Ocular cytochrome P450s and transporters: roles in disease and endobiotic and xenobiotic disposition

Heme-thiolate monooxygenase cytochrome P450 1B1, an old dog with many new tricks

Cytochrome P450 CYP1B1 is a heme-thiolate monooxygenase traditionally recognized for its xenobiotic functions and extrahepatic expressions. Recent studies have suggested that CYP1B1 is also expressed in hepatic stellate cells, immune cells, endothelial cells, and fibroblasts within the tumor microenvironment, as well as tumor cells themselves. CYP1B1 is responsible for the metabolism of a wide range of substrates, including xenobiotics such as drugs, environmental chemicals, and endobiotics such as steroids, retinol, and fatty acids. Consequently, CYP1B1 and its associated exogenous and endogenous metabolites have been critically implicated in the pathogenesis of many diseases. Understanding the mode of action of CYP1B1 in different pathophysiological conditions and developing pharmacological inhibitors that allow for systemic or cell type-specific modulation of CYP1B1 may pave the way for novel therapeutic opportunities. This review highlights the significant role of CYP1B1 in maintaining physiological homeostasis and provides a comprehensive discussion of recent advancements in our understanding of CYP1B1's involvement in the pathogenesis of diseases such as fibrosis, cancer, glaucoma, and metabolic disorders. Finally, the review emphasizes the therapeutic potential of targeting CYP1B1 for drug development, particularly in the treatment and prevention of cancers and liver fibrosis. SIGNIFICANCE STATEMENT: CYP1B1 plays a critical role in various physiological processes. Dysregulation or genetic mutations of the gene encoding this enzyme can lead to health complications and may increase the risk of diseases such as cancer and liver fibrosis. In this review, we summarize recent preclinical and clinical evidence that underscores the potential of CYP1B1 as a therapeutic target.

Aldehyde oxidase 1 activity and protein expression in human, rabbit, and pig ocular tissues

Aldehyde oxidase (AOX) is a cytosolic drug-metabolizing enzyme which has attracted increasing attention in drug development due to its high hepatic expression, broad substrate profile and species differences. In contrast, there is limited information on the presence and activity of AOX in extrahepatic tissues including ocular tissues. Because several ocular drugs are potential substrates for AOX, we performed a comprehensive analysis of the AOX1 expression and activity profile in seven ocular tissues from humans, rabbits, and pigs. AOX activities were determined using optimized assays for the established human AOX1 probe substrates 4-dimethylamino-cinnamaldehyde (DMAC) and phthalazine. Inhibition studies were undertaken in conjunctival and retinal homogenates using well-established human AOX1 inhibitors menadione and chlorpromazine. AOX1 protein contents were quantitated with targeted proteomics and confirmed by immunoblotting. Overall, DMAC oxidation rates varied over 10-fold between species (human ˃˃ rabbit ˃ pig) and showed 2- to 6-fold differences between tissues from the same species. Menadione seemed a more potent inhibitor of DMAC oxidation across species than chlorpromazine. Human AOX1 protein levels were highest in the conjunctiva, followed by most posterior tissues, whereas anterior tissues showed low levels. The rabbit AOX1 expression was high in the conjunctiva, retinal pigment epithelial (RPE), and choroid while lower in the anterior tissues. Quantification of pig AOX1 was not successful but immunoblotting confirmed the presence of AOX1 in all species. DMAC oxidation rates and AOX1 contents correlated quite well in humans and rabbits. This study provides, for the first time, insights into the ocular expression and activity of AOX1 among multiple species.

Patient Factors Influencing Intraocular Penetration of Brimonidine-Related Eye Drops in Adults: A Post Hoc Pooled Analysis

INTRODUCTION

The factors related to the ocular penetration of drugs after the administration of eye drops in humans have not been examined in detail. Therefore, this study assessed the influence of patient factors on the intraocular penetration of eye drops.

METHODS

A pooled analysis was performed on the data of 42 participants from three studies to evaluate the ocular pharmacokinetics in humans after the topical application of brimonidine-related eye drops. The patients were scheduled for vitrectomy and received brimonidine-related eye drops (0.1% brimonidine tartrate ophthalmic solution, 0.1% brimonidine tartrate and 0.5% timolol fixed-combination ophthalmic solution, or 0.1% brimonidine tartrate and 1% brinzolamide fixed-combination suspension) twice daily for 1 week. We analyzed the effects of patient factors (sex, the presence or absence of lens, age, corneal thickness, corneal endothelial cell density, tear secretion, eye axial length, height, weight and body mass index [BMI]) on brimonidine, timolol and brinzolamide concentrations in the aqueous and vitreous humor after topical application.

RESULTS

The drug concentrations in the aqueous and vitreous humor were not significantly different, regardless of sex or the presence or absence of lens. Age correlated positively with brimonidine (r = 0.3948, p = 0.012) and brinzolamide (r = 0.6809, p = 0.030) concentrations in the aqueous humor; the correlation with timolol showed a trend towards significance (r = 0.6425, p = 0.086). Corneal thickness, corneal endothelial cell density, tear secretion, eye axial length, height and BMI did not correlate with the drug concentrations in the aqueous or vitreous humor. Timolol concentration in the vitreous humor was negatively correlated with weight (r =  - 0.8333, p = 0.010).

CONCLUSION

The findings of this study emphasize the necessity of considering individual differences in ocular pharmacokinetics during drug therapy (formulation design of the eye drops and dose regimen).

Challenges and Opportunities in P450 Research on the Eye

Of the 57 cytochrome P450 enzymes found in humans, at least 30 have ocular tissues as an expression site. Yet knowledge of the roles of these P450s in the eye is limited, in part because only very few P450 laboratories expanded their research interests to studies of the eye. Hence the goal of this review is to bring attention of the P450 community to the eye and encourage more ocular studies. This review is also intended to be educational for eye researchers and encourage their collaborations with P450 experts. The review starts with a description of the eye, a fascinating sensory organ, and is followed by sections on ocular P450 localizations, specifics of drug delivery to the eye, and individual P450s, which are grouped and presented based on their substrate preferences. In sections describing individual P450s, available eye-relevant information is summarized and concluded by the suggestions on the opportunities in ocular studies of the discussed enzymes. Potential challenges are addressed as well. The conclusion section outlines several practical suggestions on how to initiate eye-related research. SIGNIFICANCE STATEMENT: This review focuses on the cytochrome P450 enzymes in the eye to encourage their ocular investigations and collaborations between P450 and eye researchers.

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.

Recent Progress of Solid Lipid Nanoparticles and Nanostructured Lipid Carriers as Ocular Drug Delivery Platforms

Sufficient ocular bioavailability is often considered a challenge by the researchers, due to the complex structure of the eye and its protective physiological mechanisms. In addition, the low viscosity of the eye drops and the resulting short ocular residence time further contribute to the observed low drug concentration at the target site. Therefore, various drug delivery platforms are being developed to enhance ocular bioavailability, provide controlled and sustained drug release, reduce the number of applications, and maximize therapy outcomes. Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) exhibit all these benefits, in addition to being biocompatible, biodegradable, and susceptible to sterilization and scale-up. Furthermore, their successive surface modification contributes to prolonged ocular residence time (by adding cationic compounds), enhanced penetration, and improved performance. The review highlights the salient characteristics of SLNs and NLCs concerning ocular drug delivery, and updates the research progress in this area.

Nanostructured Lipid Carriers for Enhanced Transscleral Delivery of Dexamethasone Acetate: Development, Ex Vivo Characterization and Multiphoton Microscopy Studies

Corticosteroids, although highly effective for the treatment of both anterior and posterior ocular segment inflammation, still nowadays struggle for effective drug delivery due to their poor solubilization capabilities in water. This research work aims to develop nanostructured lipid carriers (NLC) intended for periocular administration of dexamethasone acetate to the posterior segment of the eye. Pre-formulation studies were initially performed to find solid and liquid lipid mixtures for dexamethasone acetate solubilization. Pseudoternary diagrams at 65 °C were constructed to select the best surfactant based on the macroscopic transparency and microscopic isotropy of the systems. The resulting NLC, obtained following an organic solvent-free methodology, was composed of triacetin, Imwitor® 491 (glycerol monostearate >90%) and tyloxapol with Z-average = 106.9 ± 1.2 nm, PDI = 0.104 ± 0.019 and zeta potential = -6.51 ± 0.575 mV. Ex vivo porcine sclera and choroid permeation studies revealed a considerable metabolism in the sclera of dexamethasone acetate into free dexamethasone, which demonstrated higher permeation capabilities across both tissues. In addition, the NLC behavior once applied onto the sclera was further studied by means of multiphoton microscopy by loading the NLC with the fluorescent probe Nile red.

Ocular Delivery of Therapeutic Proteins: A Review

Therapeutic proteins, including monoclonal antibodies, single chain variable fragment (ScFv), crystallizable fragment (Fc), and fragment antigen binding (Fab), have accounted for one-third of all drugs on the world market. In particular, these medicines have been widely used in ocular therapies in the treatment of various diseases, such as age-related macular degeneration, corneal neovascularization, diabetic retinopathy, and retinal vein occlusion. However, the formulation of these biomacromolecules is challenging due to their high molecular weight, complex structure, instability, short half-life, enzymatic degradation, and immunogenicity, which leads to the failure of therapies. Various efforts have been made to overcome the ocular barriers, providing effective delivery of therapeutic proteins, such as altering the protein structure or including it in new delivery systems. These strategies are not only cost-effective and beneficial to patients but have also been shown to allow for fewer drug side effects. In this review, we discuss several factors that affect the design of formulations and the delivery of therapeutic proteins to ocular tissues, such as the use of injectable micro/nanocarriers, hydrogels, implants, iontophoresis, cell-based therapy, and combination techniques. In addition, other approaches are briefly discussed, related to the structural modification of these proteins, improving their bioavailability in the posterior segments of the eye without affecting their stability. Future research should be conducted toward the development of more effective, stable, noninvasive, and cost-effective formulations for the ocular delivery of therapeutic proteins. In addition, more insights into preclinical to clinical translation are needed.

Relationship of familial cytochrome P450 4V2 gene mutation with liver cirrhosis: A case report and review of the literature

BACKGROUND

Many genetic and metabolic diseases affect the liver, but diagnosis can be difficult because these diseases may have complex clinical manifestations and diverse clinical patterns. There is also incomplete clinical knowledge of these many different diseases and limitations of current testing methods.

CASE SUMMARY

We report a 53-year-old female from a rural area in China who was hospitalized for lower limb edema, abdominal distension, cirrhosis, and hypothyroidism. We excluded the common causes of liver disease (drinking alcohol, using traditional Chinese medicines, hepatitis virus infection, autoimmunity, and hepatolenticular degeneration). When she was 23-years-old, she developed night-blindness that worsened to complete blindness, with no obvious cause. Her parents were first cousins, and both were alive. Analysis of the patient's family history indicated that all 5 siblings had night blindness and impaired vision; one sister was completely blind; and another sister had night-blindness complicated with cirrhosis and subclinical hypothyroidism. Entire exome sequencing showed that the patient, parents, and siblings all had mutations in the cytochrome P450 4V2 gene (CYP4V2). The CYP4V2 mutations of the parents and two sisters were heterozygous, and the others were homozygous. Two siblings also had heterozygous dual oxidase activator 2 (DUOXA2) mutations.

CONCLUSION

Mutations in the CYP4V2 gene may affect lipid metabolism and lead to chronic liver injury, fibrosis, and cirrhosis.

Crinum jagus (J. Thomps. Dandy): Antioxidant and protective properties as a medicinal plant on toluene-induced oxidative stress damages in liver and kidney of rats

Crinum jagus (C. jagus; J. Thomps.) Dandy (Liliaceae) is a pantropical plant known for its medicinal values and pharmacological properties. The study assessed the protective effects and changes in oxidative stress indices due to C. jagus leaf extracts on the toluene-induced liver and kidney injuries in rats. The study was conducted on 8-week-old male Wistar rats (n = 80), weighing 243.3 ± 1.42 g. Group I, 1 ml/kg distilled water for 7 days; Group II, 4.5 ml/kg toluene once, 1 ml/kg distilled water for 7 days; Group III, 4.5 ml/kg toluene once, 500 mg/kg methanolic extract for 7 days; Group IV, 4.5 ml/kg toluene once, 500 mg/kg aqueous extract for 7 days; Group V, 500 mg/kg methanolic extract for 7 days; Group VI, 500 mg/kg aqueous extract for 7 days; Group VII, 500 mg/kg of vitamin C for 7 days; Group, VIII, 4.5 ml/kg toluene once, 500 mg/kg vitamin C for 7 days, all administrations were given by oral gavage. The phytochemical contents, absolute and relative organ weights of liver and kidneys, liver and kidney function tests, antioxidant status, as well as histological tests were analyzed using standard protocols. The tannins, flavonoids, and polyphenols were in highest concentration in both extracts, content in methanol extract (57.04 ± 1.51 mgg-1, 35.43 ± 1.03 mgg-1, 28.2 ± 0.34 mgg-1 respectively) > aqueous extract (18.74 ± 1.01 mgg-1, 13.43 ± 0.47 mgg-1, 19.65 ± 0.21 mgg-1 respectively). In the negative control group (II), bodyweights significantly (P < 0.05) reduced by 22%, liver weight and kidney weight significantly (P < 0.05) increased by 42% and 83% respectively, liver-to-bodyweight and kidney-to-bodyweight ratios increased significantly (P < 0.05); serum liver function tests (LFTs) i.e., bilirubin, alkaline phosphatase (ALP), Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), Gamma-glutamyl transferase (GGT), and serum kidney function tests (creatinine and urea) were significantly (P < 0.05) elevated; oxidant status (tissue malondialdehyde; MDA) was significantly (P < 0.05) elevated, antioxidant status i.e., tissue superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH) levels was significantly (P < 0.05) reduced; with markedly visible renal and hepatic histopathological findings, compared to the normal control group. In C. jagus extract test groups (III and IV), the parameters were significantly (P < 0.05) alleviated and reversed to normal/near normal compared to the negative control. The LFTs, kidney function tests, and antioxidant status were significantly (P < 0.05) more improved with the methanol extract test and standard control groups compared to the aqueous extract test group; Also, the methanol extract test group showed better histological features than the aqueous extract test and standard control groups. The methanolic extract shows better antioxidant potential due to the availability of more nonenzymatic antioxidants (tannins, flavonoids, and polyphenols). The findings showed that toluene is a very aggressive xenobiotic due to the promotion of oxidative stress and peroxidation of cellular lipids, but C. jagus leaves provide significant protection through the reducing power of nonenzymatic antioxidants and their ability to induce endogenous antioxidant enzymes (SOD, CAT, and glutathione reductase or GR) causing reduced cellular lipid peroxidation and tissue damages, quickened tissue repair, and improved cell biology of liver and kidneys during toluene toxicity. The methanol leaf extract provides better protection and should be advanced for more experimental and clinical studies to confirm its efficacy in alleviating oxidative stress tissue injuries, specifically due to toluene.

Human Orphan Cytochromes P450: An Update

Orphan cytochromes P450 (CYP) are enzymes whose biological functions and substrates are unknown. However, the use of new experimental strategies has allowed obtaining more information about their relevance in the metabolism of endogenous and exogenous compounds. Likewise, the modulation of their expression and activity has been associated with pathogenesis and prognosis in different diseases. In this work, we review the regulatory pathways and the possible role of orphan CYP to provide evidence that allow us to stop considering some of them as orphan enzymes and to propose them as possible therapeutic targets in the design of new strategies for the treatment of diseases associated with CYP-mediated metabolism.

Ocular metabolism and distribution of drugs in the rabbit eye: Quantitative assessment after intracameral and intravitreal administrations

Quantitation of ocular drug metabolism is important, but only sparse data is currently available. Herein, the pharmacokinetics of four drugs, substrates of metabolizing enzymes, was investigated in albino rabbit eyes after intracameral and intravitreal administrations. Acetaminophen, brimonidine, cefuroxime axetil, and sunitinib and their corresponding metabolites were quantitated in the cornea, iris-ciliary body, aqueous humor, lens, vitreous humor, and neural retina with LC-MS/MS analytics. Non-compartmental analysis was employed to estimate the pharmacokinetic parameters of the parent drugs and metabolites. The area under the curve (AUC) values of metabolites were 12-70 times lower than the AUC values of the parent drugs in the tissues with the highest enzymatic activity. The ester prodrug cefuroxime axetil was an exception because it was efficiently and quantitatively converted to cefuroxime in the ocular tissues. In contrast to the liver, sulfotransferases, aldehyde oxidase, and cytochrome P450 3A activities were low in the eye and they had negligible impact on ocular drug clearance. With the exception of esterase substrates, metabolism seems to be a minor player in ocular pharmacokinetics. However, metabolites might contribute to ocular toxicity, and drug metabolism in various eye tissues should be investigated and understood thoroughly.

Anterior pituitary, sex hormones, and keratoconus: Beyond traditional targets

"The Diseases of the Horny-coat of The Eye", known today as keratoconus, is a progressive, multifactorial, non-inflammatory ectatic corneal disorder that is characterized by steepening (bulging) and thinning of the cornea, irregular astigmatism, myopia, and scarring that can cause devastating vision loss. The significant socioeconomic impact of the disease is immeasurable, as patients with keratoconus can have difficulties securing certain jobs or even joining the military. Despite the introduction of corneal crosslinking and improvements in scleral contact lens designs, corneal transplants remain the main surgical intervention for treating keratoconus refractory to medical therapy and visual rehabilitation. To-date, the etiology and pathogenesis of keratoconus remains unclear. Research studies have increased exponentially over the years, highlighting the clinical significance and international interest in this disease. Hormonal imbalances have been linked to keratoconus, both clinically and experimentally, with both sexes affected. However, it is unclear how (molecular/cellular signaling) or when (age/disease stage(s)) those hormones affect the keratoconic cornea. Previous studies have categorized the human cornea as an extragonadal tissue, showing modulation of the gonadotropins, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Studies herein provide new data (both in vitro and in vivo) to further delineate the role of hormones/gonadotropins in the keratoconus pathobiology, and propose the existence of a new axis named the Hypothalamic-Pituitary-Adrenal-Corneal (HPAC) axis.

Hallmarks of lens aging and cataractogenesis

Lens homeostasis and transparency are dependent on the function and intercellular communication of its epithelia. While the lens epithelium is uniquely equipped with functional repair systems to withstand reactive oxygen species (ROS)-mediated oxidative insult, ROS are not necessarily detrimental to lens cells. Lens aging, and the onset of pathogenesis leading to cataract share an underlying theme; a progressive breakdown of oxidative stress repair systems driving a pro-oxidant shift in the intracellular environment, with cumulative ROS-induced damage to lens cell biomolecules leading to cellular dysfunction and pathology. Here we provide an overview of our current understanding of the sources and essential functions of lens ROS, antioxidative defenses, and changes in the major regulatory systems that serve to maintain the finely tuned balance of oxidative signaling vs. oxidative stress in lens cells. Age-related breakdown of these redox homeostasis systems in the lens leads to the onset of cataractogenesis. We propose eight candidate hallmarks that represent common denominators of aging and cataractogenesis in the mammalian lens: oxidative stress, altered cell signaling, loss of proteostasis, mitochondrial dysfunction, dysregulated ion homeostasis, cell senescence, genomic instability and intrinsic apoptotic cell death.

Comparative Proteomics Analysis of the Postmitochondrial Supernatant Fraction of Human Lens-Free Whole Eye and Liver

The increasing incidence of ocular diseases has accelerated research into therapeutic interventions needed for the eye. Ocular enzymes play important roles in the metabolism of drugs and endobiotics. Various ocular drugs are designed as prodrugs that are activated by ocular enzymes. Moreover, ocular enzymes have been implicated in the bioactivation of drugs to their toxic metabolites. The key purpose of this study was to compare global proteomes of the pooled samples of the eye (n = 11) and the liver (n = 50) with a detailed analysis of the abundance of enzymes involved in the metabolism of xenobiotics and endobiotics. We used the postmitochondrial supernatant fraction (S9 fraction) of the lens-free whole eye homogenate as a model to allow accurate comparison with the liver S9 fraction. A total of 269 proteins (including 23 metabolic enzymes) were detected exclusively in the pooled eye S9 against 648 proteins in the liver S9 (including 174 metabolic enzymes), whereas 424 proteins (including 94 metabolic enzymes) were detected in both the organs. The major hepatic cytochrome P450 and UDP-glucuronosyltransferases enzymes were not detected, but aldehyde dehydrogenases and glutathione transferases were the predominant proteins in the eye. The comparative qualitative and quantitative proteomics data in the eye versus liver is expected to help in explaining differential metabolic and physiologic activities in the eye. SIGNIFICANCE STATEMENT: Information on the enzymes involved in xenobiotic and endobiotic metabolism in the human eye in relation to the liver is scarcely available. The study employed global proteomic analysis to compare the proteomes of the lens-free whole eye and the liver with a detailed analysis of the enzymes involved in xenobiotic and endobiotic metabolism. These data will help in better understanding of the ocular metabolism and activation of drugs and endobiotics.

Sildenafil-evoked photoreceptor oxidative stress in vivo is unrelated to impaired visual performance in mice

Purpose

The phosphodiesterase inhibitor sildenafil is a promising treatment for neurodegenerative disease, but it can cause oxidative stress in photoreceptors ex vivo and degrade visual performance in humans. Here, we test the hypotheses that in wildtype mice sildenafil causes i) wide-spread photoreceptor oxidative stress in vivo that is linked with ii) impaired vision.

Methods

In dark or light-adapted C57BL/6 mice ± sildenafil treatment, the presence of oxidative stress was evaluated in retina laminae in vivo by QUEnch-assiSTed (QUEST) magnetic resonance imaging, in the subretinal space in vivo by QUEST optical coherence tomography, and in freshly excised retina by a dichlorofluorescein assay. Visual performance indices were also evaluated by QUEST optokinetic tracking.

Results

In light-adapted mice, 1 hr post-sildenafil administration, oxidative stress was most evident in the superior peripheral outer retina on both in vivo and ex vivo examinations; little evidence was noted for central retina oxidative stress in vivo and ex vivo. In dark-adapted mice 1 hr after sildenafil, no evidence for outer retina oxidative stress was found in vivo. Evidence for sildenafil-induced central retina rod cGMP accumulation was suggested as a panretinally thinner, dark-like subretinal space thickness in light-adapted mice at 1 hr but not 5 hr post-sildenafil. Cone-based visual performance was impaired by 5 hr post-sildenafil and not corrected with anti-oxidants; vision was normal at 1 hr and 24 hr post-sildenafil.

Conclusions

The sildenafil-induced spatiotemporal pattern of oxidative stress in photoreceptors dominated by rods was unrelated to impairment of cone-based visual performance in wildtype mice.

Peptidomimetics Therapeutics for Retinal Disease

Ocular disorders originating in the retina can result in a partial or total loss of vision, making drug delivery to the retina of vital importance. However, effectively delivering drugs to the retina remains a challenge for ophthalmologists due to various anatomical and physicochemical barriers in the eye. This review introduces diverse administration routes and the accordant pharmacokinetic profiles of ocular drugs to aid in the development of safe and efficient drug delivery systems to the retina with a focus on peptidomimetics as a growing class of retinal drugs, which have great therapeutic potential and a high degree of specificity. We also discuss the pharmacokinetic profiles of small molecule drugs due to their structural similarity to small peptidomimetics. Lastly, various formulation strategies are suggested to overcome pharmacokinetic hurdles such as solubility, retention time, enzymatic degradation, tissue targeting, and membrane permeability. This knowledge can be used to help design ocular delivery platforms for peptidomimetics, not only for the treatment of various retinal diseases, but also for the selection of potential peptidomimetic drug targets.

A review of nanocarrier-mediated drug delivery systems for posterior segment eye disease: challenges analysis and recent advances

Posterior segment eye disease is a leading cause of irreversible vision impairment and blindness. As the unique organ for vision, eyes are protected by various protective barriers. The existence of physiological barriers and elimination mechanisms makes it challenging to treat the posterior segment eye diseases. To achieve efficient drug delivery to the posterior segment of eyes, different drug delivery systems have been proposed. Due to their abilities to enhance ocular tissue permeability, make controlled drug release and target retina, nanocarriers, such as lipid nanoparticles, liposomes and polymeric nanomicelles, have been widely studied for posterior segment drug delivery. However, clinical applications of nanocarrier mediated drug delivery systems as non-invasive ocular drops is still not ready. The delivery of nanocarrier-mediated drug for posterior segment disease still faces the choice of being more effective or more invasive for long-term treatment. Therefore, it is necessary to have a clear understanding of the barriers and the routes of ocular drug delivery while developing the delivery systems. In this review, types of ocular barriers and drug administration routes are categorised in a more intuitive way. Recent advances in nanocarrier mediated drug delivery systems with focus on posterior segment are reviewed with illustrative examples.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

Efficacy of Sub-Tenon Micro-Perfusion of Cyclophosphamide in Rabbits with Severe Ocular Inflammation

Purpose

To explore the feasibility of cyclophosphamide (CP) via a sub-Tenon micro-perfusion system (SMS) in rabbits, and assess its therapeutic efficacy in severe ocular inflammation.

Materials and Methods

Distribution and pharmacokinetics of CP were evaluated in vivo, and the concentrations of CP in plasma, vitreous humor, and retina/choroid were quantitated by ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) at different time points. After induction of severe experimental uveitis, rabbits were divided into three groups (n=8 in each): the SMS group, subconjunctival injection (SI) group, and control group. Clinical inflammatory score was assessed in rabbits. Electroretinography and histopathology were performed on post-treatment day 8. Statistical analyses were performed using Mann–Whitney and Kruskal–Wallis tests. P-value less than 0.05 was considered significant.

Results

The concentrations of CP in vitreous humor and retina/choroid in the SMS group were significantly higher than that of the SI group at 3, 6, 10, and 24 hours (P<0.01), while plasmatic CP concentrations were comparable at all time points in the SMS group and SI group (P>0.05). The SMS group showed significantly less inflammation compared to the control group and SI group. Furthermore, the restoration of retinal structure and function were more obvious in the SMS group compared with conventional SI application.

Conclusion

Sub-Tenon micro-perfusion of CP exhibited satisfied therapeutic efficacy in rabbits with severe ocular inflammation and may provide a promising alternative for controlling ocular inflammatory disease and immune-mediated ocular diseases.

Understanding metabolism related differences in ocular efficacy of MGV354

MGV354 was being developed as a novel ocular therapy for lowering of intraocular pressure, a key modifiable risk factor for glaucoma. MGV354 is an activator of soluble guanylate cyclase, an enzyme known to be involved in the regulation of IOP. MGV354 has been shown to robustly lower IOP over 24 h after a single topical ocular drop in rabbit and monkey pharmacology models. However, MGV354 failed to produce similar results in patients with ocular hypertension or open-angle glaucoma. With an objective of explaining the lack of efficacy in the clinic, we attempted to study whether human metabolism was significantly different from animal metabolism. The present study documents the investigation of metabolism of MGV354 in an effort to understand potential differences in biotransformation pathways of MGV354 in rabbits, monkeys, and humans. Overall twenty-six metabolites, formed via oxidative and conjugative pathways, were identified in vitro and in vivo. In vitro hepatic metabolism was qualitatively similar across species, with minor but distinct differences. There were no observable interspecies differences in the hepatic and ocular metabolism of MGV354. Although ocular metabolism was not as extensive as hepatic, the results do not explain the lack of efficacy of MGV354 in clinical studies.

Cytochrome P450 4B1 (CYP4B1) as a target in cancer treatment

Cytochrome P450 4B1 (CYP4B1) plays crucial roles in biotransforming of xenobiotics. Its predominant extrahepatic expression has been associated with certain tissue-specific toxicities. However, the expressions of CYP4B1 in various cancers and hence their potential roles in cancer development were inclusive. In this work, existing knowledge on expression and regulation of CYP4B1 gene and protein, catalysis of CYP4B1, association of CYP4B1 with cancers, contradicting findings about human CYP4B1 activities as well as the employing CYP4B1 in suicide gene approach for cancer treatment were reviewed. To date, it appears that there is a wide spectrum of tissue distribution of CYP4B1 with lungs as the predominant sites. Several nuclear receptors are possibly responsible for regulating its gene expression. The involvement of CYP4B1 in cancer was considered via activation of procarcinogens and neovascularization. However, human CYP4B1 was found to be inactive due to a substitution of proline with serine at position 427. Suicide gene approach combining reengineered CYP4B1 and prodrug 4-ipomeanol (4-IPO) has shown a promising potential for targeted cancer therapy. Further studies should focus on the verification of human CYP4B1 catalytic activities. More compounds with similar structure as 4-IPO should be tested to identify more alternative agents for the suicide gene approach in cancer treatment.

Membrane transporter data to support kinetically-informed chemical risk assessment using non-animal methods: Scientific and regulatory perspectives

Humans are continuously exposed to low levels of thousands of industrial chemicals, most of which are poorly characterised in terms of their potential toxicity. The new paradigm in chemical risk assessment (CRA) aims to rely on animal-free testing, with kinetics being a key determinant of toxicity when moving from traditional animal studies to integrated in vitro-in silico approaches. In a kinetically informed CRA, membrane transporters, which have been intensively studied during drug development, are an essential piece of information. However, how existing knowledge on transporters gained in the drug field can be applied to CRA is not yet fully understood. This review outlines the opportunities, challenges and existing tools for investigating chemical-transporter interactions in kinetically informed CRA without animal studies. Various environmental chemicals acting as substrates, inhibitors or modulators of transporter activity or expression have been shown to impact TK, just as drugs do. However, because pollutant concentrations are often lower in humans than drugs and because exposure levels and internal chemical doses are not usually known in contrast to drugs, new approaches are required to translate transporter data and reasoning from the drug sector to CRA. Here, the generation of in vitro chemical-transporter interaction data and the development of transporter databases and classification systems trained on chemical datasets (and not only drugs) are proposed. Furtheremore, improving the use of human biomonitoring data to evaluate the in vitro-in silico transporter-related predicted values and developing means to assess uncertainties could also lead to increase confidence of scientists and regulators in animal-free CRA. Finally, a systematic characterisation of the transportome (quantitative monitoring of transporter abundance, activity and maintenance over time) would reinforce confidence in the use of experimental transporter/barrier systems as well as in established cell-based toxicological assays currently used for CRA.

D-cis-Diltiazem Can Produce Oxidative Stress in Healthy Depolarized Rods In Vivo

Purpose

New perspectives are needed to understand decades of contradictory reports on the neuroprotective effects of the Cav1.2 L-type calcium channel blocker d-cis-diltiazem in retinitis pigmentosa (RP) models. Here, we address, in vivo, the following two knowledge gaps regarding d-cis-diltiazem's actions in the murine outer retina: (1) do normal mouse rods contain d-cis-diltiazem-insensitive Cav1.2 L-type calcium channels? (2) Can d-cis-diltiazem modify the normal rod redox environment?

Methods

First, transretinal Cav1.2 L-type calcium channels were noninvasively mapped with manganese-enhanced magnetic resonance imaging (MRI) following agonist Bay K 8644 in C57BL/6 (B6) and in Cav1.2 L-type calcium channel BAY K 8644-insensitive mutant B6 mice. Second, d-cis-diltiazem-treated oxidative stress-vulnerable (B6) or -resistant [129S6 (S6)] mice were examined in vivo (QUEnch-assiSTed [QUEST] MRI) and in whole retina ex vivo (lucigenin). Retinal thickness was measured using MRI.

Results

The following results were observed: (1) manganese uptake patterns in BAY K 8644-treated controls and mutant mice identified in vivo Cav1.2 L-type calcium channels in inner and outer retina; and (2) d-cis-diltiazem induced rod oxidative stress in dark-adapted B6 mice but not in light-adapted B6 mice or dark-adapted S6 mice (QUEST MRI). Oxidative stress in vivo was limited to inferior outer retina in dark-adapted B6 mice approximately 1-hour post d-cis-diltiazem. By approximately 4 hours post, only superior outer retina oxidative stress was observed and whole retinal superoxide production was supernormal. All groups had unremarkable retinal thicknesses.

Conclusions

D-cis-diltiazem's unexpectedly complex spatiotemporal outer retinal oxidative stress pattern in vivo was dependent on genetic background and rod membrane depolarization, but not apparently dependent on Cav1.2 L-type calcium channels, providing a potential rationale for contradictory results in different RP models.

An Investigation into Retigabine (Ezogabine) Associated Dyspigmentation in Rat Eyes by MALDI Imaging Mass Spectrometry

Retigabine (RTG) is an antiepileptic drug approved as an adjunctive treatment for refractory partial-onset seizures in adults. In April 2013, the Food and Drug Administration issued a warning that RTG could cause changes in retinal pigmentation and discoloration of skin, resulting in a blue appearance. As part of a larger preclinical effort to gain a mechanistic understanding as to the origins of retinal pigment changes associated with RTG, we conducted a long-term repeat dosing study in rats. Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) was used to determine the distribution of RTG and its metabolites in the rat eye following 13 and 39 weeks of dosing. IMS revealed the presence of RTG, a previously characterized N-acetyl metabolite of RTG (NAMR), and several species structurally related through the dimerization of RTG and NAMR. These species were highly localized to the melanin-containing layers of the uveal tract of the rat eye including the choroid, ciliary body, and iris, suggesting that the formation of these dimers occurs from melanin bound RTG and NAMR. Furthermore, several of the RTG-related dimers have UV absorbance which give them a purple color in solution. We propose that the melanin binding of RTG and NAMR effectively concentrates the two compounds to enable mixed condensation reactions to occur when the binding provides the proper geometry in the redox environment of the uveal tissues. High lateral resolution images illustrate that the blood-retinal barrier effectively restricts retinal access to RTG-related compounds. The spatial information provided by MALDI IMS was critical in contextualizing the homogenate concentrations of key RTG-related compounds and helped provide a basis for the mechanism of dimer formation.

Differential Expression of Prostaglandin I2 Synthase Associated with Arachidonic Acid Pathway in the Oral Squamous Cell Carcinoma

Introduction

Differential expression of genes encoding cytochrome P450 (CYP) and other oxygenases enzymes involved in biotransformation mechanisms of endogenous and exogenous compounds can lead to oral tumor development.

Objective

We aimed to identify the expression profile of these genes, searching for susceptibility biomarkers in oral squamous cell carcinoma.

Patients and Methods

Sixteen oral squamous cell carcinoma samples were included in this study (eight tumor and eight adjacent non-tumor tissues). Gene expression quantification was performed using TaqMan Array Human CYP450 and other Oxygenases 96-well plate (Applied Biosystems) by real time qPCR. Protein quantification was performed by ELISA and IHC methods. Bioinformatics tools were used to find metabolic pathways related to the enzymes encoded by differentially expressed genes. Results. CYP27B1, CYP27A1, CYP2E1, CYP2R1, CYP2J2, CYP2U1, CYP4F12, CYP4X1, CYP4B1, PTGIS, ALOX12, and MAOB genes presented differential expression in the oral tumors. After correction by multiple tests, only the PTGIS (Prostaglandin I2 Synthase) gene presented significant differential expression (P < 0.05). The PTGIS gene and protein were reduced in oral tumors.

Conclusion

PTGIS presents downexpression in oral tumors. PTGIS play an important role in the arachidonic acid metabolism. Arachidonic acid and/or metabolites are derived from this pathway, which can influence the regulation of important physiological mechanisms in tumorigenesis process.

Models and Approaches Describing the Metabolism, Transport, and Toxicity of Drugs Administered by the Ocular Route

The eye is a complex organ with a series of anatomic barriers that provide protection from physical and chemical injury while maintaining homeostasis and function. The physiology of the eye is multifaceted, with dynamic flows and clearance mechanisms. This review highlights that in vitro ocular transport and metabolism models are confined by the availability of clinically relevant absorption, distribution, metabolism, and excretion (ADME) data. In vitro ocular transport models used for pharmacology and toxicity poorly predict ocular exposure. Although ocular cell lines cannot replicate in vivo conditions, these models can help rank-order new chemical entities in discovery. Historic ocular metabolism of small molecules was assumed to be inconsequential or assessed using authentic standards. While various in vitro models have been cited, no single system is perfect, and many must be used in combination. Several studies document the use of laboratory animals for the prediction of ocular pharmacokinetics in humans. This review focuses on the use of human-relevant and human-derived models which can be utilized in discovery and development to understand ocular disposition of new chemical entities. The benefits and caveats of each model are discussed. Furthermore, ADME case studies are summarized retrospectively and capture the ADME data collected for health authorities in the absence of definitive guidelines. Finally, we discuss the novel technologies and a hypothesis-driven ocular drug classification system to provide a holistic perspective on the ADME properties of drugs administered by the ocular route.

Characterization and Pharmacokinetics of Triamcinolone Acetonide-Loaded Liposomes Topical Formulations for Vitreoretinal Drug Delivery

PURPOSE

To achieve a safer alternative to intravitreal injection of corticosteroids, we developed and characterized triamcinolone acetonide-loaded liposomes formulations (TA-LFs) to be used topically for vitreoretinal drug delivery.

METHODS

Four different 0.2% TA-LFs (TA-LF1 to TA-LF4) were generated and submitted to physicochemical characterization. Posteriorly, an ex vivo diffusion assay was performed using rabbit corneas as membranes. Finally, concentrations of triamcinolone acetonide (TA) were determined by high-performance liquid chromatography in ocular tissues from New Zealand white rabbits after multiple topical doses of TA-LF2 (6 times per day, 14 days). In addition, toxicity and tolerability of TA-LF2 was evaluated by cell viability assay and eye examination of study animals, respectively.

RESULTS

TA-LF2 was the most stable formulation maintaining a stable hidrogenion potential (pH) at 30 and 40°C and even improving encapsulation with higher temperature. TA-LF2 and TA-LF3 presented the best diffusion performance in vitro reaching the highest TA concentrations after 8 h of follow-up. In vivo diffusion and pharmacokinetics analysis showed that concentrations of TA in retina and vitreous reached the highest peak at 12 h after topical administration of TA-LF2 (252.10 ± 90.00 ng/g and 32.6 ± 10.27 ng/g, respectively) and subsequently decline to 24.0 ± 11.72 ng/g and 19.5 ± 13.14 ng/g, respectively, at 14 days of follow-up. Finally, cell viability was unaffected by TA-LF2, and no increase in intraocular pressure nor ocular alterations were observed after topical administration of this formulation in rabbits.

CONCLUSION

TA-loaded liposomes, administered topically, can deliver TA in the vitreous cavity and reach the retina efficiently.

Biotransformation and bioactivation reactions - 2016 literature highlights

We are pleased to present a second annual issue highlighting a previous year's literature on biotransformation and bioactivation. Each contributor to this issue worked independently to review the articles published in 2016 and proposed three to four articles, which he or she believed would be of interest to the broader research community. In each synopsis, the contributing author summarized the procedures, analyses and conclusions as described in the original manuscripts. In the commentary sections, our authors offer feedback and highlight aspects of the work that may not be apparent from an initial reading of the article. To be fair, one should still read the original article to gain a more complete understanding of the work conducted. Most of the articles included in this review were published in Drug Metabolism and Disposition or Chemical Research in Toxicology, but attempts were made to seek articles in 25 other journals. Importantly, these articles are not intended to represent a consensus of the best papers of the year, as we did not want to make any arbitrary standards for this purpose, but rather they were chosen by each author for their notable findings and descriptions of novel metabolic pathways or biotransformations. I am pleased that Drs. Rietjens and Dalvie have again contributed to this annual review. We would like to welcome Grover P Miller as an author for this year's issue, and we thank Tom Baillie for his contributions to last year's edition. We have intentionally maintained a balance of authors such that two come from an academic setting and two come from industry. Finally, please drop us a note if you find this review helpful. We would be pleased to hear your opinions of our commentary, and we extend an invitation to anyone who would like to contribute to a future edition of this review. This article is dedicated to Professor Thomas Baillie for his exceptional contributions to the field of drug metabolism.

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.

Drug Transport by the Blood-Aqueous Humor Barrier of the Eye

The ocular barriers (cornea, blood-retinal barrier, and blood-aqueous humor barrier) make treating eye diseases with therapeutic drugs challenging. The tight capillary endothelium of the iris and the ciliary body epithelium form the blood-aqueous humor barrier. The iris and ciliary body (iris-ciliary body) express a variety of drug transporters in the ATP-binding cassette and solute carrier (SLC) families. ATP-binding cassette family drug transporters that are present in the iris-ciliary body include P-glycoprotein, breast cancer resistance protein, and several multidrug resistance-associated proteins. SLC family drug transporters that are present in the iris-ciliary body include organic anion transporters, organic anion transporting polypeptides, bile acid transporters (apical sodium-dependent bile salt transporter and sodium taurocholate cotransporter), organic cation transporters (novel organic cation transporter and multidrug and toxin extrusion transporter) and peptide transporters. Freshly dissected iris-ciliary body preparations actively accumulate a variety of substrates of SLC drug transporters that are expressed in the tissue. The ciliary body in vitro supports active transport in the aqueous humor-to-blood direction of several substrates of organic anion transporters and multidrug resistance-associated proteins, consistent with the subcellular localization of these transporters in the ciliary body epithelium. In vivo data suggest that drug transporters in the iris-ciliary body reduce the permeation of drugs in the direction of blood-to-aqueous humor, thereby reducing ocular drug bioavailability, and are also involved in active drug elimination from the aqueous humor. An understanding of the influence on pharmacokinetics of drug transporters in the blood-aqueous humor barrier should help improve drug delivery and efficacy in the eye.

The impact of Hydrogen peroxide on structure, stability and functional properties of Human R12C mutant αA-crystallin: The imperative insights into pathomechanism of the associated congenital cataract incidence

The oxidative stress in eye lens which occurs during inflammation and under chronic hyperglycemia has been already indicated in the pathogenesis of cataract disorders. The aim of this study was to examine structural and functional properties of R12C mutant αA-Crystallin (αA-Cry) in the presence of hydrogen peroxide. The study was done using different spectroscopic techniques and gel mobility shift assay. According to results of our study, H2O2 oxidation strongly compromises the chaperone function of the R12C mutant but not of wild-type αA-Cry. Also, it affects the structural properties of both wild-type and mutant proteins, albeit to different degree. The H2O2 exposure promotes extensive disulfide mediated oligomerization of the R12C mutant but not of the wild-type as revealed by gel mobility shift assay and dynamic light scattering. Moreover, in the presence of hydrogen peroxide, the mutant protein demonstrates severe conformational and protease instability and increased amyloidogenic propensity. The obtained results suggest that incubation of R12C mutant recombinant αA-Cry with hydrogen peroxide accelerates the molecular events which have been already implicated in the pathomechanism of cataract development. Taken together these results suggest that individuals carrying the R12C mutation are at an increased risk to develop early-onset cataract under condition of oxidative stress.