Abstract LB089: Effective in vivo treatment of endometrial tumor models with coexistent mutant PI3K and PTEN inactivation with a selective bi-steric mTORC1 kinase inhibitor

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Abstract Dysregulation of PI3K signaling is a common event in human cancer, but feedback inhibition of receptor kinase signaling and/or induction of PTEN translation by activated PI3K/AKT/mTOR signaling reduces pathway output and tumor dependency. In some cancers, including the majority of endometrial carcinomas, these lesions coexist and cause synergistic hyperactivation of pathway output, particularly mTORC1 kinase signaling. Although this suggests that PI3K signaling is a driver of these endometrial cancers, standard inhibitors have had only marginal activity in patients with advanced disease. We here show that, in experimental isogenic models and PDX models with PI3K and PTEN alterations, PI3K inhibitors and rapalogs are weak inhibitors of TORC1 targets and slow but do not suppress the in vivo growth of these tumors. Revolution Medicines has developed a series of bi-steric selective and potent mTORC1 inhibitors that inhibit the phosphorylation of 4EBP-1, S6K and other mTORC1 substrates at concentrations that do not affect TORC2 substrates and thus do not induce the hyperglycemia caused by downregulation of AKT signaling by pan-mTOR kinase inhibitors. We have explored the biologic and anti-tumor effects of these inhibitors in models of endometrial cancer with coexistent PI3K mutation and PTEN inactivation. The mTORC1 kinase selective bi-steric inhibitor RMC-6272 can potently suppress both mTORC1 activity and the growth of these tumor cells in vitro. Moreover, in contrast to PI3K inhibition, RMC-6272 effectively inhibited the phosphorylation of mTORC1 kinase substrates and completely suppressed the growth in vivo of multiple PDXs with this genotype without inducing hyperglycemia or causing weight loss in the mice. The results suggest that dysregulation of TORC1 activity is an important driver of endometrial carcinomas with coexistent PTEN inactivation and PI3K mutations and that selective TORC1 kinase inhibitors may prove to be useful therapeutics in this context. The bi-steric mTORC1 kinase inhibitor RMC-5552 is the first clinical candidate of this class and clinical testing is ongoing (NCT04774952). Citation Format: Hilla Solomon, Radha Mukherjee, Xiaoping Chen, HuiYong Zhao, Elisa de Stanchina, Britta Weigelt, Alison M. Schram, Carol Aghajanian, Mallika Singh, Jan Smith, Neal Rosen. Effective in vivo treatment of endometrial tumor models with coexistent mutant PI3K and PTEN inactivation with a selective bi-steric mTORC1 kinase inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB089.