Synthetic Strategies to Restore the Efficacy of Venetoclax in Acute Myeloid Leukemia

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Project Summary. Increased cell survival due to inhibited apoptosis through overexpression of anti-apoptotic BCL-2 family proteins is a hallmark of cancer that is a prominent mechanism in acute myeloid leukemia (AML), one of the deadliest human cancers. Although the FDA recently approved several new drugs for treatment of AML, these target only specific subsets of AML cases, and provide only short responses. Indeed, treatment with the BCL-2 inhibitor venetoclax (VEN), an emerging standard-of-care drug for AML, partnered with other drugs has resulted in only ~19% responses and only ~17 months median survival due to the onset of resistance. There is a clear and urgent need for more effective pharmacotherapies for AML. The BCL-2 family of proteins regulates the intrinsic apoptosis pathway, and includes both anti-apoptotic and pro-apoptotic members. The anti-apoptotic proteins, BCL-2, BCL-xL, MCL-1, BCL-w and A1, seize their pro-apoptotic partner proteins, such as BAK and BAX, via their amphipathic -helical BH3 domains. VEN is a small-molecule BH3 mimetic that is prescribed in chronic lymphocytic leukemia as well as AML, and potentially other BCL-2 dependent cancers, but its efficacy is abrogated by the development of resistance. The best characterized mechanisms of VEN resistance are upregulated expression of MCL-1 and the development of BCL-2 mutants that are no longer effectively recognized by VEN; currently, there are no clinical solutions to either of these resistance mechanisms. VEN recently demonstrated synergy in vitro and in vivo with a range of clinical drugs and advanced drug candidates that target a variety of additional proteins, with a unifying theme that the partner drug either leads to downregulation/inhibition of MCL-1 and/or upregulation of pro-apoptotic BCL-2 proteins. Since the discovery of new, highly-targeted AML drugs de novo is time- and money-intensive with no guarantees of success, we propose to utilize a polypharmacology approach, in which single drugs will be rationally designed to hit multiple targets relevant in AML, by leveraging FDA-approved drugs and those in clinical trials. Polypharmacology offers potential advantages over combination therapy, such as increased therapeutic windows and increased patient compliance. Accordingly, the solvent-exposed tetrahydropyran motif of VEN will be replaced with co- drugs that have demonstrated synergy with VEN. Similarly, we will exploit the exciting area of proteolysis targeting chimera (PROTAC) research by grafting E3 ubiquitin ligase recognition motifs onto VEN, promoting the recruitment of BCL-2 to the proteasome for degradation. This PROTAC strategy can be effective even with weaker binding ligands and thus may address the issue of resistance mutations in the BCL-2 protein. Lead compounds that potently inhibit the proliferation of VEN-sensitive and VEN-resistant AML cell lines in vitro will be evaluated in human AML xenografts. By the end of the funding period, we envisage discovering at least one VEN-based chimeric compound suitable for further preclinical evaluation in AML (and other cancers).