SELECTIVE AND COMPLETE INHIBITION OF mTORC1 BY BI-STERIC mTOR INHIBITORS DRIVES THERAPEUTIC RESPONSE IN GLIOBLASTOMA

Abstract Activation of phosphatidylinositol 3-kinase (PI3K)/PTEN pathway and oncogenic signaling via the mechanistic target of rapamycin (mTOR) occur in a majority of high-grade glial brain tumors. Allosteric mTOR inhibitors, such as rapamycin and other rapalogs, incompletely block mTORC1 by reducing phosphorylation of some substrates, including S6K1, but not 4EBP1. In contrast, ATP-competitive inhibitors, such as sapanisertib, fully inhibit mTORC1. However these inhibitors are also active against mTORC2 and lipid kinases, likely enhancing toxicity. A new class of selective mTORC1 inhibitor that interacts with both the ATP- and FKBP12/FRB-binding sites has been developed, which we term ‘bi-steric’. The prototype bi-steric inhibitor, RapaLink-1, blocks phosphorylation of many mTORC1 substrates, including 4EBP1. Importantly, RapaLink-1 showed improved efficacy in glioblastoma models in vivo as compared to rapamycin or sapanisertib (Fan et al., Cancer Cell 2017). Revolution Medicines has developed novel next-generation bi-steric mTORC1-selective inhibitors that exhibit potent and selective (>10-fold) inhibition of mTORC1 over mTORC2 in vitro in cell line models. Two of these compounds, RM-001 and RM-006, showed sustained blockade of mTORC1 signaling, including dephosphorylation of 4EBP1, following weekly ip dosing in an orthotopic U87MG-Luc model of glioblastoma. Repeated weekly administration of these agents resulted in significantly greater anti-tumor efficacy, as assessed via tumor burden (bioluminescence imaging) and overall survival in comparison to daily sapanisertib and the rapalog everolimus, and weekly dosing of the first generation bi-steric RapaLink-1, all at maximally tolerated doses. In summary, our data demonstrate that bi-steric mTORC1 selective inhibitors elict marked anti-tumor efficacy at doses that are well tolerated in a preclinical model of glioblastoma. Our study emphasizes the importance of mTOR as a central target in glioblastoma, and showcases the therapeutic potential of novel and selective clinical bi-steric mTORC1 inhibitors under development as investigational new drugs.