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Relay, Pfizer Testing CDK4, PI3Kα Inhibitors in Metastatic Breast Cancer Trial

NEW YORK – Relay Therapeutics on Wednesday announced that its PI3Kα inhibitor RLY-2608 will be tested in combination with Pfizer's selective-CDK4 inhibitor atirmociclib and fulvestrant in certain metastatic breast cancer patients in a clinical trial.

Relay and Pfizer will evaluate the triplet combination in the clinical trial, which will include patients with PI3Kα-mutant, hormone receptor (HR)-positive, HER2-negative metastatic breast cancer. The partners believe that combining atirmociclib and RLY-2608 may address toxicities typically seen with CDK4- and PI3Kα-targeting drugs that often require patients to have doses reduced or discontinue treatment.

Last year, Relay presented encouraging preliminary results from the Phase I ReDiscover trial of RLY-2608 in patients with PIK3Cα-mutated solid tumors, including patients with locally advanced or metastatic PIK3Cα-mutant HR-positive, HER2-negative breast cancer receiving fulvestrant. At the time of the data cutoff, none of the patients had experienced dose-limiting toxicities, according to a data readout at the American Association for Cancer Research's 2023 meeting.

Under the terms of the agreement, Pfizer will provide atirmociclib for the clinical trial, while Relay will conduct the study, which is expected to begin by the end of the year.

Cambridge, Massachusetts-based Relay also disclosed Wednesday plans to advance several preclinical programs and to investigate RLY-2608 for treating vascular malformations in Phase III studies in the first quarter of 2025.

Vascular malformations encompass a range of rare syndromes marked by the atypical development of lymphatic and blood vessels that lead to abnormal blood flow and cause pain, swelling, and limb discoloration. Depending on the affected vessel, the condition can be life-threatening.

According to Relay, PI3Kα mutations are key drivers of certain subtypes of the condition, affecting about 170,000 people in the US. The company is hoping that a PI3Kα inhibitor like RLY-2608 could provide better target coverage, efficacy, and tolerability.

Relay also expects to begin clinical development of an NRAS-selective inhibitor in the second half of 2025. Relay said its inhibitor is the first NRAS-selective inhibitor, which aims to overcome the off-target toxicities and efficacy limits of pan-RAS inhibitors by binding only to NRAS, and not KRAS or HRAS.