Conjugates of folic acids with zinc aminophthalocyanine for cancer cell targeting and photodynamic therapy by one-photon and two-photon excitations

Abstract

To improve the photodynamic detection and therapy of cancers (PDT), folic acid (FA) was conjugated with zinc tetraaminophthalocyanine (ZnaPc) to form ZnaPc–FA. The uptake efficiency of ZnaPc–FA to a FR-positive (folate receptor overexpressed) KB cell line (human nasopharyngeal epidermal carcinoma) was much higher than that of ZnaPc demonstrating an enhanced binding ability of ZnaPc–FA to KB cells. When KB cells were pretreated with free FA followed by incubation of ZnaPc–FA, the high uptake rate of ZnaPc–FA disappeared which demonstrated the special binding function of the FA terminal of ZnaPc–FA on KB cells. The confocal fluorescence images further showed that the affinity of ZnaPc–FA to FR-negative A549 cells (human lung epithelial carcinoma cancer cells) was very low, confirming that ZnaPc–FA can only target FR-positive cancers. The two-photon absorption cross-section of ZnaPc–FA was also higher than that of sulfonated aluminum phthalocyanine (AlPcS), an approved PS for clinical applications. With a 780 nm femto-second (fs) laser, the fluorescence image of ZnaPc–FA in KB cells under two-photon excitation (TPE) can be clearly seen, and the two-photon induced singlet oxygen in ZnaPc–FA solution was found to be proportional to the irradiation dose of the fs laser. The PDT damaging effect of ZnaPc–FA on KB cells was much effective relative to AlPcS under common one-photon excitation, and the killing efficacy of ZnaPc–FA under TPE was 10-fold higher than that of AlPcS. These results suggest that ZnaPc–FA is a promising candidate for PDT improvements and particularly for TPE PDT.