A Study Evaluating DS-1055a in Participants With Relapsed or Refractory Locally Advanced or Metastatic Solid Tumors

Inclusion Criteria: * Has a histopathologically documented locally advanced or metastatic head and neck, gastric, esophageal cancer, non-small cell lung cancer, or melanoma. Participants with other types of solid tumors may be eligible following discussion with the Sponsor. * Has a relapsed or refractory disease that is not amenable to curative standard therapy. * Is 18 years of age or older. * Has an Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0-1, with no deterioration for two weeks. * Has a measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. * Has adequate organ function within 7 days before enrollment. * Is able to provide written informed consent and is willing and able to comply with the protocol. Exclusion Criteria: * Has a concurrently active second malignancy, other than adequately treated non-melanoma skin cancers, in situ melanoma or in situ cervical cancer. Participants with history of the second malignancy have been disease-free for \<3 years. * Has a history of (non-infectious) interstitial lung disease (ILD) that required steroids, currently has ILD, or when suspected ILD cannot be ruled out by imaging at screening. * Has a history of severe pulmonary compromise or requirement of supplemental oxygen within 6 months before enrollment. * Has active hepatitis B or hepatitis C virus infection. * Has received prior immunotherapy with a Grade 3 or higher, or any unresolved ≥Grade 2 immune-related adverse event.