On October 17, 2023, Gilead Sciences and Assembly Biosciences, a biotechnology company developing innovative antiviral therapeutics targeting serious viral diseases, announced that they have entered into a 12-year partnership to advance the research and development of novel antiviral therapies, with an initial focus on Assembly Bio’s established areas of herpesviruses, hepatitis B virus (HBV), and hepatitis D virus (HDV). Wilson Sonsini Goodrich & Rosati advised Assembly Bio on the transaction.

Assembly Bio’s current portfolio of small molecule antiviral therapeutics includes both clinical and preclinical programs, including next-generation core inhibitor ABI-4334 for the treatment of HBV, long-acting helicase-primase inhibitor ABI-5366 for herpes simplex virus (HSV), an orally bioavailable HDV entry inhibitor ABI-6250, and a pan-herpes polymerase inhibitor program.

Under the terms of the agreement, Assembly Bio will receive $100 million, consisting of an $84.8 million upfront payment and a $15.2 million equity investment from Gilead. Gilead’s initial equity investment at a premium represents 19.9 percent of the outstanding voting stock of Assembly Bio as of the date of closing. In addition, subject to certain conditions, Gilead has agreed to purchase up to 29.9 percent of Assembly Bio’s outstanding voting stock at a premium.

Gilead may opt in to obtain exclusive rights for each of Assembly Bio’s current and future programs, including two preclinical programs targeting HSV and transplant-associated herpesviruses that Gilead is licensing to Assembly Bio, upon payment of an opt-in fee of at least $45 million per program after clinical proof-of-concept is achieved. If Gilead opts in to any current or future program under the collaboration, Assembly Bio is eligible to receive up to $330 million per program in potential regulatory and commercial milestones, in addition to royalties ranging from the high single digits to high teens. During the term of the collaboration, Assembly Bio will also be eligible to receive three separate $75 million collaboration extension payments at prespecified timepoints during the collaboration term to help fund future research and development.

The Wilson Sonsini team that advised Assembly Bio on the transaction includes Ian Edvalson, Jennifer Knapp, Lowell Segal, Christina Poulsen, Shefali Tandon, and Tyler Kivley.

For more information, please see the companies’ joint news release.