Inclusion Criteria:

          -  Men and women ≥ 18 years of age.

          -  Histological confirmation of urothelial carcinoma (any histology)

          -  Advanced or metastatic urothelial carcinoma.

          -  Measurable disease as defined by RECIST 1.1 criteria

          -  Has received at least 24 weeks (+/- 4 weeks) on CPI therapy per standard of care (SOC)
             for advanced urothelial carcinoma

          -  Karnofsky Performance Score (KPS) ≥70% (for more information on KPS, please see:
             http://www.npcrc.org/files/news/karnofsky_performance_scale.pdf)

          -  Willing and able to provide informed consent.

          -  Laboratory criteria for study entry must meet the following criteria:

               -  Serum creatinine ≤ 2 x ULN OR CrCl ≥ 30 mL/min (measured or calculated using the
                  Cockcroft-Gault formula).

               -  Hb ≥ 8.0g/dL

               -  AST and ALT ≤ 3.0 x ULN

               -  Bilirubin ≤ 1.5 x ULN (except subjects with Gilbert Syndrome, who can have total
                  bilirubin < 3.0 mg/dL)

        Exclusion Criteria:

          -  History of severe hypersensitivity reaction to any monoclonal antibody.

          -  Patients are excluded if they have known HIV/AIDS.

          -  Major surgery (eg, cystectomy) less than 28 days prior to the first dose of study
             drug.

          -  Any condition requiring systemic treatment with either corticosteroids (> 10 mg daily
             prednisone equivalent) or other immunosuppressive medications within 7 days prior to
             the first dose of study drug. Inhaled steroids and adrenal replacement steroid doses >
             10 mg daily prednisone equivalent are permitted in the absence of active autoimmune
             disease.

          -  Known medical condition (eg, a condition associated with diarrhea or acute
             diverticulitis) that, in the investigator's opinion, would increase the risk
             associated with study participation or study drug administration or interfere with the
             interpretation of safety results.

          -  Pregnant women are excluded from this study because animal studies have demonstrated
             that PD-1/PD-L1 inhibitors can cause fetal harm when administered to pregnant women.
             Breastfeeding women are excluded from this study because PD-1/PD-L1 inhibitors may be
             excreted in human milk and the potential for serious adverse reactions in nursing
             infants.