition. This prompted the generation of the BCL-2-selective inhibitor venetoclax (ABT-199/GDC-0199), which demonstrates robust activity in these cancers but spares platelets. Navitoclax has also been shown to enhance the efficacy of docetaxel in preclinical models of solid tumors, but clinical use of this combination has been limited by neutropenia. We used venetoclax and the BCL-XL-selective inhibitors A-1155463 and A-1331852 to assess the relative contributions of inhibiting BCL-2 or BCL-XL to the efficacy and toxicity of the navitoclax-docetaxel combination. Selective BCL-2 inhibition suppressed granulopoiesis in vitro and in vivo, potentially accounting for the exacerbated neutropenia observed when navitoclax was combined with docetaxel clinically. By contrast, selectively inhibiting BCL-XL did not suppress granulopoiesis but was highly efficacious in combination with docetaxel when tested against a range of solid tumors. Therefore, BCL-XL-selective inhibitors have the potential to enhance the efficacy of docetaxel in solid tumors and avoid the exacerbation of neutropenia observed with navitoclax. These studies demonstrate the translational utility of this toolkit of selective BCL-2 family inhibitors and highlight their potential as improved cancer therapeutics.

The anti-apoptotic protein MCL-1 is a key regulator of cancer cell survival and a known resistance factor for small-molecule BCL-2 family inhibitors such as ABT-263 (navitoclax), making it an attractive therapeutic target. However, directly inhibiting this target requires the disruption of high-affinity protein–protein interactions, and therefore designing small molecules potent enough to inhibit MCL-1 in cells has proven extremely challenging. Here, we describe a series of indole-2-carboxylic acids, exemplified by the compound A-1210477, that bind to MCL-1 selectively and with sufficient affinity to disrupt MCL-1–BIM complexes in living cells. A-1210477 induces the hallmarks of intrinsic apoptosis and demonstrates single agent killing of multiple myeloma and non-small cell lung cancer cell lines demonstrated to be MCL-1 dependent by BH3 profiling or siRNA rescue experiments. As predicted, A-1210477 synergizes with the BCL-2/BCL-XL inhibitor navitoclax to kill a variety of cancer cell lines. This work represents the first description of small-molecule MCL-1 inhibitors with sufficient potency to induce clear on-target cellular activity. It also demonstrates the utility of these molecules as chemical tools for dissecting the basic biology of MCL-1 and the promise of small-molecule MCL-1 inhibitors as potential therapeutics for the treatment of cancer.

Key Points Venetoclax monotherapy at a daily dose up to 1200 mg has an acceptable safety profile in patients with relapsed/refractory MM. Venetoclax monotherapy has demonstrated antimyeloma activity in patients with relapsed/refractory MM positive for t(11;14).