03:51 PM EST, 01/13/2025 (MT Newswires) -- GSK (GSK) has agreed to acquire Boston-based IDRx for up to $1.15 billion as the British drugmaker looks to expand its gastrointestinal cancer portfolio, the companies said Monday.

IDRx is behind IDRX-42, which is being developed as a first- and second-line therapy to treat gastrointestinal stromal tumors, or GIST.

GSK's New York Stock Exchange-listed shares were down 1.3% in Monday late-afternoon trade.

"IDRX-42 has demonstrated activity against all key primary and secondary KIT mutations, designed to improve outcomes for patients with GIST," according to a joint statement. "This breadth of mutational coverage, in addition to high selectivity which could improve tolerability, provides potential for a best-in-class profile."

The deal consideration consists of a $1 billion upfront payment, with potential for another $150 million regulatory approval milestone payment. GSK will be responsible for success-based milestone payments, as well as tiered royalties for IDRX-42 owed to Merck ( MRK ) . GSK said it expects the deal to support its growth plans through 2031 and beyond.

In November, IDRx said that IDRX-42 showed a "promising" anti-tumor activity in a phase 1/1b trial, with a 29% objective response rate across all patients and a 53% rate in second-line patients.

"We are excited by the early data from IDRX-42 and its unique ability to target all clinically relevant KIT mutations present in GIST, a major gap in the current standard of care," GSK Chief Scientific Officer Tony Wood said Monday. "We look forward to accelerating its development in 2025 to redefine treatment."

The US Food and Drug Administration has granted IDRX-42 fast track designation and orphan drug designations for the treatment of GIST, the companies said.

"We are looking forward to working with GSK to advance IDRX-42 for patients with GIST given there have been no major advances to the standard of care for almost 20 years," IDRx Chief Executive Tim Clackson said.

IDRx counts Andreessen Horowitz, Casdin Capital, Nextech Invest, Forge Life Science Partners, and RA Capital Management among its investors.

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