Modification of ultraviolet carcinogenesis by photoactive agents; preliminary report

The Aryl Hydrocarbon Receptor in the Pathogenesis of Environmentally-Induced Squamous Cell Carcinomas of the Skin

Cutaneous squamous cell carcinoma (SCC) is one of the most frequent malignancies in humans and academia as well as public authorities expect a further increase of its incidence in the next years. The major risk factor for the development of SCC of the general population is the repeated and unprotected exposure to ultraviolet (UV) radiation. Another important risk factor, in particular with regards to occupational settings, is the chronic exposure to polycyclic aromatic hydrocarbons (PAH) which are formed during incomplete combustion of organic material and thus can be found in coal tar, creosote, bitumen and related working materials. Importantly, both exposomal factors unleash their carcinogenic potential, at least to some extent, by activating the aryl hydrocarbon receptor (AHR). The AHR is a ligand-dependent transcription factor and key regulator in xenobiotic metabolism and immunity. The AHR is expressed in all cutaneous cell-types investigated so far and maintains skin integrity. We and others have reported that in response to a chronic exposure to environmental stressors, in particular UV radiation and PAHs, an activation of AHR and downstream signaling pathways critically contributes to the development of SCC. Here, we summarize the current knowledge about AHR's role in skin carcinogenesis and focus on its impact on defense mechanisms, such as DNA repair, apoptosis and anti-tumor immune responses. In addition, we discuss the possible consequences of a simultaneous exposure to different AHR-stimulating environmental factors for the development of cutaneous SCC.

Arbeitsbedingte Synkanzerogenese an der Haut - Zur kombinatorischen Einwirkung zweier Karzinogene der Berufskrankheiten-Liste

HINTERGRUND

Die Synkanzerogenese am Hautorgan, wissenschaftlich unstrittig, spiegelt sich im monokausal geleiteten Berufskrankheiten (BK)-Recht nicht wider. Nichtmelanozytäre Hauttumoren und deren Vorläuferläsionen können nur nach Einzelprüfung der BK-Nrn. 5103 und 5102 anerkannt werden.

METHODIK

Retrospektive Auswertung von 28 zwischen September 2012 und September 2015 begutachteten Fällen (Altersmedian 72,5 J.) mit nichtmelanozytären Hauttumoren und deren Vorläuferläsionen und arbeitsbedingter Exposition gegenüber natürlicher UV-Strahlung und polyzyklischen aromatischen Kohlenwasserstoffen.

ERGEBNISSE

In allen 28 Fällen sahen wir die arbeitstechnischen Voraussetzungen nach den BK-Nrn. 5103 und 5102 als erfüllt an. In 26 Fällen (93 %) empfahlen wir die Hautkrebserkrankung als BK-Tatbestand nach den Nrn. 5103 und 5102 anzuerkennen. Dem folgte der Unfallversicherungsträger in vier Fällen. Achtmal wurde die Anerkennung nur nach BK-Nr. 5103, zehnmal nur nach BK-Nr. 5102 und viermal gar nicht ausgesprochen.

SCHLUSSFOLGERUNGEN

Bei nichtmelanozytären Hauttumoren oder deren Vorläuferläsionen an UV-exponierten Hautarealen mit ausreichend kumulativer arbeitsbedingter natürlicher UV-Strahlenexposition und bei arbeitsbedingter Exposition gegenüber polyzyklischen aromatischen Kohlenwasserstoffen sollten mittels BK-Anzeige die "BK-Nrn. 5103 und 5102 im Sinne der Synkanzerogenese" gemeldet werden. Neben der gerechteren Wiedergabe der Arbeitsplatzrealitäten in einem solchen Feststellungsverfahren kann es dann im Konkreten auch zur Anerkennung von Basalzellkarzinomen kommen, die ansonsten, bei alleiniger Prüfung der BK-Nr. 5103, herausgerechnet würden.

Does Photosensitivity Predict Photococarcinogenicity?

Assessment of short-term and long-term effects of light (pho—totesting) is part of the safety evaluation of drugs. Results are incorporated into drug package inserts to advise patients and health care providers about the use of drug products on sun—exposed skin. We undertook an exhaustive literature search and a search of archived studies at the Center for Drug Evaluation and Research (CDER), Food and Drug Administration (FDA), in order to evaluate the potential of short—term photoassays to predict long—term effects of drugs used in sunlight (280–700 nm). The correlation between the findings from the photococarcinogenicity assays in mice that used exposure to simulated sunlight and those from photogenotoxicity and photosensitivity studies was examined. Results indicated that photosensitivity and photogenotoxicity assays did not necessarily predict effects in photococarcinogenicity studies in mice. Effects of drugs on skin that are not due to photoactivation of drug can be important factors in enhancement of UV—induced skin carcinogenesis.

Historical Aspects of Phototoxicology

Phototoxicology deals with the injurious effects of ultraviolet and visible radiation. The principal phototoxiceffectis the erythema, edema, and pain (“sunburn”) produced in unacclimatized skin by exposure to solar radiation. Similarly, pho-tokeratitis (“snow blindness”) is a phototoxic phenomenon which was described as early as 410 BC by Xenophon. That it was ultraviolet radiation that was the proximate cause of these photo-toxic effects was discovered by Charcot (1858) in experiments with electric arcs. Dubreuilh (1896) related outdoor solar exposure to incidence of skin cancers; and in 1899, Raab noticed that fluorescent chemicals could be activated by visible light to produce killing of microorganisms. By the middle of the twentieth century, phototoxic effects, primarily on skin, were discovered to be due to in trinsic and to topically applied materials (e.g., porphyrins, coal tar). The phototoxic effects of medicinal compounds (e.g., fluoroquinolones), have recently become important. Photoactive agents and their phototoxic effects have long been used in the therapy of such skin diseases as vitiligo and psoriasis, and more recently for the treatment of malignancies.

Phototoxicity and photoallergy

Photosensitivity may be phototoxic or photoallergic. Phototoxicity is much more common. There are 2 types of phototoxicity: photodynamic, which requires oxygen, and nonphotodynamic, which does not. Reactions induced by porphyrin molecules, coal tar derivatives, and many drugs are photodynamic. The reaction induced by psoralens, for the most part, is nonphotodynamic. Acute phototoxic reactions are characterized by erythema and edema followed by hyperpigmentation. Long-term ultraviolet phototoxicity results in chronic sun damage and skin cancer formation. Also, certain chemicals such as psoralen molecules and coal tar are photocarcinogenic. Phototoxic reactions to certain drugs produce unusual clinical patterns, that is lichenoid eruptions, dyschromia, photo-onycholysis, and pseudoporphyria. Photoallergy is an uncommon acquired altered reactivity dependent on an immediate antibody or a delayed cell mediated reaction. Solar urticaria is an example of the former, whereas photoallergy to exogenous chemicals is an example of the latter. Photoallergy to systemic drugs does occur but is difficult to characterize. The action spectrum for photoreactions to exogenous agents usually at least includes the ultraviolet A rays for both phototoxicity and photoallergy.

The role of melanocytes in the repair of UV related DNA damage in keratinocytes

Epidermal pigmentation and UV exposure are related to the incidence of skin tumors. There is a higher incidence of UV related skin tumors in populations with low pigment and in vitiligo patients, resulting from DNA damage. Normally DNA repair processes set in with the expression of PCNA in the keratinocyte. The present study was conducted on the marginal zone skin in vitiligo. Whole skin organ cultures irradiated with increasing doses of UV in the 280400 nm range show that in the depigmented area there is no expression of PCNA by the keratinocytes. In comparison, the marginal zone keratinocytes show a dose related positivity in the presence of UV responsive melanocytes. These photoresponsive melanocytes show dendricity and cytoplasmic PCNA positivity. The melanocytes interact with keratinocytes by active melanosome transfer. From this study it is suggested that this involves transfer of PCNA as well. The present study indicates the differentiating keratinocytes in skin do not express PCNA but appear to be dependent on active UV responding melanocytes for DNA repair. This factor could play an important role in the occurrence of UV-related skin tumors.

Analysis of rodent photo cocarcinogenicity models for hazard evaluation and risk assessment: industry viewpoint

The general consensus in industry is that photo cocarcinogenicity models are experimental and may prove problematic for use as toxicological tools in routine compound development. Part of the reason for this is the fact that current designs yield similar study outcomes from multiple underlying activities (e.g. photoactivation vs tumor promotion) that can be considered of very different importance in IARC and OSTP human cancer risk assessment approaches. Importantly, photoactivation concerns are significant and, therefore, a decision scheme needs to be developed based on data to ensure that activities of this type do not go undetected. Assuming that this approach includes rodent photo cocarcinogenicity models at some level, then an increased level of understanding of the role of these rodent results in determination of any possible human health impact needs to be made. Learning about the mechanism of action from historical multistage carcinogenesis models will likely prove to be important in this process. Finally, given significant current uncertainty in extrapolation from mouse cocarcinogenicity models to humans, epidemiological data need to be used whenever possible to both test the utility of the model and ensure appropriate market decisions where possible.

Biology of cutaneous squamous cell carcinoma

Nonmelanoma skin cancer is the leading cause of cancer in the United States. Cutaneous squamous cell carcinoma is second only to basal cell carcinoma in prevalence and its incidence is increasing. The biology of squamous cell carcinoma is reviewed under the broad areas of etiology, immunobiology, biochemistry, metastatic potential, and therapy, with emphasis on prevention, diagnosis, and management.

Psoriasis therapy and the risk of skin cancers

Photochemotherapy (PUVA) and methotrexate (MTX) are treatments frequently prescribed in patients with severe psoriasis. However, both treatments have carcinogenic potential. A group of 227 patients, who had been treated with PUVA, MTX or the combination, was investigated for the presence of cancer of the skin. As a control, the partners of these patients were investigated at the same visit. Cancers of the skin were observed in 15 out of 227 psoriatics, compared to one out of 95 control subjects. In the psoriatic patients a previous history of skin cancer, arsenic intake and X-ray treatment were observed more frequently. The increased frequency of skin cancers in psoriatics treated with PUVA, MTX and the combination of both, justifies regular surveillance on skin cancer in these patients.

Side-effects of psoralen photochemotherapy (PUVA)

Although psoralen photochemotherapy (PUVA) is one of the most effective forms of therapy for psoriasis, the risk of potential long-term side-effects is, as yet, not clearly determined. Chronic degenerative and pigmentary skin changes similar to those of chronic solar exposure occur after long-term PUVA treatment; PUVA also causes non-melanoma skin cancers in man, although there is, as yet, no consensus as to what cumulative phototoxic PUVA dose is carcinogenic. Long-term multicentre studies from the U.S.A. indicate a definite risk of squamous cell carcinoma for long-term PUVA-treated patients, whereas European studies reveal no overall increase in skin cancers in similar patients except for those exposed to other carcinogens. Assignment to PUVA should be based on the risk:benefit ratio for the individual patient. Careful patient selection is therefore mandatory and should be limited to those who can be monitored and controlled by informed, competent and conscientious physicians.

A case-control study of skin cancer in the tire and rubber manufacturing industry

A case-control study was conducted in the tire and rubber manufacturing industry to examine the association of squamous cell carcinoma of the skin with rubber manufacturing materials presumed to be contaminated by polycyclic aromatic hydrocarbons. Sixty-five cases were compared to 254 matched controls for exposure to carbon black, extender oils, lubricating oils, rubber solvents, and rubber stock. Both magnitude and duration of exposure were compared using data from company personnel records. Rubber stock and lubricating oils were associated with skin cancer. The relative risk (RR) associated with the highest levels of rubber stock exposure was 2.2, and with the highest level of lubricating oil exposure it was 6.5. In analysis of subgroups of study members, the associations were strongest among workers who were born after 1900 (rubber stock, RR = 11.6; lubricating oil, RR = 4.5) and among workers whose skin cancer was diagnosed before the age of seventy (rubber stock, RR = 23.2; lubricating oil, RR = 28.3).

Psoralens and related compounds in the treatment of psoriasis

PUVA therapy has radically altered the management of severe psoriasis. It is of greatest benefit in those patients with extensive involvement, and in those unresponsive to conventional therapy. The long term side effects of PUVA currently limit its use to patients with disabling disease. The full extent of long term side effects has yet to be defined. In order to reduce the toxicity and improve the efficacy of PUVA, a better understanding of the molecular aspects of psoralen-DNA interaction, DNA repair, and mutagenesis is required. The action spectrum of PUVA in clearing psoriasis has yet to be defined. By limiting the spectrum of UVA exposure it may be possible to reduce some of the toxic effects of PUVA. The recent advances in the formulation of 8-MOP preparations has yielded a drug with more predictable pharmacokinetics and clinical response. Further research with newer psoralens may produce more effective and less toxic compounds. In the last ten years, PUVA has been both a valuable addition to dermatologists' clinical armamentarium and a useful tool in increasing our understanding of cellular biology and the interaction between ultraviolet radiation and biologic systems.

Additive effects of ultraviolet B and crude coal tar on cutaneous carcinogen metabolism: possible relevance to the tumorigenicity of the Goeckerman regimen

The effect of cutaneous exposure to ultraviolet B (UVB) radiation alone, to crude coal tar (CCT) alone, and to the combination of UVB and CCT on the inducibility of the microsomal cytochrome P-450-dependent carcinogen-metabolizing enzyme aryl hydrocarbon hydroxylase (AHH) and other monooxygenases such as 7-ethoxyresorufin O-deethylase (ERD) and 7-ethoxycoumarin O-deethylase (ECD) activities in the skin of neonatal rats was studied. Exposure of the animals to UVB (400-1600 mJ/cm2) alone resulted in a dose-dependent increase in cutaneous enzyme activities. At a UVB dose of 1200 mJ/cm2 increases in AHH, ECD, and ERD were 194%, 115%, and 244%, respectively. A single topical application of CCT (10 ml/kg) 24 h before sacrifice resulted in significant induction of AHH (350%), ECD (921%), and ERD (796%) activities. Treatment of animals with the same dose of CCT followed by UVB exposure resulted in additive and/or synergistic effects on AHH (858%), ECD (1229%), and ERD (1166%) activities in the skin. In contrast, exposure of animals to UVB prior to CCT application had effects no different from those of CCT alone. Epoxide hydrolase and glutathione S-transferase activities in skin from all experimental groups were not different from those of controls. High-pressure liquid chromatographic analysis of the metabolism of benzo[a]pyrene (BP) by cutaneous microsomes prepared from animals treated with UVB alone, CCT alone, and the combination of UVB and CCT revealed increased formation of all the metabolites in each experimental group. The largest increase in metabolite formation occurred in animals receiving CCT followed by UVB exposure. The inducibility of trans-7,8-diol formation by UVB alone and CCT alone was 203% and 435%, respectively, whereas with CCT followed by UVB it was 1065%. The differential responses in AHH activity were found to parallel the capacity of skin microsomal enzymes to enhance the binding of [3H]-BP to DNA. These studies indicate that the sequence of exposure to the components of the Goeckerman regimen in rodents greatly influences metabolic activity in skin. When applied in the same sequence employed in the Goeckerman regimen (CCT followed by UVB exposure) the additive effect upon catalytic activity essential for cancer initiation suggests a possible mechanism for the enhancement of human skin cancer in individuals exposed to this therapeutic regimen.

Cutaneous phototoxicity reactions

This paper reviews some potential mechanisms of cutaneous phototoxicity and discusses selected methods of predicting and ranking phototoxic reactions. Phototoxicity is a non-immunological reaction that is induced by the action of light on a photoactive chemical. The phototoxic chemical may be either exogenous or endogenous, and if exogenous may include topical or systemic agents. Examples of phototoxic agents include psoralens (e.g. 5-methoxypsoralen, 8-methoxypsoralen, as well as the isopsoralens). Psoralens are used therapeutically for the treatment of different skin diseases and psoralen-like chemicals may be present in perfumes and cosmetics. Phototoxic reactions can be evaluated visually in animals. The hairless mouse has also proved useful with skin thickening as a marker of dermal oedema and the induction of polyamine biosynthetic enzymes, as well as other biochemical changes being indicative of cutaneous phototoxicity. In vitro tests of phototoxicity include the Candida albicans assay as well as bacterial mutagenicity. For example, in the latter category different psoralens vary in their ability to induce mutagenicity in Salmonella typhimurium strains, an assay which can provide rapid screening of potential phototoxicity of chemicals. For human testing, phototoxic chemicals can be delivered topically or orally, and in conjunction with appropriate ultraviolet radiation will produce phototoxic reactions which can be evaluated for intensity of response.

The carcinogenic properties of topical PUVA. A lifelong study in mice

The tumorigenic properties of topical methoxsalen (8-MOP) and trioxsalen (TMP) plus ultraviolet light A (UVA) were studied in NMRI female mice. The animals were treated three times weekly for 9 months, and followed up for 18 months. Acetonic solutions of TMP (0.1 mg in 0.2 ml) and 8-MOP (0.6 mg in 0.2 ml) were applied to the shaved back skin of the mice and irradiated with UVA. The doses needed to elicit equivalent phototoxic effects were 0.29 J/cm2 in the TMP group and 1.09 J/cm2 in the 8-MOP group. Papillomas, keratoacanthomas, and squamous cell carcinomas developed in the 8-MOP group only, and the first tumor was seen at 10 months after the beginning of the study. The results suggest that the carcinogenicity of these two modes of photochemotherapy may differ.

Malignant melanoma in situ in two patients treated with psoralens and ultraviolet A

Two patients are reported who were treated with 8-methoxypsoralen and ultraviolet A (PUVA) for psoriasis and developed cutaneous lesions of malignant melanoma in situ (atypical melanocytic hyperplasia). One patient received 324.5 joules/cm2 of UVA. Seven months after discontinuing therapy, he developed a superficial spreading melanoma in situ in association with an intradermal nevus on the left posterior thoracic area. The second patient received 2,802 joules/cm of UVA. While on PUVA therapy she developed an in situ lentigo melanoma on her lower lip. To our knowledge only one other psoriatic patient and one patient with vitiligo have developed malignant melanomas after PUVA therapy, so that an increased incidence of malignant melanomas after PUVA therapy, so that an increased incidence of malignant melanoma following PUVA is not documented.

Photocarcinogenesis, skin cancer, and aging

Nonmelanoma skin cancers, like most malignancies, increase in incidence with increasing age. However, in general they are not due to the aging process but are primarily due to solar radiation. Clinically, squamous cell carcinomas and basal cell epitheliomas are the most common cancers that occur in the Caucasian population in the United States. The role of radiation from the sun was suggested by a number of astute clinical observations reported around 1900 and subsequently has been established by epidemiologic and experimental studies. Action spectrum evaluations indicate that the ultraviolet B (UVB) rays are the most carcinogenic. However, recent studies indicate that the UVA rays can augment the cancer-producing effects of UVB rays. Other physical stimuli, including heat and wind, can also accelerate UVB carcinogenesis. Chemicals such as the polycyclic hydrocarbons, the nitrosoureas, and nitrogen mustard have an additive carcinogenic effect with UVB radiation. Also, some chemicals such as croton oil, the phorbol ester--TPA, and all-trans-retinoic (RA) acid can promote UVB-initiated carcinogenesis. RA can also inhibit UVB-induced cancer formation. The role of the immune status has received a great deal of attention. Both in experimental and clinical situations, nonspecific immune suppression results in increased cancer formation. Also, recent studies indicate that a specific T cell suppressor population can be induced in experimental animals with UVB which will inhibit rejection of tumors produced by UVB radiation. Finally, damage to DNA by UVB radiation is well established. Studies with the genetic disease xeroderma pigmentosum support the concept that such damage, if not repaired, will lead to cancer formation. It also has been suggested that unrepaired damage to deoxyribonucleic (DNA) and other macromolecules is at least in part responsible for the aging process in general.

Psoralen-containing sunscreen is tumorigenic in hairless mice

Sunscreens containing 5-methoxypsoralen (5-MOP) are currently being marketed to promote tanning by inducing psoralen-mediated ultraviolet (UV) A (320-400 nm) melanogenesis. The rationale is that this may prevent UVB (290-320 nm) radiation-induced skin damage. However, mouse studies have shown that 5-MOP has the same cutaneous photocarcinogenic potential as 8-methoxypsoralen. In addition, the 5-MOP--containing sunscreen Sun System III (SS III), when combined with UVA, induces epidermal ornithine decarboxylase activity, an enzyme associated with tumor promotion. Therefore, we investigated whether SS III had sufficient psoralen concentration to be tumorigenic in hairless mice exposed to chronic, intermittent UVA radiation. SS III was applied to hairless mice 5 days per week for 20 weeks. After each application the mice were exposed to 2.5 to 10 joules/cm2 UVA radiation. All test groups developed atypical squamous papillomas in direct proportion to the dosage of UVA radiation received. A shorter latency period for tumor development was seen with larger UVA doses. Test animals followed up to 1 year developed invasive squamous cell tumors. Control groups (SS III without UVA and UVA without SS III) remained free of tumors. Animals receiving SS III plus UVA developed persistent skin thickening and increased dermal cyst formation similar to that reported with chronic exposure to UVB, a known carcinogenic wavelength. Over-the-counter sunscreens containing 5-MOP do contain sufficient psoralen concentrations to cause cutaneous phototoxicity and photocarcinogenicity in mice, and their use in humans should be discouraged in the interest of preventing further UV-induced skin damage and skin cancer.

A comparison of the phototumorigenic potential of 8-MOP and 5-MOP in hairless albino mice exposed to solar simulated radiation

Hairless albino mice have been treated with topically applied 8-methoxypsoralen and 5-methoxypsoralen at two concentrations and exposed to solar simulated radiation. Both these compounds significantly increased the incidence of cutaneous tumours when compared with controls irradiated after treatment with vehicle. This effect was related to psoralen concentration and at the two concentrations investigated there was no significant difference between the compounds.

Photocarcinogenicity of 8-methoxypsoralen and aflatoxin B1 with longwave ultraviolet light

The carcinogenicity of 8-methoxypsoralen (8-MOP) and aflatoxin B1 (AFB1) with longwave ultraviolet light (UVA) to hairless mouse skin was investigated. Skin tumors were induced efficiently by 8-MOP + UVA, and the time to 50% tumor incidence was about 24 weeks. Histopathologically, some tumors were squamous cell carcinomas. AFB1 did not show any phototoxic and photocarcinogenic effects on mouse skin in this study. Although the structure and the photoreactivity of AFB1 to DNA were similar to those of 8-MOP, the photocarcinogenic response of these compounds to mouse skin was quite different.

Photochemotherapy: a new promising chemical derivative

A new synthetic psoralen (5'aminomethyl-4,4',8-trimethylpsoralen) is presented which possesses potential superior photochemotherapeutic efficacy. Results of studies conducted on the guinea pig and hairless mouse in our laboratories reveal that following oral administration there is a more rapid onset of photosensitization, a more rapid loss of photosensitization indicating apparent chemical inactivation or excretion, and a lower systemic toxicity on a milligram to milligram basis when compared to conventional psoralen derivatives. These results indicate that the use of this new derivative will minimize potential side effects such as ocular complications and inadvertent additional photosensitization that might lead to premature aging and carcinogenesis due to the slow onset and prolonged photosensitization following administration that are characteristic of conventional psoralen derivatives.

Personal solar UV-A doses received by patients undergoing oral psoralen photochemotherapy for psoriasis

Patients undergoing oral psoralen photochemotherapy (PUVA) for psoriasis receive a known amount of UV-A in the treatment cubicle and an unknown amount of UV-A from sunlight. In order to measure the solar UV-A dose, fifty-six patients received a UV-sensitive film badge to be worn from the time they took their psoralen tablet throughout the course of the day, except during treatment. The UV-A doses measured in this way varied from less than 0.2 J cm-2 to 7 J cm-2, with a median of around 1 J cm-2. These results show that the solar UV-A received by patients who have been photosensitized by 8-MOP may not be insignificant in relation to the treatment dose.

A mathematical model of the biologically effective dose of solar UVA received by patients undergoing oral psoralen photochemotherapy for psoriasis

A theoretical analysis is presented of the biologically effective dose of the solar long wave ultraviolet radiation (UVA) received by patients who are being treated for psoriasis by oral psoralen photochemotherapy. It is shown that patients treated during the summer months in the UK may well receive a UV dose which is comparable with, or even in excess of, the dose of artificial UVA received in the treatment centre and consequently should be advised about limiting their exposure to sunlight on the treatment days. In the winter months in the UK the UVA intensity in sunlight is such that no unduly restrictive precaution need apply. However, at locations between about 40 degrees N and 40 degrees S, the solar UVA intensity in midwinter is still sufficiently high that unnecessary sunlight exposure on treatment days should be avoided.

Mutagenicity in Salmonella typhimurium of some angelicin derivatives proposed as new monofunctional agents for the photochemotherapy of psoriasis

A series of angelicin derivatives were tested for their mutagenic activity with and without near-ultraviolet irradiation (NUV) in Salmonella typhimurium strains. After irradiation with NUV, the tested compounds induced different numbers of revertants in strain TA100, indicating that the mutational events involved are base substitutions. In the dark, 3 chemicals behaved as frame-shift mutagens causing reversion in strain TA98.

Inhibition of delayed hypersensitivity reaction in skin (DNCB test) by 8-methoxypsoralen photochemotherapy. Possible basis for pseudo-promoting action in skin carcinogenesis?

Fifty-five of a hundred and two subjects undergoing photochemotherapy with 8-methoxypsoralen and near ultraviolet showed an abnormally low or undetectable delayed cellular hypersensitivity reaction in the skin as judged by the dinitrochlorobenzene test. It is suggested that photochemotherapy may act as a pseudo-promotor by blocking an immunologically based control process in the skin so allowing the relatively rapid appearance of squamous skin tumours, documented elsewhere, in individuals whose skin already contains a population of potentially tumorous cells. Immune surveillance of a kind may thus operate in human skin. Impairment of delayed cellular hypersensitivity to dinitrochlorobenzene was more likely to occur with more intensive treatments and in patients with less skin pigmentation.

5-Methoxypsoralen, an ingredient in several suntan preparations, has lethal, mutagenic and clastogenic properties

Many furocoumarins found in several species of plant are potent photosensitizing agents known to cause lethal and mutagenic effects in a wide range of organisms, from viruses to man. Their role in the aetiology of cancer is debatable, but work has focused on the PUVA (psoralen-UVA) treatment of psoriasis with 8-methoxypsoralen (8-MOP) and near UV radiation. Bergaptene (5-methoxypsoralen, 5-MOP) is a major constiutent of oil of bergamot, and might be expected to have qualitatively similar photosensitizing properties to 8-MOP. Although 5-MOP is widely used as a stimulus to melanin deposition in several suntan preparations surprisingly little is known about its basic photobiology. We report here that 5-MOP has the expected properties of other biologically active furocoumarins. These properties include lethal and mutagenic photosensitization of bacteria, 'dark' induced frameshift mutagenesis in bacteria, and lethal and clastogenic effects on mammalian cells in tissue culture.

Photochemotherapy of psoriasis: DNA damage in blood lymphocytes

DNA synthesis assessed by 3H thymidine incorporation was measured in lymphocytes from patients with psoriasis receiving photochemotherapy with 8-methoxypsoralen (8-MOP) and UV-A (PUVA). The emission from the UVA source contained sufficient short wavelengths (UV-B) to impair 3H-thymidine incorporation but could be screened out with clear plastic screens. When lymphocytes were irradiated in vitro in a 1/10 dilution of plasma containing 8-MOP, increasing doses of UV-A produced progressive inhibition of phytohaemagglutinin (PHA)-induced DNA synthesis. Cells from patients given several previous treatments were inhibited significantly more than those from patients receiving their first exposure. This was also true for unirradiated cells sugesting either an accumulation of 8-MOP within cells, persistence of DNA damage or alteration of the lymphocyte population. Lymphocytes removed immediately after patients were irradiated showed less 3H thymidine incorporation than cells taken just prior to irradiation, which confirms that PUVA treatment exerts in vivo effects on circulating lymphocytes.

Mutagenicity of 8-methoxypsoralen and long-wave ultraviolet irradiation in diploid human skin fibroblasts: an improved risk estimate in photochemotherapy

Cell killing and the induction of mutation were studied in dividing and non-dividing human skin fibroblasts as a result of treatment by 8-methoxypsoralen (8-MOP) and long-wave UV irradiation (UVA). The cytotoxic effect was highly dependent upon the duration of the UVA exposure. The frequency of mutations increased linearly with the UVA dose at concentrations of 10 and 0.25 microliter 8-MOP/ml, the latter representing the concentration in the skin during PUVA treatment. The number of mutations induced per unit dose (= per microgram 8-MOP/ml per joule UVA/m2) was calculated: for dividing cells this value was 3.3 X 10(-8) per cell and for non-dividing cells 0.6 X 10.8(-8) per cell. On the basis of these values the expected number of induced mutants in the human skin per session of photochemotherapy is 1.2 X 10(-5), and per 30 years of maintenance therapy 1.3 X 10(-2) per cell. A comparison was made between this frequency and the frequency to be expected from spontaneous mutation. In addition the significance of absence in patients of SCE induction by photochemotherapy is discussed.