Photosensitizing activity of hypericin and hypericin acetate after topical application on normal mouse skin.
BACKGROUND
Hypericin, originating from Hypericum perforatum, is a potent photosensitizer known to induce skin phototoxicity when given systemically. Previously, we have examined the penetration and distribution of hypericin and its acetate ester in the skin of hairless mice after topical application.


OBJECTIVE
In this study, we assessed the time course and skin histopathology of the phototoxic response after a single topical application of hypericin and hypericin acetate, and subsequent irradiation. The amount of blood-borne photosensitizer and the skin clearance, as well the remaining photosensitizing capacity as a function of time, were evaluated. Furthermore, elicited phototoxic responses were compared with those after application of methyl aminolaevulinic acid (Me-ALA).


METHODS
At different time points after topical application of hypericin (0.1-1%) and hypericin acetate (0.015-1.5%) onto mouse ears, penetration and retention of hypericin were assessed by fluorescence microscopy. After definite application times, the ears were irradiated (10 J cm(-2), 20 mW cm(-2)). Ear thickness measurements were conducted daily, and frequently ear samples were taken for histological analysis.


RESULTS
Application of hypericin on mouse ears resulted only in limited phototoxicity, probably due to confined penetration into the epidermal layers. Extended penetration achieved by administration of hypericin acetate did give rise to a more severe and prolonged response after irradiation, characterized by intense erythema and ear swelling. Skin damage induced by 0.15% hypericin acetate application completely healed in 14 days without scar formation. After a single application of hypericin acetate, the residual photosensitizing capacity was found to decline quickly and was hardly detectable after 7 days. Under the experimental conditions used, hypericin acetate induced equal or more severe phototoxic responses compared with Me-ALA, depending on the concentration.


CONCLUSIONS
Our results indicate that hypericin is an effective photosensitizer not only after systemic administration, but also after topical application, especially when applied as its precursor acetate ester. Moreover, our data provide some insights on safety limits and the time course of skin phototoxicity following hypericin and hypericin acetate application. These data will aid in developing protocols for future photodynamic therapy in the dermatological clinic.
Boiy
A
A
Laboratory for Pharmaceutical Biology, Faculty of Pharmaceutical Sciences, K.U.Leuven, Herestraat 49 bus 824, B-3000 Leuven, Belgium.
Roelandts
R
R
van den Oord
J
J
de Witte
P A M
PA
D000287:Administration, Topical; D000622:Aminolevulinic Acid / Q000008:administration & dosage / Q000009:adverse effects; D000818:Animals; D000873:Anthracenes; D017484:Dermatitis, Phototoxic / Q000209:etiology*; D005260:Female; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D010569:Perylene / Q000008:administration & dosage / Q000009:adverse effects / Q000031:analogs & derivatives* / Q000097:blood; D017319:Photosensitizing Agents / Q000008:administration & dosage / Q000009:adverse effects; D011838:Radiation-Sensitizing Agents / Q000008:administration & dosage / Q000009:adverse effects*; D012867:Skin / Q000187:drug effects* / Q000528:radiation effects; D013997:Time Factors
D000873:Anthracenes; D017319:Photosensitizing Agents; D011838:Radiation-Sensitizing Agents; D010569:Perylene; C004965:hypericin; D000622:Aminolevulinic Acid