Daratumumab treatment is associated with decreased levels of CD38 on MM cells. (A) CD38 expression on MM cells in BM samples obtained from 21 patients, who were subsequently treated with daratumumab at a dose of 16 mg/kg in the GEN501 study. BM aspirates were obtained before start of daratumumab, before the 10th daratumumab infusion, at the time of progression (PD), and 6 months after stopping daratumumab therapy because of progressive disease (PD+6M). CD38 expression was determined by using HuMax-003-FITC, which binds to a different epitope compared with daratumumab, thereby excluding the possibility that binding of daratumumab masked the detection of CD38. Data are presented as mean ± SEM. P values between the indicated groups were calculated using a paired Student t test; **P < .01, ***P < .001, ****P < .0001, ns, not significant. (B) Flow cytometry histogram overlays depicting cell surface expression of CD38 on MM cells from 4 representative patients treated with daratumumab in the GEN501 trial at different time points: before start of treatment (green histogram), during daratumumab treatment before the 10th infusion (blue histogram), and at the time of progressive disease (red histogram). HuMax-003 FITC was used as CD38 antibody. (C) Longitudinal data representation of CD38 expression levels on MM cells from the 21 patients presented in A, according to the response achieved to daratumumab monotherapy (PR or better [gray bars] vs less than partial response [black bars]). P values between the indicated groups were calculated using a Student t test; **P < .01, ***P < .001, ****P < .0001. (D) Longitudinal data representation of absolute circulating MM cell counts over time in peripheral blood. Circulating MM cells were observed in 11 of 21 patients tested. Peripheral blood was obtained before start of treatment with daratumumab, during treatment with daratumumab, at the time of progression (PD), as well as 2, 4, and 6 months after development of progressive disease (PD). Data are presented as mean ± SEM. P values between the indicated groups were calculated using a paired Student t test; *P < .05. (E) CD38 expression on circulating MM cells before start of daratumumab treatment, during daratumumab treatment, at the time of progression (PD), as well as 2, 4, and 6 months after the development of progressive disease (PD) (n = 11 patients). CD38 expression was determined by using HuMax-003-FITC. Data are presented as mean ± SEM. P values between the indicated groups were calculated using a paired Student t test; *P < .05, **P < .01, ***P < .001, ns, not significant.