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BACKGROUND Patients with metastatic urothelial carcinoma (mUC) have poor prognosis, so further development of novel combinations for these patients is needed. OBJECTIVE To assess the safety and efficacy of eribulin mesylate (eribulin) with avelumab in mUC. DESIGN, SETTING, AND PARTICIPANTS This was an open-label, phase 1b study in which patients with mUC who were cisplatin-ineligible and treatment-naïve or platinum-resistant were treated with eribulin and avelumab. A 3 + 3 design was used. The study was prematurely terminated because the free study drug became unavailable, but we performed extended follow-up for patients enrolled in the study. INTERVENTION Patients received eribulin 1.1 mg/m2 plus avelumab 10 mg/kg on days 1 and 15 in every 28-d cycle in cohort 0, or eribulin 1.4 mg/m2 plus avelumab 10 mg/kg on days 1 and 15 in every 28-d cycle in cohort +1. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS The primary objectives were to determine the maximum tolerated dose (MTD) of eribulin with avelumab and assess the objective response rate. A key secondary endpoint was to assess efficacy by evaluating the disease control rate. Exploratory endpoints included PD-1 expression on T cells in peripheral blood and in tumor cells, and tumor DNA sequencing. RESULTS AND LIMITATIONS A total of six patients were enrolled in the MTD group (n = 3 in cohort 0 and n = 3 in cohort +1). No dose-limiting toxicity (DLT) was observed in cohort 0, whereas two DLT events were observed in cohort +1. Two patients in cohort 0 had a partial response that was durable, with one patient having a durable response for 7.8 mo. Disease control was observed in 4/6 patients (66.7%). Owing to the early termination, MTD could not be determined. CONCLUSIONS While early termination of this trial precludes any definitive conclusions, the combination of eribulin and avelumab shows promise in mUC. We observed that treatment was better tolerated and efficacious at lower doses of eribulin. Further research is warranted for this combination in mUC. PATIENT SUMMARY We evaluated different doses of eribulin (a chemotherapy drug) in combination with a fixed dose of avelumab (an antibody used to treat several different cancers) in a small group of patients with metastatic cancer of the urinary tract. The lower dose of eribulin was easier to tolerate and the combination had an anti-cancer effect. This trial is registered at ClinicalTrials.gov as NCT03502681.