This study is being done to learn more about a new medicine called PF-08634404. It is for adults with a type of bladder cancer called locally advanced or metastatic urothelial cancer (LA/mUC), meaning the cancer has spread to nearby tissues or other parts of the body.

The purpose of the study is to see if PF-08634404 is safe, how well it works, how it moves through the body, and how it affects the cancer. The study will also look at how the medicine may change certain markers in the body that are linked to cancer.

To join the study, participants must:

• Be adults (18 years or older) and
• Have locally advanced or metastatic urothelial cancer,

The study has two groups:

• Cohort A: People who have already received treatment for their cancer will get the study medicine ( PF-08634404) alone.
• Cohort B: People who have not had treatment before will get the study medicine along with another cancer medicine called enfortumab vedotin.

Everyone in the study will get the study medicine through a vein (IV infusion) with or without enfortumab vedotin. Treatment will continue as long as it helps and side effects are manageable.

Before starting, participants will go through a screening period to check if they are eligible. During the study, they will have regular visits for treatment, health checks, and tests to see how the cancer is responding. Scans will be done regularly to monitor the cancer.

If the cancer gets worse but the treatment is still helping and side effects are manageable, participants may be allowed to continue treatment with their doctor's and the sponsor's agreement.

Inclusion Criteria:

• Age ≥18 years at the time of screening.
• Histologically confirmed locally advanced or metastatic urothelial carcinoma (LA/mUC).
• Measurable disease per RECIST v1.1 criteria.
• ECOG performance status of 0 or 1.
• Adequate organ function, including hematologic, hepatic, and renal parameters.
• Willingness to comply with study procedures and provide informed consent.
• For participants of childbearing potential: agreement to use effective contraception during the study and for a defined period after the last dose.

Exclusion Criteria:

Participants will be excluded if they meet any of the following:

• History of another malignancy within 3 years before the first dose of study intervention, or any evidence of residual disease from a previously diagnosed malignancy
• Known active CNS lesions, including leptomeningeal metastasis, brainstem, meningeal, or spinal cord metastases or compression
• Active autoimmune diseases requiring systemic treatment within the past 2 years
• Participation in another investigational study within 30 days or 5 half-lives of the investigational product.
• Pregnant or breastfeeding individuals.
• Inability or unwillingness to comply with study requirements.
• Study staff or their immediate family members directly involved in the conduct of the study.

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.