Relay Therapeutics Inc. (NASDAQ: RLAY) announced plans to advance its drug combination zovegalisib plus atirmociclib into Phase 3 development for frontline patients with PI3Kα-mutated, HR+/HER2- metastatic breast cancer.

The company reported clinical data from its ReDiscover study showing a 44% objective response rate in 34 patients with measurable disease who had previously received CDK4/6 inhibitor therapy. These patients had received a median of two prior therapies in the metastatic setting.

Safety data from 62 patients showed that only 3% discontinued zovegalisib and 10% required dose reductions due to treatment-related adverse events. The overall grade 3+ treatment-related adverse event rate was 40%, with neutropenia accounting for the majority of severe events.

The planned Phase 3 trial will compare zovegalisib plus atirmociclib plus aromatase inhibitor against CDK4/6 inhibitor plus aromatase inhibitor in frontline endocrine sensitive patients. The study is expected to initiate in early 2027, subject to regulatory feedback.

Pfizer has agreed to supply atirmociclib for the experimental arm and palbociclib for the control arm through a supply agreement. Relay will sponsor and fund the Phase 3 trial while retaining global rights for zovegalisib.

Pharmacokinetic analyses showed that atirmociclib increased zovegalisib exposure by approximately 2.5-fold, while zovegalisib had no impact on atirmociclib exposure. The potential Phase 3 dose of zovegalisib will be 150mg twice daily, subject to regulatory feedback.

The company continues its ongoing ReDiscover-2 Phase 3 trial in second-line breast cancer and plans to present data for zovegalisib in vascular anomalies at ISSVA 2026 in May.