First-in-Human Trial of DS-2243a in Participants With Advanced Solid Tumors

Key Inclusion Criteria: 1. Sign and date the main ICF. 2. Adults ≥18 years at the time the biosample ICF or main ICF, whichever is signed first. Follow local regulatory requirements if the legal age of consent for trial participation is \>18 years old. 3. One of the following histologically or cytologically documented cancers: Advanced (metastatic or unresectable) SS Advanced (metastatic or unresectable) MRCLS Metastatic or unresectable locally advanced NSCLC (Ad/Sq) Metastatic or unresectable locally advanced UC 4. Relapsed from, refractory to, or intolerant to appropriate therapies \[eg, standard of care (SOC) therapy\] to provide clinical benefit for their condition as assessed by their physician and/or investigator. \[For South Korea only: Relapsed from, refractory to, or intolerant to all available therapies (eg, SOC therapy domestically approved) for their condition.\] 5. HLA-A\*02:01, 02:02, 02:03, 02:04, 02:05, 02:06, 02:09, 02:10, or 02:11 positive. 6. Has measurable disease based on RECIST v1.1 on computed tomography/magnetic resonance imaging (CT/MRI). 7. Is willing and able to provide adequate pre-treatment or archival tumor tissue sample. 8. Has Eastern Cooperative Oncology Group Performance Status (ECOG PS) score of 0 or 1 at Screening. 9. Meets the following required baseline local laboratory data within 14 days prior to start of trial intervention administration: 1. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤3 × upper limit of normal (ULN) in participants with no liver metastasis, or ≤5 × ULN in participants with liver metastasis 2. Total bilirubin (TBL) ≤1.5 × ULN (≤3 × ULN for participants with a documented history of Gilbert's syndrome) 3. Absolute neutrophil count (ANC) ≥1.5 × 10\^9/L 4. Platelet count ≥100 × 10\^9/L 5. Hemoglobin ≥8 g/dL 6. Creatinine clearance (CrCl) ≥45 mL/min as calculated using the Cockcroft-Gault equation (Section 10.3.1) Note: Any blood or blood product transfusion is allowed up to 7 days before the hematology evaluations and any dose of hematologic growth factor is allowed up to 14 days before the hematology evaluations. 10. a) A female participant of childbearing potential (CBP), as defined in Section 10.3.5.1, is eligible to participate if the following conditions are met: Participant is not pregnant as confirmed by highly sensitive pregnancy test (see Section 10.3.5.3). Participant does not breastfeed during the trial intervention period and for at least 120 days after last dose of trial intervention. Participant agrees to adhere to a contraceptive method that is highly effective (Section 10.3.5.2) and agrees not to donate eggs (ova, oocytes) to others or freeze/store eggs during the intervention period and for at least the time needed to eliminate the trial intervention after the last dose. The length of time required to continue contraception after last dose for the trial intervention is 120 days. Preservation of eggs may be considered prior to first dose of trial intervention. b) A male participant capable of producing sperm is eligible to participate if he agrees to the following during the Treatment Period and for at least the time needed to eliminate the trial intervention. The length of time required to continue contraception after the last dose of the trial intervention is 120 days. * Avoid donating sperm. Note: Preservation of sperm should be considered prior to enrollment/randomization in this trial. * Adhere to either of the following contraception methods: * True abstinence from penile-vaginal intercourse, when this is in line with the preferred and usual lifestyle of the participant, OR * Uses a penile/external condom when having penile-vaginal intercourse with a nonparticipant of childbearing potential (see Section 10.3.5) PLUS partner use of an additional contraceptive method (see Section 10.3.5.2), as a condom may break or leak. Contraceptive use should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. If the contraception requirements in the local label for any of the trial interventions are more stringent than the requirements above, the local label requirements are to be followed. Note: If the participant is azoospermic (vasectomized or secondary to medical cause, documented from the site personnel's review of the participant's medical records, medical examination, or medical history interview), no contraception is required. Key Exclusion Criteria: 1. Has received prior therapy targeting NY-ESO-1. 2. Has an inadequate treatment washout period prior to the start of trial intervention, defined as follows: 1. Radiation therapy: \<4 weeks (or \<2 weeks if palliative radiation therapy without abdominal/pelvic radiation) 2. Chemotherapy, antibody-based anticancer therapy, immunotherapy: \<4 weeks 3. Small molecules (eg, tyrosine kinase inhibitors): \<2 weeks or 5 half-lives, whichever is longer 3. Has known symptomatic central nervous system (CNS) metastases, leptomeningeal disease, or cord compression. Note: Asymptomatic or adequately treated CNS metastases are not exclusionary provided that, in the opinion of the investigator, the participant is neurologically stable. MRI/CT of the brain is required for all participants during Screening Period (see Section 8.3.1). 4. Uncontrolled or clinically significant cardiovascular disease, including the following: 1. Myocardial infarction within 6 months prior to screening 2. Uncontrolled angina pectoris within 6 months prior to screening 3. New York Heart Association (NYHA) Class III or IV congestive heart failure 4. Left ventricular ejection fraction (LVEF) ≤50% or lower than the institutional lower limit of normal 5. QT interval corrected with Fridericia's formula (QTcF) interval \>480 ms 5. Chronic steroid treatment (IV or oral) or any other immunosuppressive medication (ie, prednisone \>10 mg dail