Announcement
Assembly Biosciences Inc (NASDAQ: ASMB) announced that it exercised its option under the collaboration agreement with Gilead Sciences, Inc. to share 40 % of U.S. development costs and profits for the herpes simplex virus (HSV) helicase‑primase inhibitor program, which includes candidates GS‑1179 and GS‑5366.
Share Reaction
The news triggered a 4.9 % increase in Assembly Bio’s share price in after‑hours trading on Tuesday.
Financial Milestones and Royalties
Under the agreement Assembly Bio remains eligible to receive up to $280 million in regulatory and commercial milestones and tiered royalties on net sales outside the United States, with royalty rates ranging from the high single‑digits to low teens. If the U.S. profit‑share option is exercised and conversion rights are triggered, the potential total could rise to $330 million under the same royalty structure.
Upcoming Payments and Funding
Gilead will make a $75 million option continuation payment to Assembly Bio in the fourth quarter of 2026, coinciding with the third anniversary of the collaboration. Assembly Bio stated that its existing cash, cash equivalents and marketable securities are sufficient to fund operations through 2029.
Development Timeline
GS‑1179 (formerly ABI‑1179) is expected to enter a Phase 2 clinical trial in participants with recurrent genital herpes by the end of 2026, and may later be evaluated as part of a combination regimen with HIV pre‑exposure prophylaxis. Gilead retains sole responsibility for further clinical development and commercialization of the HSV helicase‑primase inhibitor program.
Additional Details
The collaboration allows Assembly Bio to share 40 % of U.S. development costs and profits, while Gilead retains exclusive control over the U.S. development path. The option includes opt‑out and conversion rights for either party, which could affect the ultimate milestone and royalty totals.