Background: Avelumab is a fully human anti‒PD-L1 IgG1 antibody with promising efficacy and safety in patients (pts) with metastatic urothelial carcinoma (mUC). To further characterize the clinical activity of avelumab in mUC, we report a planned interim pooled analysis of 2 cohorts from a large phase 1b trial (NCT01772004). Methods: Pts with mUC progressed after platinum-based therapy or cisplatin-ineligible, and unselected for tumor PD-L1 expression, received avelumab 10 mg/kg (1 h IV) Q2W. Response was assessed every 6 wks by independent review per RECIST v1.1. Endpoints included objective response rate (ORR), duration of response (DOR), progression-free survival (PFS), overall survival (OS), safety (NCI CTCAE v4.0), and tumor PD-L1 expression (clone 73-10). Results: As of Mar 19, 2016, 241 pts received avelumab for a median of 8 wks (range 2-80); median follow-up was 7.3 mos (range 0-18.2). Primary tumor sites were upper tract (renal pelvis/ureter [23.7%]) and lower tract (bladder/urethra [76.3%]). 95.4% of pts had progressed on prior platinum therapy, and 63.4% had received ≥ 2 prior lines for advanced disease (range 0-6). In 153 pts with ≥ 6 mos follow-up, confirmed ORR was 17.6% (95% CI 12.0-24.6) with 9 complete responses and 18 partial responses; 24/27 (88.9%) were ongoing. Median DOR was not reached, and the 24-wk DOR rate was 92.0% (95% CI 71.6, 97.9). 36 pts had stable disease as best response (disease control rate 41.2%). Median PFS was 6.4 wks (95% CI 6.1-11.4), and median OS was 7.0 mos (95% CI 5.6-11.1). Based on a ≥ 5% PD-L1 staining cutoff in evaluable pts with PD-L1+ (n = 56) and PD-L1– (n = 75) tumors, ORR was 25.0% (95% CI 14.4-38.4) vs 14.7% (95% CI 7.6-24.7; p = 0.178). Treatment-related adverse events (TRAE) of any grade occurring in ≥ 10% of pts were infusion-related reaction (22.8%) and fatigue (12.0%). 7.5% had a grade ≥ 3 TRAE; fatigue (1.2%)/asthenia (0.8%) occurred in > 1 pt. 28 pts (11.6%) had an immune-related TRAE (grade ≥ 3 in 2.5%). There was 1 treatment-related death (pneumonitis). Conclusions: Avelumab is well tolerated and shows promising clinical activity, including durable responses, in pts with mUC, regardless of tumor PD-L1 expression status. Follow-up is ongoing. Clinical trial information: NCT01772004