Background: REG, an oral receptor kinase inhibitor with activity against KIT, PDGFR, VEGFR, FGFRs, and other oncologic targets, demonstrated significant improvement in progression-free survival (PFS) over placebo (PL) in a phase III study (GRID) of patients (pts) with advanced GIST following failure of at least imatinib (IM) and sunitinib (SU). To understand the impact of pts’ baseline characteristics on outcome, we performed an exploratory analysis of REG effects across pt subgroups based on sex, age, and mitotic index of primary GIST tissue, as well as duration and number of lines of previous therapies. Methods: Adult pts with metastatic GIST (n=199) progressing after at least IM and SU were randomized 2:1 to receive oral REG 160 mg or PL once daily for the first 3 weeks of each 4-week cycle. Subgroup analysis of centrally assessed PFS was performed, based on sex, age (<65, ≥65 years), mitotic index of primary GIST (<5, ≥5 to <10, ≥10 mits/50 hpf), duration of IM and SU treatment (<6, ≥6 to <18, ≥18 months), and number of prior anticancer treatment lines (2 or ≥3). Mitotic index data on primaries were available for 119 patients. Results: REG demonstrated improvement in PFS vs PL in all subgroups, as summarized in the Table. Conclusions: REG had substantial efficacy in all patient subgroups included in this analysis. Clinical trial information: NCT01271712.