Abstract 1901: Structural basis for selective therapeutic targeting of BFL-1 in cancer by a covalent stapled peptide inhibitor

Abstract

Abstract BCL-2 family proteins are master regulators of the mitochondrial apoptotic pathway and dictate whether a cell will live or die in response to stress. Cancer cells hijack the anti-apoptotic arm of this signaling network, overexpressing BCL-2, MCL-1, and BFL-1, to enforce cellular immortality. Whereas BCL-2 has been drugged by the selective small-molecule inhibitor, venetoclax, and new MCL-1 inhibitor molecules have just entered phase I clinical testing, anti-apoptotic BFL-1 remains undrugged. Importantly, the expression of BFL-1 can cause resistance to these molecules and has been independently linked to the pathogenesis and chemoresistance of human cancers, including melanomas and hematologic malignancies. Given the emergence of BCL-2 family proteins as high-priority cancer targets, solving their structures, alone and in complex with their natural BH3 domain ligands, yields important blueprints for drug design. Whereas the structures of numerous “BH3-in-groove” complexes have been solved, the corresponding panel of apo structures remains incomplete. Here, we report the crystal structure of apo BFL-1 at 1.69 Å resolution, revealing similarities and key differences among unliganded anti-apoptotic targets. Unlike all other BCL-2 proteins, apo BFL-1 contains a surface-accessible cysteine within its BH3-binding groove, allowing for selective covalent targeting by BH3-based stapled peptide inhibitors. The crystal structure of a complex between BFL-1 and a cysteine-reactive and stapled NOXA BH3 α-helix demonstrated the sulfhydryl bond and fortuitous interactions between the acrylamide-bearing moiety and a newly formed hydrophobic cavity. Comparison of the apo and BH3-liganded structures further revealed an induced conformational change. The two BFL-1 structures expand our understanding of the surface landscapes available for selective targeting of BFL-1, and inform a new therapeutic strategy for overcoming BFL-1-dependent apoptotic blockades in human cancer. Citation Format: Edward P. Harvey, Hyuk-Soo Seo, Rachel M. Guerra, Gregory H. Bird, Sirano Dhe-Paganon, Loren D. Walensky. Structural basis for selective therapeutic targeting of BFL-1 in cancer by a covalent stapled peptide inhibitor [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 1901.