Daratumumab, a Novel Anti-CD38 Monoclonal Antibody Shows Anti-Tumor Activity in CLL and hampers Leukemia-Microenvironment Interactions

Daratumumab (DARA) is a anti-human CD38 antibody with Fc-mediated cell killing activity. DARA induces killing of tumor cells, mainly via complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC) (de Weers M. J Immunol 2011), and antibody-dependent cellular phagocytosis (ADCP) by macrophages (mΦ), both murine and human in multiple myeloma (MM) and Burkitt lymphoma cells. DARA is currently being evaluated in phase III clinical trials in patients with MM. We have previously reported that DARA induces cytotoxic activity in vitro via ADCC in primary cells and cell lines from Chronic Lymphoctic Leukemia (CLL), and significantly prolongs overall survival of animals in a systemic CLL mouse model. Here, we present additional data on in vivo mechanism of DARA and its effect on tumor-microenvironment interactions in CLL.

We first evaluated whether ADCP contributes to DARA activity both in vitro and in vivo. For in vitro ADCP, mΦ were generated from monocytes of normal PBMCs and stimulated with GM-CSF (10ng/mL, 7 days). CLL cell lines and primary cells were labeled with calcein and incubated for 4h with mΦ at an effector:target ratio of 2:1 in the presence of a fixed mAb concentration of 1 μg/mL, followed by flow cytometric analysis. The amount of remaining CLL target cells (CD19+, CD11b-) was reduced by 3-16%. ADCP defined as percentage of mφ which had phagocytosed, referred to as double positive mΦ (CD11b+, calcein+, CD19-), ranged from 3-10%. To analyze ADCP in vivo, SCID beige mice, devoid of NK cells but with active macrophages, were inoculated intraperitoneally with CLL cells (20×10⁶) and simultaneously treated with a single dose of DARA or isotype control (20mg/kg, n=3-5 per group). Forty-eight hours later, CLL cells were recovered from the intraperitoneal cavity and counted in a flow cytometer (identified as human CD45+/CD19+/CD5+cells). In DARA-treated mice the number of CLL cells recovered was reduced by 42% (n=2, p<0.05) compared to the isotype control group. Remarkably, the decrease in cell number was already detectable 2h after DARA administration.

CLL pathogenesis relies on supportive tumor-microenvironment interactions both in the bone marrow (BM) and in the lymph node (LN), and CD38 constitutes a molecular hub integrating proliferative and migratory signals for CLL (Malavasi, F. Blood 2011). We evaluated the effect of DARA on migration and adhesion. In in vitro migrations assays, we have demonstrated that DARA (10-30 μg/mL) inhibited CXCL12/SDF1α-mediated migration up to 70% (n=5). In addition, DARA reduced up to 55% (n=2) of downstream pERK activation, that peaked after 5min of CXCL12/SDF1α stimulation. We analyzed the effect of DARA on primary CLL cell migration from Peripheral Blood (PB) to BM and spleen in vivo, using NOD/SCID/gamma (NSG) null mice (lacking NK cells and effective macrophages). In this system, NSG mice were pretreated (day 0) with DARA, control IgG or anti-CXCR4 as positive control for inhibition of cell homing, prior to injection of fresh primary CLL cells (50×10⁶ cells/per mice) on day 1. PB, BM and spleen cells were isolated on day 2 and CLL cells were identified by staining for human CD45/CD19/CD5 and counted using a flow cytometer. Cell counting showed that CLL cells mainly migrate to the spleen, and that DARA significantly reduced this migration (55% inhibition on average, p<0.05).

In addition to migration, CD38 also plays a key role in cell adhesion through interaction with integrins (CD49d/CD29) and with extracellular matrix proteins. We analyzed the effect of DARA on the adhesion of CLL cells to the extracellular matrix vascular-cell adhesion molecule-1 (VCAM-1) mediated by CD49d/CD29. DARA reduced adhesion of CLL cells (n=4), to VCAM-1 by 46±13% (range 27-57) compared to isotype control. By RT-PCR we observed an up-regulation of MMP9 transcripts (average 2 fold, n=2), and DARA abrogated both constitutive MMP9 expression (90% reduction) and VCAM-derived (94% reduction) MMP9 expression.

In summary, DARA shows a positive effect on ADCP-mediated anti-tumor activity on CLL cells both in vitro and in vivo. In addition DARA exhibits a strong effect on CLL cell migration and adhesion. Based on these data, we hypothesize that DARA may exert unique and substantial effects on CLL tumor cell growth and contributes to potent therapeutic efficacy in a clinical setting.