Temperature and pH Stability Profiles of ortho and para DEET

Abstract

ABSTRACT − DEET, N,N’-diethyl-m-toluamide, is the most commonly used mosquito repellent. However, it can easilypermeate through skin leading to toxic effects. A recent study showed that the ortho analogue of DEET showed enhancedrepellency with reduced permeation compared to the commercially used meta analogue. Thus, in order to understand thedifferences in properties and effectiveness among the m-, o- and p-analogues of DEET, an HPLC-UV method was developedfor separately analyzing the three analogues. Moreover, stability profiles at temperatures ranging from 30 o C to 70C as wellas pH ranging from pH 3 to pH 9 have been determined. All three analogues were stable with no degradation observed dur-ing the 5 day period. o-DEET therefore could be further developed into a safer and more effective mosquito repellent.Key words − DEET, Temperature stability, pH stability, Mosquito repellency m-DEET is known as the golden standard of mosquito repel-lents for being highly effective, stable and relatively safe (Katzet al., 2008). In most cases it is directly applied to the humanskin protecting against tick-borne diseases (Jensennius et al.,2005) and mosquito-mediated diseases including dengue fever,West Nile virus and Eastern Equine Encephalitis (Gubler,2001). However, information about the mechanism of action ofDEET is still controversial. Studies report that DEET mayblock behavior of mosquitoes by inhibiting chemo-receptorson the mosquito antennae that are stimulated by lactic acid(Dogan et al., 1999) whereas other studies refute this phe-nomenon (Syed and Leal, 2008). Nevertheless, m-DEETexhibits mosquito repellency as shown in the complete pro-tection time (CPT) ranging from 203 to 756 minutes, varyingwith factors including climatic effects, mosquito species andphysical activities (Barnard and Xue, 2004; Bidlingmayer,1994; Bernier et al., 2000).The search for safer and more effective repellents continuesto progress, however, due to the toxicities reported of DEET(Schofield et al., 2007). Possible substitutes to DEET that havebeen reported so far include synthetically prepared Picaridin,IR3535 and Permethrin or lemon eucalyptus or citronellawhich are natural products (Katz et al., 2008). Unfortunately,none of these alternatives seem to exhibit properties that couldbe more advantageous to the current DEET products.Current research for developing repellents make use of sys-tematic bioassays or computer based tools which include aQSAR study of N-acyl-piperidines (Katritzky et al., 2008) anda genetic trial of odorant-binding protein manipulation (Li etal., 2008). One of the shortcomings of the mosquito repellentinvestigation is the use of human subjects or living vertebratesin the bioassays (Dogan et al., 1999). However, recently, anefficient in vitro bioassay system has been established whereblood substitute and collagen membrane were used instead ofhuman blood and skin (Jahn et. al., 2010). The artificial bloodsubstitute was a slight modification of that used by Kogan foran in vitro feeding assay for maintenance of mosquito colonies(Corazza et al., 2005). Unlike Kogan who utilized stretchedparafilm as a skin substitute, Jahn made use of a collagen typemembrane (Ditzen et al., 2008) since the parafilm alone couldnot mimic the physiological skin sufficiently. Moreover, inJahn’s study, skin permeation profiles of DEET and its ana-logues were examined in correlation to the mosquito repel-lency, expressed as CPT. Among the DEET analogues tested,compared to the commercially used meta DEET analogue,ortho DEET revealed enhanced repellency with reduced skinpermeation rate, implying that toxicity due to skin penetrationwould be lessened (Jahn et al., 2010).To estimate the applicability of the ortho DEET analogue, aseparation method for the three DEET isomers needed to beestablished. Because of the similar physicochemical charac-teristics of the three isomers as revealed in the capacity factors