Drug-delivery and multifunction possibilities of hypocrellin photosensitizers

Photodynamic therapy (PDT) has been a routine treatment of tumors and some microvascular diseases, but clinically available photosensitizers are still scarce. Among all kinds of photosensitizers, hypocrellins possess the most characteristics of ideal photosensitizers, such as, high photo-activity but low dark toxicity, fast clearance from tissues. This review is focused on two main topics, drug-delivery problem of hypocrellins and how the environment-sensitive fluorescence of hypocrellins was used for recognition of various biomolecules. Drug-delivery of hypocrellins was mainly achieved in two strategies - preparing the drug-delivery vehicles and finding quantitatively amphiphilic derivatives. Hypocrellin fluorescence originated from the intramolecular proton transfer is very distinct from other kinds of photosensitizers. Recently, it was proved that quantitative hypocrellin fluorescence could not only recognize various biomolecules, including proteins, polysaccharides and lipids, but also distinguish the specific binding from nonspecific binding with some kind of biomolecules. Meantime, hypocrellin uorescence was pH-sensitive. It is known that tumor cells or tissues have the features of a large amount of lipid, neonatal collagen, over-expression of polysaccharides, and lower pH values compared to normal tissues. According to the relative but not absolute specificity, further studies on quantitative recognition of various biomolecules at a cellular level, may find a new clue to treat tumors by joint usage of photodynamic diagnosis (PDD) and PDT.