Assembly Bio Shares Rise After Opting Into Profit-Sharing Deal With Gilead for Herpes Drug

Shares of Assembly Biosciences (NASDAQ: ASMB) climbed in after-hours trading Tuesday after the company announced it will deepen its partnership with Gilead Sciences, Inc. for a promising herpes simplex virus (HSV) treatment. Assembly Bio exercised its option to share in the U.S. costs and profits for the drug program, signaling confidence in its clinical and commercial prospects.

Details of the Agreement

Assembly Bio has opted to co-fund 40% of future U.S. development costs for its HSV helicase-primase inhibitor (HPI) program, according to a company statement. In return, it will receive a 40% share of the potential profits from the program in the United States. The decision was made after Assembly Bio reviewed the complete development plan and budget from its partner, Gilead.

Under the terms of the collaboration:

• Gilead retains sole responsibility for the clinical development and commercialization of the HSV HPI program, which includes candidates GS-1179 and GS-5366.
• Assembly Bio remains eligible for up to $280 million in regulatory and commercial milestone payments.
• The company is also entitled to tiered royalties on net sales outside the U.S., ranging from the high single-digits to low teens.

An opt-out clause exists for both parties. If exercised, the agreement could convert to a structure where Assembly Bio would be eligible for up to $330 million in milestones and adjusted royalty tiers.

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Clinical and Financial Outlook

The lead drug candidate, GS-1179, is set to advance into a Phase 2 clinical trial by the end of 2026 for patients with recurrent genital herpes. Gilead selected the compound for advancement based on Phase 1b results that suggested its potential for once-weekly oral dosing. The development plan also includes a potential evaluation of the drug as part of a combination strategy with HIV pre-exposure prophylaxis (PrEP).

Financially, Assembly Bio is slated to receive a $75 million option continuation payment from Gilead in the fourth quarter of 2026. The company stated that its current cash, cash equivalents, and marketable securities are expected to fund its operations into 2029.

Market Impact

Investors reacted positively to the news, with shares of Assembly Biosciences rising 4.9% in after-hours trading on Tuesday. The decision to co-invest in the U.S. market, rather than simply collect royalties, suggests management's strong belief in the HSV program's potential. This move transforms Assembly Bio from a more passive partner into an active stakeholder in the drug's largest potential market, tying its future success more closely to the clinical outcome of the collaboration with Gilead.