A Study of Valemetostat Tosylate in Combination With DXd ADCs in Subjects With Solid Tumors

This is a 2-part study of valemetostat in combination with DXd ADCs in participants with human epidermal growth factor 2 (HER2)-positive gastric cancer, non-squamous non-small cell lung cancer (NSCLC), or unresectable or metastatic HER2 low breast cancer. The study will begin with a Part 1 Dose-escalation Phase and will continue until the recommended dose for expansion "RDE" of valemetostat is determined and will then be followed by a Part 2 Dose-expansion Phase to further evaluate the safety and tolerability of the combination.