Topical application of 0.005% latanoprost increases episcleral venous pressure in normal dogs.

INTRODUCTION: Episcleral venous pressure (EVP) has an important role in intraocular pressure (IOP) homeostasis and accounts for more than 70% of the IOP in the normal dog. A frequently used species in glaucoma research is the normotensive dog especially when evaluating the efficacy of prostaglandin analogues and prostamides; however, aqueous humor dynamic studies in normal dogs are lacking, and the effect of 0.005% latanoprost on canine EVP is not known. We sought to determine the effects to the EVP of topically applied 0.005% latanoprost in the normotensive beagle dog. METHODS: Female beagle dogs (n = 14) were used and each had a normal ophthalmic examination on study entry. EVP was determined using a standard episcleral venomanometer. Animals were dosed in one eye with 0.005% latanoprost, and the effects on EVP were compared with the averaged baseline EVP's determined in the predosing phase and the fellow nondosed eye. The Mixed Model Repeated Measures method was used to analyze the EVP data. RESULTS: During the dosing phase of the study with topical 0.005% latanoprost, the mean EVPs of dosed eyes were significantly higher than that of nondosed eyes (P < 0.0001). CONCLUSIONS: The increase in EVP in the dog with exposure to topical 0.005% latanoprost has not been observed in other species that have been studied, such as in the mouse and in humans, where the drug had no significant effect on the EVP. This response may be unique to dogs and suggests that dogs may not fully mimic human aqueous humor dynamics with topical 0.005% latanoprost. Although frequently performed in human studies, EVP should not be regarded to be a constant value in aqueous humor dynamic studies in the normal beagle dog.

Effects of vitreous liquefaction on the intravitreal distribution of sodium fluorescein, fluorescein dextran, and fluorescent microparticles.

PURPOSE: The effects of vitreous liquefaction in the elderly on the distribution of drugs from intravitreal injections, depots, or devices remains unclear. The purpose of the present study was to develop a liquefied vitreous model that simulates the aged condition, to enable the study of clinically relevant drug distribution. METHODS: Dutch-belted rabbits were used to develop a study model using hyaluronidase as a vitreolytic agent. The effects of experimental vitreous liquefaction were investigated on intravitreal sodium fluorescein, fluorescein isothiocyanate-dextran (MW 150 kDa), and a suspension of 1-µm fluorescent particles. The distribution of these model compounds was monitored by retinal angiography with a confocal laser scanning system and ocular fluorophotometer. RESULTS: Hyaluronidase-treated vitreous humor (n = 6) was found to decrease the gel phase to 41% ± 9% (wt/wt; mean ± SD) compared with 81% ± 9% in the control eyes (n = 8; P < 0.05). The distribution of sodium fluorescein and fluorescein isothiocyanate dextran was greater in the liquefied vitreous than in the control. In comparison to the normal vitreous, fluorescent particles sedimented faster in the liquefied vitreous, and the distribution was more dispersed and scattered. CONCLUSIONS: A model of vitreous liquefaction in rabbits was successfully generated using intravitreal hyaluronidase. Small and large fluorescent molecules as well as particulates were distributed faster in liquefied vitreous than in the control. The results suggest enhanced convective flow and subsequent faster clearance in liquefied vitreous.

Vitreous VEGF clearance is increased after vitrectomy.

Purpose. Pars plana vitrectomy (PPV) has been reported to reduce macular thickness and improve visual acuity in patients with diabetic macular edema (ME). The hypothesis for the study was that after PPV, clearance is accelerated and VEGF concentrations are reduced. To test this hypothesis, hVEGF(165) injections were performed in rabbit eyes, with and without PPV, and vitreous VEGF levels were measured as a function of time. Methods. The PPV group rabbits had a bilateral 25-gauge PPV, and in the no-PPV group, rabbits had intact vitreous. Intravitreal injections of hVEGF(165) were performed, and the animals were euthanatized at time points up to 7 days. The vitreous was isolated and an enzyme-linked immunosorbent assay was used to measure the VEGF levels. Pharmacokinetic parameters were determined in a noncompartmental analysis approach. Results. Mean vitreous VEGF levels decreased more rapidly in eyes subjected to PPV than in no-PPV eyes. The vitreous VEGF half-life (t([)(1/2)(])) in PPV eyes was 10 times shorter than that in normal eyes. In addition, mean clearance and mean area under the curve (AUC) increased and decreased, respectively, in eyes that underwent PPV. Conclusions. VEGF clearance is increased after PPV. Reducing VEGF concentrations in the vitreous post-PPV may partially explain the improvement in macular thickness in some patients with ME. Unexpectedly, the half-life of VEGF in the vitreous, even in no-PPV eyes, was <3 hours, whereas compounds of similar molecular weight typically have longer vitreous half-lives. The back of the eye may be uniquely adapted with rapid-clearance mechanisms to regulate vitreous VEGF levels. Further study is suggested.

Recent advances in drug delivery systems for treating ocular complications of systemic diseases.

PURPOSE OF REVIEW: To examine recent advances in the development of pharmacological agents and drug delivery systems for the treatment of ocular conditions associated with systemic diseases including diabetic retinopathy, retinal vein occlusion, uveitis, and HIV-related retinitis. RECENT FINDINGS: Corticosteroids, vascular endothelial-derived growth factor antagonists, and anti-inflammatory agents have been investigated for treating ocular conditions associated with systemic diseases. Systemic pharmacotherapy for specifically treating eye diseases is discouraged as side effects may exacerbate preexisting conditions in patients with a debilitating systemic disease. Local therapy with injections into the vitreous has demonstrated varying degrees of efficacy and safety in treating certain ocular diseases; however, its usefulness in some cases can be limited by a short duration of action and the need for frequent readministration. Efforts have been underway to develop more effective drug delivery systems, such as sustained-release drug implants, to overcome these limitations. SUMMARY: Pharmacological agents are currently under investigation, and some have been FDA approved, for the treatment of ocular conditions associated with systemic disease. Advances in the development of drug delivery systems for these agents are expected to further improve the efficacy and safety of pharmacotherapy for ocular diseases in the future.

Pharmacokinetics and disposition of memantine in the arterially perfused bovine eye.

PURPOSE: To develop an improved ((1)This is to clearly acknowledge that we have tried to improve an existing model.) arterially perfused bovine eye model and investigate the general ocular disposition of memantine. MATERIALS AND METHODS: Fresh bovine eyes were prepared by exposing and cannulating one ciliary artery, placing the eye into a perfusion chamber and slowly increasing the rate of perfusion to 1.0 ml/min. Analysis of the arterial perfusion pressure (APP), intraocular pressure (IOP), venous perfusate for glucose consumption and lactate dehydrogenase (LDH) activity, and histopathology ensured viability. Memantine was administered with the perfusate (simulated systemic access), by an intravitreal injection and by topical infusion. At the appropriate time points, the cornea, aqueous humour, sclera, iris-ciliary body, choroid/RPE, retina and vitreous humour were harvested and analysed for memantine. RESULTS: The preparation remained viable for at least 9 h. At this time, histopathological examination showed mild to moderate deterioration of retinal layers. However, all retinal layers remained well defined and the integrity of the inner limiting membrane and Bruch's membrane were preserved. Glucose consumption, LDH levels and constant APP and IOP showed that correct cannulation and viability was maintained. After administration, memantine accumulated in the melanin rich iris-ciliary body and choroid/RPE. Results following topical administration indicate that substantial concentrations of memantine are present in the retina and choroid/RPE. CONCLUSIONS: The arterial perfused bovine eye system proved to be a useful system for ocular drug delivery studies. The experimental results indicate that memantine will accumulate in the posterior segment when delivered by the topical route and that melanin-binding may support sustaining significant concentrations in the retina.

Topical and systemic drug delivery to the posterior segments.

The posterior segments of the eye are exquisitely protected from the external environment. This poses unique and fairly challenging hurdles for drug delivery. It is somewhat dogmatic that topical ocular delivery is insufficient to achieve therapeutic drug levels in the posterior segments. However, some drugs are currently challenging this dogma. In this review we investigate the constraints and challenges of drug delivery to the posterior segment. Additionally, we outline several potential absorption pathways that may potentially be exploited to deliver drug to the back of the eye. Data on several compounds that achieve therapeutic posterior segment concentrations after topical dosing is presented. Finally, the issues surrounding systemic delivery to the posterior segment are reviewed.

Binding of memantine to melanin: influence of type of melanin and characteristics.

PURPOSE: The objectives of this study were to characterize sepia, synthetic, and bovine melanin and to determine their binding characteristics to the drug memantine. METHODS: Physical methods were used to characterize sepia, synthetic, and bovine melanin. Their binding properties toward memantine were determined in deionized water and phosphate-buffered saline (PBS) at 37 degrees C. Melanin-memantine binding was measured indirectly by determining the unbound fraction of memantine. Curve fitting according to the Langmuir binding isotherm for one binding site was used for the determination of binding capacity (BLmax) and dissociation constant (KD). RESULTS: Synthetic and sepia melanin had comparable Gaussian particle size distributions, whereas bovine melanin showed a heterogeneous distribution profile. The suspension medium had a small effect on the particle size distribution of synthetic and bovine melanin. There were characteristic differences in the infrared spectra of the melanins. The rank order for BLmax in deionized water was sepia > bovine > synthetic melanin. However, when the melanins were suspended in PBS, the BLmax values were lower, and the rank order was bovine > sepia > synthetic. Whereas the KD values for sepia and synthetic melanin remained largely the same in deionized water and PBS, the KD value for bovine melanin in PBS was more than twice than in deionized water. CONCLUSIONS: This study showed that the physical characteristics of the melanins investigated differ markedly. The binding of memantine to melanin is thought to be determined by the different chemistries of the melanins, particle size, and buffer electrolytes.

Development of a liquid chromatography-mass spectrometric method for measuring the binding of memantine to different melanins.

A sensitive and selective liquid chromatography-mass spectrometric method was validated for the determination of free memantine in melanin binding studies. The sources of melanin studied were sepia, synthetic and bovine melanin. Memantine was chromatographed on a reversed-phase column (Prodigy 5 microm, ODS(3), 100 A, 100 x 4.6 mm) using gradient elution with mobile phases of 0.1% formic acid in deionised water and 0.1% formic acid in methanol at a flow-rate of 0.8 ml/min. The mode of ionisation was atmospheric pressure-electrospray and detection by single ion monitoring of the memantine ion m/z 180. Validation of the method showed that the assay was linear from 0.1 to 1200 nM and 0.5 to 1200 nM memantine in deionised water and phosphate-buffered saline (PBS), respectively. Accuracy for sample preparations in deionised water was between 80 and 108% and between 80 and 123% for PBS. For both media, intra- and inter-day precision was below 1% for retention time and below 5% for analyte peak area. At the LLOQ, the variation of peak area was less than 17%. Binding of memantine to melanin was measured indirectly by determining the unbound fraction of memantine. After incubation of melanin with memantine, the sample was centrifuged and filtered to separate the memantine-melanin complex effectively from suspension. The filtrate was then assayed for free memantine from which the extent of binding was then calculated.