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

Inclusion Criteria:

1. 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.
2. Has a relapsed or refractory disease that is not amenable to curative standard therapy.
3. Is 18 years of age or older.
4. Has an Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0-1, with no deterioration for two weeks.
5. Has a measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.
6. Has adequate organ function within 7 days before enrollment.
7. Is able to provide written informed consent and is willing and able to comply with the protocol.

Exclusion Criteria:

1. 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.
2. 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.
3. Has a history of severe pulmonary compromise or requirement of supplemental oxygen within 6 months before enrollment.
4. Has active hepatitis B or hepatitis C virus infection.
5. Has received prior immunotherapy with a Grade 3 or higher, or any unresolved ≥Grade 2 immune-related adverse event.