Lee reviewed the factors affecting corneal drug penetration (73). Rojanasakul et al. showed that polylysine permeated through epithelial surface defects via an intracellular pathway when administered to the eye, whereas insulin predominates in the surface cells of the cornea (23). They noted that there was a significant amount of aminopeptidase activity present in the ocular fluids and tissues. Figure 1 summarizes the results of the metabolism of topically applied enkephalins to the eye (74). Pretreatment with the peptidase inhibitor bestatin had a significant protease inhibitory effect, albeit in the tears only.
Studies have been conducted with absorption enhancers to improve the delivery of peptides and proteins into the systemic circulation via the ocular route (75-77). Table 4 lists some penetration enhancers that have been used in the ocular delivery of peptide-like drugs. Ocular delivery of insulin to generate a therapeutic glucose-lowering response requires a penetration enhancer (78). Yamamoto et al. (79) reported that the bioavailability
Table 4 Penetration Enhancers Used to Improve Ocular Absorption
Azone Threefold increase in cyclosporine absorption
Cetrimide, cytochalasin B Increased absorption of inulin
EDTA Threefold increase in glycerol absorption
Taurocholate, taurodeoxycholate Increased permeation of insulin and
FITC-dextran of insulin could be improved in the following descending order by coadmi-nistration of the permeation enhancers: polyoxyethylene-9-lauryl ether > sodium deoxycholate > sodium glycocholate ~ sodium taurocholate.
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