Assembly Biosciences (Nasdaq: ASMB) has exercised its embedded option to co-share 40% of US development costs and profits for the herpes simplex virus (HSV) helicase-primase inhibitor (HPI) program it licensed to Gilead Sciences in December 2025, converting from a pure royalty recipient into a direct US commercial participant in the program.
The decision covers both GS-1179 (formerly ABI-1179) and GS-5366 (formerly ABI-5366), two small-molecule HPIs targeting recurrent genital herpes. Gilead retains sole responsibility for clinical development and commercialization of both candidates. Assembly Bio's opt-in followed its receipt and review of Gilead's complete development plan and budget for the program. Gilead has selected GS-1179 to advance into a Phase II clinical trial in recurrent genital herpes. Assembly Bio's 40% share of US economics would also apply to expanded uses of the program, with combination-product economics allocated according to the relative contribution of the applicable components.
Under the terms of the underlying collaboration agreement, Assembly Bio remains eligible to receive up to USD 280 million in regulatory and commercial milestones plus tiered royalties on net sales outside the US ranging from the high single digits to low teens. The US profit-share participation is subject to opt-out and conversion rights for either party, which would transition Assembly Bio to up to USD 330 million in milestones and tiered royalties on all net sales in the same royalty range.
Gilead exercised its option to license the HPI programs in December 2025, triggering a USD 35 million net payment (a USD 45 million option fee, net of USD 10 million in pre-advanced funding under a December 2024 amendment). Positive Phase Ib data for both ABI-5366 and ABI-1179 were presented publicly at ESCMID Global 2026 in April, including results from a monthly dosing cohort for ABI-5366. Assembly Bio subsequently raised USD 115 million in a public equity offering in May 2026, proceeds from which are available to fund its share of Phase II and Phase III costs.