Abstract 5760: An IgG1 version of the anti-TfR1 antibody ch128.1 shows significant antitumor activity against different xenograft models of multiple myeloma

The transferrin receptor 1 (TfR1), also identified as CD71, is a type II transmembrane homodimeric protein involved in the cellular uptake of iron and in the regulation of cell growth. TfR1 is a meaningful target for antibody-based cancer immunotherapy due to its high expression levels on the surface of cancer cells and its central role in cancer pathology. We previously developed a mouse/human chimeric IgG3 specific for human TfR1 (ch128.1), which exhibits antitumor activity in SCID-Beige mice bearing disseminated multiple myeloma (MM) KMS-11 cells. Since human IgG1 has been the isotype of choice for therapeutic antibodies targeting malignant cells and has several advantages compared to IgG3, including a more established developability, we constructed an IgG1 version of ch128.1. This novel antibody, ch128.1/IgG1, expressed in murine cells (NS0/1), is properly assembled and secreted and binds antigen (TfR1) similar to its IgG3 counterpart, as demonstrated by ELISA and flow cytometry. Administration of a single dose of the IgG1 completely blocks subcutaneous KMS-11 tumor formation in SCID-Beige mice. Importantly, a single dose of the antibody also shows significant protection, including long-term survival, in this same strain of mice bearing disseminated human MM cells KMS-11 (Asian origin) or MM.1S (African American origin), at different stages of the disease. As expected, the protection was stronger when the mice were treated at an earlier disease stage. Studies using MM.1S cells are of particular relevance given the higher incidence and mortality of MM in African Americans compared to other racial groups. Importantly, we also found that ch128.1/IgG1 confers in vivo protection against MM.1R cells, the MM.1S variant that is dexamethasone resistant. We also developed a ch128.1/IgG1 triple mutant (L234A/L235A/P329S) antibody that does not bind to FcγRs and C1q (the initial protein of the complement pathway), but retains TfR1 binding, as well as binding to FcRn, the neonatal Fc receptor (also known as the Brambell receptor), which is critical in retaining antibody bioavailability in the blood. The triple mutant failed to confer protection in vivo against KMS-11 and MM.1S cells, suggesting a critical role of the antibody Fc fragment in ch128.1/IgG1-mediated antitumor activity, consistent with a potential role of functional effector cells present in this mouse model, such as macrophages. In fact, we found that ch128.1/IgG1 elicits antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cell-mediated phagocytosis (ADCP) in the presence of murine bone marrow derived macrophages as effectors and KMS-11 cells as targets, similar to its IgG3 counterpart. Taken together, our results suggest that ch128.1/IgG1 has great potential as an effective therapy for incurable human B-cell malignancies such as MM. Citation Format: Tracy R. Daniels-Wells, Pierre V. Candelaria, Lai Sum Leoh, Otoniel Martinez-Maza, Manuel L. Penichet. An IgG1 version of the anti-TfR1 antibody ch128.1 shows significant antitumor activity against different xenograft models of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5760.