Folate-Targeted Polyethylene Glycol-Modified Photosensitizers for Photodynamic Therapy

Pyropheophorbide a (Pyro) is a promising photosensitizer; however, it has no tumor selectivity and enrichment capability. In our former work, the prepared folic acid (FA)-Pyro conjugates showed considerably improved tumor accumulation and photodynamic therapy (PDT) activity in cell- and animal-based studies. However, the targeting capability, selectivity and water solubility of the conjugate remain problematic. Here, we evaluated the installation of hydrophilic polyethylene glycol chains as the linker between Pyro and FA, by avoiding direct conjugation of Pyro with FA, aiming to improve tumor selectivity and accumulation. However, PEGylation may have negative effects on the PDT activity and cutaneous phototoxicity. Therefore, we chose various lengths of PEGs as linkers to optimize the molecular weight, hydrophilicity, and PDT activity and, thus, to balance the tumor selectivity and biological function of the conjugate. One optimized conjugate, Pyro-PEG1K-FA, exhibited excellent tumor enrichment and was able to eradicate subcutaneous tumors at a considerably reduced dose. We report the synthesis and characterization of these conjugates as well as the evaluation of their tumor accumulation ability and the corresponding PDT efficiency through in vitro and in vivo experiments.