Exposure to bright light and the concurrent use of photosensitizing drugs

Phototoxicity of herbal plants and herbal products

Plants are used by humans in daily life in many different ways, including as food, herbal medicines, and cosmetics. Unfortunately, many natural plants and their chemical constituents are photocytotoxic and photogenotoxic, and these phototoxic phytochemicals are widely present in many different plant families. To date, information concerning the phototoxicity and photogenotoxicity of many plants and their chemical constituents is limited. In this review, we discuss phototoxic plants and their major phototoxic constituents; routes of human exposure; phototoxicity of these plants and their constituents; general mechanisms of phototoxicity of plants and phototoxic components; and several representative phototoxic plants and their photoactive chemical constituents.

Phototoxicity in Human Lens Epithelial Cells Promoted by St. John's Wort¶

St. John's Wort (SJW), an over-the-counter antidepressant, contains hypericin, which absorbs light in the UV and visible ranges and is phototoxic to skin. To determine if it also could be phototoxic to the eye, we exposed human lens epithelial cells to 0.1-10 microM hypericin and irradiated them with 4 J/cm2 UV-A or 0.9 J/cm2 visible light. Neither hypericin exposure alone nor light exposure alone reduced cell viability. In contrast, cells exposed to hypericin in combination with UV-A or visible light underwent necrosis and apoptosis. The ocular antioxidants lutein and N-acetyl cysteine did not prevent damage. Thus, ingested SJW is potentially phototoxic to the eye and could contribute to early cataractogenesis. Precautions should be taken to protect the eye from intense sunlight while taking SJW.

Differential expression of the lenticular antioxidant system in laboratory animals: a determinant of species predilection to oxidative stress-induced ocular toxicity?

PURPOSE

Various animal species have been used to study oxidative stress-induced cataractogenesis; however, given that differences in the expression of the lens antioxidant system may influence species susceptibility to oxidative stress, we compared and contrasted a broad spectrum of components of the lens antioxidant system in dog, rat, marmoset, and rabbit.

METHODS

Lenses collected from beagle dogs, Sprague-Dawley rats, marmosets, and New Zealand white rabbits were assayed for reduced glutathione (GSH), and activities of copper-zinc and manganese superoxide dismutase (CuZn-SOD; Mn-SOD), catalase (CAT), glutathione peroxidase (GPX), gamma-Glutamylcysteine synthetase (GCS), glutathione reductase (GR), glutathione-S-transferases (GST), and glucose-6-phosphate dehydrogenase (GPDH), and Trolox equivalent antioxidant capacity (TEAC).

RESULTS

Expression of the lens antioxidant system varied considerably between species. Marmoset lens contained the highest levels of GSH, its respective biosynthetic and recycling enzymes GCS and GR, and the associated H(2 )O(2)-dismutation enzyme GPX. Activities of both SOD isoforms were also highest in marmoset lens. However, activities of the xenobiotic-conjugating enzyme GST and NADPH-generating enzyme GPDH were relatively low. In contrast, dog lens had the lowest levels of GSH, GCS, GR, and Cu-Zn SOD (1/2, 1/2 and 1/33, and 1/63 that in marmoset) but highest levels of GST and GPDH. Rabbit lens contained the highest CAT activity, at up to 3.5-fold that for marmoset and rat.

CONCLUSION

These results demonstrate substantial variation in lens antioxidant systems between different laboratory animal species. Given that such variation may affect relative susceptibility to oxidative stress-mediated ocular toxicity, our findings may provide useful information when choosing different animal species for lens research.

Screening for ocular phototoxicity

Normally light transmission through the eye is benign and serves to direct vision and circadian rhythm. However, with very intense light exposure, or with ambient light exposure to the aged eye and/or young or adult eye in the presence of light-activated (photosensitizing) drugs or dietary supplements, cosmetics, or diagnostic dyes, light can be hazardous, leading to blinding disorders. Light damage to the human eye is avoided because the eye is protected by a very efficient antioxidant system and the chromophores present absorb light and dissipate its energy. After middle age, there is a decrease in the production of antioxidants and antioxidant enzymes and an accumulation of endogenous chromophores that are phototoxic. The extent to which a particular exogenous photosensitizing substance is capable of producing phototoxic side effects in the eye depends on several parameters, including (1) the chemical structure; (2) the absorption spectra of the drug; (3) binding of the drug to ocular tissue (lens proteins, melanin, DNA); and (4) the ability to cross blood-ocular barriers (amphiphilic or lipophilic). For instance, compounds that have either a tricyclic, heterocyclic, or porphyrin ring structure and are incorporated into ocular tissues are potentially phototoxic agents in the eye. The extent to which these substances might damage the eye (photoefficiency) can be predicted using in vitro and photophysical techniques. With simple, inexpensive testing, compounds can be screened for their potential ocular phototoxicity at the developmental stage. It may be that a portion of the molecule can be modified to reduce phototoxicity while leaving the primary drug effect intact. Preclinical safety testing may prevent ocular side effects that can range from mild, reversible blurred vision to permanent blindness.

Photooxidation of lens alpha-crystallin by hypericin (active ingredient in St. John's Wort)

Hypericin is the active ingredient in the over the counter antidepressant medication St. John's Wort. Hypericin produces singlet oxygen and other excited state intermediates that indicate it should be a very efficient phototoxic agent in the eye. Furthermore it absorbs in the UV and visible range, which means it can potentially damage both the lens and the retina. Lens alpha-crystallin, isolated from calf lenses, was irradiated in the presence of hypericin (5 x 10(-5) M, 10 mM ammonium bicarbonate, pH 7.0) and in the presence and absence of light (> 300 nm, 24 mW/cm2). Hypericin-induced photosensitized photopolymerization as assessed by sodium dodecylsulfate-polyacrylamide gel electrophoresis. Further analysis of the oxidative changes occurring in alpha-crystallin using mass spectrometry showed specific oxidation of methionine, tryptophan and histidine residues, which increased with irradiation time. Hypericin did not damage the lens protein in the dark. Damage to alpha-crystallin could undermine the integrity of the lens directly by protein denaturation and indirectly by disturbing chaperone function. Therefore, in the presence of light, hypericin can induce changes in lens protein that could lead to the formation of cataracts. Appropriate precautions should be taken to protect the eye from intense sunlight while on this antidepressant medication.

Light-emitting diodes: a novel light source for phototherapy

High intensity light-emitting diodes (LEDs) are being studied as possible light sources for the phototherapy of hyperbilirubinemic neonates. These power-efficient, low heat-producing light sources have the potential to deliver high intensity light of narrow wavelength band in the blue-green portion of the visible light spectrum, which overlaps the absorption spectrum of bilirubin (BR). We compared the efficacy between single LEDs of different color and then constructed a prototype phototherapy device using 300 blue LEDs. The efficacy of this device was compared with that of conventional phototherapy devices by measuring the in vitro photodegradation of BR in human serum albumin. When blue, blue-green, green, and white LEDs were compared, the blue light was the most effective in degrading BR by 28% of dark control, followed by blue-green (18% of control), and then white light (14% of control). Green light was the least effective (11% of control). The prototype device with three focused arrays, each with 100 blue LEDs, generated greater irradiance (> 200 microW.cm-2.nm-1) than any of the conventional devices tested. It also supported the greatest rate of BR photodegradation. We conclude that light from LEDs should be considered a more effective treatment for hyperbilirubinemia than light from presently used phototherapy devices. Furthermore, the unique characteristics of this light source may make it especially suitable for use in safe and lightweight home phototherapy devices.

Insomnia

This article reviews issues involved in the diagnosis of insomnia and discusses treatment options, including pharmacologic treatment, which is indicated mainly in acute insomnia. Sleep hygiene is then discussed. Finally, the various behavioral treatments are reviewed, including light therapy, relaxation training, cognitive therapy, sleep curtailment, and stimulus control therapy.

Updating on intraoperative light-induced retinal injury

We are presenting the state of knowledge concerning intraoperative light-induced retinal injury, considered to be a combination of photic retinopathy and retinal photocoagulation. It may arise from retinal light exposure to the operating microscope or to the fiberoptic endoilluminator. Ultraviolet and short-wavelength visible light are more dangerous than longer wavelength light. Many risk factors may facilitate the onset of this iatrogenic disease following surgery. Many aspects of the retinal damage are still poorly understood. Many mid light-induced retinal injuries probably remain undiagnosed in routine postoperative examination. Current appropriate light filters are not the definitive solution. Appropriate precautions should be taken during both anterior segment and vitreoretinal surgery.