Phthalocyanine–Peptide Conjugates for Epidermal Growth Factor Receptor Targeting

Four phthalocyanine (Pc)–peptide conjugates designed to target the epidermal growth factor receptor (EGFR) were synthesized and evaluated in vitro using four cell lines: human carcinoma A431 and HEp2, human colorectal HT-29, and kidney Vero (negative control) cells. Two peptide ligands for EGFR were investigated: EGFR-L1 and -L2, bearing 6 and 13 amino acid residues, respectively. The peptides and Pc-conjugates were shown to bind to EGFR using both theoretical (Autodock) and experimental (SPR) investigations. The Pc–EGFR-L1 conjugates 5a and 5b efficiently targeted EGFR and were internalized, in part due to their cationic charge, whereas the uncharged Pc–EGFR-L2 conjugates 4b and 6a poorly targeted EGFR maybe due to their low aqueous solubility. All conjugates were nontoxic (IC50 > 100 μM) to HT-29 cells, both in the dark and upon light activation (1 J/cm2). Intravenous (iv) administration of conjugate 5b into nude mice bearing A431 and HT-29 human tumor xenografts resulted in a near-IR fluorescence signal at ca. 700 nm, 24 h after administration. Our studies show that Pc–EGFR-L1 conjugates are promising near-IR fluorescent contrast agents for CRC and potentially other EGFR overexpressing cancers.