Site-specific conjugation on serine → cysteine variant monoclonal antibodies

We have engineered a cysteine residue at position 442 (EU/OU numbering) in the third constant domain (C(H)3) of the heavy chain of several IgGs with different specificities, isoforms, and variants with the intent to introduce a site for chemical conjugation. The variants were expressed in NS0 mouse myeloma cells, where monomeric IgG is the major form and formation of aggregate was minimal. Monomeric IgG contained no free thiol; however, it was discovered that the engineered thiols were reversibly blocked and could be reduced under controlled conditions. Following reduction, reactive thiol was conjugated with a cysteine-specific bifunctional chelator, bromoacetyl-TMT to a humanized 323/A3 IgG4 variant. Conjugation had no significant effect on antibody affinity. To prove that the conjugation was site-specific, an antibody-TMT conjugate was labeled with lutetium-177 and subjected to peptide mapping followed by sequence analysis. Glu-C digestion demonstrated that 91% of the label was recovered in the COOH-terminal peptide fragment containing the engineered cysteine.