Background HBM4003 is a novel anti-CTLA-4 heavy chain-only antibody, designed to enhance Treg ablation and antibody-dependent cell-mediated cytotoxicity while ensuring a manageable safety profile. This phase I trial investigated the safety, pharmacokinetics, immunogenicity and preliminary efficacy of HBM4003 plus with anti-PD-1 antibody toripalimab in patients with advanced solid tumors, especially focusing on melanoma.Methods The multicenter, open-label phase I trial was divided into two parts: dose-escalation phase (part 1) and dose-expansion phase (part 2). In part 1, HBM4003 was administered at doses of 0.03, 0.1, 0.3 mg/kg in combination with toripalimab with fixed dosage of 240 mg every 3 weeks. The recommended phase II dose (RP2D) was used in the expansion phase. Primary endpoints were safety and RP2D in part 1 and objective response rate (ORR) in part 2. Biomarkers based on cytokines and multiplex immunofluorescence staining were explored.Results A total of 40 patients received study treatment, including 36 patients treated with RP2D of HBM4003 0.3 mg/kg plus toripalimab 240 mg every 3 week. 36 participants (90.0%) experienced at least one treatment-related adverse event (TRAE), of which 10 (25.0%) patients experienced grade >= 3 TRAEs and 5 (12.5%) experienced immune-mediated adverse events (irAEs) with maximum severity of grade 3. No grade 4 or 5 irAEs occurred. Efficacy analysis set included 32 melanoma patients treated with RP2D and with available post-baseline imaging data. The ORRs of anti-PD-1/PD-L1 treatment-na & iuml;ve subgroup and anti-PD-1/PD-L1 treatment-failed subgroup were 33.3% and 5.9%, respectively. In mucosal melanoma, the ORR of the two subgroups were 40.0% and 10.0%, respectively. Baseline high Treg/CD4+ratio in the tumor serves as an independent predictive factor for the efficacy of immunotherapy.Conclusions HBM4003 0.3 mg/kg plus toripalimab 240 mg every 3 week demonstrated manageable safety in solid tumors and no new safety signal. Limited data demonstrated promising antitumor activity, especially in PD-1 treatment-na & iuml;ve mucosal melanoma.Trial registration number NCT04727164.