Residual malignant and normal plasma cells shortly after high dose melphalan and stem cell transplantation. Highlight of a putative therapeutic window in Multiple Myeloma?

Multiple Myeloma (MM) is an incurable malignant plasma cell disorder. We have evaluated the counts of Multiple Myeloma Cells (MMCs) and normal plasma cells (N-PCs), seven days after high-dose melphalan (HDM) and autologous stem transplantation (ASCT). Two third of patients had detectable minimal residual disease (MRD(+)) (71.7 MMCs/μL) after induction treatment with dexamethasone and proteasome inhibitor. MMC counts were reduced by 92% (P ≤ .05) but not eradicated 7 days after HDM+ASCT. Post-HDM+ASCT MMCs were viable and bathed in a burst of MMC growth factors, linked with post-HDM aplasia. In one third of patients (MRD(−) patients), MMCs were not detectable after induction treatment and remained undetectable after HDM+ASCT. Major difference between MRD(−) and MRD(+) patients is that N-PC counts were increased 3 fold (P < .05) by HDM+ASCT in MRD(−) patients, but were unaffected in MRD(+) patients. Possible explanation could be that clearance of MMCs in MRD(−) patients makes more niches available for N-PCs. Thus, MMCs are not fully eradicated shortly after HDM, are bathed in high concentrations of MMC growth factors in an almost desert BM, are viable in short-term culture, which suggests providing additional therapies shortly after HDM to kill resistant MMCs before full repair of lesions.