This is a single group study of participants with advanced solid tumors who have not been cured by other treatments. It is the first time the drug will be used in humans, and will be in two parts.

The primary purpose of the parts are:

• Dose Escalation Part: To evaluate the safety and tolerability and to determine the maximum tolerated dose and the recommended dose for expansion of ifinatamab deruxtecan (I-DXd).
• Dose Expansion Part: To investigate the safety, tolerability and antitumor activity of I-DXd when administered as a single agent.

This study is expected to last approximately 5 years from the time the first participant is enrolled to the time the last participant is off the study.

The number of treatment cycles is not fixed in this study. Participants who continue to benefit from the study treatment may continue, unless:

• they withdraw
• their disease gets worse
• they experience unacceptable side effects.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 or 1.
• Has at least 1 measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 on computed tomography (CT) or magnetic resonance imaging (MRI) as assessed by Investigator. Measurable lesions should not be from a previously irradiated site. If the lesion at a previously irradiated site is the only selectable target lesion, a radiological assessment showing significant progression of the irradiated lesion should be provided by the Investigator
• Has adequate cardiac, hematopoietic, renal and hepatic functions
• Has an adequate treatment washout period prior to start of study treatment
• Has a pathologically documented advanced/unresectable or metastatic head and neck squamous cell carcinoma, esophageal squamous cell carcinoma, squamous and adenocarcinoma non-small cell lung cancer (NSCLC), small cell lung cancer (SCLC), bladder cancer, sarcoma, endometrial cancer, melanoma, adenocarcinoma CRPC (primary neuroendocrine or histologically confirmed neuroendocrine differentiated prostate cancer is not allowed), breast cancer that is refractory to or intolerable with standard treatment, or for which no standard treatment is available.

For Expansion Cohort 4 2L ESCC participants only:

• Has disease progression a post platinum-based and an immune checkpoint inhibitor (ICI) treatment per global or local guidelines, with a maximum of one prior line of systemic therapy for unresectable advanced or metastatic ESCC.

Exclusion Criteria:

• Has prior treatment with B7-H3 targeted agent, including I-DXd.
• Has had prior discontinuation of an antibody drug conjugate (ADC) that consists of an exatecan derivative (e.g., trastuzumab deruxtecan) due to treatment-related toxicities.
• Has multiple primary malignancies within 3 years, except adequately resected non-melanoma skin cancer, curatively treated in situ disease, superficial GI tract tumors and non-muscle invasive bladder cancer curatively resected by endoscopic surgery.
• Uncontrolled significant cardiovascular disease
• Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses including, but not limited to, any underlying pulmonary disorder, or any autoimmune, connective tissue or inflammatory disorders with potential pulmonary involvement, prior pneumonectomy, or requirement for supplemental oxygen
• Has an uncontrolled infection requiring systemic therapy.
• Has substance abuse or any other medical conditions that would increase the safety risk to the subject or interfere with participation of the subject or evaluation of the clinical study in the opinion of the Investigator.

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.