Rate control of ocular pilocarpine delivery with bispilocarpic acid diesters

Paracellular and passive transcellular permeability in immortalized human corneal epithelial cell culture model

A cell culture model of human corneal epithelium (HCE-model) was recently introduced [Invest. Ophthalmol. Vis. Sci. 42 (2001) 2942] as a tool for ocular drug permeation studies. In this study, passive permeability and esterase activity of the HCE-model were characterised. Immortalised human corneal epithelial cells were grown on collagen coated filters under air-lift. The sensitivity of transcellular permeability to lipophilicity was tested in studies using nine beta-blockers. The size selectivity of the paracellular route was investigated using 16 polyethylene glycol oligomers (PEG). An effusion-like approach was used to estimate porosity and pore sizes of the paracellular space in HCE membrane. Permeability and degradation of fluorescein diacetate to fluorescein in HCE-cells was used to probe the esterase activity of the HCE-model. Drug concentrations were analyzed using HPLC (beta-blockers), LC-MS (PEGs), and fluorometry (fluorescein). Permeabilities were compared to those in the excised rabbit cornea. Penetration of beta-blockers increased with lipophilicity according to a sigmoidal relationship. This was almost similar to the profile in excised cornea. No apical to basolateral directionality was seen in the permeation of beta-blockers. Paracellular permeability of the HCE-model was generally slightly higher than that of the excised rabbit cornea. The HCE-model has larger paracellular pores, but lower pore density than the excised cornea, but the overall paracellular space was fairly similar in both models. The HCE-model shows significant esterase activity (i.e. fluorescein diacetate was converted to free fluorescein). These data on permeability of 27 compounds demonstrate that the barrier of the HCE-model closely resembles that of the excised rabbit cornea. Therefore, the HCE-model is a promising alternative corneal substitute for ocular drug delivery studies.

Miotic effect and irritation potential of pilocarpine prodrug incorporated into a submicron emulsion vehicle

Pilocarpine prodrug, O,O'-dipivaloyl(1,2-ethylene) bispilocarpic acid diester, was introduced to a submicron emulsion vehicle in a dose equivalent to 0.5% pilocarpine base, and the formulation was studied in albino rabbits using miotic assay. Compared with pilocarpine HCl 0.5% solution delayed and prolonged miosis was observed after application of the prodrug emulsion. AUC(0-6 h) values for the prodrug emulsion and pilocarpine solution were 9252+/-1345 and 6845+/-1967%xmin, respectively. The prodrug was also administered twice daily for 5 days in the form of aqueous solution or submicron emulsion in order to study ocular irritation. Irritation potential of the prodrug was significantly reduced when submicron emulsion was used as a vehicle.

Modified beta-cyclodextrin (SBE7-beta-CyD) with viscous vehicle improves the ocular delivery and tolerability of pilocarpine prodrug in rabbits

The complexation of pilocarpine prodrug, O,O'-dipropionyl-(1,4-xylylene) bispilocarpate, with various beta-cyclodextrin (beta-CyD) derivatives was studied by the phase solubility method. The effects of coadministered sulphobutyl ether beta-CyD (SBE7-beta-CyD) with and without poly(vinyl alcohol) (PVA) on the miotic response and eye irritation of the prodrug were investigated in pigmented rabbits. The pilocarpine prodrug formed 1:1 inclusion complexes with variably substituted sulphobutyl ether derivatives of beta-CyD (SBE4-beta-CyD and SBE7-beta-CyD), and 1:1 and 1:2 complexes with hydroxypropyl-beta-CyD (HP-beta-CyD) at pH 7-4. Coadministered SBE7-beta-CyD eliminated the eye irritation due to the pilocarpine prodrug, but also decreased the miotic response. Ocular absorption of the prodrug was improved by increasing the viscosity of prodrug/SBE7-beta-CyD solution with PVA without inducing any eye irritation. Eye irritation due to viscous prodrug/SBE7-beta-CyD solutions was comparable with isotonic NaCl solution. We conclude that administration of pilocarpine prodrug in viscous SBE7-beta-CyD solution decreases substantially eye irritation while ocular absorption is not affected.