Photodynamic activity and potential usability of 14-carboxyl hypocrellin B

Abstract

Not only the photosensitivity properties but also the amphiphilicity (both hydrophilicity and lipophilicity) are important factors for hypocrellins to be clinically applicable to photodynamic therapy of vas capillary diseases. A chemically modified derivative, 14-carboxyl hypocrellin B (HBO₂H), proved to possess far better amphiphilicity than its parent hypocrellin B (HB) and hypocrellin A (HA). In this paper, the photophysical and photochemical properties of HBO₂H were investigated by spectrophotometric methods and electron paramagnetic resonance (EPR) and its photodynamic action on mouse endothelial cells was also confirmed. In oxygen-free DMSO solution, semiquinone anion radicals (HBO₂H˙⁻) are photogenerated via electron transfer between the excited triplet HBO₂H and electron donors or ground state HBO₂H (Type I mechanism). When oxygen is present, superoxide anion radicals (O₂˙⁻) are generated via electron transfer from HBO₂H˙⁻ to the ground state oxygen molecules. Singlet oxygen (¹O₂) can be produced via energy transfer from the triplet state HBO₂H to the ground state oxygen molecules (Type II mechanism). The quantum yield of singlet oxygen was estimated to be 0.63 in CHCl₃ with 0.76 for HB as a reference. Furthermore, investigations on the competition and transformation between ¹O₂ and HBO₂H˙⁻ suggested that the relative importance of Type I and Type II reactions would depend on the oxygen content in the target tissue.