Drug disposition in the mammalian eye and brain: a comparison of mechanisms

Profiling tear film enzymes reveals major metabolic pathways involved in the homeostasis of the ocular surface

The ocular surface (OS) enzymes are of great interest due to their potential for novel ocular drug development. We aimed first to profile and classify the enzymes of the OS to describe major biological processes and pathways that are involved in the maintenance of homeostasis. Second, we aimed to compare the enzymatic profiles between the two most common tear collection methods, capillary tubes (CT) and Schirmer strips (ScS). A comprehensive tear proteomic dataset was generated by pooling all enzymes identified from nine tear proteomic analyses of healthy subjects using mass spectrometry. In these studies, tear fluid was collected using CT (n = 4), ScS (n = 4) or both collection methods (n = 1). Classification and functional analysis of the enzymes was performed using a combination of bioinformatic tools. The dataset generated identified 1010 enzymes. The most representative classes were hydrolases (EC 3) and transferases (EC 2). Phosphotransferases, esterases and peptidases were the most represented subclasses. A large portion of the identified enzymes was common to both collection methods (n = 499). More enzymes were specifically detected in the ScS-extracted proteome. The major pathways in which the identified enzymes participate are related to the immune system and protein, carbohydrate and lipid metabolism. Metabolic processes for nucleosides, cellular amides, sugars and sulfur compounds constituted the most enriched biological processes. Knowledge of these molecules highly susceptible to pharmacological manipulation might help to predict the metabolism of ophthalmic medications and develop novel prodrug strategies as well as new drug delivery systems. Combining such extensive knowledge of the OS enzymes with new analytical approaches and techniques might create new prospects for understanding, predicting and manipulating the metabolism of ocular pharmaceuticals. Our study reports new, essential data on OS enzymes while also comparing the enzyme profiles obtained via the two most popular methods of tear collection, capillary tubes and Schirmer strips.

Pharmacokinetic Modeling in Nano-formulations: Concept, Implementation and Challenges

The properties of nanoparticles can be exploited to overcome challenges in drug delivery. By virtue of its design and size, the pharmacokinetics of nanoparticles are different than other small molecules. Modeling and simulation techniques have great potential to be used in nanoformulation development; however, their use in optimization of nanoformulation is very limited. This review highlights the differences in absorption, distribution, metabolism and excretion (ADME) characteristics of nanoparticles, use of modeling and simulation techniques in nanoformulation development and challenges in the implementation of modeling techniques.

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.

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.

Nitrofuran antibiotic metabolites detected at parts per million concentrations in retina of pigs—a new matrix for enhanced monitoring of nitrofuran abuse

Nitrofuran metabolite residues AOZ, AMOZ, AHD and SEM were detected at parts per million concentrations in retina of pigs fed therapeutic doses of nitrofuran antibiotics. Discovery of this residue depot may allow widespread technology transfer to laboratories lacking LC-MS/MS thus improving global monitoring of these drugs.

Nitrofurazone accumulates in avian eyes—a replacement for semicarbazide as a marker of abuse

Intact nitrofurazone is present in whole eyes of chickens fed varying levels of this banned antibiotic and may therefore be used as an alternative to the controversial marker residue, semicarbazide, to monitor for abuse of this drug in primary production.

Binding of ractopamine HCl to ocular tissues of cattle and turkeys in vivo and to melanin in vitro

Experiments were conducted to determine the total residues remaining in ocular tissues of cattle and turkeys after oral administration of [14C]ractopamine HCl. Twelve cattle were intraruminally dosed with 0.9 mg x kg(-1) x d(-1) of [14C]ractopamine HCl for 7 d. Four cattle each were slaughtered with withdrawal periods of 48,96, and 144 h. Radioactive residues were not detectable in whole-eye homogenates from the cattle. Eight male and eight female turkeys per treatment received either 7.5, 22.5, or 30 ppm dietary [14C]ractopamine HCl (0.33, 1.02, and 1.36 mg x kg(-1) x d(-1); treatment groups 1, 2, and 3, respectively) for 7 d, and the birds were slaughtered with a 0-d withdrawal period. Eyes were dissected into retina/choroid/schlera (RCS), cornea/iris (CI), and aqueous humor (AH) fractions. Residues in RCS, CI, and AH of treatment 1 turkeys were not detectable. Residues in AH were < 0.02 ppm in treatment groups 2 and 3. Mean residues in RCS ranged from 0.15 to 0.26 ppm, and mean CI residues ranged from <0.09 to 0.17 ppm for treatment groups 2 and 3, respectively. The propensities of ractopamine and synthetic ractopamine metabolites to bind to melanin were studied in vitro using radiolabeled ligands with centrifugal filtration to separate melanin from unbound ligand. In vitro studies showed that [14C]ractopamine HCl binds to melanin rapidly and was displaced from melanin by other beta-agonists. Glucuronidation of ractopamine, which produced the major biotransformation product of ractopamine in all species studied to date, prevented binding to melanin. These studies demonstrate that the propensity for the in vivo binding of ractopamine HCl to pigmented ocular tissues is less than that reported for clenbuterol.

Mechanistic analysis of S-(1,2-dichlorovinyl)-L-cysteine-induced cataractogenesis in vitro

Chronic exposure to low concentrations of the nephrotoxic cysteine conjugate S-(1,2-dichlorovinyl)-l-cysteine (DCVC) causes cataracts in mice. This study explored mechanisms of DCVC-induced cataractogenesis using explanted lenses from male Sprague-Dawley rats. Lenses placed in organ culture were exposed to 2.5 microM-1 mM DCVC for 24 hr. DCVC caused concentration and time-dependent changes in biochemical markers of toxicity (lenticular adenosine 5'-triphosphate (ATP) content, mitochondrial reduction of the tetrazolium dye MTT, and glutathione (GSH) content) at concentrations >/=25 microM. Lens clarity was adversely affected at concentrations >/=50 microM. Within 24 hr, 1 mM DCVC altered lens ATP content (-77 +/- 2%), mitochondrial MTT reduction (-40 +/- 3%), and GSH content (-19 +/- 4%) (percent difference from controls, p < 0.05). ATP was the most sensitive index of DCVC exposure in this model, while lens weight was not altered. The role of lenticular DCVC metabolism was investigated using the beta-lyase inhibitor aminooxyacetic acid (AOA) and the flavin monooxygenase (FMO) inhibitor methimazole (MAZ). AOA (1 mM) provided nearly complete protection from changes in biochemical parameters and lens transparency caused by DCVC, while MAZ (1 mM) provided only partial protection. The mitochondrial Ca2+ uniport inhibitor ruthenium red (30 microM) and the poly(ADP ribosyl)transferase inhibitor 3-aminobenzamide (3 mM) were only partially protective, whereas adverse changes in lens transparency and biochemical markers were not prevented by an antioxidant (2 mM dithiothreitol) or nontoxic transport substrates (200 microM probenecid or 10 mm phenylalanine, S-benzyl-L-cysteine or para-aminohippuric acid). Calpain inhibitors E64d (100 microM) and calpain inhibitor II (1 mM) were ineffective in preventing opacity formation caused by DCVC. In a small separate study, DCVC toxicity to explanted lenses from cynomologus monkeys was also ameliorated by coincubation with AOA. These results indicate that opacity formation by DCVC in rodent and primate lenses in vitro is primarily mediated via lenticular beta-lyase metabolism of DCVC to a reactive metabolite. Metabolism of DCVC by FMO and perturbations in mitochondrial calcium (Ca2+) homeostasis and increased poly(ADP-ribosylation) of nuclear proteins may play a limited role in opacity formation in vitro. However, opacity formation does not appear to be the result of oxidative stress or calpain activation. DCVC toxicity to the lens was not blocked with competitive inhibitors of the amino acid and organic anion transporters of DCVC as is found in the kidney.

Immunocytochemical study of cytochrome P450 (1A1/1A2) induction in murine ocular tissues

C57BL/6 and DBA/2 mice were injected intraperitoneally with beta-naphthoflavone in corn oil and killed 48 hr later. Control animals received an injection of corn oil. The immunoreactivity of cytochrome P450 1A1/1A2 expressed in different ocular tissues and liver was examined with goat anti-P450 antibody (primary antibody) and gold-conjugated anti-goat antibody (secondary antibody). DBA/2 mice, which are non-responsive to aryl hydrocarbon treatment, showed negligible levels of immunoreactivity toward anti-P450 1A1/1A2 antibody in all ocular tissues, whether or not the animals were treated with beta-naphthoflavone. In responsive C57BL/6 mice, however, the immunoreactivity of the uveal tissues, especially ciliary non-pigmented epithelium, was markedly increased by beta-naphthoflavone treatment. The time course of induction of P450 1A1/1A2 immunoreactivity was very similar for the liver and ciliary non-pigmented epithelium, although the maximum level of immunoreactivity of the ciliary epithelium reached in 48 hr after inducer treatment was about 25% of that of liver. The present results support our previous observations that the P4501A enzyme activities (e.g. aryl hydrocarbon hydroxylase) in the liver and eye of C57BL/6 mice are under the same genetic regulation. Further, this study is the first demonstration of P450 isoform induction in specific ocular tissues of the whole animal.

Properties and function of the ocular melanin--a photobiophysical view

This paper reviews the biosynthesis and physicochemical properties of the ocular melanin. Age-related changes of melanin granules and the corresponding formation of lipofuscin pigments in the retinal pigment epithelium (RPE) are also described. Adverse photoreactions of the eye and, in particular, light-induced damage to the RPE-retina are reviewed in relation to the ocular pigmentation. A hypothesis on the photoprotective role of the RPE melanin is presented that is based on the ability of the cellular melanin to bind redoxactive metal ions. Since bound-to-melanin metal ions are expected to be less damaging to the pigment cells, it is proposed that sequestration of heavy metal ions by the RPE melanin is an efficient detoxifying mechanism. It is postulated that oxidative degradation of RPE melanin may lower its metal-binding capability and decrease its anti-oxidant efficiency. Cellular and environmental factors that may contribute to possible oxidative damage of the RPE melanin are discussed in connection with the etiology of age-related macular degeneration.