The problems inherent in testing the protective efficacy of sunscreens are reviewed. Of primary importance are: the spectral power distribution of the test light source, which should simulate the ultraviolet radiation of midlatitude, midsummer sunlight; definition of minimal erythema of skin, and equilibration of light source and filters prior to testing.
Photoallergy to systemically administered quinidine was induced in the mouse, and the minimal time for sensitization as well as the duration of the allergy were studied. When groups of female albino mice, pretreated with cyclophosphamide, were exposed to quinidine 100 mg/kg i.p. followed by 0.1 J/cm2 UVB and 5.0 J/cm2 UVA, a positive challenge reaction, measured as ear edema, could be registered as early as the third day after induction and peaked on days 5-12. When animals photosensitized as above were photochallenged after 7 days, and again after 2 and 8 months, strong reactions were obtained on all 3 occasions, suggesting a duration of systemically induced photoallery to quinidine of more than 8 months. In the absence of quinidine, no photosensitivity could be demonstrated with UVA only. Phototoxicity controls were negative. These results suggest that photoallergy induced by systemic administration behaves similarly to contact and photocontact allergy.
Solar irradiance in the spectral region 280 to 800 nm was measured with a double monochromator at 2 locations in Norway, Tromsø (69.7 degrees N) and Longyearbyen (78.2 degrees N). During the observational (midnight sun) period in Longyearbyen, the maximum UVB irradiance recorded was less than 0.3 W/m2, and no radiation was detected for wavelengths below 300 nm. Such low levels are believed to be a consequence of the low solar elevation angle and the high ozone content of the Arctic ozone layer, which absorbs the incident UV light. With levels between 280 and 350 DU over the period of study, Tromsø and Longyearbyen recorded only one-ninth of the calculated UVB radiation at the equator. There is therefore a considerably higher risk of radiation damage to the skin in equatorial regions (controlling for skin type), a finding that agrees with the statistical evidence for a 7-8 times higher rate of skin cancer in the white population of equatorial countries.
During the developmental stages of sunscreen formulation it is desirable to have a simple, accurate and inexpensive biological model to test product effectiveness. Another desirable attribute is a quantitative, unbiased response endpoint for evaluation. We have developed such a test system based upon the ear swelling response of hairless albino mice. With this system, irradiation times are greatly reduced; furthermore, the response parameter is metric and can be determined noninvasively with an inexpensive micrometer. Protection factors determined with the mouse ear model show high correlation with the sun protection factors as determined on human subjects (r = 0.92) and were linearly related over a wide range of values. This new method affords a simple, accurate and inexpensive system for evaluation of efficacy and safety of new products.
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