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GIST

(P 194) UPDATED RESULTS FROM STRATEGIST 1: A PHASE 1/1B STUDY OF IDRX-42 (GSK6042981A) IN PATIENTS (PTS) WITH ADVANCED GASTROINTESTINAL STROMAL TUMORS (GIST)

Objective: Most GIST are driven by mutations in KIT and resistance to approved KIT inhibitors is largely mediated by additional mutations in KIT exons 13 and 17. IDRX-42 is a novel, highly selective KIT tyrosine kinase inhibitor (TKI) with preclinical activity across clinically relevant activating and secondary resistance KIT mutations. We present updated results from a phase 1/1b study (StrateGIST 1) evaluating IDRX-42 in pts with advanced GIST.

Methods: Phase 1 enrolled previously treated pts at doses of 120 mg to 1200 mg daily. Phase 1b enrolled pts at the recommended phase 1b dose (RP1bD) of 300 mg QD (tablet) in 4 cohorts: 1st line, 2nd line, and two ≥3rd-line cohorts (prior approved TKIs +/− investigational TKIs). This pooled phase 1/1b data focuses on clinical activity in 2nd and later-line pts at the RP1bD.

Results: As of April 28, 2025, 167 pts across all cohorts received IDRX-42 at the RP1bD (or exposure-equivalent 400-mg capsules) for a median duration of 6.4 months. Median age was 58 years old; 5%, 26%, 11%, and 57% of pts received 0, 1, 2, and ≥3 prior lines of therapy for advanced GIST, respectively. All enrolled pts had GIST harboring any KIT or nonexon-18 PDGFRA mutation(s) at baseline. Table 1 presents investigator-assessed clinical activity in 2nd and later-line pts. At the RP1bD, the most common grade ≥3 treatment-emergent adverse events were anemia (16%), neutropenia (9%), vomiting (3%), and abdominal pain (3%).

Conclusion: IIDRX-42 exhibits promising activity and safety in previously treated GIST, warranting further study in early-line pts.

Table 1: Clinical Activity per Modified RECIST v1.1
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