Phthalocyanine–Peptide Conjugates for Epidermal Growth Factor Receptor Targeting

Four phthalocyanine (<i>Pc</i>)–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 <i>Pc</i>-conjugates were shown to bind to EGFR using both theoretical (Autodock) and experimental (SPR) investigations. The <i>Pc</i>–EGFR-L1 conjugates <b>5a</b> and <b>5b</b> efficiently targeted EGFR and were internalized, in part due to their cationic charge, whereas the uncharged <i>Pc</i>–EGFR-L2 conjugates <b>4b</b> and <b>6a</b> 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 <b>5b</b> 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 <i>Pc</i>–EGFR-L1 conjugates are promising near-IR fluorescent contrast agents for CRC and potentially other EGFR overexpressing cancers