The combination of Pembrolizumab + sEphB4-HSA will be given through an intravenous infusion (into a vein). Each cycle of sEphB4-HSA will be given at Day 1, 8, and 15 of each 3 week cycle. Each cycle of Pembrolizumab will be given at Day 1 of each 3 week cycle.~Participants may continue on study protocol as long as they continue to respond and remains clinically stable on study medication.~Patients may come off treatment for the following reasons:~Disease progression.~If tumor resolves.~For participants who become pregnant.~Incidence or severity of adverse drug reaction in this or other studies indicates a potential health hazard to subjects.~Patient withdraws consent.~Study termination by the Sponsor.~Participants who are non-compliant with respect to taking drugs, keeping appointments, or having tests required for the evaluation of drug safety and efficacy.~Participant's condition renders them unacceptable for further treatment in the judgment of the investigator.
sEphB-HSA may prevent tumor cells from multiplying and blocks several compounds that promote the growth of blood vessels that bring nutrients to the tumor. The purpose of this study is to evaluate the combination of Pembrolizumab + sEphB4-HSA in the population of patients with previously untreated advanced (metastatic or recurrent) urothelial carcinoma who are chemotherapy ineligible or who refuse chemotherapy.