Cytochrome P-450 and related components of the microsomal electron transport system in the bovine ciliary body

Phase I and phase II ocular metabolic activities and the role of metabolism in ophthalmic prodrug and codrug design and delivery

While the mammalian eye is seldom considered an organ of drug metabolism, the capacity for biotransformation is present. Compared to the liver, the metabolic capabilities of the eye are minuscule; however, phase I and phase II metabolic activities have been detected in various ocular structures. The careful consideration of ocular tissue metabolic processes within the eye has important implications for controlling the detoxification of therapeutic agents and for providing the potential for site-specific bio-activation of certain drug molecules, thus enabling significant improvements in drug efficacy and the minimization of side-effect from either local or systemic drug delivery to the eye. Knowledge of these processes is important to prodrug and codrug development and to researchers involved in the design, delivery and metabolism of ophthalmic drugs. This present article reviews the progress in ocular prodrug and codrug design and delivery in light of ocular metabolic activities.

Biotransformation in review: applications in ocular disease and drug design

The purpose of this paper is to review the biochemical processes of systemic biotransformation and describe their relevance to ocular disease and drug metabolism. The diverse nature of the biochemical pathways, commonly found in enzyme metabolism, is discussed. The occurrence of these processes in the eye has significance in that the products of metabolism may accumulate locally and exert deterimental effects, presumably by altering the cellular structure and/or function of crucial visual elements. The manipulation of these metabolic pathways within the eye has ramifications in the development of novel drug design for both ocular disease treatment and, perhaps more importantly, disease prevention.

Induction of cytochrome P-450 in the rabbit eye by phenobarbital, as detected immunohistochemically

The present study was aimed to demonstrate that cytochrome P-450 was induced by drug administration in particular tissues in the eye. Phenobarbital was used as an inducer. We showed immunohistochemically that cytochrome P-450 was induced in the cornea, conjunctiva and ciliary epithelium of rabbits after four days of intraperitoneal administration of phenobarbital at a dose of 80 mg per kg per day. We did not detect significant immunofluorescence in other ocular tissues. Prolonged administration caused degeneration of the ciliary epithelium, but not pathological change was seen in other ocular tissues. In this case, immunofluorescence was not detected in the ciliary epithelium but in the cornea, conjunctiva and lens.

Cytochrome P450, drug metabolizing enzymes and arachidonic acid metabolism in bovine ocular tissues

Little information is available on drug metabolizing enzymes in ocular tissues. We investigated the presence of various cytochrome P450 isozymes by measuring different drug metabolizing enzymes, i.e., aryl hydrocarbon hydroxylase, 7 ethoxycoumarin-o-deethylase and benzphetamine demethylase activities in ciliary body, corneal epithelium and endothelium, retina and retinal pigment epithelium. Our results demonstrate that the ciliary body and the retinal pigment epithelium possess the highest activities of cytochrome P450-dependent monooxygenases in the eye. The highest activity of drug metabolizing enzymes is accompanied by high activity of NADPH cytochrome P450 (C) reductase, an integral component of this enzyme system. Heme oxygenase, a key enzyme for the regulation of heme availability to hemoproteins such as cytochrome P450 also demonstrate high activity in these two ocular tissues. Although the corneal epithelium has a lower activity of drug metabolizing enzymes, it possesses the highest activity of cytochrome P450 species capable of metabolizing arachidonic acid to biologically active compounds, whereas the other ocular tissues possess cyclooxygenase as the main microsomal enzyme that metabolizes arachidonic acid. Whether the observed catalytic activities of drug metabolizing enzymes seen in ocular tissues are associated with major or minor forms of cytochrome P450 is not yet know. However, the specialized location of cytochrome P450 isozymes in ocular tissues suggests a physiological function related to activation of endogenous compounds such as arachidonic acid, in addition to detoxification of drugs.