Clonal bone marrow plasma cells monitoring in multiple myeloma patients by flow cytometry: the first results

Summary

Background

Multiple myeloma (MM) is a malignant disease characterized by an increased number of clonal (abnormal) plasma cells (PC) in the bone marrow (BM) that secrete a monoclonal protein (M-protein) in the form of immunoglobulins or monoclonal κ and λ free light chains. High-dose chemotherapy followed by autologous stem cell transplantation (SCT) is used for the treatment of young MM patients and produces a high rate of complete remissions (CR). Recent trials with novel agent combinations alone have also resulted in high CR rates, even among old patients, high-risk patients and relapse/refractory MM. Unfortunately, most patients have recurrences of the disease. This is due to the persistence of residual tumor cells, known as minimal residual disease (MRD) which is responsible for tumor relapse. The aims of this study were: by apply multiparametric flow cytometry in order to estimate the plasma cell clones in bone marrow from MM patients who achieved partial or complete response or were resistant to chemotherapy, including high-dose chemotherapy with autologous SCT, and to assess quality of the remission, to investigate the relationship between the number of clonal bone marrow PC and other signs of MM activity.

Patients and methods

The study included 51 MM patients, average age, 54 years (36-70 years), who underwent MRD assessment from November 2014 to February 2017. According to the Durie-Salmon classification, the vast majority of patients (n=40) had III stage of the disease, 8 patients, II, and 2 patients had stage I MM. Response to treatment was assessed according to the National Clinical Recommendations of 2014. At the time of assessment, 20 patients were in CR, 8 had a partial response (PR) and 15 had a resistant disease; 5 patients had a primary MM, 3 patients were in the first relapse. Most of the patients were subject to high-dose chemotherapy with autologous SCT (n=42). Re-evaluation after therapy was managed to hold in 36 patients, at a mean of 3.1 months (1.9-5.7, min-max). Analysis was performed using a FACSCantoII flow cytometer (BD) and FACSDiva software (BD). Instrument performance was checked daily by recording fluorescence intensity with calibrating beads (Cytometer Setup and Tracking, BD Biosciences). Whole BM was estimated using combination of surface and intracellular staining with CD38/CD56/CD27/CD117/CD81/CD19/CD45/cytLambda/CD138/cyt-Kappa. The sensitivity of our panel MRD is 0.01% (i.e. 10-4).

Results

Among patients in CR (n=20) confirmed the absence of MRD in 6 patients, but 14 CR patients were MRD positive. Clonal PC was detected in all patients with PR and resistant disease (n=31). The relative content of abnormal PC in CR patients with MRD positive (n=14) was significantly lower than that in PR/resistant patients (n=31): 0.095% (0,026-0,271%) versus 1.3% (0,203-5,9%), pU = 0,000092. PR patients (n=8) had a lower relative content of abnormal PC (as expressed tendency), than patients with resistant disease (n=15): 0.286% (0.177-1.129%) versus 1.48% (0.90-8.0%), pU = 0.053. Moreover, the relative content of abnormal PC in PR/resistant patients (n=31) correlated with the serum M-gradient concentration (r=0.42; p=0.019).

Conclusion

Currently, we can presume flow cytometry as the method of choice for MRD monitoring in MM. In patients with measurable disease, the number of clonal PCs correlates with the disease status and the level of M-protein. In the absence of M-protein, the evaluation of the number of clonal PC in the bone marrow indicates the depth and quality of remission, determines the patient’s readiness for SCT and also helps regulate duration of the maintenance therapy.

Keywords

Multiple myeloma, flow cytometry, minimal residual disease.