P-040: CD138-independent strategy to predict relapse in Multiple Myeloma patients

Background CD138 has been the gold-standard surface marker to detect multiple myeloma (MM) cells for decades; however, drug resistant minimal-residual disease (MRD) and circulating tumor cells (CTCs) were shown to have lower expression of this marker. We previously published that residual MM cells following treatment in vivo were hypoxic and the combination of hypoxia and chemotherapy such as bortezomib downregulated CD138 expression, thereby making this marker unsuitable for MM detection. Needless to say, accurate number of MM cells is critical in diagnosis, autologous transplantation, MRD and drug efficiency assessment. Moreover, CTCs are considered an unfavorable prognostic factor and indicate an aggressive form of the disease, and therefore detecting CTCs can be used as a powerful prognostic tool for MM. Methods We used an alternative biomarker-set using flow cytometry defining MM cells as any cell that expresses CD38 but excluding T cell (CD3), B cell (CD19), NK cell (CD16), monocyte/macrophage (CD14), neutrophil/eosinophil (CD16), and basophil/dendritic cell (CD123). We demonstrated previously that this approach it widely available due to accessibility of flow cytometry, inexpensive, and works independently of hypoxic-, CD138 expression- and treatment status. We analyzed primary patient samples (n=50) with complete response or very good partial response for MRD and CTCs and correlated the numbers of detected MM with patients’ time-to-progression (TTP) obtained from a clinical data base. Results We found that the alternative biomarker-set identifies MM cells more precisely and at higher numbers than CD138 marker by flow or histology. Moreover, we found a correlation between the number of MM cells detected and TTP in these patients: the amount of MM cells detected by the new method ranged between 0.5-7.3%, and patients who progressed sooner than two years had 2.5-fold higher percentage of MM cells compared to patients who relapsed later than 2 years. Similarly, patients who relapsed sooner than 3 years had 4-fold higher number of MM cells than patients who relapsed later than 3 years. We further found that, among all patients who had more than 2% of MM cells detected by the new method had a mean TPP of about 20 months, while patients who had less than 2% had a mean TPP of about 38 months. Testing the prevalence of CTCs in MM patients with progressive disease using CD138 or the new method, demonstrated that CD138 detected minimal amounts of MM cells in all patients (less than 0.1%), while the new method detected a range between 0.1 - 1.8% of MM cells in the peripheral blood. Conclusion These results suggest that the alternative biomarker-set detected MM cells which correlated with relapse in MM patients. Therefore, the amount of residual cells in the bone marrow and circulating myeloma cells can be used as a prognostic marker in MM patients. Further examination to characterize this population and its role in MM relapse is warranted.