Daratumumab, a Novel Human Anti-CD38 Monoclonal antibody shows Anti-Tumor Activity In Mouse Models Of MCL, FL and CLL

Daratumumab (DARA) is a human CD38 antibody with broad-spectrum killing activity. DARA induces killing of tumor cells, mainly via complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC) (de Weers M, J Immunol 2011). DARA is currently being evaluated in phase I/II clinical trials in patients with multiple myeloma. In these clinical studies the adverse events have been manageable and marked reductions in paraprotein and bone marrow plasma cells have been observed. We have previously reported (Blood (ASH annual meeting abstracts). Nov 2012, 120 (21): 3935) that DARA induces cytotoxic activity in vitro via ADCC in primary cells and cell lines from Mantle Cell Lymphoma (MCL), Follicular Lymphoma (FL) and Chronic Lymphoctic Leukemia (CLL). CDC induction was low, which is associated to high expression of the complement inhibitors and reduced number of CD38 molecules per cell in these indications. This suggests a threshold for CD38-targeted CDC lysis. In addition, based on known interactions between CD38-CXCR4, we also demonstrated that in the CLL subtype, with high CD38 and more migratory capacity, DARA (10-30 µg/mL) inhibited in vitro CXCL12/SDF1α-mediated migration up to 70%.

Here, we present the first preclinical in vivo results of DARA in mouse models of MCL, transformed FL(tFL) and CLL. We generated heterotopic and systemic mouse models of these entities by subcutaneous or intravenous inoculation of tumor cell lines in SCID mice, that retain both NK cells and macrophages as potential effector cells.

In MCL (REC cell line) and tFL (RL cell line) subcutaneous mouse models, we tested DARA activity in both prophylactic (treatment initiation simultaneous to lymphoma cell inoculation) and therapeutic settings(treatment initiation one week after lymphoma cell inoculation, when tumors were about 100 mm³ in size). In the prophylactic setting, mice received 3 doses of DARA or control IgG bi-weekly (20/10/10 mg/kg). In the therapeutic setting, treatment was intensified to 4 doses of DARA or control IgG weekly (20/10/10/10 mg/kg). In both schedules, mice were sacrificed one week after the last dose. DARA completely abrogated tumor growth of REC or RL cells in the prophylactic setting. Moreover, in the therapeutic setting, DARA induced total tumor regression of REC tumors in 4 out of 6 mice, and prevented the splenomegaly observed in control IgG treated mice. In the case of tFL and therapeutic setting, DARA reduced 60% of tumor growth compared to control IgG treated mice at day 32, when experiment was finished. In CLL, we analyzed the effect of DARA on cell homing to lymphoid organs together with its therapeutic properties in a systemic CLL mouse model. Using NOD/SCID/gamma null mice (lacking NK cells and effective macrophages), we analyzed the effect of DARA on primary CLL cell migration from Peripheral Blood (PB) to bone marrow (BM) and Spleen. In this system, NSG mice were pretreated (day 0) with DARA, control IgG or anti-CXCR4 as positive control for inhibition of cell homing, followed by fresh CLL cell inoculation (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 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). Migration of CLL cells to BM was limited and was not affected by pretreatment of mice with DARA.

Finally, we tested DARA therapeutic activity in a systemic CLL mouse model (MEC2 cell line), following the schedule described before (4 doses of control IgG/ DARA weekly (20/10/10/10 mg/kg)), and assessed efficacy on mice overall survival. Mice treated with control IgG started to lose weight and showed signs of disease between days 30-40 and were sacrificed for ethical reasons. In the DARA treated group, in 7 out of 8 mice survival was extended up to day 90, when the experiment was stopped. In conclusion, DARA demonstrated strong in vivo activity in immunocompromised mouse models of MCL, tFL and CLL cell lines and interfered with homing of primary CLL cells to the spleen. These results warrant further investigation of DARA in clinical trials for these indications.