CHIR-12.12, an Antagonist Anti-CD40 Antibody, Exhibits Greater ADCC Than Rituximab Against a Variety of Malignant B Cells: Evaluation of FcγRIIIa Polymorphism and ADCC Response.

We have generated a novel, fully human IgG₁ anti-CD40 antagonistic monoclonal antibody, CHIR-12.12, using XenoMouse® mice (Abgenix, Inc.) and have previously demonstrated that it inhibits normal human B cell proliferation and survival and has potent ADCC against primary CLL and NHL cells. CHIR-12.12 and the anti-CD20 monoclonal antibody rituximab were compared for their relative ADCC activity against a variety of malignant human B-cell lines expressing both CD40 and CD20 antigens, including two lymphoma cell lines (Daudi, Namalwa), two multiple myeloma cell lines (ARH77, IM-9), a B-ALL cell line (CCRF-SB), and a B-CLL cell line (EHEB). All cell lines expressed both CD20 and CD40 antigens, and the number of cell surface CD20 molecules per cell were 2.6- to 30.8-fold higher than CD40. For all target cell lines, despite the greater number of CD20 receptors, CHIR-12.12 showed greater maximum cell lysis and a lower ED₅₀ than rituximab. ADCC activity of rituximab is known to correlate with the FcγRIIIa genotype of the effector cells. The homozygous valine (V/V) or heterozygous valine/phenylalanine (V/F) polymorphisms at aa158 are associated with greater cell lysis than is the homozygous F/F polymorphism. The role of the FcγRIIIa aa158 genotype as it relates to CHIR-12.12 activity was explored using Daudi lymphoma target cells and effector NK cells purified from human donors expressing the three polymorphisms. CHIR-12.12 induced potent ADCC with NK cells of all three genotypes (ED₅₀s of 4, 2, and 0.4 pM for F/F, V/F, and V/V, respectively). The rituximab ED₅₀s were 53, 21, and 9 pM for F/F, V/F, and V/V, respectively. Comparison of affinity of the FcγRIIIa F and V alleles for CHIR-12.12 and rituximab using Biacore® analysis showed that CHIR-12.12 bound the F allele with a 4.6-fold higher affinity than rituximab (2.8 μM versus 13 μM, respectively). These data demonstrate that CHIR-12.12 is a more potent ADCC mediator than rituximab, even with human NK cells of the aa158 F/F genotype. CHIR-12.12 is currently in Phase I clinical trials for B-cell malignancies.