The purpose of this study is to assess the safety and tolerability of DS-1055a in participants with relapsed or refractory locally advanced or metastatic solid tumors for which no standard treatment is available.

This Phase 1, open-label, non-randomized, dose escalation, first-in-human study will assess the safety and tolerability of DS-1055a, determine the maximum tolerated dose of DS-1055a, pharmacokinetic (PK) properties of DS-1055a, and the incidence of anti-drug antibodies (ADAs) against DS-1055a and other antibodies.

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.

De-identified individual participant data (IPD) on completed studies and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Completed studies that has reached a global end or completion with all data set collected and analyzed, and for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents on completed clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.