Enantioselectivity Transport of Timolol Maleate through Hairless Mice Skin as a Single-Valued Function of the Concentrations of Chiral Terpene Enhancer (D-Limonene)

Abstract

Objectives.  The purposes of the present study were: 1). To study the relationship between different concentrations of the chiral terpene enhancer D-Limonene (D-LM) on the solubility of individual enantiomer and racemate Timolol maleate (TM).  2).To study the preferential enhancement effect of D-LM upon individual S-TM, R-TM,and the racemates across the hairless mice skin. Method.  For the solubility studies, excess amounts of R-, S-, or racemate used using different concentrationsof D-LM (0, 0.25, 0.5, 0.75, 1.0, and 1.5% v/v).  The samples were agitated, centrifuged and filtered through a 0.45micg filter and analyzed by HPLC using chiral column at 294-nm.  For the skin transport studies, formulations containing 0.5% solutions of S-TM, R-TM, or racemate in a mixture of phosphate buffer and ethanol (3:2) with various concentrations of D-LM were studied usingside-by-side diffusion cells. The ports of each cell were capped and samples of 1 ml were withdrawn from the receptor compartment and analyzed for their drug contents by HPLC.  The steady-statefluxes (Jss), permeability coefficients (P) and the Enhancement Factor (EF) were calculated.  Results.  In the solubility studies, D-LM significantly enhances the solubility of all forms of TM in the tested formulations in aconcentration dependent manner.  In the permeation studies, presence of D-LM significantly enhances the flux values of both individual enantiomers and racemate.  However, D-LM immensely increased all permeability characteristics of the S-isomer relative to those ofR-isomer.  Conclusion.  The solubilities of the all forms of TM were found to be a single-valued function of D-LM concentration.  Moreover, addition of D-LMhas enhanced the transport of S-, R-TM enantiomers as well as that of racemate TM across hairless mice skin.  For the test formulations, the overall permeability characteristics of the therapeutically active S-TM were superior to those corresponding to the R-TM either as enantiomer or racemate.