Short Communication INSECT REPELLANT INTERACTIONS: SUNSCREENS ENHANCE DEET (N,N-DIETHYL-M-TOLUAMIDE) ABSORPTION

Toxicology studies are typically performed on single compounds, which we hypothesized would miss adverse synergies from chemical mixtures. This hypothesis was tested using an insect repellant and sunscreens because both groups include known permeation enhancers, with prior pediatric reports of toxicity from highly concentrated DEET (N,N-diethyl-m-toluamide). Using real-time mass spectroscopy in a hairless mouse skin model, we confirmed substantial penetration of a 20% DEET standard. Despite a lower (10%) DEET content, a commercially marketed sunscreen formulation had a 6-fold more rapid detection (5 versus 30 min) and 3.4-fold greater penetration at steady state. We also tested the efficacy of DEET microemulsion products and confirmed that one successfully slowed the onset of absorption, but not the steadystate permeation. Risks from mixtures of potential toxins are worthy of routine testing, which can be accomplished by simple assays, and are of utmost importance for pediatric applications. Toxicology studies are fundamental to our approach for using an enormous variety of environmental, industrial, and medicinal compounds, which are of particular importance to the pediatric population. We were concerned that mixtures of chemicals might unintentionally enhance toxicity, and chose to study a topical insect repellant in this regard. The routine use of N,N-diethyl-m-toluamide (DEET) has been widely advocated by the medical community and media after recent illnesses from West Nile encephalitis and other infectious agents. Despite DEET’s efficacy and superb safety record, we believe it is imperative that this and other topical agents not be used in commercial mixtures that unintentionally enhance their transdermal absorption. This raises a much wider and more important concern that serious adverse interactions between common potential intoxicants may be underappreciated because typical industry testing does not include that of mixtures. Some sunscreens and even DEET itself have been described as permeation enhancers (Windheuser et al., 1982; Benson, 2000; Hayden et al., 1997), and awareness of problematic formulations would be of value to the public, children in particular. Especially with case reports and media stories of alleged pediatric behavioral or neurotoxicity, there has been a longstanding concern that transdermal absorption of high-concentration products might pose a health risk (Robbins and Cherniack, 1986; Selim et al., 1995; Briassoulis et al., 2001). We hypothesized that some topical formulations might influence their component’s permeation, but could not be detected by prior steady-state models utilizing single agents. We used a mouse skin model and liquid chromatography/mass spectrometry to quantify realtime absorption kinetics of DEET in various concentrations and in a commercial sunscreen product. Materials and Methods A flow-through cell was devised that would permit mounting of 1.27-cm full-thickness hairless mouse skin and allow quantification of transdermal penetration into a small-volume well mixed receptor compartment containing 50% ethanol/water. Fresh fluid entered the device by vortex forces and was pumped out at 100 l/min using the highly regulated micropump of a highperformance liquid chromatograph (model 1100; Hewlett Packard, Palo Alto, CA) into a Finnigan/MAT LCQ liquid chromatography/mass spectrometry apparatus (Thermo Finnigan, San Diego, CA) with an atmospheric pressure chemical ionization source. All data were acquired and subsequently analyzed, with each compound identified by the mass spectroscopy. This flow cell has been extensively validated (by comparison with fractionated collections from static Franz cells) for single compounds as well as mixtures, including each member of the 4-hydroxybenzoate ester series (methyl to octyl), various parabens, caffeine, and theophylline. The experimental animal procedures were in accordance with institutional and international standards for the care and use of laboratory animals (Animal Welfare Assurance Publication A542701, Office for Protection from Research Risks, Division of Animal Welfare, National Institutes of Health). Experiments were performed in duplicate, and the skin preparation integrity was documented by the reproducibility of the caffeine standard permeation. Test solutions were applied to the skin without occlusion. We determined the times to first detection and steady state, as well as final signal intensity, with the results expressed as relative abundance: a 20% DEET standard was compared with a 9.5% product with the sunscreens octocrylene, octyl-methoxycinnamate, and benzophenone-3 (Off! Skintastic Insect Repellant with Sunscreen, S. C. Johnson and Son Inc., Racine, WI). We also tested two proprietary products marketed as having less absorption due to a microemulsion formulation: 3M Ultrathon Insect Repellant (31.58% DEET; 3M, St. Paul, MN), and Sawyer Controlled Release DEET Formula (19.00%; Sawyer Products, Safety Harbor, FL).