Ex vivo sensitivity to daratumumab is gradually recovered after the drug is discontinued. (A) Schematic depiction of the My-DST workflow. Unselected MNCs were isolated from donated patient biopsy samples, and then cells were incubated with or without drug treatment for 48 hours before flow cytometry. Viability was normalized to that of Untx controls, and our established cutoff of 80% MM cell viability compared with untreated was used to classify patient samples as sensitive (green) or resistant (red). (B) Drug-sensitivity testing results for daratumumab (Dara) sensitivity in bone marrow samples from patients who were refractory. Each data point represents the mean number of viable MM cells from 3 replicates normalized to the mean number of viable MM cells in untreated control wells for a patient sample. Pearson r value is shown with its associated P value; the solid line represents linear regression, with dashed lines for 95% confidence intervals. (C) Comparison of the Dara sensitivity in patients who had not previously been treated with an anti-CD38 antibody (Dara-naïve) with that of patients who were Dara refractory but off that treatment for less than or greater than 1 year. (D) CD38 MFI on MM cells divided by CD38 MFI on non-MM cells across settings showed that patients who were Dara-naïve had higher CD38 expression than patients who were Dara refractory, but patients with >12 months off anti-CD38 treatment did partially regain CD38 expression. (E) CD38 expression on cells of patients who were Dara refractory compared with ex vivo Dara sensitivity, showing that a fold overexpression of at least sixfold was generally necessary for sensitivity. Statistical comparisons of the mean were conducted using analysis of variance with Tukey correction for multiple comparisons. FACS, fluorescence-activated cell sorting; NK, natural killer cell; Norm %, normalized percent of controls; Untx, untreated.