Gilead Sciences Expands Antiviral Portfolio with $35M Deal for HSV Assets from Assembly Biosciences

Gilead Sciences has taken a significant step in bolstering its antiviral pipeline by exercising its option to license two promising herpes simplex virus (HSV) assets from Assembly Biosciences. The move, announced on December 22, 2025, marks the first tangible outcome of a strategic partnership formed between the two companies in October 2023.

Deal Details and Asset Overview

Gilead will pay $35 million upfront to acquire the rights to ABI-1179 and ABI-5366, two helicase-primase inhibitors designed to treat recurrent genital herpes. Both compounds are currently in phase 1b trials and have demonstrated strong antiviral activity and improvements in clinical outcomes, including a significant reduction in virus-positive lesions.

The pharmaceutical giant's decision comes two years after securing first option rights on Assembly's entire pipeline through a $100 million agreement. This earlier deal included a $15.2 million equity investment, granting Gilead a 19.9% stake in Assembly Biosciences, with the option to increase ownership to up to 49.8%.

Clinical Promise and Development Strategy

Both ABI-1179 and ABI-5366 have shown potential for once-weekly oral dosing, a feature that could significantly improve patient compliance and treatment outcomes. The assets' pharmacokinetic and safety profiles support this dosing regimen, potentially offering a competitive edge in the HSV treatment landscape.

Dr. Jared Baeten, Gilead's virology therapeutic area head, emphasized the company's commitment to developing "novel antiviral therapeutics that aim to deliver meaningful solutions that improve the lives of people affected by serious viral infections." He added, "Our research partnership with Assembly Bio has been highly fruitful, and we are excited to continue the clinical development of our herpes simplex virus candidates."

Strategic Implications and Future Prospects

This deal underscores Gilead's continued focus on expanding its antiviral portfolio beyond its well-established HIV and hepatitis C franchises. For Assembly Biosciences, the agreement represents a validation of its research capabilities and pipeline potential.

Jason Okazaki, CEO of Assembly Biosciences, expressed optimism about the collaboration, stating, "With Gilead's proven track record in developing and commercializing antiviral medicines, exercising this option is an important step forward as they take control of ABI-5366 and ABI-1179 and advance closer to delivering long-awaited new treatment options to patients suffering from recurrent genital herpes."

The agreement maintains Assembly's eligibility for up to $330 million in regulatory and commercial milestones, as well as tiered royalties on net sales. Additionally, Assembly retains the right to opt-in for a 40% share of costs and profits for these products in the U.S., offering potential for significant long-term value.