Abstract 4859: Bi-steric mTORC1 inhibitors are superior to rapamycin and induce apoptotic cell death in tumor models with hyperactivated mTORC1
Authors: Du Heng , Yang Yu Chi , Liu Heng-Jia , Yuan Min , Asara John , Wong Kwok-Kin , Singh Mallika , Kwiatkowski David 
Published: April 2023

Abstract
The PI3K-mTOR pathway is one of the most commonly dysregulated pathways in human tumors. Rapalogs have been used extensively in human clinical trials but exhibit modest clinical benefit for the most part, possibly due to their lack of effect on 4E-BP1, a key target downstream of mTORC1. ATP-competitive mTOR inhibitors can fully inhibit mTORC1 but are poorly tolerated possibly due to their inhibition of mTORC2. A new class of selective mTORC1 inhibitors has been developed and termed ‘bi-steric’, which comprise a rapamycin-like core moiety covalently linked to an mTOR active-site inhibitor. We report that bi-steric mTORC1-selective inhibitors (RMC-4627 and RMC-6272) have greater activity than rapalogs for tumors with mTORC1 hyperactivation. RMC-4627 and RMC-6272 showed more effective growth inhibition in multiple tumor cell lines with mTORC1 dysfunction compared to rapamycin. Both RMC-4627 and RMC-6272 at ~1 nM showed near complete inhibition of p4E-BP1T37/46, which was not seen with rapamycin treatment. Rapamycin and both bi-steric compounds markedly reduced kidney tumor burden and human bladder cancer PDX tumor size after four weeks of treatment. Tumor regrowth assessed after two-month treatment cessation was significantly reduced in the RMC-6272 group as compared to the rapamycin group. Finally, TUNEL staining showed RMC-6272 treatment led to a greater induction of apoptosis in kidney tumor cells and human bladder cancer PDX tumors relative to rapamycin after a single dose of each compound. This is the first time that clear evidence for cell death after mTORC1 inhibition has been seen. Integrative multi-omic analysis reveals differential global rewiring induced by RMC-6272 in comparison to rapamycin. Genes involved in cell cycle transition were downregulated by RMC-6272 vs. rapamycin. Proteins downregulated by RMC-6272 treatment relative to rapamycin were enriched for translation and ribosome-related proteins, consistent with effects of non-phosphorylated 4E-BP1 on synthesis of those proteins. Multiple purine metabolites were decreased to a greater extent in RMC-6272 treated cells as compared to rapamycin treatment, including deoxyadenosine, AICAR, AMP, IMP, and GMP. Notably, there was no significant change in R5P, suggesting that downregulation of the purine de novo synthesis pathway was the cause of these changes. Strong tumor cell apoptosis was induced by the suppression of de novo purine synthesis through mTORC1-PRPS1 axis. PRPS1, the rate-limit enzyme involving in purine synthesis was regulated by mTORC1 activity. In summary, RMC-4627 and RMC-6272 demonstrate improved in vitro and in vivo inhibition of mTORC1 in comparison to rapamycin, and induced more cell death in preclinical models, indicating the potential of bi-steric mTORC1-selective inhibitors as a novel therapeutic strategy to treat tumors with mTORC1 dysregulation.
Citation Format: Heng Du, Yu Chi Yang, Heng-Jia Liu, Min Yuan, John Asara, Kwok-Kin Wong, Mallika Singh, David Kwiatkowski. Bi-steric mTORC1 inhibitors are superior to rapamycin and induce apoptotic cell death in tumor models with hyperactivated mTORC1. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4859.