The goal of this study is to evaluate the safety and clinical efficacy of personalized chemotherapy with low doses of cyclophosphamide administered taking into account the reparative cycle of tumor cells in participants with breast cancer (stage IV or disease progression). The main question it aims to answer is: Does personalized chemotherapy of breast cancer with low doses of cyclophosphamide, taking into account the reparative cycle of tumor cells, lead to a significant reduction in tumor size and improvement in the visual status of controlled tumor foci relative to their initial state? During the preparatory stage, a tumor sample from participant is harvested intraoperatively. The primary culture is obtained from this tumor tissue sample. Repair cycle time is estimated, and the day when tumor cells are synchronized at the cell cycle G2/M phase is identified. The schedule of administering cyclophosphamide and DNAmix complex composite preparation is calculated according to the resulting time points. According to the elaborated regimen, the participant receives 4 intravenous cyclophosphamide injections at the dose of 300 mg/m2 in combination with 4 injections of 1-12 mg of DNAmix administered into prominent tumor nidi and lymph depots. The participant receives from 2 to 6 courses of therapy. The interval between courses is 21 days.