Study of Datopotamab Deruxtecan Plus Carboplatin or Cisplatin Versus Gemcitabine Plus Carboplatin or Cisplatin in Participants With Locally Advanced or Metastatic Urothelial Carcinoma

This is a global, multicenter, randomized, open-label, Phase 2/3 study of Dato-DXd plus carboplatin or cisplatin versus gemcitabine plus carboplatin or cisplatin in participants with la/mUC who progressed during or after EV plus pembrolizumab combination treatment. This trial will start with part A, Phase 2. During part A, Phase 2, preliminary efficacy and safety will be assessed, and the recommended Phase 3 dose (RP3D) will be identified when the data allow sufficient assessment of activity, safety, and tolerability. The Phase 3 part will start contingent upon the assessment in the Phase 2 part, taking into consideration the totality of information.

Key Inclusion Criteria: * Adult ≥18 years at the time the ICF is signed (if the legal age of consent is \> 18 years old, then follow the local regulatory requirements). * Histologically or cytologically confirmed unresectable locally advanced (T4b, any N; or any T, N 2-3) or metastatic (any T, any N, M1) urothelial carcinoma of the bladder, renal pelvis, ureter, or urethra. Participants with urothelial carcinoma (transitional cell) with squamous differentiation or mixed cell types are eligible if the histology is predominantly urothelial. \> Note 1: Urachal, small cell, and adenocarcinoma histology is not permitted. * Note 2: Participants with la/mUC and a history of nonclinically active prostate cancer are allowed into the trial if: 1. Participant does not have radiological metastasis of a proven prostate cancer. 2. Participant with nonmetastatic prostate cancer do not have rising PSA (as determined using local testing by a validated or approved test method) defined as follows: * Increase in PSA within 2 consecutive measurements separated by at least 1 week (completed within 4 weeks prior to consent or within Screening) and neither of the measurements with an absolute value above 2 ng/mL. 3. Participant does not currently receive androgen deprivation therapy for the treatment of prostate cancer. * Note 3: Participant with MIBC (T2-T4aN0M0 or T1-T4aN1M0) who received EV (or other agents with a vedotin payload) plus pembrolizumab (or other PD-1/PD-L1 inhibitors) as neoadjuvant/adjuvant therapy and progressed during treatment or within 12 months of treatment completion may be considered for enrollment, with approval from the Sponsor's Medical Monitor or designee. * Must provide tumor tissue sample from archival tissue or newly obtained pretreatment biopsy for exploratory biomarker testing. Tumor tissue sample should not be collected from a lesion that was irradiated unless documentation can be provided confirming that the tumor tissue was collected at least 3 months after radiation and the lesion increased/appeared since radiation occurred. Tumor tissue must be of sufficient quantity (as defined in the Laboratory Manual). a. Archival tissue collected after the most recent anticancer treatment and within 12 months before the informed consent date is preferred. * Must be considered eligible to receive cisplatin- or carboplatin-containing chemotherapy, in the investigator's judgment. Participants eligible for cisplatin will receive cisplatin. If a participant received gemcitabine, carboplatin, or cisplatin for early UC in the adjuvant/neoadjuvant setting, the decision to rechallenge the participant with platinum therapy will be at the discretion of the investigator. Participants only receive carboplatin if they are ineligible for cisplatin. Participants are cisplatin-ineligible if they meet any of the following criteria: 1. GFR \160 mmHg or diastolic blood pressure \>100 mmHg within 28 days before randomization that is not resolved despite maximal medical therapy). * Has a history of non-infectious ILD/pneumonitis including radiation pneumonitis that required steroids, has current ILD/pneumonitis, or has suspected ILD/pneumonitis that cannot be ruled out by imaging at Screening. * Has clinically severe pulmonary compromise as judged by the investigator resulting from intercurrent pulmonary illnesses including, but not limited to, any underlying pulmonary disorder (eg, severe asthma, severe chronic obstructive pulmonary disease, restrictive lung disease, pleural effusion, etc.) or any autoimmune, connective tissue, or inflammatory disorders with pulmonary involvement (eg, rheumatoid arthritis, Sjögren's syndrome, sarcoidosis, etc.), or prior complete pneumonectomy. * Toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet improved to NCI-CTCAE version 5.0 Grade ≤1 or baseline. Note: Participants may be enrolled with chronic, stable Grade 2 toxicities (defined as no worsening to Grade \>2 for at least 3 months prior to randomization and managed with standard of care treatment) which the investigator deems related to previous anticancer therapy, comprised of (including but not limited to): 1. Anticancer therapy-induced neuropathy 2. Residual toxicities from prior immunotherapy treatment: Grade 1 or Grade 2 endocrinopathies which may include: * Hypothyroidism/ hyperthyroidism * Type I diabetes * Hyperglycemia * Adrenal insufficiency * Adrenalitis c. Skin hypopigmentation (vitiligo)