P. Donald Forbes

Relevance of Animal Models of Photocarcinogenesis to Humans

8. Boring, C . C., T. S. Squires and T. Tong (1993) Cancer statistics. CA Cancer .I. Clin. 43, 18-24. Glass, A. G. and R. N. Hoover (1989) The emerging epidemic of melanoma and squamous cell skin cancer. JAMA 262, 20972102. Morison, W. L. (1988) Skin cancer and artificial sources of U V radiation. J. Dermufol. Surg. Oncol. 14, 893-898. Elmets, C . A. (1992) Cutaneous photocarcinogenesis. In Phurmucology ofthe Skin (Edited by H. Mukhtar), pp. 389416. CRC Press, Boca Raton, FL. Henriksen, T., A. Dahlback, S . Larsen and J. Moan (1990) U1traviolet radiation and skin cancer. Effect of an ozone layer depletion. Photochem. Photobid. 51, 579-584. Environmental Protection Agency, Regulatory Impact Analysis ( 1988) Protection of Strutuspheric Ozone. U S . Government Printing Office, Washington, DC.

Tumors of the Skin in the HRA/Skh Mouse after Treatment with 8-Methoxypsoralen and UVA Radiation

8-Methoxypsoralen (8-MOP) with and without UVA radiation was administered to HRA/Skh mice (36 animals per treatment group) three times a week in the feed for a total dose of 9-80 mg/kg/week for 52 weeks. Most of the animals at the top dose of 8-MOP with UVA radiation had developed skin toxicity and/or skin tumors by 52 weeks. The skin lesions seen after treatment with 8-MOP and UVA radiation were characterized as squamous cell hyperplasia, squamous cell papilloma, and squamous cell carcinoma and are similar to what has been reported in humans after exposure to 8-MOP and UVA. Squamous cell hyperplasia and acute inflammation of the cornea were also seen in some of the treated female mice. Oral administration of 8-MOP and UVA did not result in a carcinogenic response to other organ systems. There were no increases in skin neoplasms after 8-MOP or UVA radiation alone. 8-MOP given in combination with UVA was carcinogenic to the skin of mice at dose levels similar to those used to treat psoriasis in humans.

Effect of UV radiation on survival of non-haired mice.

Abstract— Patterns of mortality in a series of photocarcinogenesis experiments were examined. All experiments involved chronic irradiation of genetically non‐haired mice with simulated sunlight from a Xe lamp. Experimental variables included genetic origin of the test animals, incident dose of radiation, and the spectral quality of the radiation. In 16 experiments involving 10 genetic origins of mice the following patterns were detected: (1) survival was inversely proportional to the delivered radiation dose; (2) within origins the life‐shortening efficacy of radiation was closely correlated with its carcinogenic efficacy; (3) between genetic origins the carcinogenicity and lethality of a radiation dose were qualitatively correlated, but relative efficacy for the two effects varied; (4) altering the source spectrum by modifying the short‐wave (UVB) cutoff produced similar changes in carcinogenicity and lethality, suggesting that UVB was a significant contributor to lethal efficacy; (5) nature and relative timing of the carcinogenic response were such that carcinogenesis was not likely to have caused the observed mortality. It is speculated that systemic effects, possibly immunologic or toxic, are produced by chronic UV irradiation, and that these apparently cumulative, dose‐dependent effects result in premature death of non‐haired mice.

Quantitation of phototoxic hyperemia and permeability to protein: I. Inhibition by histamine (H1 and H2) and serotonin receptor antagonists in pig skin

Abstract Exposure of pig skin treated with a solution of anthracene to long-wave ultraviolet radiation (UVA) produced an inflammatory response which consisted of erythema and increased vascular permeability. The erythema was measured by the increase in 51 Cr-RBC content of the skin; permeability of the microvasculature was determined by measurement of the accumulation of 125 I-albumin. Within the first 100 seconds (2.6 × 10 3 J/M 2 ) of irradiation, the 51 Cr-RBC content of the skin increased to twice normal and remained at that level despite continued irradiation. The dermal vasculature became increasingly permeable through 1000 sec of irradiation at which time the 125 I-albumin content was ten times normal. Pretreatment of pigs with the histamine receptor antagonists pyrilamine (H 1 ) or cimetidine (H 2 ) had no effect in blocking the photobiologic increase in blood content but showed a significant inhibition of the increased permeability to 125 I-albumin. The serotonin receptor antagonist methysergide had the same permeability inhibiting effects but was about ten times more active. Thus, in the pig, the erythema occurring during the anthracene-UVA reaction is not mediated by receptors for histamine (H 1 or H 2 ), or by serotonin, whereas the increased vascular permeability to 125 I-albumin occurring during the same phototoxic reaction is mediated by histamine and serotonin receptors.

Ocular effects of treatment with various psoralen derivatives and ultraviolet-A (UVA) radiation in HRA/Skh hairless mice.

Abstract Hairless (HRA/Skh) mice were administered one of four dietary concentrations (50, 100, 625 or 1250 ppm) of 8‐methoxypsoralen (8‐MOP) or 5‐methoxypsoralen (5‐MOP), or molar equivalent concentrations of 5‐methylisopsoralen (5‐MIP) or 3‐carbe‐thoxypsoralen (3‐CPS) by ‘pulse feeding’ technique, 3 days per week for 13 weeks, for the final 11 weeks psoralen derivative administration was followed by exposure to 0.2 or 48 J/cm2 of unfiltered ultraviolet‐A (UVA) radient energy from FR74T12PUVA lamps. At 0 and 13 weeks eyes were dilated with 0.2% atropine solution and were examined using a binocular indirect ophthalmoscope with a +20.0 D condensing lens. The lids, cornea, anterior chamber and the lens were evaluated for pathological changes. Ocular damage consisting of dense central corneal opacification was seen at significant levels in animals given 8‐MOP or 5‐MOP and exposed to UVA. In addition, opacities in the area of the posterior lens were seen in all experimental groups and appeared to be related to drug treatment, independent of light exposure, and therefore appeared not to be related to drug‐light interaction. Some corneal and lenticular opacifiction was seen at non‐significant levels in all experimental and control groups.

Erythema and skin blood content

We have studied the inflammatory response produced by painting pigs and hairless mice with anthracene and subsequently exposing the skin to fluorescent blacklight lamps. Erythema and oedema appeared in pig skin shortly after the onset of irradiation; oedema but no erythema was evident in mouse skin. The response in both animals became increasingly severe as long as irradiation continued, and began to resolve when irradiation ceased. Hyperaemia (increased tissue blood content) was quantified by the increase in 51Cr‐RBC content (c.p.m./wet weight) of the skin. Erythema (appearance of redness) was assessed by the use of an ‘erythema grading scale’ consisting of red gelatin filters of increasing red saturation sandwiched between clear acrylic sheets. Mouse skin responded with a nine‐fold hyperaemia compared with unirradiated skin, although no erythema was produced. In contrast, pig skin responded with intense erythema with no corresponding increase in hypaercmia. It is apparent that a stimulus which produces hyperaemia will not necessarily produce erythema, and erythema can develop without hyperaemia.

Modification of photocarcinogenesis by two immunosuppressive agents

The carcinogenic effect of ultraviolet radiation (UVR) on the skin of hairless (hr) mice was modified by two immunosuppressive agents, rabbit anti-mouse lymphocytic serum (ALS), and 6-mercaptopurine (6MP). Daily exposure of mice to UVR resulted in multiple tumor production. Carcinogenesis was measured in terms of affected mice (prevalence) and numbers of tumors produced. By both criteria, photocarcinogenesis was enhanced by ALS but inhibited by 6MP.

Toxicity of 8-methoxypsoralen, 5-methoxypsoralen, 3-carbethoxypsoralen, or 5-methylisopsoralen with ultraviolet radiation in the hairless (HRASkh) mouse

An experimental design to simulate PUVA therapy (oral 8-methoxypsoralen followed by uv radiation) has been tested in a 13-week subchronic study to determine the relative toxicities of 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), 5-methylisopsoralen (5-MIP), and 3-carbethoxypsoralen (3-CEP) in inbred hairless mice (HRA/Skh). Drug was administered by 1-hr pulse feedings three times a week after mice were fasted overnight; individually housed animals were then exposed to uv radiation (320-400 nm; less than 2% output less than 320 nm). 8-MOP or 5-MOP administered orally (at doses of approximately 240 or 480 mg/m2 body surface area per week) followed one-half hour later with uv radiation of 2 J/cm2 for 13 weeks were found to cause skin toxicity including inflammation, hyperplasia, ulceration, and cellular atypia. Dose-related toxicity was not seen in other organ systems. Corresponding levels of 5-MIP or 3-CEP with uv radiation did not produce skin toxicity. These studies show that the psoralens with two potential DNA-binding sites (8-MOP and 5-MOP) were more toxic than psoralens with only one photoreactive site (5-MIP and 3-CEP).

Assays for Photocarcinogenesis: Relevance of Animal Models

In the three and a half decades since the first published reports of squamous cell carcinomas in the skin of hairless mice exposed to ultraviolet radiation, this animal model has been developed and utilized extensively in basic and applied photobiology. Quantitative aspects of photocarcinogenesis had been investigated initially in haired mice; subsequently, the discipline of photoimmunology has developed largely on the basis of information derived from haired animals. Classical selective breeding methods, a few fortuitous mutations along the way, and the advent of genetic engineering have all enhanced the utility of the laboratory animal models. The benefits have included advances in our understanding of many influences on photocarcinogenesis such as genetic diversity, animal age, epidermal DNA damage and repair, wavelength dependency (action spectrum), ultraviolet radiation dose and its delivery, interactions with chemicals, and nutrition. As in other fields of basic and biomedical research, photobiological data have not been considered journalistically newsworthy or particularly contentious until they acquired potential economic impact. Thus, photocarcinogenesis became a more familiar word (at least along the Washington Beltway) when stratospheric ozone first appeared to be threatened by chlorofluorocarbons, and trends in photocarcinogenesis seemed less arcane when it appeared that the trends might be modified by consumer products and pharmaceuticals. The greatest challenge has not been in finding imaginative ways to exploit the models and to push the frontiers of science, but rather has been the uncertainty about quantitatively extrapolating the findings to humans. Research models would hardly be useful if they were not different from people (i.e., smaller, quicker to respond, shorter lived), and that fact encompasses differences in anatomy, physiology, metabolism, surface-to-weight ratios, etc. Selected examples from photomedicine (e.g., chronic solar damage, occupational exposure to tar, psoriasis phototherapy) tend to confirm that several basic principles about photocarcinogenesis are shared by man and mouse; other risk assessments await development and refinement, or remain to be corrected by experience.

Comparative Histopathologic Appearance of UV-Induced Sicin Tumors in Hairless Mice, Crwtothrix Mice, and Fuzzy Rats

AbstractSquamous cell papillomas and carcinomas occur spontaneously with very low frequency in the skin of laboratory mice and rats. The incidence of tumors is markedly increased with ultraviolet (UV) irradiation. Skin tumors can be produced in animals from varied genetic backgrounds when subjected to simulated sunlight. The incidence within a given stock/strain may be altered by UV dose or by chemical enhancers; less information is available on tumor types as a function of the genetic origin (stock/strain) of animals. The hypotrichotic condition of hairless (hr) mice, cryptothrix (crh) mice, and fuzzy (fz) rats is attained by different mechanisms and the adult skin differs histologically. Others have reported that UV-irradiated skin of shaved haired mice produces a spindle cell tumor of epithelial origin; hr mice develop squamous cell papillomas and carcinomas. Crh mice and fz rats have tumors with histologic features that are influenced by their genetic origin.