Reducing mAb Immunogenicity
Several different experimental approaches are being used for humanizing mAbs. Chimerization, introduced over 25 years ago, recombines mouse variable chains onto human Fc scaffolds. Chimerization has been used to generate five FDA-approved mAbs to date, including abciximab (ReoPro) and rituximab (Rituxan).
Chimeric mAbs have proper Fc effector function but can exhibit significant immunogenicity due to nonhuman sequences in the variable domains. To circumvent this, many labs have turned instead to CDR grafting, where the six complementarity determining regions (CDRs), rather than the full mouse variable chains, are transplanted onto a human IgG scaffold that includes human variable chain frameworks and constant chains. CDR grafting has been used to generate nearly half of all FDA-approved therapeutic antibodies, including trastuzumab (Herceptin), which elicits HAHA responses in <1% of patients.
Despite looking more like human antibodies, some CDR-grafted mAbs (e.g., alemtuzumab (Campath)) elicit more significant immune responses due in part to murine CDR residues, not all of which contribute to antibody specificity. Moreover, CDR grafting can result in epitope drift, a change in antibody specificity due to subtle structural changes in the altered variable chains. Thus, even the best humanization approaches usually require additional engineering to introduce back mutations (reversions to murine origin) into antibody frameworks.
Fully human antibodies derived entirely of human sequences are typically less immunogenic than humanized counterparts and are an increasingly popular starting point using display technologies (phage, yeast, and ribosome), humanized mice, or human B cells. These approaches have been used to generate nine FDA-approved mAbs to date, including belimumab (Benlysta), the first new treatment for lupus in 50 years.
Despite being of human origin, more than half of these human mAbs elicit detectable immune responses in humans, while responses to many other fully human mAbs in clinical trials are unknown. One of the reasons for the immunogenicity of human mAbs is that large randomized libraries can result in the selection of hypermutated binders that may contain nonessential changes or foreign looking residues. Thus, even fully human mAbs often require further humanization to reduce potential immunogenicity.