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:~Dose Escalation (Part 1)~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.
This study will assess the safety, efficacy, and pharmacokinetics of DS-6157a in participants with advanced gastrointestinal stromal tumors (GIST).