Progress and Challenges in the Design and Clinical Development of Antibodies for Cancer Therapy

Abstract

The remarkable progress in engineering and clinical development of therapeutic antibodies in the last 40 years, after the seminal work by Köhler and Milstein, has led to the approval by the United States Food and Drug Administration (FDA) of 21 antibodies for cancer immunotherapy. We review here these approved antibodies, with emphasis on the methods used for their discovery, engineering, and optimization for therapeutic settings. These methods include antibody engineering via chimerization and humanization of non-human antibodies, as well as selection and further optimization of fully human antibodies isolated from human antibody phage-displayed libraries and immunization of transgenic mice capable of generating human antibodies. These technology platforms have progressively led to the development of therapeutic antibodies with higher human content and, thus, less immunogenicity. We also discuss the genetic engineering approaches that have allowed isotype switching and Fc modifications to modulate effector functions and bioavailability (half-life), which together with the technologies for engineering the Fv fragment, have been pivotal in generating more efficacious and better tolerated therapeutic antibodies to treat cancer.

Keywords: Fc engineering; chimerization; humanization; oncology; phage display; therapeutic antibodies; transgenic mice.

Figure 1

Intact human IgG1, Protein Data Bank (PDB) ID: 1HZH (23). Heavy chain is shown in blue. Light chain in cyan, and the N-glycan in red. Fv (top right) with the antigen-binding site seen from the antigen perspective. V_L complementarity-determining regions (CDRs) in yellow; V_H CDRs in red. Fc (bottom right) rotated with respect to the antibody to better show the location of the N-glycan (in red). Notice that one of the hinge peptides is missing in the figure. Due to its flexibility it was not solved since coordinates for this region are not available in the PDB file. This figure was generated using Discovery Studio.

Figure 2

Diverse mechanisms of actions described for antibody-based drugs. Antibodies such as IgG1 can activate immune effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cell-mediated phagocytosis (ADCP), and complement-mediated cytotoxicity (CDC) via specific binding to membrane targets on the cancer cell and binding to the Fc receptors on the surface of effector cells. Antibodies can also elicit protective activity by targeting a soluble ligand or their receptors on the surface of cancer cells blocking their interaction. In addition, targeting a cell surface receptor may trigger events that result in cytotoxic activity independent of blocking its ligand, such as receptor dysfunction due to cross-linking.

Figure 3

Human content of chimeric, humanized, and fully human antibodies listed in Table 1. See Figure 4 caption for a detailed description of the human content calculation.

Figure 4

Sequence alignment of the V regions of the antibodies listed in Table 1. Only the amino acids encoded in the IGHV and IGKV genes are reported. The amino acid sequences were taken from DrugBank (66). In those cases where more than one sequence per therapeutic antibody is reported at DrugBank or no sequence was available in this source, we used the sequences compiled by Jain and collaborators (67). The sequences were compared with the repertoire of human germlines compiled at IMGT using IgBLAST (https://www.ncbi.nlm.nih.gov/igblast/) and the percentage of identities as reported in IgBLAST’s output is listed in the second column of the Figure. The third column lists the number of identities divided the length of the amino acid sequence. Some antibodies, in particular the sequence of the chimeric antibodies, matched more than one human germline gene sequence with equal percentage of identities. In these cases, we report the first germline gene in the IgBLAST’s output. In other instances, the antibody sequence matched more than one human germline gene allele with equal number of identities, but with amino acid mismatches at different positions of the V region. In these cases we report the “01” allele. The numbering on top of the sequences corresponds with Chothia’s definition (68). CDRs are delimited with boxes, following Kabat’s definition (69), except at the CDR-H1, which is a combination of Chothia’s and Kabat’s definition. The color code corresponds to mismatches with respect to the closest human germline gene; green, chimeric antibodies; yellow, humanized antibodies; blue, fully human antibodies.