Novel site-specific chemical delivery system as a potential mydriatic agent: formation of phenylephrine in the iris-ciliary body from phenylephrone chemical delivery systems

The objective of this study was to test the three novel ester derivatives of phenylephrone (isovaleryl, phenylacetyl, and pivalyl esters) as potential site-specific chemical delivery systems. The mydriatic effect and ocular distribution/metabolism of these compounds were studied by topical application to the eyes of normal rabbits. It was assumed that a reduction-hydrolysis sequence could produce the active phenylephrine in the iris-ciliary body tissues. All the derivatives showed a more pronounced mydriatic effect than that of phenylephrine, whereas phenylephrone was completely devoid of any mydriatic activity. Phenylacetyl ester was the most potent drug, with short duration of action, and showed maximum activity in the presence of 0.01% benzalkonium chloride without causing any visible irritation to the rabbit eye. Administration of the novel compounds to the eyes of the rabbits showed no traces of phenylephrine in the systemic circulation, contrary to topical administration of phenylephrine. Phenylephrone was detected in different compartments of the eye, whereas phenylephrine was present only in the iris-ciliary body tissues following administration of phenylacetyl ester. The conversion of phenylephrone esters to the active drug, phenylephrine, and thus their subsequent activity was dependent on the physicochemical characteristics of the drugs. The results suggest the potential use of phenylacetyl ester as a potent short-term mydriatic agent without systemic side effects.