V.H.J.vdV. obtained using the EuroFlow approach, including CD38-multiepitope KDU691 (ME) vs MRD data obtained using VS38c. The correlation coefficient for all samples together was 0.998 (< .001); similar numbers were obtained if the 2 2 groups (with and without daratumumab) were analyzed separately. The samples obtained from patients treated without (n = 14) or with daratumumab (n = 15) are shown in closed and open circles, respectively. Fifteen samples (8 from patients not treated with daratumumab and 7 from patients treated with daratumumab) were MRD negative by both approaches and are shown in the bottom left this plot. B, Mean fluorescence intensities (MFIs) of CD38-ME KDU691 and VS38c on B cells (B), normal plasma cells (PC), and multiple myeloma (MM) cells (if present at levels equal to or above 0.01%) in patients without (n = 14) or with daratumumab treatment (+ dara; n = 15). For VS38c, no significant differences were observed for the MFI values of PC and MM cells, independent of daratumumab treatment. For CD38, expression was highest on normal PC from patients not treated with daratumumab; MM cells from such patients showed a tendency for lower CD38 expression levels (= .06). In patients treated with daratumumab, CD38 expression on B cells and normal cells was KDU691 significantly lower compared with these cells in patients not treated with daratumumab (< .05), and the same tendency was present for MM cells (= .09). All values were based on 2-tailed Mann-Whitney tests. The red lines represent the median MFI value of the group. The black lines on top of the figure indicate statistically significant differences between the 2 groups. Because VS38c has been reported to be easier for MM cell identification than using CD38, we next evaluated whether this approach is also valid for the CD38-ME. As shown in FIGURE 1B, CD38-ME and VS38c both showed high and comparable mean fluorescence intensities (MFIs) on normal plasma cells from patients not treated with daratumumab. In contrast, in these patients, VS38c showed significantly lower MFI values on KDU691 normal B cells compared with CD38. Consequently, the ratio between normal plasma cells and B cells was significantly higher for VS38c than for CD38-ME (VS38c: median [range], 202 [12.8-364]; CD38-ME: median [range], 24.5 [8.6-67.6]; < .01). In daratumumab-treated patients, VS38c expression levels on B cells and normal plasma cells were comparable with those in patients not treated with daratumumab, whereas CD38 expression on B cells and normal plasma cells was significantly lower FIGURE 1B. The ratio between normal plasma cells and B cells in daratumumab-treated patients was significantly higher for VS38c compared with CD38-ME (VS38c: median [range], 148 [3.3-511]; CD38-ME: median [range], 19.2 [1.4-55.7]; < .01). VS38c was expressed in MM cells at similar levels as on normal plasma cells, without a clear effect of daratumumab. In contrast, CD38 expression in MM cells tended to be lower than in normal plasma cells (= .056) and decreased even more in the majority of daratumumab-treated patients FIGURE 1B. An example of a patient with virtually complete loss of CD38 expression is shown in FIGURE 2. Despite the differences in expression levels between VS38c and CD38-ME, identification of MM cells was felt to be comparably easy for both approaches in the context of the other markers. Open in a separate window FIGURE 2 Flow cytometric data from Mouse monoclonal antibody to Rab2. Members of the Rab protein family are nontransforming monomeric GTP-binding proteins of theRas superfamily that contain 4 highly conserved regions involved in GTP binding and hydrolysis.Rabs are prenylated, membrane-bound proteins involved in vesicular fusion and trafficking. Themammalian RAB proteins show striking similarities to the S. cerevisiae YPT1 and SEC4 proteins,Ras-related GTP-binding proteins involved in the regulation of secretion a patient with multiple myeloma (MM) treated with daratumumab. Upper row: Data obtained using the EuroFlow MM minimal residual disease (MRD) tube, including the CD38-multiepitope (ME) antibody. Whereas CD138 is brightly KDU691 expressed by the MM cells (in red), CD38 expression is hardly detectable. Bottom row: Data obtained using the EuroFlow MM MRD tube, in which the CD38-ME antibody has been replaced by the VS38c antibody. Both CD138 and VS38c are brightly expressed by the MM cells (in red). In both tubes, the MM cells could easily be distinguished by low CD45 expression in combination with CD56 expression. FITC, fluorescein isothiocyanate; PerCP Cy5.5, peridinin-chlorophyll-protein complex-cyanine 5.5. DISCUSSION There have been concerns that current MM MRD assays do not work well in patients treated with targeted therapies, such as daratumumab. Within the EuroFlow consortium, a CD38-ME was.