Daratumumab (DARA) is a fully human antibody against CD38, a membrane-associated antigen, which is considered to be a highly relevant target for the treatment of hematological malignancies such as multiple myeloma (MM) and chronic lymphocytic leukemia (CLL). DARA has the ability to effectively kill target cells by CDC, ADCC and by induction of apoptosis. While CD38 is frequently overexpressed on MM cells, it has a variable degree of expression on the malignant CLL cells. To gain detailed insight into the potential therapeutic power of DARA in tumors with different CD38 expression levels, we now investigated the correlation between the level of CD38 expression and DARA-induced kill by CDC and ADCC. To this end, we first used the human MM cell lines L363 and UM9, expressing low and intermediate CD38 levels, respectively. While both cell lines could be lysed via ADCC, they were not susceptible to killing via CDC. To delineate a possible correlation between CD38 expression and induction of CDC, we lentivirally transduced the CD38 gene into L363 and UM9 cells to generate cell lines ranging in CD38 expression from 50,000 to 800,000 molecules per cell. Hereby we covered the range of CD38 expression found in CLL and MM patients. The L363 and UM9 cell lines were also transduced with the luciferase-GFP marker genes to enable their in vivo quantitative monitoring by bioluminescence imaging (BLI). In CDC assays, up to 95% lysis of CD38-transduced L363 and UM9 cells was achieved and showed an excellent correlation with the level of CD38 expression (r2=0.90). In ADCC assays, however, killing already reached maximum levels at DARA concentrations as low as 1 ng/ml at the lowest CD38 expression levels tested. These in vitro assays indicated that cell killing via CDC is enhanced with increasing CD38 expression levels, while CDC resistant, low CD38-expressing tumors could be targeted by DARA via ADCC. To explore this in an in vivo setting, we used a newly developed MM model in Rag2−/−gc−/− mice and we inoculated the mice with nontransduced, CD38low, UM9 cells, which can be lysed by ADCC but not by CDC. Treatment of the mice with DARA one day after tumor inoculation completely prevented the outgrowth of these UM9 cells; treatment of established UM9 tumours at week 3 resulted in a significant delay in tumor growth. These results confirmed that the CDC resistant UM9 could be readily targeted by DARA in vivo, possibly via ADCC mediated by monocytes in this model. In vivo experiments with variants that express high levels of CD38 are in progress. Taken together, these in vitro and in vivo data underscore the potency of DARA as a novel antibody for CD38-targeted therapy in hematological malignancies, and warrant further exploration in clinical trials.
de Weers:Genmab bv: Employment. Parren:Genmab: Employment.
Asterisk with author names denotes non-ASH members.