San Francisco biotech Vir eyes hepatitis D rival to Gilead drug

A San Francisco biotech race is now playing out against one of the deadliest forms of viral hepatitis, with Vir Biotechnology trying to build a challenger to Gilead Sciences just as the Foster City drugmaker won the first U.S. approval for hepatitis D.

Hepatitis D virus, or HDV, only infects people who already have hepatitis B. That overlap makes the disease rare, but especially dangerous: HDV can speed liver fibrosis, liver cancer, liver failure and death. Until May 22, there were no FDA-approved treatments in the United States. On that date, the U.S. Food and Drug Administration cleared Gilead’s Hepcludex, also known as bulevirtide-gmod, for adults without cirrhosis or with compensated cirrhosis, calling it the first and only approved therapy for chronic hepatitis delta in the country.

The approval came with a sharp warning. The FDA added a boxed warning because stopping Hepcludex may trigger severe acute exacerbations of both HDV and HBV infection. The agency based the decision in part on the pivotal MYR301 trial, where the combined response rate reached 48% at week 48 in the Hepcludex group, compared with 2% in the delayed-treatment group.

Vir Biotechnology, based in San Francisco, has been working on a different approach. Its Phase 2 SOLSTICE trial of tobevibart and elebsiran showed early activity in a de novo combination cohort, with 12 of 27 patients meeting the combined response endpoint at 12 weeks and 7 of 11 at 24 weeks. Vir said it planned to meet regulators in the third quarter of 2024 and expected a future trial to use bulevirtide as the comparator, a sign that it was preparing to challenge the Gilead standard head-on. The company later received FDA breakthrough therapy designation for the combination, and as of May 18, 2026, said it would present complete week 96 SOLSTICE data at the European Association for the Study of the Liver congress in Barcelona.

South San Francisco-based Assembly Biosciences is also pressing into the space with ABI-6250, an orally bioavailable HDV entry inhibitor. In December 2024, Gilead put about $20.1 million into new equity in Assembly and added another $10 million in accelerated funding, lifting its stake to 29.9% and helping extend Assembly’s cash runway to mid-2026. Assembly said the money would speed ABI-6250 development. By May 2026, it said chronic toxicology studies were complete and Phase 2 was expected to begin in the fourth quarter.

The broader stakes reach beyond one rare disease. A 2025 study estimated HDV prevalence at 1.2% among hepatitis B surface antigen-positive samples in a national laboratory sample, while the Hepatitis B Foundation says U.S. studies have put the rate at 1% to 5% of people living with chronic hepatitis B, or fewer than 200,000 people nationwide. The CDC notes that hepatitis B vaccination protects against both HBV and HDV. For San Francisco, the race matters because it shows whether the region’s next life-sciences leaders can still produce drugs that move from local labs to national standards of care.