Serabelisib, sapanisertib (PIKTOR) and paclitaxel: A combinatorial approach to improve PI3K/mTOR/AKT pathway inhibition and treatment response in solid tumors.

Abstract

e15134 Background: Despite high mutation rates in genes of the PI3K/mTOR/AKT pathway in cancer, therapeutic strategies to inhibit this pathway featuring single targeted therapies have achieved limited clinical success. An alternate approach inhibiting multiple parts of the pathway simultaneously demonstrated improved activity (Starks et al, GynOnc 2022). The Phase 1 study combined the targeted PI3K-alpha and mTOR inhibitors serabelisib and sapanisertib (PIKTOR) with paclitaxel showing reasonable safety and promising clinical efficacy in patients with advanced endometrial, breast and ovarian tumors that had progressed on prior taxane. The ORR was 47% including 3 complete responses, with a PFS of 11 months in 15 evaluable subjects (NCT03154294). We sought to better understand the mechanistic basis for the efficacy of this triplet drug combination in models of endometrial and breast cancer to support successful clinical development of this regimen. Methods: Signaling activity in PI3K/mTOR/AKT pathway was probed by western blot for phospho-S6 and phospho-4EBP1 in human endometrial and breast cancer cell lines with PI3K/mTOR/AKT mutations. Single and combinatorial dose-response curves were established for serabelisib, sapanisertib and paclitaxel versus FDA-approved pathway inhibitors. Mechanisms of cytotoxicity and paclitaxel sensitization were probed with cell death assays. The impact of single and combined agents on tumor growth was assessed with in vivo xenograft studies. Results: In human endometrial and breast cancer cell lines, the combination of serabelisib and sapanisertib achieved markedly improved PI3K/mTOR/AKT pathway inhibition versus FDA-approved single agents. Serabelisib and sapanisertib increased the cytotoxicity of paclitaxel, supporting prior work that identifies PI3K/mTOR/AKT pathway activation as a key mediator of taxane resistance. Further, interrogation of a range of clinically relevant doses and scheduling led to an optimized regimen in which serabelisib and sapanisertib substantially improved the response to paclitaxel in vivo. Conclusions: Simultaneous inhibition of PI3K-alpha, mTORC1 and mTORC2 with PIKTOR offers potential for superior PI3K/mTOR/AKT pathway inhibition, suggesting that rational drug combination approaches may offer two important benefits: (1) more complete PI3K/mTOR/AKT pathway inhibition by targeting multiple signaling nodes, and (2) greater tumor cell cytotoxicity by sensitization to taxanes. These data, combined with the promising efficacy and safety signal from PIKTOR plus paclitaxel in a Phase 1 trial (NCT03154294), provide a compelling rationale for further clinical investigation. Future studies will focus on tumors known to have high rates of PI3K mutations including gynecologic tumors where there is a high unmet need.