Inclusion Criteria:

          -  Be willing and able to provide written informed consent/assent for the trial.

          -  Age ≥ 18 years.

          -  Be treatment naïve and have advanced (metastatic or recurrent) pathologically proven
             urothelial carcinoma. Patients progressing more than 12 months of their last dose of
             platinum-based neoadjuvant or adjuvant chemotherapy are eligible.

          -  Have a performance status of 0 or 1 on the ECOG Performance Scale.

          -  Demonstrate adequate organ function as defined in Table 1., all screening labs should
             be performed within 10 days of treatment initiation.

          -  Be ineligible for chemotherapy in the assessment of the treating physician or refusing
             chemotherapy in frontline setting. Cisplatin ineligibility is defined as Cresatinine
             Clearance (CrCl) < 60cc/min, NYHA class III hear failure, grade II neuropathy and
             grade II hearing loss, the latter two also apply to carboplatin-based chemotherapy.

          -  Be approved to begin treatment with pembrolizumab per standard of care and
             pembrolizumab must be available to the patient. Patient must enroll on this study
             prior to the receiving the second dose of pembrolizumab and prior to having restaging
             imaging after receiving pembrolizumab. Other PD1/PDL1 antibodies are now allowed.

          -  Have measurable disease based on RECIST 1.1. We request an OPTIONAL core biopsy from
             an accessible metastatic site after a minimum of 2 cycles of treatment AND prior to
             progression of disease to help the investigators better understand the activity of
             these drugs in tumor tissue.

        In addition, we request an OPTIONAL collection of any surgical specimens obtained per
        standard of care during the study. For instance, specimens from metastasectomy while
        patient is undergoing therapy on this study.

        Exclusion Criteria:

          -  Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of trial
             treatment.

          -  Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA
             [qualitative] is detected).

          -  Has a known history of active TB (Bacillus Tuberculosis)

          -  Has active autoimmune disease that has required systemic treatment in the past 2 years
             (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a
             form of systemic treatment.

          -  Has a known additional malignancy that is progressing or requires active treatment.
             Exceptions include basal cell carcinoma potentially cured after surgery or prostate
             cancer that is under control by hormonal agents.

          -  Has known active central nervous system (CNS) metastases and/or carcinomatous
             meningitis. Treated brain metastases are allowed.

          -  Is pregnant or breastfeeding, or expecting to conceive or father children within the
             projected duration of the trial, starting with the screening visit through 120 days
             after the last dose of trial treatment.

          -  Has New York Heart Association (NYHA) class 3 or 4, myocardial infarction, acute
             coronary syndrome, diabetes mellitus with ketoacidosis or chronic obstructive
             pulmonary disease (COPD) requiring hospitalization in the preceding 6 months; or any
             other intercurrent medical condition that contraindicates treatment with sEphB4HSA or
             pembrolizumab or places the patient at undue risk for treatment related complications.

          -  Has received a live vaccine within 30 days of planned start of study therapy. Note:
             Seasonal influenza vaccines for injection are generally inactivated flu vaccines and
             are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are live
             attenuated vaccines, and are not allowed.

          -  Uncontrolled hypertension is excluded- systolic blood pressure >140mmHg or diastolic
             >90mmHg. Patients experiencing white coat hypertension the office, may be considered
             eligible if blood pressure log at home is within acceptable limits AND upon review and
             agreement from the Sponsor.