This study will enroll participants with urothelial cancer (UC). UC can include cancer of the bladder, kidney, or the tubes that carry pee through the body (ureter, urethra). This study will try to find out if the drugs disitamab vedotin with pembrolizumab works better than platinum-containing chemotherapy to treat patients with UC. This study will also test what side effects happen when participants take these drugs together. A side effect is anything a drug does to the body besides treating the disease.

Participants in this study will have cancer that has spread through the body (metastatic) or spread near where it started (locally advanced).

In this study, there are 2 different groups. Participants will be assigned to a group randomly. Participants in the disitamab vedotin arm will get the study drug disitamab vedotin once every two weeks and pembrolizumab once every 6 weeks. Participants in the standard of care arm will get gemcitabine once a week for 2 weeks with either cisplatin or carboplatin once every 3 weeks.

Inclusion Criteria:

• Histopathological confirmation of locally advanced unresectable or metastatic urothelial carcinoma (LA/mUC), including UC originating from the renal pelvis, ureters, bladder, or urethra.
• Measurable disease by investigator assessment per RECIST v1.1.
• Participant must not have received prior systemic therapy for LA/mUC. Exception will be made for neoadjuvant or adjuvant therapy, if disease recurrence/progression occurred more than 12 months after the last dose of therapy.
• Eligible to receive cisplatin- or carboplatin-containing chemotherapy.
• Able to provide archived formalin-fixed paraffin-embedded tumor tissue blocks from a muscle-invasive or metastatic UC lesion or biopsy of metastatic UC prior to treatment initiation. If archival tissue is not available a newly obtained baseline biopsy of an accessible tumor lesion is required within 28 days of cycle 1 day 1.
• HER2 expression of 1+ or greater on immunohistochemistry (IHC).
• Eastern Cooperative Oncology Group (ECOG) performance score of 0, 1, or 2 within 7 days prior to randomization.

Exclusion Criteria:

• Known hypersensitivity to disitamab vedotin, cisplatin, carboplatin, gemcitabine, or pembrolizumab or any of their components.

• History of severe/life threatening immune-related adverse event (irAE) with PD-(L)1 inhibitors are excluded.

• Central nervous system (CNS) and/or leptomeningeal metastasis. Participants with treated CNS metastases are permitted if all of the following are met.

  • CNS metastases have been clinically stable for at least 4 weeks and baseline scans show no evidence of new or worsening CNS metastasis.
  • Participant is on a stable dose of ≤ 10 mg/day of prednisone or equivalent for at least 2 weeks.

• History of or active autoimmune disease that has required systemic treatment in the past 2 years.

• Prior treatment with an agent directed to another stimulatory or co-inhibitory T cell receptor (including but not limited to CD137 agonists, CAR-T cell therapy, CTLA-4 inhibitors, or OX-40 agonists).

• Prior solid organ or bone marrow transplantation.

• Pleural effusion or ascites with symptoms or requiring symptomatic treatment.

• Estimated life expectancy <12 week

• Prior treatment with an MMAE agent or anti-HER2 therapy

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.