Increased corneal hydration induced by potential ocular penetration enhancers: assessment by differential scanning calorimetry (DSC) and by desiccation
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Abstract
The corneal toxicity of some surfactants of possible use as ocular penetration enhancers
was investigated by measuring their effect on hydration of rabbit corneas 'in vitro'.
The tested substances were benzalkonium chloride (BAC), cetylpyridinium chloride (CPC),
ethylenediaminetetraacetic acid disodium salt (EDTA), polyoxyethylene-20-stearyl ether
(Brij 78, PSE), polyethoxylated castor oil (Cremophor EL, PCO) and sodium deoxycholate
(DC). Freshly excised corneas, mounted in perfusion cells, were kept in contact for
1 h with solutions of these agents; corneal hydration was then evaluated by measuring:
(a) their total (free+bound) water content by desiccation (gravimetric analysis);
and (b) their free water content by differential scanning calorimetry (DSC). The DSC
measurements also provided a rough quantitative estimate of corneal solutes. All tested
agents significantly influenced corneal hydration, evidently as a consequence of alteration
of the corneal epithelium. Although a brief contact with the precorneal tissues 'in
vivo' may not prove harmful, the use of these compounds as potential ocular permeation
enhancers or otherwise as ingredients of topical ocular formulations for long-term
use should be considered with caution.