Abstract C084: The farnesyl transferase inhibitor KO-2806 constrains mTORC1 activity to enhance the antitumor efficacy of mutant-selective PI3Kα inhibitors

Abstract Several mutant-selective PI3Kα inhibitors have recently entered clinical development. Although these agents may have improved therapeutic indices compared to isoform-selective predecessors, feedback reactivation of PI3K-AKT-mTORC1 or compensatory parallel pathways may hinder their efficacy as monotherapies and necessitate combination strategies to improve clinical outcomes. We have previously demonstrated farnesyl transferase inhibitors (FTIs) enhance the antitumor activity of multiple targeted therapies, including the α-selective PI3K inhibitor alpelisib, by blocking RHEB-mediated mTORC1 activation. We hypothesized feedback reactivation of mTORC1 would remain a factor limiting the efficacy of mutant-selective PI3Kα inhibitors, and FTI KO-2806 could potentiate this efficacy by sustained inhibition of mTORC1 signaling. In this study, we assessed the potential therapeutic utility of combining KO-2806 with mutant-selective PI3Kα inhibitors STX-478 or RLY-2608 in a panel of in vitro cell line and in vivo xenograft models spanning the breadth of solid tumor indications with the highest frequency of PIK3CA mutation. We first evaluated the effect of these combinations on cell viability. PIK3CA-mutant breast, endometrial, ovarian, bladder, and colorectal carcinoma cell lines were cultured in monolayer or as 3D tumor spheroids and exposed to KO-2806 and STX-478 or RLY-2608 in dose-response matrices. Although the sensitivity of individual models to single-agent PI3Kα inhibition varied, KO-2806 deepened the antiproliferative effects of either PI3K inhibitor across the cell line panel. To examine the mechanism underlying this synergy, we probed cell signaling kinetics following PI3Kα inhibitor exposure in the presence or absence of KO-2806. Single-agent STX-478 or RLY-2608 rapidly inhibited AKT and mTORC1 activity, as evidenced by dephosphorylation of AKT/PRAS40 and S6K/S6/4EBP-1 at 1 hr, but phosphorylation partially or completely rebounded by 24-48 hrs. In contrast, when cells were simultaneously exposed to KO-2806 and a PIK3α inhibitor from the outset, initial mTORC1 inhibition was deeper, and rebound of activity was blocked. This correlated with apoptosis (PARP cleavage) and cell cycle arrest (inhibition of Rb phosphorylation), consistent with our cell viability results. Given promising in vitro findings, we asked whether this combination activity translated to in vivo models. In PIK3CA-mutant ER+ breast, triple-negative breast, gynecological, colorectal, and bladder xenograft models, KO-2806 enhanced the antitumor effects of STX-478 or RLY-2608, with responses ranging from tumor stasis to deep regressions. In the ER+ model MCF7, combination of KO-2806 and RLY-2608 was comparable to combination of fulvestrant and RLY-2608, resulting in tumor regressions, while the triplet further deepened tumor regression. These data suggest that, due to its ability to constrain mTORC1 signaling, KO-2806 holds promise as a combination agent for mutant-selective PI3Kα inhibitors across a broad range of PIK3CA-mutant solid tumor indications. Citation Format: Alison E. Smith, Ayuna Jombik, Linda Kessler, Francis Burrows, Shivani Malik, Alison Smith. The farnesyl transferase inhibitor KO-2806 constrains mTORC1 activity to enhance the antitumor efficacy of mutant-selective PI3Kα inhibitors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr C084.