Influence of alpha-cyclodextrin and hydroxyalkylated beta-cyclodextrin derivatives on the in vitro corneal uptake and permeation of aqueous pilocarpine-HCl solutions

Interactions in aqueous solution between pilocarpine hydrochloride (P-HCl), a rather hydrophilic drug with good water solubility, and various cyclodextrins (CDs) were described recently. To assess the influence of CDs on the diffusion behavior of pilocarpine, in vitro studies were performed using porcine or bovine corneas as diffusion barriers. The affinity of P-HCl for porcine cornea in the presence of alpha-cyclodextrin (alpha-CD) and (hydroxyethyl)-beta-cyclodextrin (HE-beta-CD) was determined by drug uptake experiments. Additionally, in vitro permeation experiments through bovine corneas were conducted with a modified diffusion device optimized for corneal perfusion studies. The results obtained from the corneal uptake studies indicate that the addition of alpha-CD led to increased tissue drug levels. The increase in permeability of pilocarpine in the presence of alpha-CD was similar to 10-fold (log P-app = -4.87 +/- 0.03) in comparison with plain P-HCl solution (log P-app = -5.89 +/- 0.06). Permeation studies with corneas pretreated with alpha-CD solution revealed enhanced corneal permeability of pilocarpine due to alpha-CD-induced membrane effects. The hydroxyalkylated beta-CD derivatives HE-beta-CD (log P-app = -6.27 +/- 0.09) and (hydroxypropyl)-beta-cyclodextrin (HP-beta-CD; log P-app = -6.40 +/- 0.03), however, seemed to cause slightly decreased permeation rates, supporting the concept of an interaction between pilocarpine and the hydroxyalkylated-beta-CD derivatives. Considering physiological compatibility, the addition of CDs seems to be an effective tool to modify and optimize the ocular availability of pilocarpine.