Antagonist Anti-CD40 Antibody, CHIR-12.12, Induces ADCC, Inhibits Tumor Growth, and Prolongs Survival in a Human Multiple Myeloma Xenograft Model.

CD40 is expressed on all B-cell malignancies, including multiple myeloma, and represents an attractive target for antibody therapy. CHIR-12.12 is a fully human, highly potent, IgG₁ antagonistic anti-CD40 monoclonal antibody generated using XenoMouse^® mice (Abgenix, Inc). CHIR-12.12 can mediate antitumor activity by at least two mechanisms: blocking CD40-ligand-mediated survival signals and killing tumor cells by antibody-dependent cellular cytotoxicity (ADCC). We have previously reported that CHIR-12.12 mediates stronger in vitro killing of CD40⁺- and CD20⁺-expressing human non-Hodgkin’s lymphoma and lymphoblastoid B cells by ADCC than rituximab and significantly inhibits the growth of rituximab-responsive (Daudi) and rituximab-resistant (Namalwa) human lymphoma and lymphoblastoid B-cell (IM-9) xenografts in vivo. In this study, we examined the in vitro and in vivo efficacy of CHIR-12.12 against the human multiple myeloma cell line KMS-12-BM. CHIR-12.12 induced lysis of KMS-12-BM cells by ADCC in a dose-dependent manner, reaching maximum cell lysis at 0.1μg/ml with an EC₅₀ of 17.5 pM. CHIR-12.12 efficacy in vivo was evaluated in orthotopic and subcutaneous KMS-12-BM xenograft models. In the staged orthotopic model, tumor cells were delivered intravenously and treatment was initiated 7 days post cell implantation. CHIR-12.12 significantly prolonged the median survival of tumor-bearing mice in a dose-dependent manner, with a median survival of 78 and 98 days in the groups treated with 1 mg/kg and 10 mg/kg CHIR-12.12 weekly, respectively, compared to a median survival time of 68 days in the control IgG₁ group (P<0.0001). Bortezomib administered i.v. twice weekly at 0.5 or 1 mg/kg showed no survival benefit. In the staged subcutaneous model, CHIR-12.12 was administered weekly at 1 and 10 mg/kg after the mean tumor volume reached 100mm³. CHIR-12.12 significantly inhibited tumor growth, with a tumor volume reduction of 42% (P<0.05) and 63% (P<0.01), respectively. Bortezomib and melphalan/prednisone did not inhibit KMS-12-BM tumor growth at the doses and schedules reported for other human multiple myeloma xenograft models. Western blot analysis and immunohistochemical staining showed significantly increased levels of cleaved PARP in KMS-12-BM s.c. tumors 7 days after the initiation of CHIR-12.12 treatment, suggesting the induction of cell death by CHIR-12.12. Taken together, these data demonstrate that the anti-CD40 mAb CHIR-12.12 has potent activity against human multiple myeloma cells in vitro and in xenograft models in vivo. Currently CHIR-12.12 is in Phase I clinical trials for the treatment of B-cell malignancies.