Background: YH001 is a humanized anti -hCTLA-4 IgG1 mAb that relieves CTLA-4-mediated immunosuppression, and thereby enhances the T-cell-mediated antitumor immune response. Pre-clinical data have shown potent anti-cancer activity when combined with anti-PD-1 mAb. Methods: This is an ongoing phase 1 dose-escalation study. Patients (pts) with advanced solid tumors received YH001 by IV administration at 0.05 to 6.0 mg/kg for 1 cycle (21 days) then in combination with Toripalimab (anti-PD-1 mAb) at 240 mg Q3W for 4 cycles. An accelerated titration method followed by the standard “3+3” design was utilized to evaluate safety, tolerability and preliminary efficacy. Results: As of 31-Dec-2020 data cut-off, 10 pts were enrolled and treated at 0.05 mg/kg (n = 2), 0.1 mg/kg (n = 3), 0.3 mg/kg (n = 3) and 1 mg/kg (n = 2). The median age was 62 years (range 46-74). Baseline ECOG scores were 0 (n = 8), 1(n = 2) with all pts progressed after a median of 2 prior lines of available standard therapy (range 1-4) including 1 pt progressed after immunotherapy of pembrolizumab. There were no dose limiting toxicities (DLT) observed. No severe adverse events (SAEs), Grade (G) 3 or above adverse events (AEs) and AEs leading to treatment discontinuation were reported. Twelve drug related AEs were all G1/2 events including 2 G2 AEs (1 rash maculopapular at 0.05mg/kg, 1 hypothyroidism at 0.1mg/kg), 10 G1 AEs (1 hypotension, 1 dry skin, 1 pruritus at 0.05mg/kg; 1 rash, 1 rash macular, 1 hyperthyroidism, 2 rash pruritus at 0.1mg/kg, 2 fatigues at 0.3mg/kg). Among 7 patients having imaging tumor assessment by RECIST v1.1, there were 4 SD, including 1 at 0.05 mg/kg with tongue carcinoma at week 8 assessment, 1 at 0.1 mg/kg with nasopharyngeal carcinoma at week 8 and 15 assessment, 2 at 0.3 mg/kg with gastroesophageal junction cancer and uterus leiomyosarcoma at week 8. Conclusions: YH001 combined with Toripalimab is safe and tolerable up to 1 mg/kg dose level. Updated safety and preliminary efficacy data will be presented. Clinical trial information: NCT04481009