Background: In the Phase 3 IMmotion151 trial, atezo + bev showed improved PFS vs sun in untreated mRCC pts expressing PD-L1. Here we report results of a prespecified subgroup analysis in pts whose tumors have sarc histology, an independent predictor of poor survival. Methods: Pts were randomized to receive atezo 1200 mg IV q3w + bev 15 mg/kg IV q3w or sun 50 mg po qd for 4w on, 2w off. Coprimary endpoints were reported previously (Motzer ASCO GU 2018). Secondary endpoints included INV-PFS and OS in sarc pts and are shown here with INV-ORR, safety, PRO and biomarker data. Results: 142 randomized pts (16%) from IMmotion151 had tumors with any component of sarc histology; mPFS was 8.3 vs 5.3 mo with atezo + bev vs sun and mOS was NR vs 15.0 mo, respectively (see Table for PD-L1+). ORR was 49% vs 14% and CR rate was 10% vs 3% in the atezo + bev vs sun arms. Grade 3-4 AEs occurred in 27 pts (40%) with atezo + bev and 34 (49%) with sun. Using the MDASI scale, sarc pts reported longer median time to deterioration (TTD) of symptom interference with daily activities with atezo + bev vs sun (11.3 vs 4.9 mo). Prevalence of Angiogenesis^High gene expression (GE) signature subset was lower (34% vs 65%) and T-effector^High GE subset was higher (54% vs 40%) in sarc vs non-sarc tumors. PD-L1+ disease was more common in sarc vs non-sarc tumors (63% vs 39%). Conclusions: mRCC pts with sarc histology had longer OS and PFS and a higher ORR/CR rate when treated with atezo + bev vs sun, regardless of PD-L1 status. Biomarker data support a biological correlate for the increased responsiveness to atezo + bev in sarc pts. Clinical trial information: NCT02420821

m, median NR, not reached Parentheses denote 95% CI Stratification factors: MSKCC risk score, liver mets, PD-L1 status