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Aim: In the present study, we investigated the therapeutic efficacy of trastuzumab labeled with lutetium-177 (177Lu) in mice bearing breast tumors. Materials and methods: Trastuzumab was labeled with 177Lu via D OTA as a chelator. In vitro and in vivo quality control tests of incorporated activity, immunoreactivity, stability of 177Lu-trastuzumab, biodistribution, and imaging studies were performed. Therapeutic efficacy and dosimetry studies were carried out in mice with breast tumors. Results: The good in vitro and in vivo quality control results suggested that 177Lu-trastuzumab could be used as a radioimmunotherapy agent. Reductions of 81% and 98% in the mean tumor volume for the group that received 177Lu-trastuzumab [7.4 MBq] were observed at 42 and 45 days after treatment, respectively. Conclusion: The results showed that 177Lu-trastuzumab could be considered as a possible new radiopharmaceutical agent for use in radioimmunotherapy of breast cancer in humans. [ABSTRACT FROM AUTHOR]