Abstract 5779: Enhancing selectivity and overcoming tissue-specific toxicity in drug discovery: A novel BET inhibitor with targeted delivery for prostate cancer treatment

Abstract Improving selectivity and circumventing tissue-specific toxicity remain significant challenges in the realm of contemporary drug discovery. Despite the advanced development of highly selective inhibitors, their systemic distribution often stimulates off-tissue unwanted side-toxicity. To address this, we engineered a highly selective inhibitor that specifically targets and releases a small molecule drug payload to a desired cell type. As a proof of concept, we apply this method to epigenetic inhibitors of BET proteins, which are crucial regulators of gene transcription implicated in the progression and survival of cancer. We engineered a novel BET-selective chemotype conjugated to a Prostate-Cancer specific targeting component in order to overcome the off-tissue effects of state-of-the-art BET inhibitors. To achieve delivery of the compound, we modified the benzodiazepine (BzD) scaffold of the well-established BET inhibitor I-BET762, conjugating the tertiary amine with a self-immolating motif, allowing for intracellular compound release upon an enzymatic trigger. This new chemotype, RT53, maintains the biochemical properties of I-BET762, showing potent and selective binding to BET bromodomains in a range of established biophysical assays. In cells, RT53 phenocopied I-BET762 on cancer cell lines, including prostate cancer. We validated the established strong anti-proliferative effect following BET inhibition using clonogenic assays, flow cytometry, and monitored the suppression of MYC expression. Moreover, we established the transcriptional rationale for loss of cellular fitness using RNA sequencing, discovering strong down-regulation of solute carriers, depriving cells of nutrient flux and disrupting normal processes. Furthermore, RT53 exhibits antitumor activity in vivo, in localized models of prostate cancer established in male athymic nude mice. The targeted delivery of RT53 demonstrated superior efficacy in vivo ameliorating on-target, off-tissue toxicity, as low-dose treatment led to almost complete elimination of grafted tumor sites, significant extension of survival, and no signs of toxicity on non-targeted tissues. Taken together, our data establish a novel class of BET-specific inhibitors offering a handle for selective delivery in vivo, enhancing the therapeutic index while avoiding on-target off-tissue toxicity. Citation Format: Cong Tang, Rui Traquete, Elizabeth Henderson, João Conde, Sara Picaud, Panagis Filippakopoulos, Gonçalo Bernardes. Enhancing selectivity and overcoming tissue-specific toxicity in drug discovery: A novel BET inhibitor with targeted delivery for prostate cancer treatment abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 5779.