Hypericin photoactivation

The hypericin constituent of St. John s wort is photolabile and can be activated by exposure to certain wavelengths of visible or UVA light. Parenteral formulations of hypericin (photoactivated just before administration) have been used investigationally to treat HIV infection (given... 

As well as these beneficial (either for the plant or for humankind) uses of photoactivation of plant compounds, there are equally numerous and varied examples of harmful phototoxic responses to plant compounds. Many plants potentially ingested by livestock can initiate photosensitizing responses [38]. In particular, plants that produce sufficient concentrations of furanocoumarins, hypericin, and cercosporin have been documented to affect livestock in this manner [39]. A specific example of non-target species photosensitization by plant compounds is the dramatic erythemic response of agricultural workers exposed to the sap of celery (Apium graveolens) plants during harvest [40]. 

Phototoxicity manifested as elevated, itching, erythematous lesions has been described (Golsch et al., 1997). Phototoxicty may also present as neuropathy, presumably caused by demyelination of cutaneous axons by photoactivated hypericins (Bove, 1998). After taking ground St. John s wort extract for 4 wk, a 35-yr-old woman complained of stinging pain on sun-exposed areas, worsened by cold, minimal mechanical stimuli, and sun exposure. The neuropathy improved over 2 mo after she discontinued the product (Bove, 1998). 

Virus inactivation by hypericin seems to involve inhibition of various stages of virus replication depending on the integrity of the viral envelope. Photoactivation of hypericin leads to the formation of active oxygen species that likely react with viral components making them unable to assembly into complete virus particles. 

Subcutaneous tumors that developed after implantation of human mammary carcinoma cells (MX-1) in athymic mice regressed following local injection of hypericin 1 and exposure to visible light [167]. Tissue uptake and distribution of hypericin 1 was measured in rabbits and in nude mice xenografted with P3 human squamous cell carcinoma to assess the value of this naphthodianthrone as in vivo sensitizer for laser photoactivation of solid tumors. Maximum Hypericum levels were seen in... 

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