Myeloma cell contamination of peripheral blood stem-cell grafts can predict the outcome in multiple myeloma patients after high-dose chemotherapy and autologous stem-cell transplantation

Purpose High-dose chemotherapy with hematopoietic stem-cell rescue is increasingly being used in the treatment of multiple myeloma. Bone marrow and also peripheral blood stem-cell (PBSC) grafts contain measurable quantities of plasma cells, the biological significance of which is unknown.Methods Patients with multiple myeloma were mobilized with chemotherapy and filgrastim. The number of CD38++/CD138+ cells/kg in the grafts for autologous transplantation was determined by flow cytometry. Patients were stratified into two groups (threshold 4.5×10⁵ plasma cells kg^-1) of reinfused plasma cells in the first autologous graft.Results The median statistical progression-free survival was 14 months (4–34 months) in the high-contamination group (>4.5×10⁵ plasma cells kg^-1) compared to 26 months (4–43 months) in the low-contamination group (<4.5×10⁵ plasma cells kg^-1, P =0.0096). Patients with 13q deletion were more frequently found to have a high contamination of the stem-cell graft with malignant plasma cells.Conclusions Patients with graft contamination of more than 4.5×10⁵ plasma cells kg^-1 had a high risk of early disease progression after high-dose chemotherapy. In vivo tumor cell purging prior to mobilization chemotherapy might be one strategy to improve the time to progression of high-risk patients.