Synthesis and evaluation of N-acetylprolinate esters - novel skin penetration enhancers

A series of N-acetylproline esters (alkyl side chain length, 5-18) were synthesized and tested for potential skin penetration enhancement activity using modified Franz diffusion cells and hairless mouse skin as the penetration barrier. Benazepril and hydrocortisone were used as model drugs and were applied as saturated solutions in propylene glycol. The enhancers were added at a concentration of 5% (w/v). Drug flux, permeability coefficient and enhancement ratios for permeability coefficient were determined. Azone was used as the positive control. While all the compounds tested increased the skin penetration of hydrocortisone, the 5- and 8- carbon esters had no significant effect on the skin penetration of benazepril. The highest fluxes were obtained with 11, 12, and 18-carbon esters and they were comparable to Azone. There was no significant difference between the fluxes obtained with 2 and 5% (w/v) concentrations of the 12-carbon ester on hydrocortisone permeation. The 16-carbon ester, where ethanol was used as a cosolvent, significantly increased the fluxes of both the drugs compared to the control. Differential scanning calorimetric studies suggested that the enhancers may be acting on the lipids of the stratum corneum and their effect was similar to that of Azone. The membrane/vehicle partition coefficient studies indicated an increase in benazepril partition coefficient with enhancer treatment compared to the control. Maximum flux increase was obtained with the 11 and 12 carbon (alkyl chain length) esters for both benazepril and hydrocortisone. The 18- carbon ester which has a cis-double bond in the alkyl side chain, also increased the flux significantly.