Gilead Sciences bought a large block of shares of antiviral-therapeutics developer Assembly Biosciences as part of an amended investment and funding agreement.

Gilead paid $20.1 million on Dec. 19 for 940,500 Assembly shares, an average price of $21.37 each. Gilead now owns 2.21 million Assembly shares, a 29.9% stake, according to a form Gilead filed with the Securities and Exchange Commission. The shares were newly issued by Assembly, and represented a 35% premium to a calculated 30-day volume-weighted average price immediately before the date of purchase.

Under the amendment, Gilead will also provide $10 million in accelerated funding to support Assembly’s pipeline of antiviral therapeutic candidates for herpesviruses, hepatitis D virus, and hepatitis B virus. Assembly expects to release data “from multiple ongoing clinical studies in the coming year,” including the interim Phase 1b proof-of-concept data readout for ABI-5366, a long-acting helicase-primase inhibitor for treatment of recurrent genital herpes, projected for the first half of 2025.

The amendment adjusts the option timepoints and option payment structure for ABI-6250, an orally bioavailable viral entry inhibitor for hepatitis D virus.

Assembly said the stock sale and accelerated funding will extend its cash runway to mid-2026.

“Gilead’s further investment strengthens our balance sheet as we look ahead to multiple key clinical data readouts for our novel antiviral candidates in 2025,” said Assembly CEO Jason Okazaki in a news release.

Gilead didn’t respond to a request for comment on the stock purchase and amended agreement. It remains Assembly’s largest shareholder.