DEET release

Ethanol evaporates rapidly from the skin and can leave an increased concentration of available DEET. The manner of the evaporation creates a significant concentration gradient on the skin and promotes more absorption of the DEET than does a 100% DEET (no solvent) formulation.

In hopes of diminishing the potential for absorption toxicity, researchers from the University of Arizona (Tucson) and the University of Kansas (Lawrence), led by James Blanchard from Arizona, recently looked at using cyclodextrins (CDs) as an alternative solvent to ethanol in DEET formulations. Past work on CDs has illustrated that they can have marked effects on the skin’s permeability to various drugs. For DEET, not only is skin permeability an issue, but volatility is as well, as DEET must evaporate in order to repel insects.

The scientists used diffusion cells to measure concentration gradients in the samples of straight DEET, DEET in ethanol, and DEET in two different CDs—-cyclodextrin and hydroxypropyl--cyclodextrin (HPBCD). DEET release in the CD samples was much slower, indicating that DEET remained as part of the lotion longer, which could mean that less DEET would be absorbed through the skin (J. Pharm. Sci. 2002, 91 (1), 101–110). It was suggested that this effect was produced by the CDs’ stabilization of emulsions and not by a direct CD complex with DEET.

The team tested the formulations in vials to determine the amount of DEET that evaporated from the surface. DEET evaporation in the CD formulations was higher than in ethanol, and DEET in HPBCD was even more volatile than neat DEET.

Thus, using CDs instead of ethanol to make topical DEET preparations is a promising, potentially safer alternative. DEET in CD, the researchers concluded, is less likely to be absorbed, and it remains volatile enough to be an effective pest repellant. Furthermore, the study determined that long-term stability of the CD-based formulations was good, something essential to any commercial product.