Abstract 1120: Tipifarnib potentiates the antitumor effects of PI3Ka blockade in HNSCC via convergent inhibition of mTOR activity

The PI3K-AKT-mTOR signaling cascade is the most frequently activated pathway in the head and neck squamous cell carcinomas (HNSCC). PIK3CA (encoding PI3K’s α catalytic subunit), is activated by gain-of-function mutation or amplification in approximately 30% of HNSCCs, making PI3Kα an attractive therapeutic candidate. While the PI3Kα inhibitor alpelisib has shown some promise in HNSCC in a phase I setting, its single agent efficacy will likely be limited by feedback reactivation of PI3K or compensatory parallel pathways, necessitating the development of rational combination strategies. Here, we utilize cell line and patient-derived xenograft (PDX) models to evaluate the therapeutic potential of the farnesyltransferase inhibitor (FTI) tipifarnib, in combination with alpelisib in the PIK3CA-dysregulated subset of HNSCC. Because FTIs potentially block hyperactivated growth factor signaling at multiple nodes, including HRAS and RHEB, we examined tipifarnib’s impact on growth of PIK3CA-altered HNSCC models in vitro and in vivo. In cell lines harboring PIK3CA mutation or amplification, tipifarnib reduced proliferation of both monolayer and spheroid cultures and when combined with alpelisib, induced cytotoxicity. Consistently, in PIK3CA mutant/amplified PDX models, the tipifarnib-alpelisib doublet led to deeper antitumor responses compared to alpelisib monotherapy. Simultaneous administration was superior to split intermittent dosing, hinting at cooperativity between the mechanistic targets of tipifarnib and alpelisib. To interrogate the mechanistic underpinnings of this synergy, we exposed HNSCC cell lines to tipifarnib, alpelisib, or the combination, and assessed their effect on RAS/PI3K pathway activity. In PIK3CA dysregulated lines, single agent tipifarnib or alpelisib reduced phosphorylation of p90 RSK, and mTOR substrates, particularly S6 kinase and ribosomal protein S6. Combination treatment effects were more robust and induced rapid apoptosis. In cells exposed to alpelisib alone, marked rebound of RSK and mTOR substrate phosphorylation occurred after 24 hours, correlating with restored AKT activity. In contrast, although AKT activity rebounded in cells treated with the combination, RSK phosphorylation and markers of mTOR activity (including 4EBP1) remained suppressed. Thus, tipifarnib appears to blunt both MAPK and mTOR reactivation following PI3K inhibition. This dual effect implies that the efficacy of tipifarnib in this context may stem from simultaneou