Improved corneal pilocarpine permeability with O,O'-(1,4-xylylene) bispilocarpic acid ester double prodrugs

O,O'-(1,4-Xylylene) bispilocarpic acid esters are pilocarpine prodrugs containing two pilocarpic acid monoesters linked with one pro-moiety. Each mole of prodrug forms two pilocarpine moles in the presence of esterases. Corneal uptake and permeability of various bispilocarpic acid diesters were investigated in vitro using isolated albino rabbit corneas. The permeability coefficient of pilocarpine was 2.8 x 10(-6) cm/sec, whereas for bispilocarpic acid diesters, despite their large molecular weights (between 638 and 722), permeability coefficients were 6.5-20.2 x 10(-6) cm/sec. Only pilocarpine, and no intact prodrug, was observed at the endothelial side. Corneal uptake was increased with increasing lipophilicity, but a parabolic relationship between the logarithm of the apparent partition coefficient (1-octanol-pH 7.4 phosphate buffer) (log PC) and the corneal permeability was noticed. Corneal permeability and the rate of enzymatic hydrolysis of the compounds correlated well. The corneal permeability of pilocarpine given as lipophilic bispilocarpic acid diester (log PC greater than or equal to 3) prodrugs seems to be controlled by the formation of pilocarpine in the corneal epithelium rather than by the absorption of prodrugs into the epithelium or their epithelium-stroma transport rate.