Abstract only TPS42 Background: Cabozantinib (CABO) is an oral receptor tyrosine kinase inhibitor of MET, VEGFR, and TAM family receptors (TYRO3, AXL, and MER). It is approved for patients (pts) with RCC after prior therapy with antiangiogenic therapy, and has demonstrated clinical activity in UC. In clinical studies, CABO exposure increased circulating CD8+ T cells and reduced immune-suppressive monocytes and Tregs. In preclinical tumor models, CABO increased MHC class 1 expression on tumor cells and reduced myeloid-derived suppressor cells. CABO may facilitate an immune-permissive tumor environment and may enhance response to immune checkpoint inhibitors. Atezolizumab (ATEZO), an anti-PD-L1 mAb, is approved for: locally advanced/metastatic UC in pts who are cisplatin-ineligible or have disease progression during/following platinum-containing chemo; pts with metastatic NSCLC and disease progression during/following platinum-containing chemo. We present the study design of an ongoing phase 1b study combining CABO with ATEZO in pts with locally advanced/metastatic UC or RCC. Methods: This multicenter, phase 1b, open-label study aims to assess safety, tolerability, preliminary efficacy, and pharmacokinetics of CABO in combination with ATEZO (NCT03170960). The study will enroll pts with advanced UC (bladder, renal pelvis, ureter, urethra) or RCC. It consists of two stages: dose escalation and expansion. In the dose-escalation stage (3+3 design), a recommended CABO dose for the combination will be established. In the expansion stage, four tumor-specific cohorts will be enrolled: 1) pts with UC who have progressed on/after platinum-containing chemo; 2) chemo-naïve pts with UC who are cisplatin ineligible; 3) chemo-naïve pts with UC who are cisplatin eligible; and 4) untreated pts with RCC with clear cell histology; the primary objective is to determine the objective response rate in each cohort. Exploratory objectives include correlation of tumor and plasma biomarkers, and changes in immune cell profiles with clinical outcome. The study has been initiated and enrollment target is 120 pts across the 4 expansion cohorts. Clinical trial information: NCT03170960.