Relationship between steroid permeability across excised rabbit cornea and octanol-water partition coefficients

Topical carbonic anhydrase inhibitors. III: Optimization model for corneal penetration of ethoxzolamide analogues

An analogue series representing modification to the benzene ring of ethoxzolamide has been evaluated for solubility, pKa, partitioning, and permeability across excised rabbit corneas. These physical parameters were correlated to Hammett sigma (para) and/or Hansch pi parameter values for each compound. From these correlations, a mathematical model was developed relating corneal permeability to molecular modifications of ethoxzolamide. A three-dimensional plot of maximum attainable penetration rate versus sigma (para) and pi yielded an optimal range of pi and sigma values from which an optimally penetrating analogue could be designed.

Relationship of chemical structure to corneal penetration and influence of low-viscosity solution on ocular bioavailability

Current understanding of the mechanism of corneal penetration by organic molecules proposes the epithelial layer as the rate-limiting membrane for water-soluble compounds and the stromal layer as rate limiting for lipid-soluble compounds. This suggests that the relationship between corneal permeability and the logarithm of oil/water partition coefficients, for a series of drugs, should not be the typical, single, continuous, parabolic-shaped curve. Corneal penetration studies have been conducted in unanesthetized albino rabbits using various organic compounds, representing five orders of magnitude in partition coefficient, at a constant concentration of 4 X 10(-5) M dispensed in either a 1- or 90-centipoise (cps) solution. It has been shown that for non-ionizable compounds, a pair of bell-shaped curves were generated, one for lipid-soluble and one for water-soluble compounds. Small water-soluble species demonstrate very high apparent permeabilities, which may relate to the presence of aqueous pores or other paracellular drug movement. Penetration of ionizable compounds does not appear to correlate well with the structural relationships invoked for un-ionized compounds. Consistent with the proposed mechanisms of corneal penetration, oil-soluble drug substances show no improvement in drug bioavailability when dosed from a 90-cps solution, and water-soluble drugs show a modest improvement in ocular drug bioavailability. Small water-soluble substances demonstrate no improvement due to their already high bioavailability. The importance of nonproductive absorption and precorneal drainage on bioavailability is addressed.

Corneal penetration behavior of beta-blocking agents I: Physiochemical factors

Rabbit corneas were excised and mounted in a chamber to determine the permeability characteristics of a group of beta-blocking agents which varied in octanol-water partitioning over a fourfold logarithmic range. From the permeability rate at steady state, permeability coefficients (pH 7.65) were determined. For each drug the distribution coefficient and pKa were measured, permitting the partition coefficients to be estimated. Various correlations were determined for the log permeability coefficient as a sum of log functions of the partition (or distribution) coefficient, molecular weight, and/or degree of ionization. The best fit, as judged by a high correlation coefficient (r = 0.9756) and lack of systematic deviation, was represented by: log PT = 0.623 log DC - 0.108(log DC)2 - 5.0268.

Disposition of cyclophosphamide in the rabbit and human cornea

Rabbit (dark irides and albino) and human corneas were excised and mounted in a Lucite chamber in order to determine the permeability of cyclophosphamide-14C and to identify its metabolites. The chromatographic scans obtained from the developed thin-layer plates showed only one peak for either rabbit or human corneas which corresponded to cyclophosphamide. By measuring the permeability of cyclophosphamide across the intact cornea, stroma alone, epithelium/stroma, and stroma/endothelium, it was possible to determine the resistance to penetration for each layer. The epithelium was the rate determining barrier, whereas both the stroma and endothelium offered little resistance. The stroma behaves as an inert sieve with drug diffusing through an aqueous media of gel-like mucopolysaccharide interspersed by a matrix of collagen fibrils.