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5013 Background: Cabozantinib (C), an inhibitor of MET, AXL, and VEGFR, has been shown to promote an immune-permissive environment and has shown promising clinical activity in combination with immune checkpoint inhibitors (ICIs) in solid tumors including renal cell carcinoma and urothelial carcinoma (UC). ICI monotherapy is approved for patients (pts) with locally advanced or metastatic UC with disease progression after platinum-containing chemotherapy. COSMIC-021, a multi-center phase 1b study, is evaluating the combination of C with atezolizumab (A) in various solid tumors (NCT03170960). We report results from Cohort 2 in UC pts with prior platinum-containing chemotherapy. Methods: Eligible pts had ECOG PS 0-1 and had progressed on or after a platinum-containing chemotherapy (including pts with disease recurrences < 12 months after the end of perioperative chemotherapy). Pts received C 40 mg PO QD and A 1200 mg IV Q3W. CT/MRI scans were performed Q6W for first year and Q12W thereafter. The primary endpoint is objective response rate (ORR) per RECIST v1.1 by investigator. Other endpoints include safety, duration of response (DOR), PFS, and OS. Results: As of Dec 20, 2019, 30 pts with advanced UC were enrolled with a median follow-up of 16.5 mo (range 12, 21). Median age was 66 yrs (range 44, 84), 73% were male, and 60% had ECOG PS 1. Primary tumor sites were bladder (80%), renal pelvis (10%), and ureter (10%); the most frequent metastatic sites included lung (40%) and liver (27%). Fourteen pts (47%) had received ≥2 prior systemic anticancer therapies. The most common treatment-related AEs (TRAEs) of any grade were asthenia (37%), diarrhea (27%), decreased appetite (23%), increased transaminases (23%), and mucosal inflammation (20%). Grade 3/4 TRAEs occurred in 57% of pts, with no grade 5 TRAEs. Confirmed ORR per RECIST v1.1 was 27% (8 of 30 pts), including 2 pts with CR. DCR (CR+PR+SD) was 64%. Median DOR was not reached, with the longest DOR ongoing at 14.3+ mos. Median PFS was 5.4 mo (range 0.0+, 17.3+). Conclusions: C in combination with A demonstrated encouraging clinical activity in pts with advanced UC with an acceptable safety profile. Additional cohorts of pts with advanced UC are being explored in the study. Clinical trial information: NCT03170960 .