Background: CS1001 is the first full-length, fully human anti-PD-L1 mAb developed by the OMT transgenic rat platform, which mirrors natural IgG4 human antibody with expected PK profiles, and may potentially reduce the risk of immunogenicity and toxicity in pts. This first-in-human Phase Ia/Ib study of CS1001 was conducted to evaluate the safety, tolerability, PK profile, and anti-tumor activity of CS1001 in pts with advanced solid tumors or lymphomas. Methods: Pts with advanced solid tumors or lymphomas were enrolled in the dose escalation Phase Ia, receiving CS1001, Q3W, IV, at escalating doses from 3, to 10, 20, 40 mg/kg and 1200 mg. Dose escalation was aided by a 3+3 dose escalation scheme. DLT was evaluated within 3 weeks after the initial dose. Pts with various tumor types were enrolled in the dose expansion Phase Ib to assess anti-tumor activity and safety, including NSCLC, esophageal carcinoma, GC, HCC, cholangiocarcinoma, etc. Safety was assessed by monitoring AEs and the associated grades per NCI CTCAE v4.03, tumor assessed per RECIST v 1.1 (solid tumors) or Lugano 2014 (lymphomas). Results: As of 30 Nov 2018, 29 pts, median age of 53 (23-75) yrs, were enrolled in Phase Ia, 3 mg/kg (N = 3); 10 mg/kg (4); 20 mg/kg (3); 40 mg/kg (3) and 1200 mg flat dose (16). A total of 20 pts discontinued treatment due to disease progression (14), death (2), withdrawal by pts (2) and AEs (2; Grade [G] 4 hepatic function abnormal and G3 pulmonary tuberculosis, both were not related to treatment). 9 pts remain on treatment. Median treatment duration was 126 (21-408+) days. No DLTs were observed. 27 of 29 pts had TRAEs with the most frequent TRAEs including anaemia (14), proteinuria (13) and blood bilirubin increased (8). G3 TRAEs include anaemia (2) and platelet count decreased (1). SAEs were reported in 6 pts and they were TRAEs. Three G4 AEs were reported: anaemia (1), hypokalaemia (1) and hepatic function abnormal (1), they were not TRAEs as determined by the investigators. irAEs occurred in 7 pts (24%). Among the 29 evaluable pts, 7 pts had PR and 8 had SD, mDoR was not reached. In Phase Ib, 97 pts were enrolled, with 65 pts on treatment and 32 pts discontinued from treatment. The most frequent reason on the discontinuation was disease progression (21). Phase Ib enrollment is still ongoing. Conclusions: CS1001 is well tolerated without DLT across tested dose levels. Evidence of anti-tumor activities was observed. Currently, 1200 mg flat dose Q3W is being explored in various tumor types in Phase Ib, and safety and efficacy results will be displayed in the presentation. Clinical trial information: NCT03312842