- Documentation of disease

      - Histologic documentation: Histologically or cytologically confirmed urothelial
      carcinoma (UC) with predominantly transitional-cell features

      - Stage: Locally advanced or metastatic disease prior to starting immune checkpoint
      blockade

      - Tumor Site: Bladder, renal pelvis, ureter, urethra, or prostate

   - Patients must have received at least one cycle of current active treatment with
   standard of care (SOC) Food and Drug Administration (FDA) approved PD-1/L1 immune
   checkpoint inhibitor (ICI)-containing therapy for locally advance or metastatic UC

   - Patients without progressive disease per Response Evaluation Criteria in Solid Tumors
   (RECIST) version (v) 1.1 guidelines (i.e., with an ongoing [complete response] CR,
   partial response [PR], or stable disease [SD]) following completion of 12-15 months of
   ICI treatment

   - No history of tuberculosis, active hepatitis B (HBV) or hepatitis C (HCV), or
   uncontrolled human immunodeficiency virus (HIV)

      - Patients with resolved HBV infection, defined as positive hepatitis B core
      antibody (anti-Hb) and negative hepatitis B surface antigen (HbsAg), are eligible

      - Patients with positive HCV antibody are eligible if HCV ribonucleic acid (RNA)
      polymerase chain reaction (PCR) is negative

      - Patients with HIV who are compliant with highly active antiretroviral therapy
      (HAART) and have normal CD4 count and undetectable viral load are eligible

   - No history of allogeneic organ transplantation

   - No current immunosuppressive medication exceeding 10 mg/day of prednisone or its
   equivalent

   * Patients with pre-existing or treatment-emergent autoimmune or inflammatory
   disorders which do not require systemic immunosuppressive treatment exceeding 10
   mg/day of prednisone or its equivalent may be included

   - No history of another primary malignancy except for malignancy treated with curative
   intent with no known active disease for >= 2 years, and adequately treated
   non-melanomatous skin cancer or carcinoma in situ (e.g. cervical carcinoma in situ
   [CIS]) without evidence of disease

   - No female patients who are pregnant or breastfeeding, or male or female patients of
   reproductive potential who are not willing to employ effective birth control, because
   this study involves investigational agents whose genotoxic, mutagenic, and teratogenic
   effects on the developing fetus and newborn are unknown. Therefore, for women of
   childbearing potential only, a negative urine or serum pregnancy test pregnancy test
   done =< 14 days prior to registration is required

REGISTRATION ELIGIBILITY CRITERIA:

   - Patients without progressive disease per RECIST v 1.1 guidelines (i.e., with an
   ongoing CR, PR or SD) following completion of 12-15 months of ICI treatment

   - No toxicity from ICI therapy that makes continuation of treatment clinically
   unacceptable

   - Adequate bone marrow and organ functions to continue PD-1/L1 ICI as judged by the
   treating physician

   - Age >= 18 years

   - Eastern Cooperative Oncology Group (ECOG) performance status 0-2

   - Central nervous system (CNS) metastasis is allowed if radiographically stable,
   clinically asymptomatic, and prior local therapy (if received) was completed > 6
   months before registration

   - No history of tuberculosis, active hepatitis B (HBV) or hepatitis C (HCV), or
   uncontrolled human immunodeficiency virus (HIV)

      - Patients with resolved HBV infection, defined as positive hepatitis B core
      antibody (anti-Hb) and negative hepatitis B surface antigen (HbsAg), are eligible

      - Patients with positive HCV antibody are eligible if HCV RNA PCR is negative

      - Patients with HIV who are compliant with highly active antiretroviral therapy
      (HAART) and have normal CD4 count and undetectable viral load are eligible