Effect of drug chirality on the skin permeability of ibuprofen

The in vitro passive diffusion of S-ibuprofen (S-IB) and RS-ibuprofen (RS-IB) through human epidermis was determined to study the effects of drug chirality. S-IB has a lower melting point (Tm = 54 °C) than RS-IB (Tm = 77 °C) and, therefore, a greater solubility (S-IB: 127 ± 1 μg/mL; RS-IB: 81 ± 1 μg/mL). Supersaturated plasters were prepared by using a poly(dimethylsiloxane) adhesive and Eugragit® RL and propylene glycol as antinucleant agents. The in vitro skin permeation profiles were determined by Franz cells and human epidermis obtained from three different donors. The permeation profiles of S-IB from saturated solutions resulted statistically higher than those of RS-IB (p < 0.002). When plasters were used, no differences were noticeable between the enantiomer and racemate (p > 0.17). The latter unexpected results could be explained considering that the RS-IB or S-IB in vitro release rate constants, determined using 3% w/w or 6% w/w loaded plasters, were not statistically different, suggesting that the drug diffusivity within the adhesive matrix represented the rate limiting step to the skin absorption.