Assembly Biosciences, based in South San Francisco, has exercised its option to participate in 40% of the U.S. profit share for its herpes simplex virus (HSV) helicase-primase inhibitor program, a collaboration with Gilead Sciences. The agreement centers on two drug candidates, GS-1179 and GS-5366, both designed to disrupt viral replication through novel mechanisms compared to existing nucleoside analog therapies. Gilead selected GS-1179 for its potential to deliver once-weekly oral dosing, with Phase 2 trials expected to begin by the end of 2026 for recurrent genital herpes. The program may also explore combination strategies with HIV pre-exposure prophylaxis (PrEP).
Under the terms, Gilead retains full responsibility for clinical development and commercialization of the HSV program. Assembly Bio is eligible to receive up to $280 million in regulatory and commercial milestones, alongside tiered royalties on net sales outside the U.S., ranging from high single-digits to low teens. The U.S. profit-sharing arrangement is subject to opt-out or conversion rights, which could shift terms to include up to $330 million in milestones and royalties on net sales, also spanning high single-digits to low teens. A fourth-quarter 2026 payment of $75 million extends the collaboration’s financial runway through 2029.
The deal underscores Assembly Bio’s strategic focus on advancing therapeutic options for chronic viral infections, while Gilead’s involvement strengthens its pipeline in infectious disease treatments. The milestone structure and royalty terms reflect a balanced risk-sharing model, aligning incentives between the parties to drive commercial success.
