Background: Patients with GIST often relapse after first line therapy with imatinib due to secondary resistance mutations in KIT. As the KIT mutation targeting profiles of bezuclastinib (type I TKI) and sunitinib are distinct and complementary, when combined they inhibit a broad spectrum of secondary KIT mutations. In a prior Phase 1b/2a study (NCT02401815), the combination of the original formulation (Form. A) of bezuclastinib + sunitinib had an acceptable safety profile and was associated with clinical activity at the RP2D (Wagner, JAMA Oncol 2021;7(9):1343-50). An optimized formulation (Form. B) of bezuclastinib with improved bioavailability was developed to reduce pill burden for GIST pts. PK and safety results from the Part 1a lead-in of Peak (NCT05208047) have been previously presented (Wagner, CTOS 2022, P320). Herein we intend to report comprehensive clinical results from Part 1a including duration of therapy, initial response assessment and updated safety. Methods: Peak, a randomized Phase 3, open-label study, aims to evaluate efficacy and safety of bezuclastinib + sunitinib vs sunitinib in pts with imatinib-resistant or intolerant GIST. In Part 1a of the 3-part study, bezuclastinib (Form. B) dosing was escalated in serial cohorts based on PK results until the target exposure, comparable to those achieved at the RP2D established in the Phase 1b/2a study, was achieved. Key inclusion: adult with locally advanced, metastatic and/or unresectable GIST, ≥1 measurable lesion according to modified RECIST v1.1, and ECOG PS 0 to 2. Results: Part 1a enrolled 19 pts. Five pts (Cohort 1) received the starting dose of once daily (QD) bezuclastinib 300 mg+ sunitinib 37.5 mg; dose was escalated to 600 mg with 14 pts receiving bezuclastinib 600 mg + sunitinib 37.5 mg. Median age - 60 yrs (range: 42-77); 68% male; 95% ECOG PS 0-1; 95% metastatic and 5% locally advanced. As of Sept 2022 data cutoff, the median (range) treatment duration was 6 weeks (3.1, 23.9). Due to AEs, one pt required dose reduction of bezuclastinib (Gr 3 diarrhea) and 1 pt discontinued (Gr 2 rash). The majority of TEAEs were low grade, with no ≥ Gr4. Most common TRAEs were diarrhea (37%), neutropenia (37%), ALT (32%) and AST (26%) increases. One pt experienced SAEs of Gr 2 neutropenia and pyrexia and Gr 3 thrombocytopenia. Steady state exposure in pts receiving QD doses of bezuclastinib 600 mg + sunitinib 37.5 mg in this study were similar to that at the RP2D established in the prior Phase 1b/2a study. Conclusions: Initial data from Peak shows an encouraging safety and tolerability profile with no unique safety signals when compared to the known safety profile for sunitinib monotherapy. A dose of bezuclastinib 600 mg QD + sunitinib 37.5 mg QD was confirmed for use in Peak study Part 2, for which enrollment is ongoing. Updated data from Part 1a, including response data, will be presented. Clinical trial information: NCT05208047.