Photosensitivity

Abstract: Extracts of St. John's wort (Hypericum perforatum) are used in treatment of depression. They contain various substances with the naphthodianthrones hypericin and pseudohypericin as characteristic ingredients. These compounds were shown to cause phototoxicity in cell culture and in animals. A placebo-controlled randomized clinical trial with monitoring of hypericin and pseudohypericin plasma concentration was performed to evaluate the increase in dermal photosensitivity in humans after application of high dose hypericum extracts. The study was divided into a single dose and a multiple dose part. In the single dose period, each of 13 volunteers received in a double blind fourfold complete crossover design, either placebo, or 900, 1800 or 3600 mg of a standardized hypericum extract (LI 160) containing zero, 2.81, 5.62 and 11.25 mg of total hypericin (total hypericin is the sum of hypericin and pseudohypericin). Maximum total hypericin plasma concentrations were observed about 4 h after dosage and were 0, 0.028, 0.061 and 0.159 mg/L, respectively. Before and 4 h after drug intake, the subjects were exposed at small areas of their back to increasing doses of solar simulated irradiation (SSI, with combined ultraviolet A, UV-A, and UV-B light) and another part was exposed to selective UV-A light irradiation. Minimal erythema dose was determined 5, 20 and 68 h after irradiation. Comparison of SSI sensitivity without and with hypericum extract did not show and difference and there was no dose-related trend in light sensitivity. Sensitivity to selective UV-A light was increased only after the highest dose from a minimal tanning dose of 10.8 J/ cm2 (mean) after placebo to 8.7 J/cm2 after 3600 mg extract with marginal statistical significance (p = 0.03 by one sided paired t-test). There was no correlation between total hypericin plasma concentrations and photosensitivity. In the multiple dose part, 50 volunteers received 600 mg hypericum extract t.i.d. with a daily dose of 5.6 mg of total hypericin. Comparison of UV light sensitivity before dosing with day 15 of treatment showed a slightly increased SSI sensitivity expressed by decrease of the MED from 0.17 to 0.16 J/cm2 (p = 0.005 by Wilcoxon test), and similarly, sensitivity to UV-A light increased (the mean tanning dose decreased from 9.9 to 7.8 J/cm2, p < 0.0001). This increase in cutaneous light sensitivity could be compensated by reducing irradiation time by 21%. Doses used in this study were higher than typical doses in current commercial preparations. In spite of these high doses in the double blind single dose part, frequency of side effects was equal to placebo medication and UV light sensitivity was not or only marginally increased. The study does not, however, exclude phototoxic reactions with doses above 11.25 mg of total hypericin and plasma levels above 100 micrograms/L. Furthermore, phototoxicity may be different after application of pure hypericin, since some constituents in the plant extract may exhibit protective effects.

Abstract: Photo-cross-linking has received a considerable attention for the design of intelligent materials in biochemical and biomedical applications. In this report, we describe the synthesis and properties of a novel photoreversible poly(ethylene glycol)- (PEG-) based hydrogel system. 9-Anthracenecarboxylic acid was used to modify the hydroxyl groups of an eight-armed PEG polymer (molecular weight 20 000) and the degree of substitution was determined to be 87.4%. The PEG−anthracene macromers (PEG−AN) exhibited high photosensitivity at wavelengths close to visible light (absorption maxima at 366 and 380 nm) and underwent rapid and reversible photo-cross-linking upon exposure to alternating wavelengths of irradiation (365/254 nm) in the absence of photoinitiators or catalysts. Changes in light exposure and wavelength of irradiation reversibly altered the physicochemical properties of the PEG−AN hydrogel, including swellability, absorption spectrum, and topography.

Abstract: Localized heating with a flame is shown to be a simple and effective method for substantially augmenting the photosensitivity of high-silica optical waveguides to (UV) light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than 10 (photoinduced ΔnUV > 10−3) and renders strongly photosensitive the cores of high-quality Ge:SiO2-on-Si and Ge:SiO2-on-SiO2 planar waveguides that were negligibly photosensitive before treatment. We have written large-modulation-depth Bragg gratings, in both fiber and planar optical waveguides photosensitized by our method, using KrF (249-nm) radiation incident upon the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.

Abstract: Burchard, Robert P. (University of Minnesota, Minneapolis), and Martin Dworkin. Light-induced lysis and carotenogenesis in Myxococcus xanthus. J. Bacteriol. 91:535-545. 1966.-Myxococcus xanthus, grown vegetatively in the light, developed an orange carotenoid after the cells entered stationary phase of growth; pigment content increased with age. Cells grown in the dark did not develop carotenoid and could be photolysed by relatively low-intensity light only during stationary phase; rate of photolysis increased with age. Photolysis adhered to the reciprocity law, was temperature-independent and oxygen-dependent, and required the presence of nonspecific, monovalent cations; it was inhibited by one of several divalent cations. Logarithmic-phase cells were photosensitized by 100,000 x g pellet preparations of sonic-treated stationary-phase cells grown in the light and dark. A porphyrin with a Soret band at 408 mmu was isolated from photosensitive cells; logarithmic-phase cells contained about 1/16 the amount of porphyrin of stationary-phase cells. The purified material had spectral and chemical properties of protoporphyrin IX and photosensitized logarithmic-phase cells. Its spectrum was similar to the action spectrum for photolysis. We concluded that protoporphyrin IX is the natural endogenous photosensitizer. Carotenogenesis was stimulated by light in the blue-violet region of the visible spectrum and was inhibited by diphenylamine, resulting in photosensitivity of the cells. Photoprotection by carotenoid was lost in the cold. A mutant which synthesized carotenoid in the light and dark was photosensitive only after growth in diphenylamine. The ecological significance of these phenomena is discussed.