Ocular non-P450 oxidative, reductive, hydrolytic, and conjugative drug metabolizing enzymes

Metabolism in the eye for any species, laboratory animals or human, is gaining rapid interest as pharmaceutical scientists aim to treat a wide range of so-called incurable ocular diseases. Over a period of decades, reports of metabolic activity toward various drugs and biochemical markers have emerged in select ocular tissues of animals and humans. Ocular cytochrome P450 (P450) enzymes and transporters have been recently reviewed. However, there is a dearth of collated information on non-P450 drug metabolizing enzymes in eyes of various preclinical species and humans in health and disease. In an effort to complement ocular P450s and transporters, which have been well reviewed in the literature, this review is aimed at presenting collective information on non-P450 oxidative, hydrolytic, and conjugative ocular drug metabolizing enzymes. Herein, we also present a list of xenobiotics or drugs that have been reported to be metabolized in the eye.

Investigation of Ocular Bioactivation Potential and the Role of Cytochrome P450 2D Enzymes in Rat

BACKGROUND

Timolol is clinically administered topically (ocular) to reduce intraocular pressure and treat open-angle glaucoma. Ocular administration of timolol in low doses (0.5% w/v in the form of eye drops) has led to challenges for in vivo metabolite identification. An understanding of drug metabolism in the eye is important for clinical ocular therapeutics and potential drug candidates.

METHODS

We aimed to investigate the metabolism of timolol in rat ocular and liver S9 fractions, as well as rat ocular tissue and plasma following a 0.5% topical (ocular) dose of timolol. We explored the potential in vitro metabolic bioactivation in the eye/liver by conducting trapping studies for putative aldehyde and iminium ion intermediates that may arise from the morpholine functionality.

RESULTS

Oxidative metabolism of timolol to its major metabolite (M4) in ocular S9 and recombinant rat cytochrome P450 (CYP) isoforms supports the possible role of rat ocular CYP2D2, 2D4, and/or 2D18. Observation of N-acetyl-timolol (M5) is suggestive that the ocular N-acetyltransferases may also play a larger role in ocular disposition of timolol, a previously unreported finding. This research is the first comprehensive report of in vitro ocular metabolism of timolol in rat.

CONCLUSION

This study also indicates that in vitro hepatic metabolism is over-predictive of ocular metabolism following topically ocular dosed timolol. The research, herein, highlights the eye as an organ capable of first pass metabolism for topical drugs. Thus, new ophthalmologic considerations for studying and designing long term topical therapies in preclinical species are needed in drug discovery.