Photobleaching of Hypericin Bound to Human Serum Albumin, Cultured Adenocarcinoma Cells and Nude Mice Skin¶

Abstract

Hypericin is a promising photosensitizer for photodynamic therapy (PDT) characterized by a high yield of singlet oxygen. Photobleaching of hypericin has been studied by means of absorption and fluorescence spectroscopy in different biological systems: in human serum albumin solution, in cultured human adenocarcinoma WiDr cells and in the skin of nude mice. Prolonged exposure to light (up to 95 min, 100 mW/cm2) of wavelength around 596 nm induced fluence‐dependent photobleaching of hypericin in all studied systems. The photobleaching was not oxygen dependent, and singlet oxygen probably played no significant role. Emission bands in the spectral regions 420–560 nm and above 600 nm characterize the photoproducts formed. An emission band at 615–635 nm was observed after irradiation of cells incubated with hypericin or of mouse skin in vivo but not in albumin solution. The excitation spectrum of these products resembled that of hypericin. Hypericin appears to be more photostable than most sensitizers used in PDT, including mTHPC and Photofrin.

Evidence for Intracellular Aggregation of Hypericin and the Impact on its Photocytotoxicity in PAM 212 Murine Keratinocytes¶

“…Moreover, even at an administered light fluence of 20 J/cm 2 (dose far beyond LD 90 as can be seen from Fig. 7) no photobleaching of the drug was observed, suggesting that hypericin is a very stable photosensitizer in agreement with other work ( 27 ). It should be noted, however, that photoinduced disaggregation may mask the extent of photodegradation.…”

Photosensitizing activity of hypericin and hypericin acetate after topical application on normal mouse skin

“…The combination of limited PpIX levels and rapid photobleaching yields a low PDT dose which may fail to achieve sufficient cell killing during a light exposure that photobleaches all accumulated PpIX. In contrast to PpIX, hypericin is very photostable 21 and laboratory studies have indicated that, when activated by artificial light, it has enhanced antitumour activity 22–24 . Moreover, tests investigating the topical application of hypericin have shown promising results 13 .…”

High Level of Low‐density Lipoprotein Receptors Enhance Hypericin Uptake by U‐87 MG Cells in the Presence of LDL

“…The interaction of Hyp with HSA has already been profoundly studied (11)(12)(13)(14)(15)(16)(17)(18). It was found that Hyp specifically interacts and forms a 1:1 complex with HSA (binding constant K b = 7.5 · 10 5 M M )1 ) (12).…”