Free amino acids in the pre-retinal vitreous space. Effect of high potassium and nipecotic acid

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.

Vitreal pharmacokinetics of peptide-transporter-targeted prodrugs of ganciclovir in conscious animals

PURPOSE

To delineate the vitreal pharmacokinetics of dipeptide monoester prodrugs of ganciclovir (GCV) with conscious rabbit model using ocular microdialysis and to compare with published results from anesthetized model.

METHODS

New Zealand albino male rabbit was selected as the animal model. Conscious animal ocular microdialysis technique with permanently implanted probes was employed to delineate the pharmacokinetics of GCV, L-valine-GCV (Val-GCV), and dipeptide monoester GCV prodrugs [val-val and L-glycine-val (Gly-Val)] after intravitreal administration.

RESULTS

This work employs conscious model to evaluate vitreal pharmacokinetic parameters and compares the results with previously published data from anesthetized animal, thereby demonstrating the effect of anesthesia on the vitreal disposition of dipeptide prodrugs of GCV. Results have revealed that area under curve (AUC), clearance, and last measured plasma concentration (C(last)) for all 4 compounds were significantly altered in a conscious animal relative to the anesthetized model, while mean residence time (MRT) was significantly reduced. However, the AUCs of regenerated Val-GCV and GCV from Gly-Val-GCV and Val-Val-GCV were found to be unchanged, suggesting higher ocular metabolism in conscious animals.

CONCLUSION

This study for the first time delineates the vitreal pharmacokinetics of a GCV prodrug in conscious animals and compares the data with anesthetized animals. Lower vitreal exposure levels were obtained in case of conscious animal model; however, the elimination rates were not influenced by anesthesia.

Ascorbic acid concentration in rabbit vitreous measured by microdialysis with HPLC-electrochemical detection before and after vitreous surgery

Microdialysis with high performance liquid chromatography and electrochemical detection (HPLC-ECD) was used to measure ascorbic acid (AA) concentrations in rabbit vitreous before and after vitrectomy. A cellulose microdialysis probe was implanted in the vitreous humor, and after stabilization, AA measurements were made daily over a 10 day period. The effect of removing two-thirds of the vitreous by vitrectomy was examined. The effect of triamcinolone acetonide (TA) was evaluated in four groups of rabbits: Group 1, sub-tenon TA (20 mg) alone; Group 2, intravitreous TA (4 mg) alone; Group 3, sub-tenon TA (20 mg) after vitrectomy, and Group 4, intravitreous TA (4 mg) after vitrectomy. The results showed that the AA concentration after vitrectomy was significantly lower from days 2 to 10 with a maximum reduction of 49.5% (P < 0.005) on day 7. No significant changes in the AA level was observed in Groups 1 and 2, a mild recovery of AA concentration reduction after vitrectomy was detected in Group 3. The highest recovery of the AA concentration reduction was observed in Group 4. The attenuating effect of TA treatment on the reduction of AA in the vitreous after vitrectomy was significant. This attenuating effect of the TA may be due to prevention of the disruption of the blood-aqueous barrier by its anti-inflammatory action.

Pharmacokinetics of erythromycin in rabbit corneas after single-dose infusion: role of P-glycoprotein as a barrier to in vivo ocular drug absorption

Efflux pump like P-glycoprotein (P-gp) is known to be a major barrier to drug delivery. Functional P-glycoprotein has been recently identified in cornea and corneal cell lines. Thus, it is probable that P-glycoprotein may restrict in vivo ocular drug absorption, resulting in low ocular bioavailability. Experiments were designed using New Zealand albino (New Zealand White) rabbits to assess inhibitors of P-gp efflux to increase drug absorption. Anesthetized rabbits were given constant topical infusions of [(14)C]erythromycin in the presence and absence of inhibitors. Testosterone, verapamil, quinidine, and cyclosporine A were selected as P-gp inhibitors. Transport experiments were conducted in Madin-Darby canine kidney cells transfected with the human mdr1 gene (MDCK-MDR1). Erythromycin exhibited significant efflux out of MDCK-MDR1 cells, suggesting that erythromycin is a good substrate for P-gp. Ocular pharmacokinetic studies were conducted using a topical single-dose infusion method. Maximum inhibition of P-gp mediated efflux was observed with 500 microM testosterone. Area under the curve (AUC)(0- infinity ) of erythromycin with 500 microM testosterone was almost 4 times higher than AUC(0- infinity ) without any inhibitor. Rate of elimination (k(10)) for erythromycin and those with inhibitors was found to be similar (141 +/- 23 min), suggesting that elimination pathways were not altered. All the inhibitors were found to be nontoxic. Verapamil also inhibited the efflux pump with moderate change in AUC(0- infinity ) and C(max) compared with control. Thus, P-gp is found to be active in vivo, and it restricts topical erythromycin absorption across the cornea, which can be inhibited by known P-gp inhibitors. Therefore, ocular bioavailability of P-gp substrates can be significantly enhanced by proper selection of P-gp inhibitors.

Microdialysis measurement of ascorbic acid in rabbit vitreous after photodynamic reaction

A method for long-term intravitreous microdialysis was used to measure endogenous reduced ascorbic acid in the vitreous of rabbits by HPLC-ECD before and after exposure to intense visible light in the presence of fluorescein. Cellulose microdialysis probes were implanted into the vitreous humor of each eye and after stabilization ascorbic acid measurements were recorded over a 14 day period. Under this experimental condition, normal ascorbic acid concentrations in vitreous varied from 98.0 +/- 9.8 to 106.9 +/- 20.3 microM(mean +/- S.D.). The eyes received light irradiation (25 000 lux) for 2 hr and fluorescein was used as the photosensitizer once or twice. No immediate effects on ascorbic acid concentrations could be observed in the eyes irradiated twice without fluorescein i.v. injections and in the twice fluorescein injected without irradiation. However, in the eyes irradiated once with fluorescein (30 mg kg(-1)), ascorbic acid concentration after irradiation significantly decreased from day 2 and continued over a period of 10 days compared with that before irradiation and maximal reduction was 32.6% (P < 0.005) on day 6 after irradiation. By day 13, the ascorbic acid concentration returned to control levels (P > 0.01). In the eyes irradiated twice with fluorescein injections, ascorbic acid concentration after irradiation decreased even more over the experimental period and the maximal reduction was 65.5% (P < 0.005) on day 5 after irradiation and did not recover over the next 9 days. In the eyes irradiated twice with fluorescein injections plus administration of ascorbic acid (150 mg kg(-1)) 30 min before irradiation, a significant increase (52.5%) of ascorbic acid (P < 0.005) was found on day 1 and control levels of ascorbic acid were maintained from day 2 onward. The protective role of ascorbic acid in the vitreous humor against photodynamic reaction is suggested.

An in vivo eyecup preparation for the rat

A method for the preparation of an in vivo eyecup and a complex stimulating-sampling device are described; these are suitable for long-term parallel neurochemical and electrophysiological experiments on the rat retina without any additives into the eyecup. In this in vivo eyecup the extracellular microenvironment is under the normal homeostatic control of the vascular system; no continuous exchange of the eyecup fluid and no addition of glutamate is necessary to maintain stable retinal electric responses and amino acid concentrations. The eyecup viability was tested by monitoring the electroretinogram (ERG) and the amino acid contents of the eyecup fluid sampled from the preretinal space by means of microdialysis. After the initial increase the b-wave of the ERG changed by less than 10% in maximal amplitude during experiments lasting 5 h. The glutamate, glutamine, and glycine levels proved comparatively, whereas the taurine level rose continuously throughout the experimental protocol. Recovery of ERG was achieved following exposure to bright background illumination. Total exchange of the eyecup volume requires 20 min at a flow rate of 1 microl/min. The effect of L-AP4 on the ERG was successfully reproduced, which suggests the applicability of this in vivo eyecup for pharmacological experiments on the rat retina.

Microdialysis and drug delivery to the eye

The eye presents unique challenges in both the development of tools for elucidating drug disposition as well as for the development of modes of drug delivery for treatment of ocular diseases. In this paper, we present a discussion of the anatomical and physiological characteristics and limitations present in the eye for microdialysis sampling of endogenous substrates and xenobiotics. To date, over twenty papers describing microdialysis approaches for assessment of ocular drug delivery and endogenous substrate characterization have been published. Although the majority of papers describe sampling of vitreous humor, recent efforts have been directed towards ocular anterior segment sampling using microdialysis. With this approach, an appreciable reduction in animal use has been realized. In addition, simultaneous examination of administered drug and endogenous substrates modulated by the drug is possible with this approach, facilitating construction of ocular pharmacokinetic/pharmacodynamic relationships through use of relevant surrogate markers.

An eyecup preparation for the rat and mouse

The eyecup preparation has traditionally been used to study retinal physiology in lower vertebrates and in some mammals. The procedures for preparing eyecups of the rat and mouse have not been described, however. We now describe methods for preparing and maintaining viable eyecups for these two species. Eyecups were everted over a plastic dome and held in place between the two halves of a superfusion chamber. Fluid exchange in the chamber was rapid, with near total exchange occurring in 9 s. Eyecup viability was tested by monitoring light-evoked retinal responses as the preparation aged. In both rat and mouse, the amplitude of the electroretinogram (ERG) b-wave decreased slowly, declining to 1/2 maximal amplitude in approximately 70 min. Light-evoked spike activity of neurons in the ganglion cell layer remained stable for approximately 3 h and attenuated responses were recorded for an additional 1-2 h. Eyecups were able to dark adapt. Retinal sensitivity, tested by monitoring b-wave amplitude, recovered following exposure to an adapting light.

Electrophysiological properties of a new isolated rat retina preparation

A piece of rat retina was mounted in an open chamber and perfused with a Ringer solution at 37 degrees C. The electroretinogram (ERG) was recorded between an extracellular microelectrode in contact with the rod outer segments and a reference electrode under the retina. The addition of 250-500 microM of glutamate to the media prevented the b-wave from decaying in amplitude with time. Minor components of the ERG, the scotopic threshold response (STR) and oscillatory potentials (OPs), were well maintained with glutamate in the media. Experiments on the spatial properties of the recordings indicated that a small area immediately around the microelectrode contributes most strongly to the response. The similarity of ERGs recorded in vivo from the cornea to the transretinal ERGs from the isolated retina of the same animal indicated that the functional integrity of the isolated retina was well preserved in the media with glutamate.

Pharmacokinetics of topical beta-adrenergic antagonists in rabbit aqueous humor evaluated with the microdialysis method

The microdialysis method was used to evaluate the pharmacokinetics of the beta-adrenergic antagonists carteolol and timolol and the new ophthalmic solution WP-934 in rabbit aqueous humor, following instillation. A probe with a microdialysis membrane (length, 5 mm; diameter, 0.2 mm) was implanted in the anterior chamber of the pigmented rabbit and perfused with Ringer's solution. Twenty microliters of 0.5% timolol maleate (0.5% Timoptol(R)), 2% carteolol hydrochloride (2% Mikelan(R)), or a novel preparation of 0.5% timolol maleate (WP-934) that gels after instillation were then instilled. The concentrations of these drugs in dialysates were measured using high-performance liquid chromatography and an electrochemical detection system. In vitro relative recovery of the membrane with timolol and carteolol was approximately 17.5% and 21. 6%, respectively. Timolol and carteolol levels in aqueous humor increased rapidly after instillation of Timoptol and Mikelan and reached maximal levels (Cmax) within 60 minutes. The Cmax of carteolol (4.25 microg ml-1) was lower than that of timolol (5.52 microg ml-1), suggesting that the corneal permeability of timolol is higher than that of carteolol. After instillation of WP-934, the Cmax of timolol (12.32 microg ml-1) was 2.2-fold higher than that after instillation of Timoptol. However, t1/2 values of beta-adrenergic antagonists after instillation of the three preparations were not significantly different. These data suggest that the microdialysis technique is useful for continuous monitoring of aqueous levels of beta-blockers and for analysis of their pharmacokinetic parameters while requiring much fewer animals than conventional sampling with paracentesis.

Passage of drugs through different intraocular microdialysis membranes

BACKGROUND

Since drug penetration from the blood to the vitreous body is very poor, it is important to find means other than systemic delivery to reach necessary intraocular concentrations of drugs. This study represents a step in this direction.

METHOD

Microdialysis probes implanted intraocularly in rabbits were perfused with different substances, mainly drugs. The substances belonged to three groups, antibiotics, corticosteroids and cytostatics, and were: benzylpenicillin and cefuroxim; triamcinolone and dexamethasone; daunomycin and 5-fluorouracil. In addition, three substances of different molecular weights were tested: formic acid (MW 70), glucose (MW 189) and insulin (MW ca. 5200).

RESULTS

When used in tracer concentrations, some lipophilic drugs stick to polycarbonate but not to polyamide membranes. The latter material has therefore been used in all intraocular perfusions. All substances except inulin were found to diffuse through the polyamide membrane into the vitreous at a rate of about 10-20% of the perfusate concentration. Membranes with different dimensions and the above-mentioned two materials have also been screened for their transport properties in vitro. No differences were found between the two membrane materials, polycarbonate and polyamide. The net dialysis is strongly dependent on the probe geometry.

CONCLUSIONS

We have shown that the above-mentioned substances penetrate into the vitreous body of rabbits through an implanted microdialysis membrane. This is of importance for the development of new means of intraocular drug administration.

A removable ocular microdialysis system for measuring vitreous biogenic amines

A microdialysis system with a removable probe and a fixed scleral entry port is presented. The probe can be inserted several times with minimal trauma, permitting repeated sampling in one animal. The functioning of the setup is illustrated by the measurement of dopamine, dihydroxyphenyl acetic acid, and noradrenaline in the vitreous of healthy rabbits under physiological conditions.

Intraocular microdialysis with permanently implanted probes in rabbit

The technique of microdialysis in vivo, much used in brain experiments, has been adapted for intraocular use. A new probe was designed, made from a soft tube with the dialysis membrane mounted in a fenestrated protecting sleeve, facing one side. A special surgical procedure was developed for the long-term implantation of the probes. They stay functional in the eyes for several weeks or more. Previously published intraocular microdialysis studies were short-term experiments. The acute surgical trauma is likely to affect the concentration of many compounds in the vitreous, and this effect is minimized with long-term probes.