Improved Speciation Characteristics of PEGylated Indocyanine Green-Labeled Panitumumab: Revisiting the Solution and Spectroscopic Properties of a Near-Infrared Emitting anti-HER1 Antibody for Optical Imaging of Cancer

A water-soluble amine-reactive PEGylated analogue of near-infrared emitting dye indocyanine green (5) was synthesized and used to label the anti-HER I antibody panitumumab (Vectibix) at various equivalents. These conjugates were compared with non-PEGylated analogue conjugate products and the solution speciation analyzed with UV-vis spectrophotometry, size exclusion HPLC, and SDS-PAGE. PEGylation of the bioconjugates was successful in preventing aggregation in solution, a phenomenon observed with the non-PEGylated bioconjugates presumably clue to the hydrophobicity of indocyanine green. Competitive radioimmunoassay demonstrated that the targeting moiety of the PEGylated bioconjugates was conserved. Fluorescence microscopy also demonstrated membrane binding of the bioconjugate to HER1-expressing A431 cells. Hence, these bioconjugates are suitable candidates for the in vivo optical imaging of HER1-expressing tumors.