A Study of DS-1103a Combination Therapy in Participants With Advanced Solid Tumors

Inclusion Criteria: * Sign and date the informed consent form (ICF), prior to the start of any study-specific qualification procedures * Adults ≥18 years of age at the time the ICF is signed (please follow local regulatory requirements if the legal age of consent for study participation is \>18 years old) * Pathologically documented HER2-expressing (immunohistochemistry \[IHC\] 1+ or greater) or HER2-mutated (activating mutation) solid tumor that is unresectable or metastatic (further detailed in inclusion criteria specific for Part 1 and Part 2 below) * Is willing and able to provide adequate baseline tumor samples. If an adequate archival tumor tissue is not available, a fresh tumor tissue biopsy is required * Presence of at least 1 measurable lesion based on computed tomography (CT) or magnetic resonance imaging (MRI) per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) by investigator assessment * Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1 * Has a left ventricular ejection fraction (LVEF) ≥50% by either an echocardiogram (ECHO) or multigated acquisition (MUGA) scan within 28 days before enrollment * Has adequate organ and bone marrow function within 28 days before the start of study treatment. Transfusion (red blood cell or platelet) or granulocyte-colony stimulating factor (G-CSF) administration is not allowed within 14 days prior to the start of study treatment. * If the participant is a female of childbearing pote…