Gilead Sciences has handed back a next-gen hepatitis B (HBV) antiviral to Assembly Biosciences, while the two companies continue to partner on other assets.
It means Assembly is now back in sole control of ABI-4334, a highly potent capsid assembly modulator candidate the biotech took into a phase 1b trial in 2024.
Gilead first took an interest in Assembly back in October 2023, when the drugmaker paid $100 million upfront for opt-in rights on all of Assembly’s current and future programs. That 2023 pact—which tied Assembly to Gilead for 12 years—included two treatments for recurrent genital herpes the pharma went on to license for $35 million in December.
At the time, Gilead’s virology therapeutic area head Jared Baeten, M.D., Ph.D., put the decision to license the two herpes assets in the context of the pharma’s work to “develop novel antiviral therapeutics that aim to deliver meaningful solutions that improve the lives of people affected by serious viral infections.”
But Gilead clearly didn’t feel as strongly about ABI-4334. In Assembly’s full-year 2025 earnings release, Assembly disclosed that its partner had “declined to exercise or defer its option” to bag the rights to the HBV candidate.
As a result, Assembly said it has “initiated a structured process to find a partner for ABI-4334 after regaining sole rights to the program.”
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Gilead’s decision will chip away at the $330 million in regulatory and commercial milestones Assembly could have potentially received from its original 2023 agreement.
While Gilead may have lost interest in ABI-4334, the pharma has another HBV bet in the works via a separate long-running partnership with Hookipa Pharma. Last year, Gilead paid $10 million to secure the rights to arenavirus immunotherapies for both HBV and HIV. The partners launched a phase 1a/1b trial of HB-400, an arenaviral therapeutic vaccine for chronic HBV, back in 2023.
Gilead already markets Vemlidy for patients with chronic HBV who have stable liver disease. The antiviral brought in $1.07 billion in global sales (PDF) last year.
