A Novel Anti-IgM/HLA-DR Bispecific Antibody for Treatment of Refractory B Cell Malignancies

Introduction: IgM composes a part of the B cell receptor, and its expression is restricted to B cell lineage cells. There have been several reports that refractory B cell malignant cells frequently express IgM. For example, activated B cell-like (ABC) diffuse large cell lymphoma (DLBCL) is a subtype with a poor prognosis following R-CHOP treatment (Lenz et al., N Engl J Med, 2008), and the tumor cells generally express IgM-type immunoglobulin (Ruminy et al., Leukemia, 2011). In addition, IgM-expressing chronic lymphocytic leukemia (CLL) was also found to have a poor prognosis (D'Avola et al., Blood, 2016). Therefore, IgM is a promising therapeutic target for refractory B cell malignancies. However, anti-IgM antibodies may not function fully in the body due to the presence of soluble IgM. To improve the selectivity to the IgM expressing cells, we produced a bispecific antibody bound to IgM and HLA-DR, which are both expressed by B cells, and it was able to bind IgM+HLA-DR+ cells in the presence of soluble IgM. Thus, the aim of the present study was to examine the in vitro anti-tumor activity of the anti-IgM/HLA-DR bispecific antibody against B cell lymphoma, and to clarify its efficacy and tolerability in non-human primates (NHP).

Methods: BTA106 is the bispecific antibody that can bind to IgM and HLA-DR, and BTA111 is glycoengineered BTA106. To investigate their direct inhibitory effects and Fc-mediated effects against B cell lymphoma cell lines, a growth inhibition assay, antibody-dependent cell cytotoxicity (ADCC) assay and complement dependent cytotoxicity (CDC) assay were performed. In the growth inhibition assay and CDC assay, cell proliferation was measured by WST-8. In the ADCC assay, human peripheral blood mononuclear cells were used as effector cells. Dead cells were stained with propidium iodide and measured by flow cytometry. JeKo-1 (Mantle cell lymphoma, MCL), Ramos (Burkitt's lymphoma, BL), RL (DLBCL), B104 (B cell lymphoma) and two rituximab-resistant cell lines, RL-4RH (Czuczman et al., Clin Cancer Res., 2008) and RRBL1 (generously provided by Dr. Tomita, Tomita et al., Int J Hematol., 2007), were used. To clarify the efficacy and tolerability of BTA106 in NHP, a dose escalation study and single-dose administration study were performed using cynomolgus monkeys.

Results: BTA106 and BTA111 demonstrated direct growth inhibition activity, and CDC and ADCC activity, although their intensities differed among cell lines. In the case of rituximab-resistant cell lines, RRBL1 and RL-4RH, BTA106 and BTA111 exhibited weak or no growth inhibitory effects, but their CDC and ADCC activity was superior to that of rituximab. Development of resistance to rituximab is also a poor prognostic factor and remains problematic. Thus, BTA106 and BTA111 are expected as therapeutic antibodies to overcome rituximab resistance.

A dose escalation study and single dose study were performed in cynomolgus monkeys. BTA106 greatly reduced peripheral B cells. Moreover, after the administration of BTA106, lymph follicles disappeared in axillary lymph nodes. In these studies, decreased locomotor activity and vomiting were observed; however, these symptoms resolved 2 hrs after injection. Other BTA106-related clinical toxicities or changes in body weight were not observed.

Conclusions: In the in vitro studies, BTA106 and BTA111 demonstrated significant anti-tumor effects on rituximab-sensitive and -resistant B cell lymphoma cell lines. The efficacy and toxicity studies in cynomolgus monkeys revealed that BTA106 depletes peripheral and lymph node B cells, and it was well tolerated. BTA111 was considered to be significantly more effective than BTA106 due to glycoengineering, which induced high ADCC activity. These data suggest that BTA111 is an effective and potent therapeutic antibody for B cell malignancies, especially rituximab-resistant B cell lymphoma. Currently, a repeated administration study of BTA111 with cynomolgus monkeys to evaluate its efficacy and toxicity is ongoing.