Improving effector functions of antibodies for cancer treatment: Enhancing ADCC and CDC

Abstract

As platforms for therapeutic agents, monoclonal antibodies (MAbs) have already been approved, and several MAbs have demonstrated clinical effectiveness in a variety of malignancies. However, several issues have also been emerging in antibody therapy, such as high cost and insufficient drug action. Recently, to improve MAb activity in humans, effector functions have been subjects of focus, especially antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). Extensive efforts have been made to enhance these effector functions of MAbs, and successful approaches have been reported by us and others, wherein the binding activity of MAbs to FcgammaRIIIa or C1q is increased by introducing amino acid mutations into heavy chain constant regions or through glyco-modification of Fc-linked oligosaccharides. In addition, one of the next approaches to optimizing therapeutic antibodies would be to combine multiple enhancing modifications into a single antibody platform to overcome the diverse mechanisms of clinical resistance of tumor cells. For this aim, we have recently developed a successful combination composed of ADCC-enhancing modification by the fucose depletion from Fc-linked oligosaccharides and CDC-enhancing modification by IgG1 and IgG3 isotype shuffling in heavy chains, which could be of great value for the development of third-generation antibody therapeutics.

Keywords: ADCC; CDC; Fc oligosaccharides; IgG isotypes; effector functions; nonfucosylated IgG.

Figure 1

Structure of human IgG1 antibody. The Mabs of human IgG1 isotype consists of two immunoglobulin light chains and two immunoglobulin heavy chains. Heavy chains are covalently paired by disulfide bonds in hinge regions, and each heavy chain is connected to a light chain by a disulfide bond between CH1 and CL. A pair of VH and VL in Fab regions makes an antigen binding site. In the CH2 domains of Fc regions, an oligosaccharide is covalently attached to the both domains at asparagine 297 (Asn-297).

Figure 2

Fc-Linked oligosaccharides of antibodies. A conserved oligosaccharide core, linked to the Asn-297, is composed of a mannosyl-chitobiose core structure in the presence or absence of a core fucose (Fuc), a bisecting N-acetylglucosamine (GlcNAc) and terminal galactose (Gal) and sialic acid; this structure gives rise to heterogeneity with a mixture of 30 or more glycoforms. Abbreviations: Man, mannose monosaccharide residue.

Figure 3

Schematic diagram of CDC. CDC is a cytolytic cascade mediated by a series of complement proteins abundantly present in serum. It is triggered by the binding of C1q, a subunit of C1, to the constant region of cell-bound antibody molecules. Finally, activated complements form menbrane-attack complex, perforating membrane.

Figure 4

Scheme of the mixed isotype heavy chain. In the mixed isotype heavy chain variant, a part of the Fc portion of human IgG1 heavy chain is converted into the corresponding part of human IgG3 (Complegent).