Background: Lymphocyte-activation gene 3 (LAG-3) is an immune checkpoint receptor protein that functions to control T cell response, activation and growth. Dual inhibition of PD-1 and LAG-3 may improve anti-tumor effect synergistically. In this first-in-human dose-escalation study, we report the preliminary safety and anti-tumor activity of IBI110 ± sintilimab in pts with advanced solid tumors. Methods: Enrolled pts, ECOG PS 0-1, had locally advanced, recurrent or metastatic solid tumors for whom standard therapy had failed. Pts received escalating doses of IBI110 (0.01/0.1/0.3/1/3/10/20mg/kg) IV Q3W in phase Ia and escalating doses of IBI110 (0.3/0.7/1.5/3/5 mg/kg) in combination with sintilimab 200 mg IV Q3W in phase Ib. Crossover from mono to combo was allowed at progression. The objectives were safety and tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and anti-tumor activity of IBI110 alone or IBI110+sintilimab (per RECIST v1.1). Results: Phase Ia: 21 pts (median age: 62 yr [range 43-72]; ECOG PS: 0 [n = 11], 1 [n = 10]) were enrolled. Dose escalation has completed and no dose-limiting toxicity (DLT) was observed in all dose cohorts. The most common treatment-related adverse event (TRAE) was anaemia (19.0%). TRAEs ≥G3 included anaemia (4.8%), ascites (4.8%) and hepatic function abnormal (4.8%). By investigator-assessment, best response was 1 confirmed partial response (PR) (ovarian cancer, 3 mg/kg IBI110 single agent) and 5 stable disease (SD) in monotherapy. After crossing from mono to combo at progression, 5 pts were observed to have SD. Phase Ib: 12 pts (median age: 60 yr [range 33-72]; ECOG PS: 0 [n = 7], 1 [n = 5]) were enrolled. All dose cohorts in dose escalation except IBI110 5mg/kg+ sintilimab have completed DLT observation and no DLT was observed. The most common TRAE was AST increased (41.7%). TRAEs ≥G3 included hyperglycaemia (8.3%), bilirubin conjugated increased (8.3%) and hepatic function abnormal (8.3%). By investigator-assessment, best response was 2 PR (small cell lung cancer and endometrial cancer) and 6 SD. Conclusions: IBI110 alone or plus sintilimab has acceptable toxicity and shows preliminary antitumor activity. Clinical trial information: NCT04085185