A Phthalocyanine-Peptide Conjugate with High In Vitro Photodynamic Activity and Enhanced In Vivo Tumor-Retention Property

A novel zinc(II) phthalocyanine conjugated with a short peptide with a nuclear localization sequence, Gly-Gly-Pro-Lys-Lys-Lys-Arg-Lys-Val, was synthesized by click chemistry and a standard Fmoc solid-phase peptide synthesis protocol. The conjugate was purified by HPLC and characterized with UV/Vis and high-resolution mass spectroscopic methods. Both this compound and its non-peptide-conjugated analogue are essentially non-aggregated in N,N-dimethylformamide and can generate singlet oxygen effectively with quantum yields (phi(delta)) of 0.84 and 0.81, respectively, relative to unsubstituted zinc(II) phthalocyanine (phi(delta) = 0.56). Conjugation of the peptide sequence, however, can enhance the cellular uptake, efficiency in generating intracellular reactive oxygen species, and photocytotoxicity of the phthalocyanine-based photosensitizer against HT29 human colorectal carcinoma cells. The IC50 value of the conjugate is as low as 0.21 mu M. In addition, the conjugate shows an enhanced tumor-retention property in tumor-bearing nude mice. After 72 h post-injection, the dye concentration in the tumor was significantly higher than that in other organs. The results suggest that this phthalocyaninepeptide conjugate is a highly promising photosensitizer for photodynamic therapy.