SUNSCREEN PRODUCT TESTING: APPROPRIATE ENDPOINTS FOR ASSESSING EFFICACY
SUNSCREEN PRODUCT TESTING: APPROPRIATE ENDPOINTS FOR ASSESSING EFFICACY include a determination of phototoxic protection factors (PPFs) and melanogenic protection factors (MPFs), FDA's Dermatologic Drugs Advisory Cmte. agreed at its Nov. 18 meeting. The two factors were derived as a measure of sunscreen protection against UVA radiation in two studies presented to the cmte. by Allergan's Herbert Labs division. Allergan representative Sydney Dromgoole, PhD, explained that the phototoxic protection factor is a ratio of the dose of energy that causes minimally perceptible phototoxicity (MPD) in sunscreen treated skin over that of unprotected skin. Similarly, he said the melanogenic protection factor is the "dose of energy that causes minimally perceptible melanogenesis. The ratio then becomes minimum melanogenic dose in the sunscreen treated skin divided by that in the unprotected skin." The two factors were determined in two double-blind, randomized studies comparing Padimate 0, Parsol 1789, a combination of these two, and a currently marketed sunscreen containing Padimate 0 and oxybenzone. Dromgoole reported that in the first study, subjects were administered .6 mg/kg of oral 8-methoxypsoralen in order to accelerate the response to UVA light. He said sunscreens were applied and test sites were exposed to UVA radiation in incremental doses 1-1/2 hours later. Erythema scores, used to derive the phototoxic protection factor, were "rated at 48 and 72 hours on the standard scale of 0 to 4," Dromgoole stated. Pigmentation scores, used to derive the melanogenic protection factor, were evaluated at 12 to 18 days, he added. In the second study, subjects were treated with topical 8-methoxypsoralen and 15 minutes later test sites were covered with sunscreens and irradiated with UVA light. Dromgoole said the two studies "clearly demonstrate that the combination of Padimate 0 and Parsol is significantly greater at protecting against the effects of UVA radiation than currently marketed sunscreens containing Padimate 0 and oxybenzone." He concluded that the studies demonstrate the utility of using PPFs and MPFs to measure the effects of UVA. Although concurring that PPFs and MPFs were appropriate endpoints, cmte. member Robert Stern, MD, Beth Israel Hospital, Boston, Mass., stated that pilot research has to be done to determine the effect of "UVA alone versus PUVA sensitized subjects in terms of the dose response per UVA radiation so we can . . . tell people what the protection factor is, not so much expressed as erythemia units as expressed as essentially the reduction in proportion of ambient radiation given below the stratum cornea, or getting to a level where it will have some biologic effect." He added that just looking at PUVA erythemia reduction "would be overestimating beneficial effects of the sunscreen, whatever that endpoint might be in terms of beneficial effects, reduction in pigmentation, erythemia, or whatever." FDA also asked the cmte. whether use of methoxsalen (8-methoxypsoralen) is an acceptable method of determining sunscreening effect since it will not be used during the normal uses of a sunscreen. The cmte. unanimously agreed that it was acceptable. Stern commented that "given the substantial amounts of UVA radiation it takes to yield erythemia, using an oral synthesized substance with psoralen makes sense." With regard to the necessity of outdoor testing of sunscreen products, the cmte. voted six in favor and two opposed. Frederick Urbach, MD, Temple University, asked his opinion by the cmte., stated that outdoor testing is almost impossible for a product with a sunscreen protection factor beyond two.
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