Photochemistry and Photobiology, 2008, 84: 450 462 Review Effects of Visible Light on the Skin Bassel H. Mahmoud, Camile L. Hexsel, Iltefat H. Hamzavi and Henry W. Lim* Multicultural Dermatology Center, Department of Dermatology, Henry Ford Hospital, Detroit, MI Received 1 October 2007, accepted 5 December 2007, DOI: 10.1111 j.1751-1097.2007.00286.x ABSTRACT Electromagnetic radiation has vast and diverse effects on human skin. Although photobiologic studies of sunlight date back to Sir Isaac Newton in 1671, most available studies focus on the UV radiation part of the spectrum. The effects of visible light and infrared radiation have not been, until recently, clearly elucidated. The goal of this review is to highlight the effects of visible light on the skin. As a result of advances in the understanding of skin optics, and comprehensive studies regarding the absorption spectrum of endogenous and exogenous skin chromophores, various biologic effects have been shown to be exerted by visible light radiation including erythema, pigmentation, thermal damage and free radical production. It has also been shown that visible light can induce indirect DNA damage through the generation of reactive oxygen species. Furthermore, a number of photodermatoses have an action spectrum in the visible light range, even though most of the currently available sunscreens offer, if any, weak protection against visible light. Conversely, because of its cutaneous biologic effects, visible light is used for the treatment of a variety of skin diseases and esthetic conditions in the form of lasers, intense pulsed light and photodynamic therapy. INTRODUCTION Studies in photodermatology have focused mainly on the UV part of the electromagnetic radiation spectrum. It is known that UV radiation contains sufficient energy that produces biologic effects in the skin. Until recently, visible light (400 700 nm) has been regarded to have no significant cutaneous photobiologic effect. Recent developments in photodynamic and laser therapy have led to further investigations to be conducted on the cutaneous effect of visible light; yet, until recently, technologies in the incoherent, nonlaser light sources have had great difficulty in fractionating light to produce pure visible spectrum without a UV This paper is part of a special issue dedicated to Professor Hasan Mukhtar on the occasion of his 60th birthday. *Corresponding author email: hlim1@hfhs.org (Henry W. Lim) Ó 2008TheAuthors. JournalCompilation. TheAmericanSocietyofPhotobiology 0031-8655/08 or infrared components. This review will focus on the effects of visible light on the skin. HISTORY The earliest study on photobiology was reported in 1671 by Sir Isaac Newton, who fractionated sunlight into different colors of the rainbow using a prism (1). Radiation outside of the visible light spectrum was discovered 125 years later by Sir William Herschel and by Johann Ritter. In 1800, Herschel (2) found that a thermometer registered a higher temperature beyond the visible red end of the spectrum than within it, a spectrum now known as infrared radiation. In 1801, Ritter showed a stronger chemical action on silver chloride beyond the visible violet end of the spectrum (3), hence the term UV spectrum. In 1798, Robert Willan described sensitivity to sunlight under the term eczema solare (4), a condition which was also reported by Rayer (5). Sir Everard Home s (6) experimental observation in 1820 in England reported that some components of sunlight, other than heat, affected the skin, which was confirmed in 1829 by Davy (7), who investigated whether different rays of the solar spectrum produce different effects. Since ancient times, it has been known that electromagnetic radiation has injurious as well as useful effects; however, the science of photobiology only started to fully develop in the past few decades. As the ability to produce specific wavelengths was developed, the biologic implications of these wavelengths were identified. BIOLOGIC EFFECTS OF LIGHT Electromagnetic radiation is classified based on wavelength. Visible light (400 760 nm) is the wavelength range of general illumination. Other regions of this spectrum include radio waves, microwaves, infrared radiation (heat), UV, X-rays and gamma radiation. Definition of visible radiation The visible spectrum is the portion of electromagnetic radiation visible to the human eye, which responds to wavelengths from 400 to 700 nm; some individuals can perceive wavelengths from 450
Photochemistry and Photobiology, 2008, 84 451 380 to 780 nm. A photopic eye, namely the eye that has been exposed to a relatively high intensity light which permits photochemical changes in the retina and constriction of the pupil, has its maximum sensitivity at 555 nm, the green region of the optical spectrum. The frequently used term, UV light, is technically incorrect. The term light is best used for wavelengths of radiation that results in a sensation of vision. Hence, the preferred term is UV radiation (8). Skin optics Once photons enter the skin one of two processes may occur. The first process that takes place when UV and visible photons enter the skin is absorption, which initiates chemical changes in the cells. When absorbed, the energy of the photon is transferred to the chromophore and the photon no longer exists; hence the depth of penetration of radiation is affected by position and absorption spectrum of the chromophore in the skin. The second process to occur is scattering, which is dependent on the wavelength of the photon, which affects the depth of penetration of radiation into the skin (8). Absorption spectrum is the probability of absorption of photons against the wavelength. A number of chromophores only absorb UVB and UVA, and others absorb throughout the UV and visible wavebands. b-carotene has absorption maxima at 465 and 490 nm in the visible spectrum, but also absorbs in the UV range. Protoporphyrin IX has an absorption maximum at 405 nm but absorbs UVA strongly; the other major absorption bands include 410, 504, 538, 576 and 630 nm. Other endogenous chromophores which absorb visible light include melanin, water, riboflavin, hemoglobin and bilirubin. In photodynamic therapy (PDT), the photosensitizing dyes absorb longer visible wavelengths (>650 nm), while 5-aminolaevulinic acid (ALA) PDT results in endogenous protoporphyrin IX production which absorbs light in the aforementioned spectra. Exceptionally, melanin absorbs throughout the UVB, UVA and visible wavebands (8). Figure 1 illustrates absorption spectra of some of the chromophores in the skin. Erythema UVB can penetrate through the epidermis to the upper dermis. It exerts its biologic effects, in part, via mediators released by epidermal cells (9,10). Erythema and sunburn are mainly caused by UVB radiation, although UVA and visible light can also cause skin erythema, requiring much higher doses than Figure 1. Skin chromophore absorption spectra. UVB. Mediators released under the effect of visible light and action spectrum of erythema in the visible range are not yet fully studied. A 1960 study (11) described the erythema caused by UV radiation and visible light. It found that the UV erythema caused by 250 and 297 nm is the result of capillary dilatation, and such wavelengths penetrate only very superficially into the upper dermis. On the other hand, as the longwave UV (366 nm) and visible light (405 and 436 nm) penetrate much deeper into the dermis, it is suggested that the erythema caused by these longer wavelengths radiation is the result of dilatation of the vessels of the subpapillary plexus. Radical production Ascorbate is a nonenzymatic antioxidant, which contains oxidative products including ascorbate free radical (12). A 2006 study (13) of ex vivo irradiation of human skin with solarsimulated radiation, showed that the ascorbate radical signal was directly proportional to the irradiance. Radical production in the substratum corneum by UV and visible light components were approximately 67% and 33%, respectively. This study indicated that visible light contribution to radical production, which has obvious implications in the design of organic sunscreens and the prevention of free-radical, induces damage. Pigmentation Limited information is available regarding the role of visible light on pigmentation. As a result, most information is extrapolated from UVA wavelengths upon pigmentation. Skin pigmentation induced by radiation occurs in three phases immediate pigment darkening (IPD), persistent pigment darkening (PPD) and delayed tanning (DT) (14). IPD occurs as a response to low doses of UVA (1 5 J cm )2 ), appears immediately after exposure, and fades within 20 min to 2 h (15). IPD results from oxidation and redistribution of pre-existing melanin (14). At higher UVA doses (>10 J cm )2 ), PPD occurs and persists from 2 to 24 h (15). PPD also results from oxidation and redistribution of pre-existing melanin (14). In contrast to IPD and PPD, DT, occurring days after exposure, is associated with the synthesis of new melanin. Both UVB and UVA are capable of inducing DT, However, DT induced by UVA is preceded by IPD and PPD without noticeable redness, while that induced by UVB is always preceded by erythema. UVB is more efficient than UVA in inducing erythema and DT (16). In 1983, Kollias and Baqer (17) conducted an in vivo study on the changes in pigmentation induced by visible and nearinfrared light using a polychromatic light source of 390 1700 nm, which simulates the solar spectrum without the UV part, with powers of up to 0.35 W cm )2. Their aim was to determine the changes in color occurring during irradiation and to also record them using remittance spectroscopy. They found that pigmentation could occur without any UV component, and that there were no erythema or temperature changes greater than 0.4 C, even after 3 h of irradiation, associated with the pigmentary change. They objectively defined detectable erythema as changes in the remittance at 542 and 575 nm of more than 5%. IPD was observed with energy greater than 720 J cm )2, and the pigmentation lasted for up to 10 weeks.
452 Bassel H. Mahmoud et al. Other studies showed that exposure of normal skin to visible light can result in the induction of IPD, immediate erythema and DT. Porges et al. (18) exposed 20 healthy individuals with skin Types II, III and IV to visible light source of a compact 150 W xenon-arc solar simulator, with a spectral distribution between 385 and 690 nm. Both IPD and immediate erythema faded over a 24-h period; whether these represent thermal effects is unclear. The residual tanning response remained unchanged for the remaining 10-day observation period. The threshold dose for IPD with visible light was between 40 and 80 J cm )2, while the threshold dose for DT was closer to 80 120 J cm )2. Because of lack of standardization pertaining to the spectrum of visible light producing sources in the aforementioned studies by Kollias and Porges, it is hard to compare their results. The filter used in the latter study was a 3-mm Schott GG385, which should remove most of the short wavelength UV radiation; however, a part of the long UVA spectrum, from 385 to 400, together with the visible light, is still emitted by this filtered light source. While currently there is no standardized visible light source used consistently in all studies, there are several ways of obtaining visible light radiation. Broadband visible light could be produced by filtering out the UV and infrared energy in most broadband light sources. For example, the solar simulator could be used with proper filters to attenuate the UVA, UVB and IR radiation. Long-pass glass filters could be used to attenuate UVA and UVB radiation and heat-absorbing hot mirrors could be used to block the infrared energy. A low-cost option is to use an incandescent lamp with a Plexiglas that blocks most of the UV component. The more expensive option is to use a xenon-arc lamp with modern high-throughput interference filters (Semrock, Inc., Rochester, MN) to obtain the broadband visible light (400 700 nm) while efficiently blocking the UV radiation as much as 6 orders of magnitude. To minimize the effect of heat resulting from infrared radiation, the ideal choice could be a water filter (Y. Liu, personal communication). In the future, it would be ideal to use an identical type of light source in all studies. Kollias et al. (19) reported the development of erythema and melanogenesis in skin Type V individuals following exposure to 295, 305, 315 and 365 nm monochromatic radiation, and then compared erythema and melanogenesis observed in individuals with skin Types I and II. The ratio of values for the minimum erythema dose between skin Type V and skin Types I and II was 2.29 ± 0.8 (mean ± SD), which is close to the ratio of constitutional pigment in these skin types, as measured by diffuse reflectance spectroscopy in the visible range. Interestingly, the minimum melanogenic dose at 295, 315 and 365 nm was independent of skin type. It would be interesting to correlate the above data presented by Kollias and colleagues with future studies in the longer visible light spectrum aiming at determining the minimal erythema dose and minimum melanogenic dose at different wavelengths. CELLULAR AND MOLECULAR PHOTOBIOLOGY Radiation has a variety of biologic effects on human skin, depending on the wavelength. The effects of UVB (290 320 nm) on the skin are mediated predominantly by direct DNA damage, while the effects of UVA (320 400 nm) are dominated by indirect damage caused by reactive oxygen species (ROS) such as singlet oxygen. UVA has been divided into UVA1 (340 400 nm) and UVA2 (320 340 nm), with UVA2 inducing damages similar to that of UVB (20). UVB dominates the carcinogenic effects of sunlight; however, UVA has been estimated to contribute around 10 20% to the carcinogenicity of sunlight (21). Similar to the continuum of the biologic effects of UVB to UVA2, because the division between UVA1 and visible light is arbitrary, the photobiologic effects of UVA1 and visible light may also be similar (22). The effects of UVA1 and visible light on human skin have not been comprehensively investigated, although it has been shown that UVA1 (365 nm) induces squamous cell carcinomas (SCC) in hairless mice which have been exposed to a daily radiation at three different doses 240, 140 and 75 kj m )2 for 620 days (23). Due to the lack of in-depth investigations regarding the longterm effects of visible light on skin, and as it is widely used in laser and PDT, further studies on its long-term effects are needed. Edstrom et al. (22) studied the effects of repetitive low doses of UVA1 on 12 healthy individuals, utilizing an output spectrum between 340 and 400 nm, a peak emission at 365 nm, and visible light using an Osram xenon-arc lamp with two filters (Schott KG 3 and GG 420), which gives a transmission of only visible light. A segment of the buttock was exposed to 20 J cm )2 UVA1 and another segment to 126 J cm )2 of visible light three times a week for 4 weeks. Edstrom et al. observed a dispersed pattern of p53 (involved in several cell-cycle checkpoints, in G1, G2 and at mitosis) positive cells in the epidermis, which may be a reversible reaction to both UVA1 and visible light. Ren et al. (24) observed a compact pattern, which reflected a clonal multiplication of keratinocytes with mutated p53; no expression of p21 WAF)1 (the p53 mediator responsible for the arrest in G1) in keratinocytes was noted, which is in contrast to that reported after UVA and UVB irradiation (25). Interestingly, p53 was immunohistochemically detectable in the biopsy specimens of healthy individuals after only few irradiations with UVA1 in suberythemal doses. The increase was observed even after the first UVA1 exposure, and there were more positive cells after each consecutive irradiation. The number of p53-positive cells tended to accumulate and remained elevated 2 weeks after the last irradiation. An increased expression of cyclin A and a preferential expression of Ki67 was also observed, indicating proliferation of epidermal cells, which is a known effect of UVA1. These findings suggest that there was an increase in epidermal cells in the G1 phase. p53 downregulates the bcl-2 gene, whose product acts as an antiapoptotic agent. However, these investigators noted a small increase in bcl-2 expression, the opposite of what was expected, indicating that p53 may be mutated. Visible light also caused an increase in p53-positive cells and proliferation in the epidermis, but to a lesser degree than after irradiation with UVA1 (22). Kielbassa et al. (26) studied action spectrum for the formation of dimers and oxidative DNA modification in mammalian cells, using a monochromatic radiation. Oxidative DNA damage formation was observed extending from the
Photochemistry and Photobiology, 2008, 84 453 UVA1 range into visible light, with a peak between 400 and 450 nm. Similar to long-wave UVA, visible light produces DNA damage indirectly through the generation of ROS. Hoffmann- Dorr et al. (27) have analyzed the generation of micronuclei associated with the induction of oxidative DNA damage by visible light (>395 nm) in melanoma cells and primary culture of human skin fibroblasts. Using cyclobutane pyrimidine dimers as a marker of the direct effect of electromagnetic radiation, and the presence of modified DNA bases sensitive to repair enzyme glycosylase as a marker of electromagnetic radiation-induced oxidative damage, it was concluded that indirectly generated oxidative DNA modifications can contribute significantly to the adverse effects of visible light. Consequently, the wavelength dependence of DNA mutation induction does not correlate with that of pyrimidine dimer formation. The ratio of pyrimidine dimers and oxidative DNA modifications at various wavelengths depends on both the direct and indirect mechanisms which have an impact on the assessment for the risk of mutation related to solar irradiation. In addition to a peak at 313 nm, DNA base modifications by solar radiation has a second peak between 400 and 450 nm because of the excitation of endogenous photosensitizers. Oxidative damage induced by 400 500 nm accounts for approximately 10% of the total endonuclease-sensitive base damage in cells exposed to sunlight (26). In an in vivo study by Kvam and Tyrrell (28), the total amount of oxidation of guanine induced by monochromatic radiation ranging from a UVB wavelength (312 nm) up to wavelengths in the near-visible (434 nm) range in dermal fibroblasts was the same or greater than the amount of cyclobutane pyrimidine (the major type of direct DNA damage). The action spectra for oxidative damage in skin fibroblasts were UVA (above 334 nm) and near-visible radiations. In rapidly dividing lymphoblastoid cells, no oxidative guanine damage was induced. However, in melanoma cells, almost as much damage as in nongrowing fibroblasts (1.1 per 10 4 guanine bases after 1200 kj m )2 UVA) were found. The authors concluded that oxidative DNA base damage can most likely contribute to the induction of both nonmelanoma and melanoma skin cancer by sunlight. ROLE OF ELECTROMAGNETIC RADIATION IN THE PATHOGENESIS OF PHOTODERMATOSES UVB (290 320 nm) is the major spectrum responsible for cutaneous erythema, whereas UVA (320 400 nm) is involved in the majority of photodermatoses. Photodermatoses are a broad group of skin disorders primarily caused or exacerbated by exposure to UV or visible light. Photodermatoses can be classified into five general categories idiopathic, most likely immune mediated; secondary to exogenous agents; secondary to endogenous agents; photoexacerbated dermatoses; and genodermatoses (29). Photodermatoses can have action spectrum in the UVB, UVA and or visible light range, with UVA being the most common. Those with action spectrum in the visible light range, solar urticaria, chronic actinic dermatitis (CAD) and cutaneous porphyrias will be discussed in the following section. Solar urticaria Solar urticaria is a rare photosensitivity disorder, most commonly occurring in early adulthood, but with variable age of onset. The disease has been well described from the beginning of the 20th century. Hundreds of cases have been reported worldwide. In 1988, Champion (30) reviewed 2310 cases of urticaria which his group had seen over a period of 30 years, of which nine (0.4%) developed solar urticaria. In a recent survey of 390 patients with urticaria, only two cases (0.5%) of solar urticaria were found. In a study conducted in a photodermatology referral center, the percentage of patients with solar urticaria in relation to other photodermatoses ranged from 4% to 18% (31). Solar urticaria is considered to be a Type I immediate hypersensitivity response, mediated by mast cells. It is characterized by the onset of a pruritic, erythematous and urticarial eruption on sun-exposed skin within minutes following sun exposure, and is usually resolved within 2 h, if further exposure is avoided. Solar urticaria is usually more marked on skin that is normally covered, with relative sparing of the face and dorsa of the hands. Episodes may be accompanied by systemic symptoms such as headache, nausea, bronchospasm, faintness and syncope (32). Action spectrum of solar urticaria. The activating wavelengths responsible for solar urticaria vary from UVB to visible light. Monochromator phototesting for solar urticaria was performed by Stratigos et al. (33) in 23 patients, the majority exhibiting induction of lesions with visible light. In a study by Uetsu et al. (34), 24 patients (60%) were sensitive only to visible light. On the other hand, patients with solar urticaria seen in the UK and Europe have been reported to be activated most frequently by UV radiation. The majority (77%) of patients reported by Frain-Bell (35) reacted to a wide action spectrum from UVB to visible light, and only 5 of 26 patients responded to visible light alone. Ryckaert and Roelandts (36) reported that from a series of 25 patients, 5 were sensitive only to visible light. Figure 2 displays the results of the aforementioned studies in relation to the action spectrum of solar urticaria. In solar urticaria, a certain spectrum outside of the activating wavelengths may affect wheal formation. An inhibition spectrum was first demonstrated by Hasei and Ichihashi (37) in a patient who developed urticaria in the 400 500 nm range. Longer wavelengths inhibited wheal formation when irradiation was performed immediately after exposure to radiation at the action spectrum. The wavelengths of the inhibition spectrum are commonly longer than those of the action spectrum. However, in one of their patients and also in a patient studied by Leenutaphong et al. (38), the urticarial reaction to visible light was inhibited by pre-exposure and postexposure to UVA, respectively. In addition, augmentation spectrum has also been described in one patient with an action spectrum in the 320 420 nm range; exposure to longer wavelengths (450 500 nm) before exposure to 320 420 nm enhanced wheal formation (34). In a study of 14 patients, an augmentation spectrum was detected in four cases examined (29%), while an inhibition spectrum was found in 68%. The mechanisms and clinical relevance of inhibition and augmentation spectra remain unknown (34).
454 Bassel H. Mahmoud et al. Chronic actinic dermatitis Chronic actinic dermatitis, a term originally proposed by Hawk and Magnus (39), is a spectrum of clinical entity covering conditions previously known as persistent light reactivity, actinic reticuloid (40), photosensitive eczema (41) and photosensitivity dermatitis (42). It affects most commonly elderly males and manifests as a chronic eczematous photodistributed eruption (43). In several studies conducted in photodermatology referral centers, the percentage of patients with CAD in relation to other photodermatoses ranged from 5% to 17% (31). Action spectrum of chronic actinic dermatitis. While the action spectrum of CAD is primarily in the UVB or UVB and UVA range, action spectrum at the visible light range has been reported. Stratigos et al. (33) found that phototesting in diagnosed cases of CAD showed abnormal erythemal reactions to UVA (5 15, 33.3%), UVA and UVB (5 15, 33.3%), UVB (2 15, 13.3%), and to UVA, UVB and visible light (2 15, 13.3%). Dawe and Ferguson (44) described a study involving 507 CAD patients investigated by monochromator phototesting in the Dundee photobiology unit. UVB, UVA and visible light induced lesions in 95%, 92% and 67% of cases, respectively. Phototesting is always abnormal in moderate to severe CAD, confirming the disorder. Papular or eczematous lesions are commonly seen after exposure to the UVB wavelengths, often also the UVA but rarely with visible light. Occasional abnormalities have been reported to just the UVA, and also very rarely to just 600 nm visible light (45). Phototoxic and photoallergic skin reactions Phototoxic reactions are significantly more common than photoallergic reactions; they mimic exaggerated sunburn. Photoallergic reactions are delayed hypersensitivity response resembling allergic contact dermatitis on sun-exposed areas, but may also extend into covered areas (46). Management includes topical and systemic corticosteroids; avoidance of precipitating agents is essential. Figure 2. Studies showing action spectrum of solar urticaria: (a) Stratigos et al. (33). (b) Uetsu et al. (34). (c) Frain-Bell (35). (d) Ryckaert and Roelandts (36). Porphyrias Congenital erythropoietic porphyria was described using different terminology such as pemphigus leprosus by Schultz in 1874 (47) (who noticed the dark coloration of urine); xeroderma pigmentosum by Gagey in 1896 (48); hydroa vacciniforme by M Call Anderson in 1898 (49) (who was the first to recognize that the lesions were induced by light); hereditary syphilis by Vollmer in 1903 (50); hydroa aestivale by Ehrmann in 1905 (51) (who first suggested that the lesions resulted from the sensitization of the skin to light exposure by porphyrins); and also by Linser in 1906 (52); until Gu nther described the condition, in 1911, as a porphyria. The effect of porphyrins in leading to acute photosensitivity was demonstrated by Meyer-Betz when, in 1912, he injected himself with 200 mg hematoporphyrin and became acutely photosensitive (53). The name porphyria cutanea tarda was first used in 1937 by Waldenstrom (54), who also extensively studied acute intermittent porphyria. The other porphyrias were described later, some after World War II. In 1951,
Photochemistry and Photobiology, 2008, 84 455 erythropoietic protoporphyria was recognized. Variegate porphyria was recognized in South Africa in the 1940s and 1950s (55). Porphyrias are caused by accumulation of endogenous phototoxic agents (porphyrins) because of enzyme defects in heme biosynthesis. Characteristic porphyrin profiles in plasma, erythrocytes, urine and stool allow for diagnosis. Porphyrin molecules are tetrapyrrole ring structures which absorb visible light, generating excited states. Exposure of porphyrins to sunlight results in the generation of free radicals, which cause lipid peroxidation and protein cross-linking leading to cell membrane damage and death. The type of cellular damage depends on the solubility and tissue distribution of porphyrins (56). Most types of porphyrias are inherited in an autosomal dominant manner with incomplete penetrance, but autosomal recessive and more complex patterns of inheritance are also seen in some. The genes coding for the enzymes have been characterized and localized; mutations in them have been identified in patients with porphyria. Individual porphyrias are genetically heterogeneous but the clinical phenotype is uniform; the seven different porphyrias can be divided into two subgroups hepatic or erythropoietic according to the organ in which accumulation of porphyrins and their precursors primarily appears (57). Porphyrias occur from the deficiencies of seven enzymes in heme biosynthesis. Aminolevulinate dehydratase deficiency porphyria and acute intermittent porphyria do not have cutaneous findings. Cutaneous findings in the other porphyrias could be subdivided into acute phototoxicity and subacute phototoxicity (58). Prenatal diagnosis is possible for congenital erythropoietic porphyria (59), and in vitro gene therapy has been successfully performed for hepatoerythropoietic porphyria, congenital erythropoietic porphyria and erythropoietic protoporphyria (60). Pathogenesis of skin symptoms in hepatic porphyrias. Porphyria cutanea tarda, hereditary coproporphyria and variegate porphyria have chronic manifestations usually days after sun exposure. Blistering and scarring occur on the backs of the hands. Hypertrichosis and hyperpigmentation arise on the face. Skin symptoms are marked in summer after sun exposure. Skin symptoms develop as a result of interaction of solar radiation (around 400 nm) with high amounts of circulating porphyrins, which originate from the liver and accumulate in the skin (61). One of the properties of porphyrins is their intense spectral absorption at about 400 nm. This is known as the Soret band, which is a very strong absorption band in the blue region of the optical absorption spectrum of porphyrin. It is intense and narrow and is most well developed in acid solution; its position and magnitude differ for each porphyrin (62). A study by Stolik et al., on the kinetic of porphyrins in mice using photoacoustic and fluorescence spectroscopies, showed that the highest porphyrin concentration in skin, determined from the optical absorption of the Soret band at 410 nm, occurred 2 h after delta-aminolevulinic acid administration and corresponds to porphyrin production in skin tissue. In addition, a first peak was observed at 15 min, which possibly represents the porphyrin content in blood vessels within the skin (63). Skin manifestations of porphyrias include two types accumulation of water-soluble uroporphyrins and coproporphyrins with blister formation (which is the most commonly seen type of cutaneous porphyrias) and accumulation of the lipophilic protoporphyrin which is characterized by an immediate burning sensation in the skin to exposure of light. This can be followed by swelling, redness, purpura and sometimes erosions. These clinical features occur more commonly in erythropoietic protoporphyria (64). PHOTOPROTECTION Sunscreens Sunscreens are the mainstay of treatment for photodermatoses. In some cases, patients are sensitive to visible as well as UV radiation or may be sensitive to isolated visible wavelengths, particularly in the blue region. Unfortunately, currently available sunscreens are not effective for photodermatoses that have action spectrum in the visible light range. Patients taking systemic photosensitive drugs such as Photofrin for PDT have photosensitivity in the visible light range for up to 6 weeks (65). UV filters are divided into organic (also known as chemical) and inorganic (also known as physical) filters; currently there is no effective organic filter for visible light. Chemical agents such as dibenzoylmethanes and camphor derivatives have been used to provide extended protection for the UVA region. The term total sunblock is imperfectly used to describe them. Only optically opaque filters are able to absorb visible light; therefore, only optically opaque inorganic filters can protect against visible light. The two generally available inorganic sunscreen agents are zinc oxide (ZnO) and titanium dioxide (TiO 2 ) (66). When an incident visible light enters particles of TiO 2 and ZnO, it is reflected by some facets of the particles in the direction of our eyes and consequently it appears white (67). Absorption range from visible to UVA to UVB depends on the particle size. To enhance their cosmetic acceptability, inorganic sunscreen agents are more commonly used in micronized or microfine form, and coated with dimethicone or silica to maintain their effectiveness as sunscreen. Micronization results in particle size of less than 50 nm in diameter, which makes them much less visible on the skin; however, as the pigment is micronized, absorption range shifts toward UVB, especially with TiO 2, which makes them poor absorbers of visible light. Micronized ZnO has better UVA1 protection and lower refractive index compared to microfine TiO 2 ; therefore it looks less white (66). Micronized forms of metal oxides not only scatter and reflect light, but also absorb UV as they are capable of mobilizing electrons within their atomic structure. They are stable, nontoxic and safe in their coated form (68). Nano-sized formulations are able to enhance or reduce skin penetration at a limited rate. Insoluble TiO 2 or ZnO nanoparticles (NP) which are colorless do not penetrate into the viable portion of human epidermis. In vitro cytotoxicity, genotoxicity and photogenotoxicity studies on insoluble NP which report uptake by cells, oxidative cell damage, or genotoxicity should be interpreted with caution as these toxicities may be secondary to phagocytosis of mammalian cells exposed to high concentrations of insoluble particles. Other NP may have characteristics enabling skin penetration.
456 Bassel H. Mahmoud et al. There is little evidence that NP have greater effects on the skin or produce toxicities relative to micro-sized materials. Current evidence suggests that nano materials such as nano-sized vesicles or TiO 2 and ZnO NP (currently used in sunscreens) have no risk to human skin (69). In one study, diffuse spectral transmittance of various thicknesses (7 260 lm) of opaque sunscreen formulations were measured in the 350 800 nm range using a spectrophotometer. Transmission through 20% ZnO paste was high and decreased minimally despite large increases in the sunscreen layer thickness (up to 260 lm). Adding another visible light absorber, iron oxide, substantially lowered transmittance below that predicted by the product of the transmittances for each component alone. The addition of pigments greatly enhances photoprotection and can produce a better match with the patient s skin color, which consequently increases cosmetic acceptability (70). Results of the study of Moseley et al. (71) confirmed the observation that pigmentary TiO 2 plus ZnO exhibits relatively uniform transmission in the UVB and UVA region, which extends protection into the visible region; this preparation has been shown to provide protection for patients with sensitivity to the blue light region of the visible light. Window glass Standard glass filters out UVB, but transmits longer wavelengths radiation: UVA, visible light and infrared. Currently, filters are added for UVA and infrared radiation to provide different levels of UV and infrared protection (72). Common types of glass used in residential and commercial buildings are summarized in Table 1. Antioxidants Antioxidants are less potent than sunscreens in preventing sunburn (73). The advantage of oral antioxidants is that they protect the entire skin surface without being affected by external factors such as washing, perspiration or rubbing. Topical antioxidants diffuse poorly into the epidermis and are unstable (74). UVA visible light (VIS) has been evaluated with only ())-epigallocatechin-3-gallate (EGCG), which is the most effective chemopreventive component of green tea. Different concentrations of EGCG were effective in preventing DNA damage induced by UVR VIS in cultured human lung Table 1. Common types of glass used in residential and commercial buildings. Glass types Transmission in the UV range (%)* Transmission in the visible light range (%) Clear glass >72 >90 Tinted (heat-absorbing) glass 40 62 Reflective glass 17 19 Low-emissivity glass 20 71 Laminated glass <1 79 UV-blocking coated glass <1 80 Insulating glass <1 69 Source: Modified from Tuchinda et al. (72). *Measured from 300 to 380 nm. Measured from 400 to 780 nm. fibroblasts. Pre-exposure incubation with this concentration of EGCG also significantly reduced UVA VIS-induced damage in skin fibroblasts and epidermal keratinocytes. Data also show that green tea polyphenols can exert a protective role through dietary supplementation (75). PHOTOTHERAPY USING VISIBLE LIGHT Intense pulsed light therapy In 1976, Muhlbauer et al. (76) described the thermocoagulation of capillary hemangiomas and port-wine stains by polychromatic infrared light. It was not until 1990 that Goldman and Eckhouse developed new high-intensity flashlamps for treating vascular anomalies of the skin; and in 1994 the first intense pulsed light (IPL) technology was marketed. Theoretically, lasers are also a form of IPL; however, in clinical practice, the term IPL is commonly used to refer to a high-intensity polychromatic incoherent light with a wavelength range from 515 to 1200 nm; by using different filters, a variety of wavelengths can be obtained with IPL devices (77). The first law of photobiology, the Grotthus Draper law, states that light must be absorbed by tissue for a biologic effect to take place, whereas transmitted or reflected light has no effect. The mechanism of action of light systems corresponds to the selective photothermolysis that Anderson and Parrish (78) described for the pulsed dye laser (PDL). The different absorption maximums of the respective target structures allow the right wavelengths to be selected for heating (>80 C) and destruction to occur. Hemoglobin absorbs at a wavelength of 580 nm and melanin absorbs the entire visible spectral range (400 750 nm). The wavelength determines the absorption behavior and the depth of penetration of light, which increases with the wavelength. With different cutoff filters (515 755 nm), the optimal wavelength spectrum can be filtered out to correspond to the depth of the target structure and to the patient s skin type (77). Advantages of IPL. The three main chromophores (hemoglobin, water and melanin) in human skin have broad absorption peaks. Therefore laser monochromaticity is not a prerequisite for selective heating (79). As a result, IPL can be a versatile device to treat a variety of conditions in a cost-effective manner. Disadvantages of IPL. Significant experience is needed when working with IPL because of its wide range of wavelengths, pulse durations and fluences. Because of lack of monochromaticity, the spectrum may not be consistent from pulse to pulse. The large handpieces and spot sizes used with IPL make the device harder to maneuver than the laser, especially over irregular skin surfaces and discrete lesions (80). The decision to use an IPL or laser depends on the physician s expertise with a specific device and its availability. Low-level light therapy Low-level light therapy (LLLT) means to deliver doses lower than the optimum value for a certain indication; this dose would produce a diminished therapeutic outcome. This can be done using either coherent-light sources (lasers) or
Photochemistry and Photobiology, 2008, 84 457 noncoherent light sources (light-emitting diodes [LEDs]). Whether the coherent monochromatic light of a laser is better than noncoherent light in a defined wavelength range is still controversial. LLLT is different from other types of laser by having a low intensity, which causes low temperature changes; still, it can induce biologic effects, with minimal discomfort to the patient (81). The mechanism of LLLT is absorption of red and nearinfrared light by chromophores present in the protein components of the respiratory chain located in mitochondria, mainly cytochrome c oxidase. This is followed by subsequent photodissociation of inhibitory nitric oxide from cytochrome c oxidase (82) leading to increased enzyme activity (83), increased electron transport (84) and increased production of ATP (85). In addition, LLLT stimulates the expression of genes related to cellular migration and proliferation; it also alters the production of growth factors and cytokines (86). The currently two main indications for LLLT are wound healing and stimulation of hair growth. A study by Demidova- Rice et al. (81) suggested that LLLT stimulates mouse wound healing by promoting contraction. They hypothesized that the illumination delivered at 30 min postwounding induces fibroblast myofibroblast transition and, hence, wound contraction. They compared coherent monochromatic light from Helium Neon laser (632.8 nm) and light from a broadband noncoherent light source (635±15 nm) using the same spot size and the same fluence rate. They observed improved wound healing under both experimental conditions (He Ne laser and 635±15 nm lamp) compared to controls; but the difference between the light from different sources was not significant. They also compared four different wavelength ranges of red and near-infrared light centered at 635, 670, 720 and 820 nm. The most pronounced stimulation of wound healing was obtained with 820 nm delivered from the same noncoherent polychromatic light source. Regarding fluences, 2 J cm )2 had the largest positive effect, 1 and 10 J cm )2 improved healing to a lesser extent while 50 J cm )2 had a negative effect on wound healing. They concluded that the wavelength is the main variable that defines the wound healing in response to light. The other indication of LLLT is male and female pattern hair loss. Bernstein (87) noticed thick terminal hair developed in the treated area following 810 nm diode laser treatment. He concluded that this phenomenon should be studied to better understand hair growth cycles and to help develop more effective treatments for hair loss and hair growth. There are still no controlled trials to assess the efficacy of LLLT for hair growth, that is why its clinical use remains controversial. Phototherapy for hyperbilirubinemia Bilirubin alteration in the treatment of neonatal hyperbilirubinemia with phototherapy includes three different mechanisms photooxidation and photooxygenation to biliverdin, maleimides and propentdyopents (88); photoaddition to protein-bound bilirubin (89); and constitutional, configurational and structural photoisomerization. Production and excretion of structural photoisomers through bile and urine is considered to be the main mechanism (90). Phototherapy lamps with output in the blue to green part of the spectrum are most effective in lowering serum bilirubin levels. At these wavelengths, light penetrates the skin well and is absorbed strongly by bilirubin (91). Different light sources exist for treatment of hyperbilirubinemia. Blue fluorescent tubes, with an output in the blue spectrum (430 490 nm), are an effective light source for lowering serum bilirubin (92). LEDs provide high-intensity light in the blue portion of the visible spectrum that emit light through a narrow wavelength band with a peak emission between 450 and 470 nm. They do not emit significant infrared or UV radiation and produce minimal radiant heat; the latter must be taken into account when administered to a naked infant (93). Photodynamic therapy While studying the influence of acridine orange on protozoa, Oscar Raab, a medical student at the Department of Pharmacology at the University of Munich, Germany, was the first to discover that the cell-killing effects of a drug were potentiated by the presence of light. In 1904, von Tappeiner used the term photodynamic reaction (94). Photodynamic therapy is a therapeutic technique based on the delivery of a photosensitizer followed by irradiation with visible light, as the light by itself is not sufficient to produce significant photochemical reaction in the tissue. This reaction activates molecular oxygen to generate ROS, such as singlet oxygen, the hydroxyl radical, the superoxide anion and hydrogen peroxide, which have a cytotoxic effect (95). Depending on the amount and localization in the target tissue, ROS either modifies cellular functions or induces cell death by necrosis or apoptosis (96). Most of the currently approved clinical photosensitizers belong to the porphyrin family. Photosensitizers developed in the 1970s and early 1980s are called first-generation photosensitizers (e.g. Photofrin). The photosensitizers made since the late 1980s are called secondgeneration photosensitizers such as ALA, which is the most common photosensitizing agent used in dermatology, first described by Kennedy and colleagues in 1990. New secondgeneration photosensitizers (e.g. benzoporphyrin derivatives, phthalocyanines and chlorins) were then developed; they have the advantage of producing a photosensitivity that lasts for a limited duration. Benzoporphyrin derivative has a delay before irradiation of up to 2.5 h and duration of photosensitization of up to 7 days. Third-generation photosensitizers refer to the modifications such as biologic conjugates (e.g. antibody conjugate, liposome conjugate) (97). Photosensitizers. Topical sensitizers. Eosin red or erythrosine were the first topical photosensitizers used to treat conditions such as pityriasis versicolor, psoriasis, molluscum contagiosum, syphilis, lupus vulgaris and skin cancer (94). ALA is not a photosensitizer by itself but it is metabolized to photosensitizing protoporphyrin IX (PpIX) through the intrinsic heme pathway. It penetrates through the stratum corneum when applied topically as a prodrug, and into dystrophic skin cells and the sebaceous gland where it is transformed into a highly photoactive porphyrin derivative, PpIX (98). ALA synthesis and accumulation is higher in malignant and premalignant cells than in their normal tissues. The ratio of PpIX accumulation in epidermal tumors in comparison with surrounding normal skin is 10:1 (99). The absorption spectrum of PpIX is within the visible spectrum. Different light sources can be used to activate it, including those that emit blue (410 nm), yellow
458 Bassel H. Mahmoud et al. (595 nm) or red (630 nm) light. PpIX has a high absorption peak corresponding to the Soret band at about 405 nm and other absorption maxima, the Q-bands, at approximately 510, 545, 580 and 630 nm. Although the Q-bands are 10- to 20-fold smaller than the peak in the Soret band, most clinical studies have used 625 633 nm red light, which allows for a deeper penetration into the skin (100). Systemic sensitizers. Systemic photosensitizers are delivered intravenously as they do not penetrate well into the skin. Hematoporphyrin and Photofrin were the first photosensitizers to be studied, which have absorption spectra with a small peak at 625 633 nm. Therefore, by using a red light source, they would produce a biologic effect at a depth of 5 10 mm. These photosensitizers also have an absorption peak in the far red (660 700 nm) or near-infrared (700 850 nm) regions where the depth of penetration into the tissues is up to 20 mm (100). In addition, near infrared-absorbing photosensitizers should allow for the treatment of highly pigmented tumors, such as metastatic lesions of melanoma, which do not respond in the visible range, but not above 850 nm. Although wavelengths above 850 nm have good depth of penetration in the tissue, they are not commonly used because their photon energy is too low to trigger a photochemical reaction (101). Ideal photosensitizers. An ideal photosensitizer should be available in the market with low toxicity without irradiation, but powerful photocytotoxicity, selective toward target cells, long-wavelength absorbing, rapid removal from the body and easily administered through various routes (102). Light sources. Light sources used for PDT belong to three groups broadband lamps, diode lamps and lasers. Their activity depends on the emission spectrum, irradiance, spatial distribution and constancy of the output. Broadband lamps emit radiation in the visible and infrared ranges but with insignificant UV emissions. LEDs have a high and reliable emission in a narrow bandwidth of 20 50 nm without infrared emission. Lasers produce high-energy monochromatic light of a specific wavelength. They provide for the exact selection of wavelengths matching the absorption peak of the photosensitizer and a highly homogeneous light beam but with a limited treatment field (101). Applications of PDT in dermatology. Premalignant skin lesions such as actinic keratosis became the first approved dermatologic indication of PDT, and nonmelanoma skin cancers, including Bowen s disease, are amenable to treatment using ALA-PDT. The exception is SCC, especially the nonsuperficial types, because of the high recurrence rates and metastatic potential and lack of evidence to support its routine treatment with ALA-PDT (103). A number of additional nonmalignant conditions (e.g. psoriasis, viral warts and hair removal) are currently under clinical investigation worldwide (104,105). Studies of ALA-PDT have also been conducted on basaloid follicular hamartomas and cutaneous T-cell lymphoma (106), also on both Mediterranean and HIV-related Kaposi s sarcomas, as it is effective, can be repeated and is not associated with immunosuppression (107). PDT differs from other cancer therapies in that the individual components of drug and light are inert unless they are combined (108). Other common dermatologic conditions treated include acne vulgaris, through accumulation of 5-ALA in the pilosebaceous unit, particularly the sebaceous gland; rosacea, by selective occlusion of abnormal dermal capillaries; sebaceous hyperplasia, as 5-ALA converts to PpIX in sebaceous glands; in photodamage (dyspigmentation poikiloderma), as ALA enhances the effectiveness of laser and light treatment; and in actinic cheilitis (109). Fluorescence diagnosis. The selectivity of porphyrin following topical application of ALA is used for diagnostic purposes (through illumination of tissues with blue light at the Soret band) to induce the emission of pink fluorescent light for delineation of the tumor because of high ratio of porphyrin content in the tumor against the surrounding tissue (110). The dermatologist can then perform either a guided biopsy or a complete resection of the tumor, sparing healthy tissue (111). Laser emitting light in the visible range The first laser was developed by Maiman (112) in 1959 using a ruby crystal to produce red light with a 694 nm wavelength. Cutaneous laser surgery was changed in the 1980s with the introduction of the theory of selective photothermolysis by Anderson and Parrish, which was used for the destruction of a target in the skin with minimal thermal injury (78). Lasers emitting light in the visible range are shown in Table 2. Once laser energy is absorbed into the skin, three basic effects are possible photothermal, photomechanical or photochemical. Selective photothermolysis is an example of photothermal damage, in which light of a wavelength (which is absorbed by a target chromophore) selectively destroys the chromophore if the fluence is high enough and the pulse duration is less than or equal to the thermal relaxation time (TRT). The TRT is the time taken for the target to dissipate half of the incident thermal energy. Photoaccoustic effect is another means by which a laser can destroy a chromophore, in which extremely rapid thermal expansion can lead to acoustic waves and subsequently photomechanical destruction of the absorbing tissue. Laser-induced initiation of a chemical process is another consequence of laser therapy. Photochemical effects serve as the basis of PDT (113). The main endogenous chromophores which absorb visible light include melanin, oxyhemoglobin and dexohemoglobin. The three primary absorption peaks of oxyhemoglobin are within the visible range of the electromagnetic spectrum 418, 542 and 577 nm. Vascular lesions are commonly treated with 585 and 595 nm lasers which have the advantage of deeper dermal penetration without significant loss of absorption Table 2. Classification of lasers emitting light in the visible range based on their wavelengths. Lasers emitting light in the visible range Wavelength (nm) Argon-pumped tunable dye 577 585 nm Copper vapor laser 511 (green) 578 (yellow) fd Nd:YAG and KTP 532 PDL 585 Ruby 694 Alexandrite 755 Nd = neodymium; YAG = yttrium-aluminum-garnet; PDL = pulsed dye laser; fd = frequency doubled; KTP = potassium-titanylphosphate.