PF579 TUMOR CIRCULATING PLASMA CELLS DETECTED BY FLOW CYTOMETRIC SINGLE PLATFORM METHOD ARE RELATED TO NEGATIVE PATIENTS‘ CHARACTERISTICS IN MULTIPLE MYELOMA
Authors: V.E. Muccio, M. Gilestro, E. Saraci, S. Spada, A. Capra, A. Costa, M. Ruggeri, P. Galieni, P. Curci, N. Pescosta, L. De Rosa, V. Pavone, S. Ronconi, C. Cellini, I.D. Vincelli, C. Musolino, A.M. Cafro, E. Piro, S. Caltagirone, D. Oddolo, C. Mussatto, F. Gay, M. Boccadoro, P. Omedè
Journal: HemaSphere (2019)

Background: Multiple myeloma (MM) is a clinically heterogeneous disease and several classification systems have been developed to stratify patients in low, intermediate or high risk. Tumor Circulating Plasma Cells (TCPC) are considered a marker of high‐risk disease in all plasma cell disorders. To date, different methods are available to detect TCPC in peripheral blood, but they are barely reproducible and time consuming. Aims: For the first time the amount of TCPC was quantified in newly diagnosed MM patients with a single platform flow cytometric method and was evaluated their correlation with patients‘ baseline characteristics. Methods: Whole peripheral blood (PB) samples from 413 newly diagnosed young MM patients enrolled in the UNITO‐MM‐01/FORTE trial were included in the study. For the single platform technique, the antibody combination CD38PC7/CD138PC5.5/CD45KO/ CD56PE/CD19PB was mixed with 100 μL of EDTA PB, dispensed with reverse pipetting, and incubated for 15 min at RT, added with 500 μL of lysing solution and, after 15 min, 100 μL of flow count fluorospheres were dispensed with reverse pipetting and cells acquired with Navios flow cytometer. Intracytoplasmic kappa and lambda were tested to confirm the TCPC clonality. Results: Circulating Plasma Cells (CPC) were found in 390 out of 413 samples (94.4%), with median values of 0.03% (range: 0%&gt;51%) and 2.37/mm 3 (range: 0/mm 3 –6272/mm 3 ). White blood cells were 5710/mm 3 (range: 1752/mm 3 –26102/mm 3 ); median CPC events 58 (range: 0–441.000); cellular events acquired were 190.000 (range: 4.428–1.300.000). TCPC were found in 272 samples (66%) with a median value of 1.24/mm 3 (range 0.06/mm 3 ‐ 6272/mm 3 ). Patients were stratified according to different baseline characteristics and median values of absolute TCPC were compared. The most significant statistical differences ( p &lt; 0.001) were: Hb &lt;10 (12.9/mm 3 ) vs ≥10 (0.81/mm 3 ); ISS I (0.30/mm 3 ) vs ISS II (2.85/mm 3 ) vs ISS III (5.14/mm 3 ); R‐ISS I (0.25/mm 3 ) vs II (2.76/mm 3 ) vs III (5.14/mm 3 ); Albumin &lt;3.5 g/dL (2.76/mm 3 ) vs ≥3.5 g/dL (1.05/mm 3 ); β2‐microglobulin &lt;3.5 mg/dL (0.67/mm 3 ) vs 3.5 mg/dL‐5.5 mg/dL (3.88/mm 3 ) vs &gt;5.5 mg/dL (16.47/mm 3 ); LDH ≤upper limit (1.14/mm 3 ) vs &gt;upper limit (7.36/mm 3 ); PC in biopsy &lt;60% (0.60/mm 3 ) vs ≥60% (2.76/mm 3 ); with 1q gain (3.18/mm 3 ) vs without (1.18/mm 3 ); Morgan risk Standard (1.16/mm 3 ) vs High (3.00/mm 3 ) (presence of del17 and/or t(4;14) and/or t(14;16) and/or 1q gain). Moreover, the TCPC were sorted in quartiles according to the absolute values (0/mm 3 –0/mm 3 , 0/mm 3 –1.24/mm 3 , 1.24/mm 3 –8.3/mm 3 , 8.3/mm 3 –6272/mm 3 ) and a correlation between the increasing levels of TCPC and the parameters mentioned above was observed. The most significant correlations expressed by Cramer's V were observed between TCPC and: hemoglobin (V = 0.41), ≥60% of plasma cells in biopsy (V = 0.28), ISS (V = 0.26), β2‐microglobulin &lt;3.5 mg/dL vs 3.5 mg/dL‐5.5 mg/dL vs &gt;5.5 mg/dL (V = 0.26), R‐ISS (V = 0.24), Morgan risk (V = 0.24 p &lt; 0.001), 1q gain (V = 0.24) all with a p &lt; 0.001. Summary/Conclusion: The single platform flow cytometric method allowed us to quantify TCPC in a high number of MM patients and the values obtained were strongly related to the worst clinical and cytogenetic features. This method is fast, not expensive, feasible with a small amount of PB and can be easily performed in a high number of patients.