A marked effect of charge modification on the uptake and phototoxicity of a photoimmunoconjugate (PIC) was demonstrated. A site-specific conjugation strategy was developed to attach the photosensitizer chlorine6 (ce6) to the F(ab′)2 fragment of the murine antiovarian cancer monoclonal antibody OC125. Poly-l-lysine linkers carrying ce6 with a cationic charge or by polysuccinylation with an anionic charge were used and covalently attached to partially reduced antibody via a heterobifunctional reagent. PICs were purified by column chromatography and were also radiolabeled with 125I. PIC binding and uptake were studied with a human ovarian cancer cell line, NIH-OVCAR-5, and a nonantigen-expressing colon cancer cell line, SW1116, and the data were compared with the binding and uptake of nonspecific rabbit IgG PICs. PICs with both cationic and anionic charges preserved antigen binding as shown by competition studies with native antibody, but the cationic PIC had up to 17 times higher cellular uptake of ce6, probably due to enhanced internalization. The ratio of ce6 to 125I retained by the cells varied with the likelihood of internalization and lysosomal degradation. The phototoxicity of the PICs generally varied with their uptake, but a correlation was found between lysosomal hydrolysis as measured by an increased cellular ratio of ce6:125I and increased relative phototoxicity. These data suggest cationic PICs may have advantages for photoimmunotherapy of disseminated intracavity cancer following local administration.
This work was supported by NIH Grant RO1 AR40352 and in part by the Department of Defense Free Electron Laser Program facilities.