EXTH-117. Bitopic inhibitors of mTORC1 cooperate with inhibitors of mitogen-activated protein kinase kinase to drive cytotoxicity in glioblastoma

Abstract We previously described RapaLink-1, a bisteric mTORC1 inhibitor that links the allosteric mTORC1 inhibitor rapamycin to the mTOR kinase inhibitor sapanisertib. RapaLink-1 showed increased efficacy compared to either parent inhibitor in orthotopic xenograft models for glioblastoma. The bitopic clinical derivative of this agent, RMC-5552 (Revolution Medicines/RevMed), is currently in a phase 1/2, UCSF SPORE-supported clinical trial in glioblastoma. We show that RMC-5552, RapaLink-1, and RevMed’s bioequivalent bitopic mTORC1 inhibitor RMC-6272 are cytostatic rather than cytotoxic preclinically. Here, identify combination strategies that drive cell death in glioblastoma. We screened RapaLink-1 and RMC-6272 in combination with a library of FDA-approved drugs in two glioblastoma lines. Among 2332 drugs, the clinical mitogen-activated protein kinase kinase (MEK) inhibitor trametinib was the most active agent in driving cytotoxicity in glioblastoma cells. Like trametinib, the MEK inhibitor selumetinib similarly cooperated with RMC-6272 to block proliferation and increase apoptosis in isogenic T98G glioblastoma lines knocked-down for NF1 compared to NF1 wild-type T98G cells. We rescued NF1 deficiency in NF1MT lines by expressing full length NF1. Combination therapy with trametinib and RMC-6272 drove apoptosis in both lines, more efficiently in NF1-rescued lines. RMC-6272 induced autophagy. Combining RMC-6272, trametinib and the autophagy ULK1 tool inhibitor MRT68921 augmented apoptosis compared to RMC-6272 and trametinib alone. RMC-6272 cooperated with trametinib in orthotopic xenografts of GBM43 in vivo, blocking proliferation, driving apoptosis and improving survival compared to either monotherapy. Biochemically, RMC-6272 cooperated with trametinib to block MAP kinase and mTORC1 signaling and to drive apoptosis in vivo. We conclude that bitopic inhibitors of mTORC1 cooperate with inhibitors of MEK to drive cytotoxicity in both NF1WT and NF1MT glioblastoma. Combining RMC-5552 and trametinib represents a translatable strategy to improve survival in patients with glioblastoma.