The purpose of this study is to test the safety of a drug called CA-4948 in combination with pembrolizumab for patients with metastatic urothelial cancer. This study tests different doses of the drug to determine the highest dose with manageable side effects in people.

CA-4948, an investigational product, will be used in combination with Pembrolizumab (MK-3475). This combination isn’t approved by the U.S. Food and Drug Administration (FDA) for the treatment of this disease. 
   
There are two parts in this study: a dose escalation (dose finding) part and a dose expansion part (where the established dose is tested further). In the dose escalation part of this study, different people will get different doses of the study drug CA-4948 and all people taking part on the study will get the same dose of pembrolizumab.

Study participation entails receiving pembrolizumab through a vein in the arm on the first day of each 21-day cycle for 30 minutes. CA-4948 will be taken by mouth twice a day at the same time of the day every day during each cycle. 

There are also additional exams, tests, and procedures that will be done during the study.

Patients can continue treatment with CA-4948 and pembrolizumab for up to 2 years. After finishing the study treatment, the study doctor will continue to follow participants' condition every 3 months for up to 2 years.