Vitreal kinetics of quinidine in rabbits in the presence of topically coadministered P-glycoprotein substrates/modulators

Protein and polypeptide mediated delivery to the eye

Hybrid or recombinant protein-polymers, peptide-based biomaterials, and antibody-targeted therapeutics are widely explored for various ocular conditions and vision correction. They have been noted for their potential biocompatibility, potency, adaptability, and opportunities for sustained drug delivery. Unique to peptide and protein therapeutics, their production by cellular translation allows their precise modification through genetic engineering. To a greater extent than drug delivery to other systems, delivery to the eye can benefit from the combination of locally-targeted administration and protein-based specificity. Consequently, a range of delivery platforms and administration methods have been exploited to address the ocular delivery of peptide and protein biomaterials. This review discusses a sample of preclinical and clinical opportunities for peptide-based drug delivery to the eye.

In Situ Gel of Triamcinolone Acetonide-Loaded Solid Lipid Nanoparticles for Improved Topical Ocular Delivery: Tear Kinetics and Ocular Disposition Studies

Triamcinolone acetonide (TA), an intermediate acting corticosteroid, is used in the treatment of posterior ocular diseases, such as inflammation, posterior uveitis, and diabetic macular edema. The objective of this investigation was to prepare TA-loaded solid lipid nanoparticles (TA-SLNs) and in situ gel (TA-SLN-IG) formulations for delivery into the deeper ocular tissues through the topical route. TA-SLNs were prepared by hot homogenization and ultrasonication method using glyceryl monostearate and Compritol® 888ATO as solid lipids and Tween®80 and Pluronic® F-68 as surfactants. TA-SLNs were optimized and converted to TA-SLN-IG by the inclusion of gellan gum and evaluated for their rheological properties. In vitro transcorneal permeability and in vivo ocular distribution of the TA-SLNs and TA-SLN-IG were studied using isolated rabbit corneas and New Zealand albino rabbits, respectively, and compared with TA suspension, used as control (TA-C). Particle size, PDI, zeta potential, assay, and entrapment efficiency of TA-SLNs were in the range of 200⁻350 nm, 0.3⁻0.45, -52.31 to -64.35 mV, 70⁻98%, and 97⁻99%, respectively. TA-SLN-IG with 0.3% gellan gum exhibited better rheological properties. The transcorneal permeability of TA-SLN and TA-SLN-IG was 10.2 and 9.3-folds higher compared to TA-C. TA-SLN-IG showed maximum tear concentration at 2 h, indicating an improved pre-corneal residence time, as well as higher concentrations in aqueous humor, vitreous humor and cornea at 6 h, suggesting sustained delivery of the drug into the anterior and posterior segment ocular tissues, when compared to TA-SLN and TA-C. The results, therefore, demonstrate that the lipid based nanoparticulate system combined with the in situ gelling agents can be a promising drug delivery platform for the deeper ocular tissues.

P-glycoprotein Restricts Ocular Penetration of Loperamide across the Blood-Ocular Barriers: a Comparative Study in Mdr1a Knock-out and Wild Type Sprague Dawley Rats

The current research was undertaken to determine the existence and magnitude of P-glycoprotein (P-gp) expression on the blood-ocular barriers by studying the ocular penetration of loperamide, a specific P-gp substrate, in P-gp (Mdr1a) knock-out (KO) and wild type (WT) Sprague Dawley rats. A clear, stable, sterile solution of loperamide (1 mg/mL), for intravenous administration, was formulated and evaluated. Ocular distribution was studied in P-gp KO and WT rats following intravenous administration of loperamide (at two doses). The drug levels in plasma, aqueous humor (AH), and vitreous humor (VH) samples were determined with the aid of UHPLC-Q-TOF-MS/MS, and the AH/plasma (D _AH ) and VH/plasma (D _VH ) distribution ratios were estimated. Electroretinography (ERG), ultrastructural analyses, and histology studies were carried out, in both KO and WT rats, to detect any drug-induced functional and/or structural alterations in the retina. Dose-related loperamide levels were observed in the plasma of both WT and KO rats. The loperamide concentrations in the AH and VH of KO rats were significantly higher compared to that observed in the WT rats, at the lower dose. However, a marked increase in the D _AH and D _VH was noted in the KO rats. ERG, ultrastructure, and histology studies did not indicate any drug-induced toxic effects in the retina under the test conditions. The results from these studies demonstrate that P-gp blocks the penetration of loperamide into the ocular tissues from the systemic circulation and that the effect is more pronounced at lower plasma loperamide concentrations.

Retina Compatible Interactions and Effective Modulation of Blood Ocular Barrier P-gp Activity by Third-Generation Inhibitors Improve the Ocular Penetration of Loperamide

Effective drug delivery to the deeper ocular tissues remains an unresolved conundrum mainly due to the expression of multidrug resistance efflux proteins, besides tight junction proteins, in the blood ocular barriers (BOBs). Hence, the purpose of the current research was to investigate the ability of the third-generation efflux protein inhibitors, elacridar (EQ), and tariquidar (TQ), to diminish P-glycoprotein (P-gp) mediated efflux transport of loperamide (LOP), a P-gp substrate, across the BOB in Sprague Dawley rats. Initially, Western blot analysis confirmed the expression of P-gp in the iris-ciliary bodies and the retina choroid in the wild type rats. Next, the ocular distribution of LOP, in the presence and absence of EQ/TQ (at 2 doses), was evaluated. The significantly higher aqueous humor/plasma (D_AH) and vitreous humor (VH)/plasma (D_VH) distribution ratios of LOP in the rats pretreated with EQ or TQ demonstrated effective inhibition of P-gp activity in the BOB. Interestingly, the modulation of P-gp activity by EQ/TQ was more pronounced at the lower dose. The normal functioning and architecture of the retina, as indicated by electroretinography studies, confirmed the cytocompatibility of LOP and EQ/TQ interactions at the doses tested.

Vitreous humor analysis for the detection of xenobiotics in forensic toxicology: a review

Vitreous humor (VH) is a gelatinous substance contained in the posterior chamber of the eye, playing a mechanical role in the eyeball. It has been the subject of numerous studies in various forensic applications, primarily for the assessment of postmortem interval and for postmortem chemical analysis. Since most of the xenobiotics present in the bloodstream are detected in VH after crossing the selective blood-retinal barrier, VH is an alternative matrix useful for forensic toxicology. VH analysis offers particular advantages over other biological matrices: it is less prone to postmortem redistribution, is easy to collect, has relatively few interfering compounds for the analytical process, and shows sample stability over time after death. The present study is an overview of VH physiology, drug transport and elimination. Collection, storage, analytical techniques and interpretation of results from qualitative and quantitative points of view are dealt with. The distribution of xenobiotics in VH samples is thus discussed and illustrated by a table reporting the concentrations of 106 drugs from more than 300 case reports. For this purpose, a survey was conducted of publications found in the MEDLINE database from 1969 through April 30, 2015.

Utility of transporter/receptor(s) in drug delivery to the eye

The eye is a highly protected organ, and designing an effective therapy is often considered a challenging task. The anatomical and physiological barriers result in low ocular bioavailability of drugs. Due to these constraints, less than 5% of the administered dose is absorbed from the conventional ophthalmic dosage forms. Further, physicochemical properties such as lipophilicity, molecular weight and charge modulate the permeability of drug molecules. Vision-threatening diseases such as glaucoma, diabetic macular edema, cataract, wet and dry age-related macular degeneration, proliferative vitreoretinopathy, uveitis, and cytomegalovirus retinitis alter the pathophysiological and molecular mechanisms. Understanding these mechanisms may result in the development of novel treatment modalities. Recently, transporter/receptor targeted prodrug approach has generated significant interest in ocular drug delivery. These transporters and receptors are involved in the transport of essential nutrients, vitamins, and xenobiotics across biological membranes. Several influx transporters (peptides, amino acids, glucose, lactate and nucleosides/nucleobases) and receptors (folate and biotin) have been identified on conjunctiva, cornea, and retina. Structural and functional delineation of these transporters will enable more drugs targeting the posterior segment to be successfully delivered topically. Prodrug derivatization targeting transporters and receptors expressed on ocular tissues has been the subject of intense research. Several prodrugs have been designed to target these transporters and enhance the absorption of poorly permeating parent drug. Moreover, this approach might be used in gene delivery to modify cellular function and membrane receptors. This review provides comprehensive information on ocular drug delivery, with special emphasis on the use of transporters and receptors to improve drug bioavailability.

Evaluation of the intravenous and topical routes for ocular delivery of hesperidin and hesperetin

PURPOSE

The objective of this study was to determine the ocular bioavailability of hesperidin and hesperetin, especially with respect to their distribution into the posterior segment of the eye, following systemic and topical administration in rabbits.

METHODS

Hesperidin and hesperetin were administered either intravenously or topically to male New Zealand white (NZW) rabbits. Vitreous humor and plasma samples were collected after intravenous administration and analyzed to estimate the concentrations of the parent compounds and their metabolites. Ocular tissue concentrations, obtained on topical administration of hesperidin and hesperetin, were also determined.

RESULTS

In the systemic circulation, hesperidin and hesperetin were rapidly metabolized into their glucuronides, which are extremely hydrophilic in nature. Vitreal samples did not demonstrate any detectable levels of hesperidin/hesperetin following intravenous administration. Topical administration produced significant concentrations of hesperidin/hesperetin in all the ocular tissues tested at the 1 and 3 hours time points postdosing, with hesperetin showing higher levels compared to hesperidin. However, only low levels were generated in the vitreous humor. Inclusion of a penetration enhancer, benzalkonium chloride (BAK), improved the back-of-the-eye hesperetin levels.

CONCLUSIONS

Ocular delivery of hesperidin/hesperetin via the systemic route does not seem to be feasible considering the rapid generation of the hydrophilic metabolites. Topical application appears to be more promising and needs to be further developed/refined.

Intravitreal kinetics of hesperidin, hesperetin, and hesperidin G: effect of dose and physicochemical properties

Hesperidin, a flavanone glycoside, and its aglycone hesperetin are potential candidates for the treatment of diabetic retinopathy and macular edema. The objective of this study was to delineate vitreal pharmacokinetics of hesperidin and hesperetin and the hydrophilic derivative glucosyl hesperidin (hesperidin G) following intravitreal administration in anaesthetized rabbits. Concentration changes in vitreous humor were monitored using microdialysis sampling procedure. All three molecules were administered intravitreally at three dose levels (50 µL injection volume containing 1.5, 4.5, and 15 µg of the drug, resulting in a final vitreal concentration of 1, 3, and 10 µg/mL). Vitreal microdialysis samples were collected every 20 min over a period of 10 h. All three molecules exhibited linear pharmacokinetics within the dose range tested because area under the curve and maximum concentration (C(max) ) increased linearly with increasing dose and a significant difference in the elimination parameters such as clearance or half-life was not observed. The vitreal elimination half-life of these three compounds was observed to correlate with the molecular weight and lipophilicity of the molecules. The findings from this study provide practical information that will be useful in the future design of ocular drug delivery strategies for bioflavonoids.

Barrier analysis of periocular drug delivery to the posterior segment

Periocular administration is a potential way of delivering drugs to their targets in posterior eye segment (vitreous, neural retina, retinal pigment epithelium (RPE), choroid). Purpose of this study was to evaluate the role of the barriers in periocular drug delivery. Permeation of FITC-dextrans and oligonucleotides in the bovine sclera was assessed with and without Pluronic gel in the donor compartment. Computational model for subconjunctival drug delivery to the choroid and neural retina/vitreous was built based on clearance concept. Kinetic parameters for small hydrophilic and lipophilic drug molecules, and a macromolecule were obtained from published ex vivo and in vivo animal experiments. High negative charge field of oligonucleotides slows down their permeation in the sclera. Pluronic does not provide adequate rate control to modify posterior segment drug delivery. Theoretical calculations for subconjunctival drug administration indicated that local clearance by the blood flow and lymphatics removes most of the drug dose which is in accordance with experimental results. Calculations suggested that choroidal blood flow removes most of the drug that has reached the choroid, but this requires experimental verification. Calculations at steady state using the same subconconjunctival input rate showed that the choroidal and vitreal concentrations of the macromolecule is 2-3 orders of magnitude higher than that of small molecules. The evaluation of the roles of the barriers augments to design new drug delivery strategies for posterior segment of the eye.

Interaction between topically and systemically coadministered P-glycoprotein substrates/inhibitors: effect on vitreal kinetics

The objective of the present study was to investigate the effect of topically coadministered P-glycoprotein (P-gp) substrates/inhibitors on the vitreal kinetics of a systemically administered P-gp substrate. Anesthetized male rabbits were used in these studies. The concentration-time profile of quinidine in the vitreous humor, after intravenous administration, was determined alone and in the presence of topically coadministered verapamil, prednisolone sodium phosphate (PP), and erythromycin. The vitreal pharmacokinetic parameters of quinidine in the presence of verapamil [apparent elimination rate constant (λ(z)), 0.0027 ± 0.0002 min(-1); clearance (CL_F), 131 ± 21 ml/min; area under the curve (AUC(0-∞)), 39 ± 7.0 μg · min/ml; and mean residence time, 435 ± 20 min] were significantly different from those of the control (0.0058 ± 0.0006 min(-1), 296 ± 46 ml/min, 17 ± 3 μg · min/ml, and 232 ± 20 min, respectively). A 1.7-fold decrease in the vitreal λ(z) and a 1.5-fold increase in the vitreal AUC of quinidine were observed in the presence of topical PP. Statistically significant differences between the vitreal profiles of the control and erythromycin-treated group were also observed. Plasma concentration-time profiles of quinidine, alone or in the presence of the topically instilled compounds, remained unchanged, indicating uniform systemic quinidine exposure across groups. This study demonstrates an interaction between topically and systemically coadministered P-gp substrates, probably through the modulation of P-gp on the basolateral membrane of the retinal pigmented epithelium, leading to changes in the vitreal kinetics of the systemically administered agent.