Skin penetration/permeation enhancers are compounds that improve (trans)dermal drug delivery. We designed hybrid terpene-amino acid enhancers by conjugating natural terpenes (citronellol, geraniol, nerol, farnesol, linalool, perillyl alcohol, menthol, borneol, carveol) or cinnamyl alcohol with 6-(dimethylamino)hexanoic acid through a biodegradable ester linker. The compounds were screened for their ability to increase the delivery of theophylline and hydrocortisone through and into human skin ex vivo. The citronellyl, bornyl and cinnamyl esters showed exceptional permeation-enhancing properties (enhancement ratios up to 82) while having low cellular toxicities. The barrier function of enhancer-treated skin (assessed by transepidermal water loss and electrical impedance) recovered within 24 h. Infrared spectroscopy suggested that these esters fluidized the stratum corneum lipids. Furthermore, the citronellyl ester increased the epidermal concentration of topically applied cidofovir, which is a potent antiviral and anticancer drug, by 15-fold. In conclusion, citronellyl 6-(dimethylamino)hexanoate is an outstanding enhancer with an advantageous combination of properties, which may improve the delivery of drugs that have a limited ability to cross biological barriers.
Terpenes are a class of natural compounds with strong permeation-enhancing potential and have been generally recognized as safe (GRAS) adjuvants with relatively low and transient irritation. For example, the acyclic monoterpene alcohols citronellol, geraniol, and linalool, enhanced the permeation of ondansetron, caffeine and haloperidol, respectively. The cyclic monoterpenes borneol, carveol, menthol, and limonene were reported as enhancers for ibuprofen, curcumin, indomethacin, and valsartan, respectively. In addition, sesquiterpene farnesol increased the permeation of haloperidol. (Sci Rep. 2019; 9: 14617.)