Assembly Biosciences shares gained traction in after-hours trading on Tuesday, rising 4.9% after the company said it had exercised its contractual option to participate in a 40% split of U.S. development costs and profits for a herpes simplex virus (HSV) helicase-primase inhibitor program run in collaboration with Gilead Sciences.
The program under the agreement focuses on two investigational agents - GS-1179 and GS-5366 - aimed at treating recurrent genital herpes. Assembly said its decision followed a review of Gilead's complete development plan and budget for the program.
By electing to join the profit-sharing arrangement for U.S. activity, Assembly stands to receive a material portion of any potential commercial returns in the United States, while Gilead will continue to carry sole responsibility for clinical development and global commercialization efforts.
Assembly also retains eligibility for up to $280 million tied to regulatory and commercial milestones, in addition to tiered royalties on net sales outside the United States. The company reiterated that its cash runway extends into 2029, a position supported in part by a $75 million collaboration extension fee from Gilead that is due in the fourth quarter of 2026.
The wider market did little to influence the move, with the S&P 500 nearly unchanged, the Dow Jones Industrial Average marginally lower, and the Nasdaq slightly in the red - indicating the after-hours lift in Assembly's stock was driven by company-specific developments rather than broad market momentum.
Gilead selected the lead HSV candidate, GS-1179, for advancement based on Phase 1b data and because of its potential to allow once-weekly oral dosing, a profile the companies say would differentiate it from current nucleoside analog therapies.
Observers pointed to three factors that combined to bolster investor sentiment: Assembly's opt-in to a 40% U.S. share of development costs and profits, the company reaffirming its funding runway into 2029, and the near-term Phase 2 catalyst for GS-1179. Those elements helped push shares to $37 in after-hours trade, above the regular-session high of $36.68 and nearer the stock's 52-week high of $39.705.
Contextual note: The company-specific announcement and financial reaffirmation were cited as the primary drivers of the after-hours move.