Purpose Daratumumab treatment results in a marked reduction of CD38 expression on multiple myeloma (MM) cells. The aim of this study was to investigate the clinical implications and the underlying mechanisms of daratumumab-mediated CD38 reduction. Experimental design We evaluated the effect of daratumumab alone or in combination with lenalidomide-dexamethasone, on CD38 levels of MM cells and non-tumor immune cells in the GEN501 study (daratumumab monotherapy) and the GEN503 study (daratumumab combined with lenalidomide-dexamethasone). In vitro assays were also performed. Results In both trials daratumumab reduced CD38 expression on MM cells within hours after starting the first infusion, regardless of depth and duration of the response. In addition, CD38 expression on non-tumor immune cells, including NK-, T- B-cells and monocytes, was also reduced irrespective of alterations in their absolute numbers during therapy. In-depth analyses revealed that CD38 levels of MM cells were only reduced in the presence of complement or effector cells, suggesting that the rapid elimination of CD38high MM cells can contribute to CD38 reduction. In addition, we discovered that daratumumab-CD38 complexes and accompanying cell membrane were actively transferred from MM cells to monocytes and granulocytes. This process of trogocytosis was also associated with reduced surface levels of some other membrane proteins including CD49d, CD56, and CD138. Conclusion Daratumumab rapidly reduced CD38 expression levels, at least in part, through trogocytosis. Importantly, all these effects also occurred in patients with deep and durable responses, thus excluding CD38 reduction alone as a mechanism of daratumumab resistance.

<p>Supplemental Methods, Supplementary Tables, Supplementary Figure Legends</p>

<p>Supplemental Methods, Supplementary Tables, Supplementary Figure Legends</p>

<p>Daratumumab-mediated CD38 reduction on NK cells is abrogated by depleting monocytes from PBMCs.</p>

<div>Abstract<p><b>Purpose:</b> Daratumumab treatment results in a marked reduction of CD38 expression on multiple myeloma cells. The aim of this study was to investigate the clinical implications and the underlying mechanisms of daratumumab-mediated CD38 reduction.</p><p><b>Experimental Design:</b> We evaluated the effect of daratumumab alone or in combination with lenalidomide-dexamethasone, on CD38 levels of multiple myeloma cells and nontumor immune cells in the GEN501 study (daratumumab monotherapy) and the GEN503 study (daratumumab combined with lenalidomide-dexamethasone). <i>In vitro</i> assays were also performed.</p><p><b>Results:</b> In both trials, daratumumab reduced CD38 expression on multiple myeloma cells within hours after starting the first infusion, regardless of depth and duration of the response. In addition, CD38 expression on nontumor immune cells, including natural killer cells, T cells, B cells, and monocytes, was also reduced irrespective of alterations in their absolute numbers during therapy. In-depth analyses revealed that CD38 levels of multiple myeloma cells were only reduced in the presence of complement or effector cells, suggesting that the rapid elimination of CD38^high multiple myeloma cells can contribute to CD38 reduction. In addition, we discovered that daratumumab–CD38 complexes and accompanying cell membrane were actively transferred from multiple myeloma cells to monocytes and granulocytes. This process of trogocytosis was also associated with reduced surface levels of some other membrane proteins, including CD49d, CD56, and CD138.</p><p><b>Conclusions:</b> Daratumumab rapidly reduced CD38 expression levels, at least in part, through trogocytosis. Importantly, all these effects also occurred in patients with deep and durable responses, thus excluding CD38 reduction alone as a mechanism of daratumumab resistance.</p><p>The trials were registered at <a href="http://www.clinicaltrials.gov" target="_blank">www.clinicaltrials.gov</a> as NCT00574288 (GEN501) and NCT1615029 (GEN503). <i>Clin Cancer Res; 23(24); 7498–511. ©2017 AACR</i>.</p></div>