Development of a high-affinity antibody-binding peptide for site-specific modification.

Immunoglobulin G (IgG)-binding peptides such as 15-IgBP are convenient tools for site-specific modification of antibodies and the preparation of homogeneous antibody-drug conjugates. A peptide such as 15-IgBP can be selectively crosslinked to the fragment crystallizable region of human IgG in an affinity-dependent manner via the e-amino group of Lys8. Previously, we found that the peptide 15-Lys8Leu has a high affinity (Kd = 8.19 nM) due to the presence of the γ-dimethyl group in Leu8. The primary amino group required for the crosslinking to the antibodies has however been lost. We report the design and synthesis of a novel unnatural amino acid, 4-(2-aminoethylcarbamoyl)leucine (Aecl), which possesses both the γ-dimethyl fragment and a primary amino group. A peptide containing Aecl8 (15-Lys8Aecl) was synthesized and showed a binding affinity 10-fold higher (Kd = 24.3 nM) than that of 15-IgBP (Kd = 267 nM). Fluorescein isothiocyanate (FITC)-labeled 15-Lys8Aecl with an N-hydroxy succinimide ester at the side chain of Aecl8 (FITC-15-Lys8Aecl(OSu)) successfully labeled an antibody (Trastuzumab, Herceptin®) with the fluorophore. This peptide scaffold has both strong binding affinity and crosslinking-capability and could be a useful tool for the selective chemical modification of antibodies using molecules of interest such as drugs.