Background: CD73 is implicated in tumor resistance to checkpoint immunotherapy (CPI) and plays a critical role in adenosine-mediated immune suppression. Uliledlimab, a differentiated CD73 antibody, inhibits the adenosine pathway in a non-competitive and unique intra-dimer binding mode. Uliledlimab suppresses tumor growth when combined with a PD-(L)1 inhibitor in multiple pre-clinical models. Methods: This 3+3 dose-escalation phase 1 study (NCT03835949) evaluated safety, tolerability, PK, PD and preliminary efficacy in cancer patients. Uliledlimab was administered intravenously at doses of 5, 10 or 15 mg/kg weekly (QW) or 15 or 20 mg/kg every 3 weeks (Q3W) alone in the first cycle and in combination with atezolizumab (1,200 mg Q3W) starting on week 4. Soluble CD73 in serum and CD73 receptor occupancy (RO) in circulating CD19⁺ B cells were measured. Expression of PD-L1, CD73 and A2A receptor was analyzed in baseline tumor specimens (n = 14). Tumor responses were assessed by RECIST/iRECIST. Results: As of 17 January 2021, 20 patients with advanced solid tumors were enrolled (M:F 8:12; mean age = 64; median prior regimens = 3 (range 1-9)). Uliledlimab was well-tolerated with no dose limiting toxicity. The most common treatment-related adverse events were first dose infusion related reactions (65%, n = 13) most commonly comprising chills/rigors, nausea, and vomiting (Grade 1 or 2) that resolved in subsequent infusions. PK appears linear at doses ≥ 10 mg/kg and modelling indicated a mean derived effective half-life of ̃19 days. Soluble CD73 was undetectable and complete RO was achieved in all patients after the first dose at ≥ 10 mg/kg. Anti-drug antibody was detected in 3/20 patients (15%). Among 13 efficacy-evaluable patients dosed at ≥ 10 mg/kg, complete response (CR = 1) and partial response (PR = 2) were observed in 3 patients (ORR = 23%) together with 3 stable disease (SD) patients (DCR = 46%). One PD-(L)1 inhibitor naïve patient with clear cell ovarian cancer achieved CR at 10 mg/kg QW and remains on study after 12 months. Two patients with NSCLC dosed at 15 mg/kg QW and 20 mg/kg Q3W, respectively, achieved PR. One patient failed nivolumab and the other received no prior PD-(L)1 inhibitor treatment. CD73 was expressed on 78% (mean) of malignant cells from archival tumor specimens in responders compared to 23% in non-responders. Conclusions: Uliledlimab is safe and well tolerated up to 20 mg/kg Q3W and 15 mg/kg QW. Full saturation of circulating and cell-bound CD73 was achieved at doses ≥ 10 mg/kg. Uliledlimab exhibited evidence of clinical activity in both PD-(L)1 treatment naïve and refractory cancer patients with high archival tumor expression of CD73. The results of this phase 1 study encourage further clinical investigation to evaluate the efficacy of uliledlimab in the treatment of solid tumors. Clinical trial information: NCT03835949