Key Points• Response to the CD38-targeting antibody daratumumab is significantly associated with CD38 expression levels on the tumor cells.• Resistance to daratumumab is accompanied by increased expression of complementinhibitory proteins.The anti-CD38 monoclonal antibody daratumumab is well tolerated and has high single agent activity in heavily pretreated relapsed and refractory multiple myeloma (MM). However, not all patients respond, and many patients eventually develop progressive disease to daratumumab monotherapy. We therefore examined whether pretreatment expression levels of CD38 and complement-inhibitory proteins (CIPs) are associated with response and whether changes in expression of these proteins contribute to development of resistance. In a cohort of 102 patients treated with daratumumab monotherapy (16 mg/kg), we found that pretreatment levels of CD38 expression on MM cells were significantly higher in patients who achieved at least partial response (PR) compared with patients who achieved less than PR. However, cell surface expression of the CIPs, CD46, CD55, and CD59, was not associated with clinical response. In addition, CD38 expression was reduced in both bone marrow-localized and circulating MM cells, following the first daratumumab infusion. CD38 expression levels on MM cells increased again following daratumumab discontinuation. In contrast, CD55 and CD59 levels were significantly increased on MM cells only at the time of progression. All-trans retinoic acid increased CD38 levels and decreased CD55 and CD59 expression on MM cells from patients who developed daratumumab resistance, to approximately pretreatment values. This resulted in significant enhancement of daratumumab-mediated complement-dependent cytotoxicity. Together, these data demonstrate an important role for CD38 and CIP expression levels in daratumumab sensitivity and suggest that therapeutic combinations that alter CD38 and CIP expression levels should be investigated in the treatment of MM. These trials were registered at www.clinicaltrials.gov as #NCT00574288 (GEN501) and #NCT01985126 (SIRIUS). (Blood. 2016;128(7):959-970)

Daratumumab is an anti-CD38 monoclonal antibody with lytic activity against multiple myeloma (MM) cells, including ADCC (antibody-dependent cellular cytotoxicity) and CDC (complement-dependent cytotoxicity). Owing to a marked heterogeneity of response to daratumumab therapy in MM, we investigated determinants of the sensitivity of MM cells toward daratumumab-mediated ADCC and CDC. In bone marrow samples from 144 MM patients, we observed no difference in daratumumab-mediated lysis between newly diagnosed or relapsed/refractory patients. However, we discovered, next to an expected effect of effector (natural killer cells/monocytes) to target (MM cells) ratio on ADCC, a significant association between CD38 expression and daratumumab-mediated ADCC (127 patients), as well as CDC (56 patients). Similarly, experiments with isogenic MM cell lines expressing different levels of CD38 revealed that the level of CD38 expression is an important determinant of daratumumab-mediated ADCC and CDC. Importantly, all-trans retinoic acid (ATRA) increased CD38 expression levels but also reduced expression of the complement-inhibitory proteins CD55 and CD59 in both cell lines and primary MM samples. This resulted in a significant enhancement of the activity of daratumumab in vitro and in a humanized MM mouse model as well. Our results provide the preclinical rationale for further evaluation of daratumumab combined with ATRA in MM patients.

The unrestrained growth of tumor cells is generally attributed to mutations in essential growth control genes, but tumor cells are also influenced by signals from the environment. In multiple myeloma (MM), the factors and signals coming from the bone marrow microenvironment are possibly even essential for the growth of the tumor cells. As targets for intervention, these signals may be equally important as mutated oncogenes. Given their oncogenic potential, WNT signals form a class of paracrine growth factors that could act to influence MM cell growth. In this paper, we report that MM cells have hallmarks of active WNT signaling, whereas the cells have not undergone detectable mutations in WNT signaling genes such as adenomatous polyposis coli and ␤-catenin (CTNNB1). We show that the malignant MM plasma cells overexpress ␤-catenin, including its N-terminally unphosphorylated form, suggesting active ␤-catenin͞T cell factor-mediated transcription. Further accumulation and nuclear localization of ␤-catenin, and͞or increased cell proliferation, was achieved by stimulation of WNT signaling with either Wnt3a, LiCl, or the constitutively active S33Y mutant of ␤-catenin. In contrast, by blocking WNT signaling by dominant-negative T cell factor, we can interfere with the growth of MM cells. We therefore suggest that MM cells are dependent on an active WNT signal, which may have important implications for the management of this incurable form of cancer.␤-catenin ͉ T cell factor ͉ lymphoma ͉ plasma cell

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.

No abstract