Effect of hydroxypropyl-β-cyclodextrin on the solubility, photostability and in-vitro permeability of alkannin/shikonin enantiomers

The influence of hydroxypropyl-β-cyclodextrin (HPCD) on the solubility, photostability, and the in-vitro permeability of alkannin/shikonin enantiomers (A/S) was investigated. Solubility of A/S increased approximately 200 fold at 50% HPCD. The phase-solubility diagram revealed the formation of a 1:1 inclusion complex of A/S-HPCD and a stability constant (k(s)) of 260.5 M^-1. Addition of HPCD significantly decreased the photochemical decomposition rate of A/S at pH 9.0. The apparent-first-order photochemical decomposition rate constants of free and complexed A/S were calculated from the Lineweaver-Burk plot to be 0.329 and 0.156 h^-1, respectively. The in-vitro permeability study showed that the release of A/S from four test bases followed a Higuchi release model. The presence of HPCD enhanced the A/S release rate by 34 and 54 fold in cream and gel form, respectively, at one-tenth A/S concentration of the control preparation-Shiunko ointment.