BET degraders for improving colorectal cancer therapy

PROJECT SUMMARY/ABSTRACT Colorectal cancer (CRC) is one of the leading causes of cancer-related death in the US. Developing novel and more effective CRC therapies is an unmet biomedical need as most of advanced and metastatic CRCs that progress after initial therapies respond poorly to therapeutic treatment. The bromodomain and extra-terminal domain (BET) family proteins such as BRD4 are epigenetic readers that control expression of key oncogenic proteins that drive CRC initiation and progression. Targeting the BET family proteins using small-molecule inhibitors has emerged as a promising therapeutic approach. However, BET inhibitors (BETi) as single agents are generally ineffective against epithelial cancers including CRCs. The molecular mechanisms underlying the anticancer activity of BET-targeting agents are not well understood. Recently, a new class of agents that induce rapid degradation of BET proteins has been developed. Our preliminary studies reveal that two such BET degraders (BETd), BETd260 and BETd246, are much more potent than other BET-targeting agents in CRC cells and patient-derived xenografts (PDXs). We identified a novel, on-target mechanism of action of BETd in transcriptionally activating Death Receptor 5 (DR5), a key component of the extrinsic apoptotic pathway. Importantly, the induction of DR5 is essential for the cell-killing and chemosensitization effects of BETd, and responsible for increased BETd sensitivity in a subset of CRCs with an activating mutation in Speckle-type POZ protein (SPOP), a subunit of the E3 ubiquitin ligase of BET proteins. Furthermore, our data suggest BETd have robust immunogenic effects by inducing DR5-mediated immunogenic cell death (ICD). A combination of BETd260 and anti-PD-1 antibody was well tolerated and nearly eradicated mouse CT26 syngeneic tumors in a DR5-dependent manner. Based on these findings, we hypothesize that BETd improve CRC therapies by inducing DR5-mediated CRC cell killing and antitumor immunity. Aim 1. Identify the mechanism and biomarkers of the potent anticancer activity of BETd in CRC cells; Aim 2. Determine the therapeutic efficacy of BETd against therapy-refractory and metastatic CRCs; Aim 3. Delineate and harness the immunogenic effects of BETd to improve CRC therapies. The proposed studies are expected to provide new mechanistic insights and establish key preclinical parameters for using BETd to develop precision and personalized therapies against therapy-refractory and incurable CRCs. In the long run, these studies may lead to new and improved therapies against CRCs and other types of cancer.