Malignant Plasma Cells Responsible for Multiple Myeloma Relapse Are Detectable and Survive Nine Days After High Dose Melphalan: Highlight of An Optimal Therapeutic Window ?

Multiple myeloma (MM) is still an incurable plasma cell dyscrasia. In an attempt to further improve MM patients' outcome, we have characterized malignant plasma cells resistant to treatments as well as their cellular and molecular environment. The aim is to highlight interactions that could be targeted at optimal time points of the treatment schedule.

Twenty-four newly diagnosed MM patients treated in first line by bortezomib and dexamethasone induction followed by high dose melphalan (HDM) and autologous stem cell transplantation were included. Tumor and normal plasma cell were detected and counted in peripheral blood and bone marrow using 7-color multiparameter flow cytometry at different time points of treatment: at the end of induction, 9 days, 3 months and 6 months after high dose melphalan and autologous stem cell reinjection. The sensitivity of detection reached was 10⁻⁵. The cellular content of the leukapheresis product was also analyzed after thawing before being reinjected. Plasma cells were characterized in terms of tumor markers (CD200, CD56, CD117, CD27, CD19, CD45), proliferation status (KI67) and expression of CCR2 chemokine receptor. Using multiplex technique, we also evaluated the medullary cytokine / chemokine environment (Interleukin-1b (IL-1b), IL-1RA, IL-2, IL-2R, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12, IL-13, IL-15, IL-17, Epidermal Growth Factor (EGF), eotaxin, Fibroblast Growth Factor-basic (FGF-b), Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), G-CSF, Hepatocyte Growth Factor (HGF), Interferon-a (IFN-a), IFN-g, Inducible Protein 10 (IP-10, CXCL10), Monocyte Chemotactic Protein-1 (MCP-1, CCL2), Monokine Induced by IFN-g, Regulated on Activation Normally T-cell Expressed and Secreted (RANTES, CCL5), Tumor Necrosis Factor a (TNF- a), and Vascular Endothelial Growth Factor (VEGF) at the end of induction, 9 days and 3 months post HDM. IGF1, IL21, BAFF and APRIL were measured by Elisa.

Two thirds of the 24 patients had detectable tumor plasma cells after induction treatment and in all these patients, tumor plasma cells (median 5.9 cells/ mm3, range 0.1–76.9 cells/mm3) as well as normal plasma cells (median 3.3 cells/mm3, range 0.4–32.2 cells/mm3) could be detected 9 days after HDM treatment in an aplastic bone marrow (median cell count 400 cells/mm3). Nine days post HDM, a burst of cytokines / chemokines was detected in bone marrow plasma which could prompt survival, growth and homing of tumor plasma cells. CCL2 chemokine (chemokine C-C motif ligand 2) which is a chemotactant and survival factor for myeloma cells was the only cytokine / chemokine differentially expressed in immuno-phenotypic complete response patients compared to patients with persistent tumor plasma cells, with a concentration 2.1 times higher in the last ones. In parallel, preliminary data show an enrichment in CCR2 positive tumor plasma cells 9 days after HDM, CCR2 being the receptor for CCL2. This interaction could be a clinically significant target to get rid of minimal residual disease.

In conclusion, we show that the early post intensification (HDM) period could be an interesting therapeutic window to target resistant myeloma cells and their interactions with the pro-survival micro-environment. The CCL2-CCR2 interaction could be of particular interest, an anti CCR2 antibody already being in clinical development in rheumatoid arthritis (Van Driel et al. 2008).

No relevant conflicts of interest to declare.