Background: In clinical trials of radiolabeled monoclonal antibodies targeting metastatic breast carcinoma, myelosuppression has been the initial dose-limiting toxicity. We previously have shown that mobilized peripheral blood stem cell transfusions ameliorate this toxicity of radioimmunotherapy (RIT). Because of the difficulty we experienced harvesting adequate numbers of precursor cells from some of these heavily pretreated patients, we have compared the mobilization results and clinical histories of the metastatic breast carcinoma patients referred for RIT with those of metastatic breast carcinoma patients referred for high dose chemotherapy (HDCT) with transplantation.
Methods: Mobilization of stem cells into the peripheral blood was accomplished using granulocyte-colony stimulating factor with or without chemotherapy. Granulocyte macrophage colony-forming assays (CFU-GM) and flow cytometric analysis for CD34 positive cells were used to evaluate the number of hematopoietic precursors mobilized and collected with each apheresis procedure. Clinical characteristics, including prior chemotherapy and external beam radiotherapy, tumor bulk, and performance status, were determined by chart review.
Results: Significantly fewer hematopoietic precursors (both CD34 positive cells and CFU-GM) were harvested per procedure from the six RIT patients compared with a group of six patients with metastatic breast carcinoma who had stem cells harvested prior to HDCT (P = 0.002). There was no significant difference between the groups with regard to age or prior radiotherapy. All the RIT patients had received more chemotherapy, had a less favorable performance status (1 vs. 0), and had measurable tumor, whereas all the HDCT patients had minimal residual disease.
Conclusions: Patients enrolled in RIT studies had lower stem cell yields than metastatic breast carcinoma patients scheduled to receive HDCT with stem cell transplantation. Poor mobilization is unlikely to be due to the mobilizing regimen alone and may be related to the intensity of prior therapy and/or tumor bulk. Mobilizing adequate stem cells for multiple treatment cycles from patients on Phase I/II RIT trials may require new, more effective mobilizing regimens, but referral of patients earlier in their disease course, prior to chronic bone marrow damage and disease progression, is recommended strongly.