Background: There is still urgent medical needs for new therapeutics in the patients with metastatic breast cancer (MBC) and other solid tumors with HER2 overexpression. RC48-ADC is an antibody-drug conjugate drug with a novel humanized anti-HER2 antibody conjugated to monomethyl auristatin E (MMAE) through a cleavable linker. Preclinical data in various animal models, including breast cancer, gastric cancer and ovarian cancer, suggested excellent antitumor efficacy. Methods: It was an open-label, single-center, phase I study. Eligible pts (18-65 years) were confirmed by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) as HER2-positive (IHC 2+/FISH+ or IHC 3+) MBC. A 3+3 dose-escalation was conducted with a 28-day window to evaluate dose limiting toxicity (DLT). DLT was mainly defined as Grade 4 neutropenia effectively managed with symptomatic treatment, Grade 3 neutropenia accompanied by infection and Grade 3 non-hematological toxicity. Tumor response was accessed per RECIST 1.1 every 6 weeks. Results: As of 29 Jan, 2018, 23 female pts with MBC were treated in 5 dose escalation cohorts (dose levels 0.5, 1.0, 1.5, 2.0, 2.5 mg/kg) once every two weeks (Q2W). Median age was 57 (32-65), and the course of disease was more than 3 years in 15 pts (65.2%). 16 pts (69.7%) were previously treated with trastuzumab. Median prior lines of therapy in metastatic setting was 3 (1-6). 14 pts (60.9%) had≥3 visceral metastatic sites of disease. MTD was not reached at doses up to 2.0 mg/kg Q2W. The most common treatment-related AEs (TRAEs) were leucopenia (43.5% Grade 1-2; 4.3% Grade 3), AST elevation (43.5% Grade 1-2; 4.3% Grade 3), followed by neutropenia (30.4%, Grade 1-2, 13% Grade 3-4), which could be readily managed. 20 pts were evaluable for response. Of 14 pts at doses≥1.5mg/kg , 8 (57.1%) achieved PR and 4 (28.6%) SD. For 11 trastuzumab-pretreated pts, ORR was 72.7%. Conclusions: RC48-ADC administrated with Q2W has demonstrated good tolerability and promising antitumor activity in HER2-positive pts with MBC. The MTD has not been determined and 2.5 mg/kg Q2W dose escalation is ongoing. Clinical trial information: NCT02881138. Clinical trial information: NCT02881138