GSK plc ( GSK ) agreed to acquire IDRx, Inc., a biopharmaceutical company focused on precision therapeutics for gastrointestinal stromal tumors (GIST), for $1 billion.

Under the agreement, GSK could pay an additional $150 million based on regulatory approval milestones.

The acquisition includes the lead molecule IDRX-42. It’s a highly selective KIT TKI being developed as a first and second-line therapy for the treatment of GIST.

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GIST typically presents in the GI tract, with 80% of cases driven by mutations in the KIT gene that lead to tumor cells’ growth, proliferation, and survival (primary or activating mutations).

90% of patients treated in the first-line develop new KIT mutations (secondary or resistance mutations) that typically lead to relapse with limited therapeutic options.

Currently, there are no approved TKIs that inhibit the full spectrum of clinically relevant primary and secondary mutations in KIT.

IDRX-42 has demonstrated activity against all key primary and secondary KIT mutations, designed to improve outcomes for patients with GIST.

Updated clinical data from StrateGIST 1, an ongoing phase 1/1b trial of IDRX-42 in patients with advanced GIST, were presented in an oral presentation at the Connective Tissue Oncology Society 2024 Annual Meeting.

Across patients with second-line or greater GIST, and amongst all KIT mutation subsets, the objective response rate (ORR) in the total efficacy evaluable population was 29% (n=87), including one complete response (CR) and 24 partial responses (PRs).

Among patients with one prior line of therapy, the ORR was 53% (n=15), including one CR and 7 PRs.

Across all patients, two of the PRs were awaiting confirmation at the time of the data cut, both of which were subsequently confirmed. The emerging durability data from StrateGIST 1 was also favorable. IDRX-42 was generally well-tolerated, and treatment-related adverse events (TRAEs) were mainly low-grade at the recommended phase Ib dose.

The transaction supports GSK’s ambitions for growth through 2031 and beyond.

GSK will also be responsible for success-based milestone payments and tiered royalties for IDRX-42 owed to Merck KGaA ( MKGAF ) (OTC:MKKGY) in Darmstadt, Germany.

Price Action: GSK stock is down 1.33% at $32.65 during the premarket session on the last check Monday.

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