Purpose: Granulocyte-macrophage colony-stimulating factor (GM-CSF) –secreting tumor vaccines have demonstrated bioactivity but may be limited by disease burdens and immune tolerance. We tested the hypothesis that cyclophosphamide (CY) and doxorubicin (DOX) can enhance vaccine-induced immunity in patients with breast cancer.

Patients and Methods: We conducted a 3 x 3 factorial (response surface) dose-ranging study of CY, DOX, and an HER2-positive, allogeneic, GM-CSF–secreting tumor vaccine in 28 patients with metastatic breast cancer. Patients received three monthly immunizations, with a boost 6 to 8 months from study entry. Primary objectives included safety and determination of the chemotherapy doses that maximize HER2-specific immunity.

Results: Twenty-eight patients received at least one immunization, and 16 patients received four immunizations. No dose-limiting toxicities were observed. HER2-specific delayed-type hypersensitivity developed in most patients who received vaccine alone or with 200 mg/m² CY. HER2-specific antibody responses were enhanced by 200 mg/m² CY and 35 mg/m² DOX, but higher CY doses suppressed immunity. Analyses revealed that CY at 200 mg/m² and DOX at 35 mg/m² is the combination that produced the highest antibody responses.

Conclusion: First, immunotherapy with an allogeneic, HER2-positive, GM-CSF–secreting breast tumor vaccine alone or with CY and DOX is safe and induces HER2-specific immunity in patients with metastatic breast cancer. Second, the immunomodulatory activity of low-dose CY has a narrow therapeutic window, with an optimal dose not exceeding 200 mg/m². Third, factorial designs provide an opportunity to identify the most active combination of interacting drugs in patients. Further investigation of the impact of chemotherapy on vaccine-induced immunity is warranted.