Objective We aimed to investigate the pharmacokinetics, safety, efficacy, and immunogenicity of different dosing regimens (weekly and every three weeks) of inetetamab in combination with vinorelbine in human epidermal growth factor receptor 2 (HER2)+ patients with metastatic breast cancer who had received one or more chemotherapy regimens.Methods HER2+ patients with metastatic breast cancer who had received one or more chemotherapy regimens were included. Eligible patients received inetetamab administered weekly or every three weeks in combination with vinorelbine injection chemotherapy. Pharmacokinetics, safety, efficacy, and immunogenicity were compared between the groups.Results Sixty HER2+ patients were randomized into a single-week administration group ( n = 29) and a three-week administration group ( n = 31). After the final dose in the single-week administration group and the three-week administration group, the mean Cmax values were 79.773 mu g/mL and 146.083 mu g/mL; the mean Cmin values were 30.227 mu g/mL and 11.926 mu g/mL; the mean AUCtau values were 7328.443 mu g<middle dot>h/mL and 22647.101 mu g<middle dot>h/mL; and the mean Cav values were 43.622 mu g/ mL and 44.935 mu g/mL, respectively. The best overall response (BOR) rates at 24 weeks and unconfirmed BOR rates at 24 weeks were both 40.7% in the single-week dosing group and 40.7% in the three-week dosing group, and the 24-week confirmed disease control rates (DCRs) were 88.9% and 81.5%, respectively. The incidence of adverse events (AEs) was generally consistent across all levels.Conclusion There were slight differences in the mean Cmax, Cmin, AUCtau and Cav between the three-week dosing group and the single-week dosing group, and the mean steady-state concentrations of Cav were comparable; however, there were no differences in efficacy, safety or immunogenicity between the two groups.