Rabbit as an animal model for intravitreal pharmacokinetics : Clinical predictability and quality of the published data

Corrigendum: Experimental Eye Research 169 (2018) 60 doi: 10.1016/j.exer.2018.01.006 WOS: 000430158400008Intravitreal administration is the method of choice in drug delivery to the retina and/or choroid. Rabbit is the most commonly used animal species in intravitreal pharmacokinetics, but it has been criticized as being a poor model of human eye. The critique is based on some anatomical differences, properties of the vitreous humor, and observed differences in drug concentrations in the anterior chamber after intravitreal injections. We have systematically analyzed all published information on intravitreal pharmacokinetics in the rabbit and human eye. The analysis revealed major problems in the design of the pharmacokinetic studies. In this review we provide advice for study design. Overall, the pharmacokinetic parameters (clearance, volume of distribution, half-life) in the human and rabbit eye have good correlation and comparable absolute values. Therefore, reliable rabbit-to-man translation of intravitreal pharmacokinetics should be feasible. The relevant anatomical and physiological parameters in rabbit and man show only small differences. Furthermore, the claimed discrepancy between drug concentrations in the human and rabbit aqueous humor is not supported by the data analysis. Based on the available and properly conducted pharmacokinetic studies, the differences in the vitreous structure in rabbits and human patients do not lead to significant pharmacokinetic differences. This review is the first step towards inter-species translation of intravitreal pharmacokinetics. More information is still needed to dissect the roles of drug delivery systems, disease states, age and ocular manipulation on the intravitreal pharmacokinetics in rabbit and man. Anyway, the published data and the derived pharmacokinetic parameters indicate that the rabbit is a useful animal model in intravitreal pharmacokinetics. (C) 2015 The Authors. Published by Elsevier Ltd.Peer reviewe

Mechanisms of cellular retention of melanin bound drugs : Experiments and computational modeling

Melanin binding of drugs is known to increase drug concentrations and retention in pigmented eye tissues. Even though the correlation between melanin binding in vitro and exposure to pigmented eye in vivo has been shown, there is a discrepancy between rapid drug release from melanin particles in vitro and the long in vivo retention in the pigmented tissues. We investigated mechanisms and kinetics of pigment-related drug retention experimentally using isolated melanin particles from porcine retinal pigment epithelium and choroid, isolated porcine eye melanosomes, and re-pigmented ARPE-19 cells in a dynamic flow system. The experimental studies were supplemented with kinetic simulations. Affinity and capacity of levofloxacin, terazosin, papaverine, and timolol binding to melanin revealed Kd values of asymptotic to 50-150 mu M and B-max asymptotic to 40-112 nmol.mg(-1). The drugs were released from melanin in < 1 h (timolol) or in 6-12 h (other drugs). The drugs were released slower from the melanosomes than from melanin; the experimental differences ranged from 1.2-fold (papaverine) to 7.4-fold (timolol). Kinetic simulations supported the role of the melanosomal membrane in slowing down the release of melanin binders. In release studies from the pigmented ARPE-19 cells, drugs were released from the cellular melanin to the extra -cellular space in asymptotic to 1 day (timolol) and asymptotic to 11 days (levofloxacin), i.e., much slower than the release from melanin or melanosomes. Simulations of drug release from pigmented cells in the flow system matched the experimental data and enabled further sensitivity analyses. The simulations demonstrated a significant prolongation of drug retention in the cells as a function of decreasing drug permeability in the melanosomal membranes and increasing melanin content in the cells. Overall, we report the impact of cellular factors in prolonging drug retention and release from melanin-containing cells. These data and simulations will facilitate the design of melanin binding drugs with prolonged ocular actions.Peer reviewe

Corneal and conjunctival drug permeability: Systematic comparison and pharmacokinetic impact in the eye

On the surface of the eye, both the cornea and conjunctiva are restricting ocular absorption of topically applied drugs, but barrier contributions of these two membranes have not been systemically compared. Herein, we studied permeability of 32 small molecular drug compounds across an isolated porcine cornea and built a quantitative structure-property relationship (QSPR) model for the permeability. Corneal drug permeability (data obtained for 25 drug molecules) showed a 52-fold range in permeability (0.09-4.70x10(-6) cm/s) and the most important molecular descriptors in predicting the permeability were hydrogen bond donor, polar surface area and halogen ratio. Corneal permeability values were compared to their conjunctival drug permeability values. Ocular drug bioavailability and systemic absorption via conjunctiva were predicted for this drug set with pharmacokinetic calculations. Drug bioavailability in the aqueous humour was simulated to be <5% and trans-conjunctival systemic absorption was 34-79% of the dose. Loss of drug across the conjunctiva to the blood circulation restricts significantly ocular drug bioavailability and, therefore, ocular absorption does not increase proportionally with the increasing corneal drug permeability.Peer reviewe

Role of retinal pigment epithelium permeability in drug transfer between posterior eye segment and systemic blood circulation

Retinal pigment epithelium (RPE) is a major part of blood-retinal barrier that affects drug elimination from the vitreous to the blood and drug distribution from blood circulation into the eye. Even though drug clearance from the vitreous has been well studied, the role of RPE in the process has not been quantified. The aim of this work was to study the role of RPE clearance (CLRpE) as part of drug elimination from the vitreous and ocular drug distribution from the systemic blood circulation. We determined the bidirectional permeability of eight small molecular weight drugs and bevacizumab antibody across isolated bovine RPE-choroid. Permeability of small molecules was 10(-6) -10(-5)cm/s showing 13-15 fold range of outward and inward permeation, while permeability of bevacizumab was lower by 2-3 orders of magnitude. Most small molecular weight drugs showed comparable outward (vitreous-to-choroid) and inward (choroid-to-vitreous) permeability across the RPEchoroid, except ciprofloxacin and ketorolac that had an over 6 and 14-fold higher outward than inward permeability, respectively, possibly indicating active transport, Six of seven tested small molecular weight drugs had outward CLRPE values that were comparable with their intravitreal clearance (CLIvr) values (0.84-2.6 fold difference). On the contrary, bevacizumab had an outward CLRPE that was only 3.5% of the CLIvt, proving that its main route of elimination (after intravitreal injection) is not RPE permeation. Experimental values were used in pharmacokinetic simulations to assess the role of the RPE in drug transfer from the systemic blood circulation to the vitreous (CLBv). We conclude that for small molecular weight drugs the RPE is an important route in drug transfer between the vitreal cavity and blood, whereas it effectively hinders the movement of bevacizumab from the vitreous to the systemic circulation.Peer reviewe

Liposomal sunitinib for ocular drug delivery : A potential treatment for choroidal neovascularization

Choroidal neovascularization (CNV) is a prevalent vision-threatening vascular disorder in aging population. CNV is associated with several diseases in the posterior segment of the eye such as age-related macular degeneration (AMD). In this study we developed sunitinib-loaded liposomes to block the neovascularization signalling pathway through inhibition of tyrosine kinase of vascular endothelial growth factor receptors (VEGFRs). Liposomal sunitinib formulations were prepared by thin film hydration method and studied for their encapsulation efficiency (EE), loading capacity (LC) and drug release profile in buffer andvitreous. Our finding showed that the liposomes (mean size 104 nm) could effectively entrap sunitinib (EE approximate to 95%) at relatively high loading capacity (LC approximate to 5%) and release sunitinib over at least 3 days. Intravitreal sunitinib-loaded liposomes revealed inhibitory effect on established neovascularization in laser-induced CNV mouse model while the intravitreal injection of sunitinib solubilized with cyclodextrin was inefficient in management of neovascularization. Accordingly, liposomal sunitinib is a promising drug delivery system that should be further studied to inhibit the CNV related to AMD.Peer reviewe

Applying Linear and Non-Linear Methods for Parallel Prediction of Volume of Distribution and Fraction of Unbound Drug

Volume of distribution and fraction unbound are two key parameters in pharmacokinetics. The fraction unbound describes the portion of free drug in plasma that may extravasate, while volume of distribution describes the tissue access and binding of a drug. Reliable in silicopredictions of these pharmacokinetic parameters would benefit the early stages of drug discovery, as experimental measuring is not feasible for screening purposes. We have applied linear and nonlinear multivariate approaches to predict these parameters: linear partial least square regression and non-linear recursive partitioning classification. The volume of distribution and fraction of unbound drug in plasma are predicted in parallel within the model, since the two are expected to be affected by similar physicochemical drug properties. Predictive models for both parameters were built and the performance of the linear models compared to models included in the commercial software Volsurf+. Our models performed better in predicting the unbound fraction (Q2 0.54 for test set compared to 0.38 with Volsurf+ model), but prediction accuracy of the volume of distribution was comparable to the Volsurf+ model (Q2 of 0.70 for test set compared to 0.71 with Volsurf+ model). The nonlinear classification models were able to identify compounds with a high or low volume of distribution (sensitivity 0.81 and 0.71, respectively, for test set), while classification of fraction unbound was less successful. The interrelationship between the volume of distribution and fraction unbound is investigated and described in terms of physicochemical descriptors. Lipophilicity and solubility descriptors were found to have a high influence on both volume of distribution and fraction unbound, but with an inverse relationship.Peer reviewe

Understanding dexamethasone kinetics in the rabbit tear fluid : Drug release and clearance from solution, suspension and hydrogel formulations

Rapid precorneal loss of topically applied eye drops limits ocular drug absorption. Controlling release and precorneal residence properties of topical formulations may improve ocular drug bioavailability and duration of action. In this study, we evaluated in vivo ocular pharmacokinetics of dexamethasone in rabbits after application of a drug solution (0.01%), suspension (Maxidex (R) 0.1%), and hydrogels of 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AAc) copolymers. The rabbits received a single eyedrop (solution or suspension) or dexamethasone-loaded hydrogel topically. Dexamethasone in tear fluid was sampled with glass capillaries and quantitated by LC-MS/MS. Higher dexamethasone exposure (AUC) in the tear fluid was observed with the suspension (approximate to 3.6-fold) and hydrogel (12.8-fold) as compared to the solution. During initial 15 min postapplication, the highest AUC of dissolved dexamethasone was seen after hydrogel application (368 min*mu g/ mL) followed by suspension (109.9 min*mu g/mL) and solution (28.7 min*mu g/mL. Based on kinetic simulations, dexamethasone release from hydrogels in vivo and in vitro is comparable. Our data indicate that prolonged exposure of absorbable dexamethasone in tear fluid is reached with hydrogels and suspensions. Pharmacokinetic understanding of formulation behavior in the lacrimal fluid helps in the design of dexamethasone delivery systems with improved ocular absorption and prolonged duration of action.Peer reviewe

Distribution of Small Molecular Weight Drugs into the Porcine Lens : Studies on Imaging Mass Spectrometry, Partition Coefficients, and Implications in Ocular Pharmacokinetics

Lens is the avascular tissue in the eye between the aqueous humor and vitreous. Drug binding to the lens might affect ocular pharmacokinetics, and the binding may also have a pharmacological role in drug-induced cataract and cataract treatment. Drug distribution in the lens has been studied in vitro with many compounds; however, the experimental methods vary, no detailed information on distribution between the lens sublayers exist, and the partition coefficients are reported rarely. Therefore, our objectives were to clarify drug localization in the lens layers and establish partition coefficients for a wide range of molecules. Furthermore, we aimed to illustrate the effect of lenticular drug binding on overall ocular drug pharmacokinetics. We studied the distribution of 16 drugs and three fluorescent dyes in whole porcine lenses in vitro with imaging mass spectrometry and fluorescence microscopy techniques. Furthermore, we determined lens/buffer partition coefficients with the same experimental setup for 28 drugs with mass spectrometry. Finally, the effect of lenticular binding of drugs on aqueous humor drug exposure was explored with pharmacokinetic simulations. After 4 h, the drugs and the dyes distributed only to the outermost lens layers (capsule and cortex). The lens/buffer partition coefficients for the drugs were low, ranging from 0.05 to 0.8. On the basis of the pharmacokinetic simulations, a high lens-aqueous humor partition coefficient increases drug exposure in the lens but does not significantly alter the pharmacokinetics in the aqueous humor. To conclude, the lens seems to act mainly as a physical barrier for drug distribution in the eye, and drug binding to the lens affects mainly the drug pharmacokinetics in the lens.Peer reviewe

Ocular intracameral pharmacokinetics for a cocktail of timolol, betaxolol and atenolol in rabbits

The mechanisms of drug clearance from the aqueous humor are poorly defined. In this study, a cocktail approach was used to simultaneously determine the pharmacokinetics of three β-blocker agents after intracameral (ic) injection into the rabbit eyes. Aqueous humor samples were collected and analyzed using LC–MS/MS to determine drug concentrations. Pharmacokinetic parameters were obtained using a compartmental fitting approach, and the estimated clearance, volume of distribution, and half-life values were the following: atenolol (6.44 μL/min, 687 μL, and 73.87 min), timolol (19.30 μL/min, 937 μL, and 33.64 min), and betaxolol (32.20 μL/min, 1421 μL, and 30.58 min). Increased compound lipophilicity (atenolol < timolol < betaxolol) resulted in higher clearance and volume of distributions in the aqueous humor. Clearance of timolol and betaxolol is about 10 times higher than the aqueous humor outflow, demonstrating the importance of other elimination routes (e.g., uptake to iris and ciliary body and subsequent elimination via blood flow)

Topical ocular pharmacokinetics and bioavailability for a cocktail of atenolol, timolol and betaxolol in rabbits

Ocular bioavailability after eye drops administration is an important, but rarely determined, pharmacokinetic parameter. In this study, we measured the pharmacokinetics of a cocktail of three beta blockers after their topical administration into the albino rabbit eye. Samples from aqueous humour were analysed with LC-MS/MS. The pharmacokinetic parameters were estimated using compartmental and non-compartmental analyses. The ocular bioavailability was covering broad range of values: atenolol (0.07 %), timolol (1.22%, 1.51%) and betaxolol (3.82%, 4.31%). Absolute ocular bioavailability presented a positive trend with lipophilicity and the values showed approximately 60-fold range. The generated data enhances our understanding for ocular pharmacokinetics of drugs and may be utilized in pharmacokinetic model building in ophthalmic drug development.Peer reviewe