Abstract PO3-20-04: A first-in-human study of the highly selective PI3Kα inhibitor RLY-5836 in patients with advanced breast cancer and other solid tumors

Abstract Background: Oncogenic activation of PI3Kα is a common driver event in solid tumors. Although PI3Kα is a validated therapeutic target, there is no approved highly selective inhibitor that targets mutant PI3Kα. Alpelisib, a non-selective orthosteric PI3Kα inhibitor, is approved in combination with fulvestrant for patients with PIK3CA-mutant, ER+, HER2– advanced breast cancer. Inhibition of wild-type PI3Kα causes hyperglycemia, limiting the tolerability, dosing, and efficacy of alpelisib. RLY-5836 is the second allosteric, selective pan-mutant PI3Kα inhibitor under clinical investigation. It is molecularly distinct with differentiated pharmaceutical properties compared to RLY-2608. This first-in-human study aims to further explore the tolerability and efficacy of highly selective PIK3CA-mutant inhibition as monotherapy and in combination treatment. Methods: This multicenter, open-label study is designed to evaluate the safety, maximum tolerated dose (MTD), recommended Phase 2 dose (RP2D), pharmacokinetics (PK) and preliminary anti-tumor efficacy of RLY-5836 as a single agent and in various combinations. A Bayesian Optimal Interval design is used for dose escalation, followed by a dose expansion part. As monotherapy, RLY-5836 will be investigated in pts with advanced solid tumors harboring a PIK3CA mutation (Arm 1) per local assessment (blood and/or tumor). In pts with PIK3CA-mutant, HR+, HER2– advanced or metastatic breast cancer, RLY-5836 combination arms will include anti-estrogen therapy with or without CDK4/6 inhibition: fulvestrant (Arm 2), and fulvestrant + CDK4/6 inhibitor(s) (Arms 3–5). RLY-5836 is administered orally, continuously in 28-day cycles until disease progression or study discontinuation. Enrollment criteria include age ≥18 years, ECOG PS 0–1, and measurable disease (Arm 1) or evaluable disease (Arms 2–5) per RECIST version 1.1. Participants in the combination arms are required to have received prior treatment with ≥1 CDK4/6 inhibitor and ≥1 anti-estrogen therapy. Prior treatment with a PI3Kα inhibitor is an exclusion criterion for the dose expansion parts of all arms. Planned overall sample size is ~220 pts (~145 pts for the dose escalation, and ~15 pts in each dose expansion arm). Primary endpoints are the MTD, RP2D and overall safety profile of RLY-5836, either as a single agent or in combination. The MTD will be determined as the dose with a dose-limiting toxicity rate closest to the target toxicity rate (30%) in Cycle 1. Secondary endpoints include PK and efficacy parameters. Statistical analyses will be descriptive. This study (NCT05759949) is enrolling in the United States. For further information, please contact clinicaltrials@relaytx.com. Funding source: This study is funded by Relay Therapeutics, Cambridge, MA, United States. Citation Format: Andreas Varkaris, Steven Isakoff, Cesar A. Perez, Bert O'Neil, Erika Hamilton, Erin Conlin, Gege Tan, Xiaoyan Li, Alison Timm, Ramin Samadani, Erika Puente-Poushnejad, Eunice Kwak, Brenton G. Mar, Alison M. Schram. A first-in-human study of the highly selective PI3Kα inhibitor RLY-5836 in patients with advanced breast cancer and other solid tumors abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-20-04.