Novel Strategy to Circumvent P–Glycoprotein Mediated Efflux on Rabbit Cornea by Prodrug Derivatization

Abstract

Purpose: : Efflux pump like P–glycoprotein (P–gp) is known to be a major barrier to drug delivery. Functional P–glycoprotein has been recently identified in rabbit cornea and corneal epithelial cells in our laboratory. It was also proved that P–glycoprotein restricts the ocular drug absorption resulting in lower ocular bioavailability. The objective of this study is to investigate whether transporter–targeted prodrug derivatization of a model P–glycoprotein (P–gp) substrate, quinidine can modulate P–gp mediated efflux on cornea.

Methods: : The L–valine, L–Valine–Valine esters of quinidine, val–quinidine (VQ) and val–val–quinidine (VVQ) respectively were synthesized in our laboratory. [3H] ritonavir, [¹⁴C] Erythromycin were chosen as model P–gp substrates to delineate the kinetics of quinidine. [³H] Glysar was chosen as a model PepT substrate to delineate the kinetics of val–quinidine and val–val–quinidine. The uptake studies were performed on rPCEC (Rabbit Primary Corneal Epithelial Culture) using 12–well plates. Transport studies were conducted with isolated rabbit corneas at 34°C.

Results: : The permeability of quinidine was highly improved in the presence of verapamil, a p–gp inhibitor indicating quinidine as p–gp substrate. The efflux of [¹⁴C] Erythromycin was found to be saturable at higher concentrations of quinidine having a Ki value of 1.16 nM and the efflux was not affected significantly in presence of val–quinidine and val–val–quinidine.. The transport of val–quinidine and val–val–quinidine was inhibited with GS and the permeability values of val–quinidine and val–val–quinidine were 1.3 and 3 times higher than the permeability of quinidine respectively.

Conclusions: : Results from this study clearly indicate that prodrug derivatization of quinidine into val–quinidine or val–val–quinidine can modulate P–gp–mediated efflux on rabbit cornea. These prodrugs not only have a reduced or diminished affinity towards P–glycoprotein but were also recognized by an active mediated process. The increased permeabilities of the prodrugs indicate that transporter targeted prodrug derivatization can be a viable strategy for overcoming P–gp mediated efflux on cornea.

Keywords: cornea: epithelium • pump/barrier function • anterior chamber