Proteins in the Bcl-2 family are central regulators of programmed cell death, and members that inhibit apoptosis, such as Bcl-X(L) and Bcl-2, are overexpressed in many cancers and contribute to tumour initiation, progression and resistance to therapy. Bcl-X(L) expression correlates with chemo-resistance of tumour cell lines, and reductions in Bcl-2 increase sensitivity to anticancer drugs and enhance in vivo survival. The development of inhibitors of these proteins as potential anti-cancer therapeutics has been previously explored, but obtaining potent small-molecule inhibitors has proved difficult owing to the necessity of targeting a protein-protein interaction. Here, using nuclear magnetic resonance (NMR)-based screening, parallel synthesis and structure-based design, we have discovered ABT-737, a small-molecule inhibitor of the anti-apoptotic proteins Bcl-2, Bcl-X(L) and Bcl-w, with an affinity two to three orders of magnitude more potent than previously reported compounds. Mechanistic studies reveal that ABT-737 does not directly initiate the apoptotic process, but enhances the effects of death signals, displaying synergistic cytotoxicity with chemotherapeutics and radiation. ABT-737 exhibits single-agent-mechanism-based killing of cells from lymphoma and small-cell lung carcinoma lines, as well as primary patient-derived cells, and in animal models, ABT-737 improves survival, causes regression of established tumours, and produces cures in a high percentage of the mice.

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.

Overexpression of the prosurvival Bcl-2 family members (Bcl-2, Bcl-xL, and Mcl-1) is commonly associated with tumor maintenance, progression, and chemoresistance. We previously reported the discovery of ABT-737, a potent, smallmolecule Bcl-2 family protein inhibitor. A major limitation of ABT-737 is that it is not orally bioavailable, which would limit chronic single agent therapy and flexibility to dose in combination regimens. Here we report the biological properties of ABT-263, a potent, orally bioavailable Bad-like BH3 mimetic (K i 's of <1 nmol/L for Bcl-2, Bcl-xL, and Bcl-w). The oral bioavailability of ABT-263 in preclinical animal models is 20% to 50%, depending on formulation. ABT-263 disrupts Bcl-2/Bcl-xL interactions with pro-death proteins (e.g., Bim), leading to the initiation of apoptosis within 2 hours posttreatment. In human tumor cells, ABT-263 induces Bax translocation, cytochrome c release, and subsequent apoptosis. Oral administration of ABT-263 alone induces complete tumor regressions in xenograft models of small-cell lung cancer and acute lymphoblastic leukemia. In xenograft models of aggressive B-cell lymphoma and multiple myeloma where ABT-263 exhibits modest or no single agent activity, it significantly enhances the efficacy of clinically relevant therapeutic regimens. These data provide the rationale for clinical trials evaluating ABT-263 in small-cell lung cancer and B-cell malignancies. The oral efficacy of ABT-263 should provide dosing flexibility to maximize clinical utility both as a single agent and in combination regimens. [Cancer Res 2008;68(9):3421-8]

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.