P159 | CIRCULATING TUMOUR CELLS QUANTIFICATION IN NEWLY-DIAGNOSED TRANSPLANT INELIGIBLE REAL WORLD MULTIPLE MYELOMA PATIENTS: COMPARISON OF SINGLE PLATFORM FLOW CYTOMETRY VS. NEXT GENERATION FLOW

Introduction. Circulating tumor plasma cells (CTCs) are emarging as an independent prognostic factor in Multiple Myeloma. Flow Cytometry (FC) is a sensitive, rapid, and cost-effective method for measuring CTCs. Previous studies reported heterogeneous prognostic CTCs cut-off values, and these differences may be due in part to the techniques used to quantify them. The aim of our study is to compare the percentage of CTCs measured with single-platform FC vs Next Generation Flow (NGF). Methods. CTCs % was prospectively analyzed in the peripheral blood at screening by single-platform FC in 163 patients receiving daratumumab-containing regimens enrolled in the REAL MM trial (NCT03829371). In 31/163 patients, CTCs % was obtained with both single-platform FC and NGF. An antibody panel consisting of CD38, CD138, CD45, CD56, and CD19 was used with single-platform FC. To confirm CTCs clonality, a second tube containing the same antibodies plus cytoplasmic Kappa and Lambda light chains was used. For NGF-based evaluation of CTCs, a two-tube, eight-color panel was employed following EuroFlow standardized protocols for sample preparation and cell acquisition (www.euroflow.org). Results. A median of 960,000 nucleated cells (range 320,000–1,630,000) with single-platform FC and a median of 10,600,000 nucleated cells (range 4,310,000–11,480,000) with NGF were analyzed. Single-platform FC detected CTCs in 27/31(87%) patients, while NGF detected CTCs in 29/31 (93%) patients, including 2 cases that were negative by single-platform FC. A strong correlation was observed between the percentages measured with the two methods (R²=0.95, p<0.0001, Figure 1A). CTC % obtained with NGF technique (median 0.0049%, range 0-0.22%) was significantly and systematically lower to that obtained with single platform FC (median 0.009%, range 0-0.45%) (p<0.0001, Figure 1B). No significant differences in baseline CTCs were observed grouping patients by international staging system or by the presence of high-risk chromosomal abnormalities. A trend of correlation was observed between % of CTCs and % of monoclonal plasma cells in the bone marrow biopsy (R2=0,2841, p=0.09) Conclusions. Both NGF and single-platform FC are valid and applicable techniques to measure CTCs levels but NGF has higher sensitivity. The difference of CTCs % obtained with the 2 techniques should induce caution in interpreting CTCs levels across studies, especially if values are near the prognostic cut-off.