DS-6157a in Participants With Advanced Gastrointestinal Stromal Tumor (GIST)

This study is a two-part, multicenter, open-label, multiple-dose, first-in-human study of the antibody-drug conjugate (ADC) DS-6157a given as a single agent to participants with gastrointestinal stromal tumor (GIST). This study will include 2 parts: 1. Dose Escalation (Part 1) 2. Dose Expansion (Part 2) Dose Escalation: Participants with histopathologically documented advanced GIST not amenable to curative therapy may be included in which the maximum tolerated dose (MTD) and/or recommended dose for expansion (RDE) of DS-6157a monotherapy will be determined. Dose Expansion: Once the RDE(s) is established for DS-6157a (Part 1), enrollment in Dose Expansion (Part 2) will commence in 2 cohorts. Participants with GIST who have progressed on or are intolerant to imatinib (IM) and at least one post-IM treatment will be enrolled in Cohort 1, and participants with GIST who progressed on IM and had not received a post-IM treatment (2nd line) will be enrolled in Cohort 2. The study was terminated after Dose Escalation and the study never proceeded to the Dose Expansion part.