Enantioselectivity Transport of Timolol Maleate Through Hairless Mice Skin is A Single-Valued Function of the Concentrations of Chiral Terpene Enhancer(D-Limonene)

The purposes of the present study were: 1).To study the relationship between different concentrations of the chiral enhancer D-Limonene (D-LM) on the solubility of individual enantiomer and racemate Timolol maleate (TM). 2). To study the preferential enhancement of D-LM upon S-TM, R-TM, and racemate across the hairless mice skin. For solubility studies, excess of R-, S-, or racemate with wide-range of different concentrations of D-LM were prepared.  Samples were agitated, centrifuged and filtered and analyzed by HPLC using chiral column at 294nm.  For skin transport studies, formulations containing 0.5% solutions of S-TM, R-TM, or racemate in buffer solution with predetermined concentrations of D-LM were studied. Samples of 1-ml were withdrawn and quantitatively analyzed for their TM contents.  The steady-state fluxes (Jss), permeability coefficients and the enhancement factor were calculated. In solubility studies, D-LM significantly enhances solubility of all forms of TM in a concentration dependent manner.  In permeation studies, presence of D-LM significantly enhances the flux values of both enantiomers and racemate.  However, D-LM immensely increased all permeability characteristics of the S-isomer compared to those of R-isomer. Solubilities of all forms of TM were found to be a single-valued function of D-LM concentration. Moreover, addition of D-LM has enhanced the transport of S-, R-TM enantiomers and that of racemate across hairless mice skin.  For all tested formulations, the overall permeability characteristics of the therapeutically active TM (i.e., S-TM) were superior results obtained with the R-TM either as enantiomer or racemate.