Number of Participants with Dose Limiting Toxicities within a period of 35 days will be analyzed reviewing the aggregate of adverse events (AEs) and serious adverse events (SAEs) by the B-701 program Safety Oversight Committee and will result in a recommended Phase 2 dose. Six subjects at a time are enrolled and observed for 35 days after the initial dose. If 2 or more subjects experience a DLT that dose will be declared intolerable and de-escalation of the dose will occur.

Evaluate the safety and tolerability of B-701 (vofatamab) plus pembrolizumab in subjects with UCC as assessed by number of subjects experiencing adverse events (AEs and SAEs), physical examination findings, laboratory test results, and vital signs over time. This outcome is measured by a safety monitoring committee who regularly met and reviewed aggregate trends of reports AEs, lab ranges, physical exams etc. and determined if the drug was safe to continue.

Evaluate the efficacy of B-701 (vofatamab) plus pembrolizumab in subjects with UCC as measured by objective response rate (ORR) by RECIST 1.1. ORR is defined as the percentage of subjects who have baseline measurable disease and who achieve a best response of either complete response (CR) or partial response (PR).

Whole blood (PBMCs), serum, and plasma samples for biomarker analyses will be obtained prior to infusion of B-701 at pre-defined visit days.

The effects of B-701 on the downstream signaling of the FGFR3 pathway, tumor sub-type and on the immune surveillance of UCC tumors will be monitored using techniques that include gene expression profiling (such as whole transcriptome RNAseq), sequencing of T-cell receptors, and immunohistochemistry.

The study was terminated early and this outcome was not analyzed because the data were not collected.

Evaluate the efficacy of B-701 (vofatamab) in combination with pembrolizumab in the treatment of subjects with UCC as measured by duration of objective response (DOR), defined as the time from first occurrence of a documented, objective response until the time of relapse or death from any cause (RECIST 1.1).

The study was terminated early and this outcome was not analyzed because the data were not collected.

Evaluate the efficacy of B-701 in combination with pembrolizumab in the treatment of subjects with UCC as measured by disease control rate (DCR), defined as the percentage of subjects who achieve either complete response (CR) or partial response (PR) or stable disease (SD) according to RECIST 1.1.

The study was terminated early and this outcome was not analyzed because the data were not collected.

Evaluate the efficacy of B-701 (vofatamab) in combination with pembrolizumab in the treatment of subjects with UCC as measured by progression-free survival (PFS), defined as the time from a first study treatment dose to first occurrence of disease progression (per RECIST 1.1) or death from any cause, whichever occurs first.

The study was terminated early and this outcome was not analyzed because the data were not collected.

Evaluate the efficacy of B-701 (vofatamab) in combination with pembrolizumab in the treatment of subjects with UCC as measured by overall survival (OS), defined as the time from first study drug administration to death from any cause (RECIST 1.1)

The study was terminated early and this outcome was not analyzed because the data were not collected.

Evaluate the efficacy of B-701 (vofatamab) in combination with pembrolizumab in the treatment of subjects with UCC as measured by the change over time in subject reported quality of life as measured by the European Organization for Research and Treatment Quality of Life Questionnaire (EORTC QLQ-C30).

The study was terminated early and this outcome was not analyzed because the data were not collected.

PK will be analyzed by measuring B-701 C(trough) levels. B-701 C(trough) levels then will be summarized over time throughout the study and will be compared to predicted B-701 C(trough) levels, whose prediction is based on data observed in previous studies with B-701.

Determine the immunogenicity of B-701 as measured by anti-B-701 antibody titers at several time points throughout the study.