CDR substitutions of a humanized monoclonal antibody (CC49): contributionsof individual CDRs to antigen binding and immunogenicity

One of the major obstacles in the successful clinical application of monocl onal antibodies has been the development of host immune responses to murine Ig constant and variable regions. While the CDR grafting of MAbs may allev iate many of these problems, the potential remains that one or more murine CDRs on the human Ig backbone of a "humanized" MAb may still be immunogenic . Studies were undertaken employing a MAb of potential clinical utility, CC 49, to define those CDRs that are essential for antigen binding and those t hat may be immunogenic in humans. We previously developed a humanized CC49 (HuCC49) by grafting the MAb CC49 hypervariable regions onto frameworks of human MAbs. To identify those CDRs essential for binding, a panel of varian t HuCC49 MAbs was generated here by systematically replacing each of the mu rine CDRs with their human counterparts. The relative affinity constant of each variant was determined. Serum from a patient who received murine CC49 was used to determine the potential immunogenicity of each CDR in humans. T he serum was shown to react with the anti-CC49 variable region. Results sho wed that patients' anti-idiotypic responses are directed mainly against LCD R3 and moderately against LCDR1 and HCDR2. These studies demonstrate for th e first time that variants containing individual CDR substitutions of a hum anized MAb can be constructed, and each CDR can be defined for the two most important properties for potential clinical utility: antigen binding and i mmunogenicity. Published by Elsevier Science Ltd.