Real-time polymerase chain reaction in multiple myeloma: Quantitative analysis of tumor contamination of stem cell harvests

Objective. Autologous transplantation of bone marrow (BM) and peripheral blood progenitor cells (PBPC) is commonly used for treatment of multiple myeloma (MM). Although both stem cell sources harbor residual clonal cells, a quantitative evaluation of their level of tumor contamination (LTC) still needs to be performed through highly accurate and reproducible approaches. In this study, we used a validated real-time polymerase chain reaction (PCR) strategy to evaluate LTC of BM and PBPC samples obtained from MM patients. Materials and Methods. The patients underwent two different mobilization courses (defined as early or late course) following two cycles of cyclophosphamide 5 g/m². LTC was evaluated by measuring the number of clonal immunoglobulin heavy-chain rearrangements followed by normalization of samples using the GAPDH gene. Results. Overall, 26 PBPC and 12 BM samples were analyzed. Main results are as follows. 1) PBPC harvests are less contaminated than BM samples taken immediately after each mobilization course (median difference 2.68 logs; range 1.7 to 4.6) (p < 0.0001). 2) LTC of PBPC harvests has only minimal variation among different leukaphereses performed during the same mobilization course (median difference 0.45 logs; range 0.22 to 1.2). 3) No difference was observed among PBPC and BM samples obtained after the late mobilization course as compared to the early mobilization course (median reduction 0.21 logs; range -0.39 to 1.3) (p = 0.84). 4) In PBPC but not in BM samples, there is a clear overestimation of the percentage of plasma cells when flow cytometric evaluation of CD38^bright cells is compared to real-time PCR results. This suggests that in PBPC, most CD38^bright cells do not belong to the neoplastic clone. Conclusions. Real-time PCR using the IgH rearrangement proved an effective tool for monitoring LTC in stem cell harvests from MM patients. The smaller LTC of PBPC harvests supports the role of PBPC as stem cell rescue for MM patients compared to BM cells.