Bernard Plazonnet

Topical use of chitosan in ophthalmology : tolerance assessment and evaluation of precorneal retention

The mucoadhesive polysaccharide chitosan was evaluated as a potential component in ophthalmic gels for enabling increased precorneal drug residence times. This cationic vehicle was expected to slow down drug elimination by the lacrymal flow both by increasing solution viscosity and by interacting with the negative charges of the mucus. The molecular weight (Mw) and concentration of polysaccharide were studied in four types of chitosan as parameters that might influence ocular tolerability and precorneal residence time of formulations containing tobramycin as therapeutic agent. An ocular irritation test, using confocal laser scanning ophthalmoscopy (CLSO) combined with corneal fluorescein staining, clearly demonstrated the excellent tolerance of chitosan after topical administration onto the corneal surface. Gamma scintigraphic data showed that the clearance of the formulations labelled with 99mTc-DTPA was significantly delayed in the presence of chitosan with respect to the commercial collyrium (Tobrex(R)), regardless of the concentration and of the molecular weight of chitosan in solution. At least a 3-fold increase of the corneal residence time was achieved in the presence of chitosan when compared to Tobrex(R).

Gelrite®: a novel, ion-activated, in-situ gelling polymer for ophthalmic vehicles. Effect on bioavailability of timolol

Abstract Gelrite® solution, a novel ophthalmic vehicle, gels in the presence of mono or divalent cations. In the conjunctival sac ‘ion-activation’ of the sol/gel transition is accomplished by the lacrimal fluid. A 0.6% Gelrite® vehicle has been compared to an equiviscous solution of hydroxyethylcellulose (HEC) using timolol maleate as a drug probe. In vitro release rates of timolol from HEC and Gelrite® gel were similar. In vivo, the formation of the gel prolonged precorneal residence time and increased ocular bioavailability of timolol in the cornea, aqueous humor and iris + ciliary body of albino rabbits.

Application of in vivo confocal microscopy to the objective evaluation of ocular irritation induced by surfactants

An ocular irritation test using confocal laser scanning ophthalmoscopy has been developed in which corneal lesions subsequent to instillation of surfactants are specifically marked by fluorescein and assessed by digital image processing. The sum of the observed fluorescent corneal areas is taken into account as an endpoint of ocular irritation. Eight currently used nonionic, cationic and anionic surfactants were applied onto the cornea of rabbits and mice, four times per day during 3 days at various concentrations. Benzalkonium chloride, a cationic surfactant, at a concentration range of 0.01-0.5%, was tested in the same manner. The cornea was evaluated in vivo for ocular tolerance by confocal microscopy. In both rabbits and mice, the test revealed following irritation rankings: cationic>anionic>nonionic surfactants. Furthermore, in both animal models, the ocular damage increased with the concentration of benzalkonium. The test was sensitive enough to detect ocular microlesions at concentrations of surfactants as low as 0.01% for benzalkonium. These findings demonstrate the usefulness of confocal microscopy for the non-invasive, in situ evaluation of ocular tolerance.

Scintigraphic assessment of an ophthalmic gelling vehicle in man and rabbit

Studies of the rate of clearance of a gellan gum formulation (Gelrite) radiolabelled by the inclusion of technetium-99m labelled diethylenetriaminepentaacetic acid were carried out in volunteer subjects and in rabbits. Disposition was followed by gamma scintigraphy and compared with 0.5% w/v hydroxyethylcellulose (HEC) solution and isotonic saline administered to the same subjects. Clearance of all solutions was found to follow bi-exponential kinetics with differences in clearance rates between the two species studied. A significant retention of the gellan gum formulation compared to HEC (p = 0.006) or saline (p = 0.009) was noted in man, but not in the rabbit. In this latter species the HEC showed greater retention compared to Gelrite. The species-specific differences in the precorneal residence of the formulations are attributed to the different physiological responses following instillation of solutions into the eye.

Functionality testing of gellan gum, a polymeric excipient material for ophthalmic dosage forms

Low acetyl gellan gum, originally a food ingredient, has been used to devise novel ophthalmic formulations, significantly improving drug ocular bioavailability. This increase in drug bioavailability results from the unique gelling property of gellan gum in the presence of tear fluid cations. The aim of this study was to develop a functionality test ensuring the consistency of the dosage forms gelling property and a reproducible pharmacological effect. The rupture strength of the gel was shown to be a reliable indicator of the ocular drug bioavailability in the albino rabbit. The test parameters susceptible to influence the test results were identified, evaluated and optimized. The influence of the raw material characteristics and of the processing parameters on the final product gel strength were determined and optimized, and finished product specifications established.

Gamma scintigraphic study of precorneal drainage and assessment of miotic response in rabbits of various ophthalmic formulations containing pilocarpine

Topical drug delivery to the eye is generally characterized by poor intraocular bioavailability. Hydrocolloids are used to increase viscosity and hence to slow down drug dilution and elimination by the lacrimal flow. The purpose of this work was to monitor cornel contact time by gamma scintigraphy and to compare the miotic response induced by viscosified vehicles containing pilocarpine nitrate. New Zealand White rabbits were used as test animals. The polysaccharides tested were hydroxyethylcellulose (HEC), xanthan gum and gellan gum (Gelrite®). The present comparative study using a solution containing no viscosifying agent as reference showed that Gelrite® and xanthan gum prolonged significantly the constriction of the pupil, whereas no prolongation of the miotic response could be shown with HEC. The clearance of the solutions labelled with 99mTc-DTPA was measured over a 10 min period and was shown to follow a two-phase pattern. The zero-order rate constant was determined for the initial 2 min and was significantly different for all the viscosified formulations in comparison with the reference solution.

Bioadhesion: The effect of polyacrylic acid on the ocular bioavailability of timolol

Abstract The bioadhesive polymer, polyacrylic acid, was added to ophthalmic formulations and the effect on the ocular distribution of timolol evaluated. Ocular bioavailability of 0.5% Timoptol ® was measured in cornea, aqueous humor and iris + ciliary body, of albino rabbits and was compared to that of 0.5% timolol in isoviscous solutions of polyvinyl alcohol (PVA), polyacrylic acid (PAA) and timolol-polyacrylic acid salt (PAA salt). Ocular bioavailability of timolol was increased by each of the viscous solutions. These increases, assessed by measurement of AUC (0–4 h) in cornea, aqueous humor and iris + ciliary body ranged from 1.4- to 2.8-fold. The largest increases were obtained with the non-mucoadhesive polymer PVA. The bioadhesive PAA polymers modified the concentration vs time profiles of timolol and gave the highest timolol concentrations in iris + ciliary body at later sampling times.

Ocular tolerance of preservatives on the murine cornea.

We investigated the effects of instilling 13 commonly used preservatives on the murine cornea in vivo. Due to the instillation of preservatives, micro-lesions are formed on the cornea and can be selectively marked by fluorescein. The sum of the resulting fluorescent areas was measured using an episcopic microscope coupled to an image processing system. All the tested preservatives proved to be well-tolerated by the eye at commonly used concentrations. However, in some cases, increased concentrations of preservatives or combinations resulted in significant increase of the amount of corneal damage. With increasing the concentration, corneal lesion increased the most in the case of cetylpyridinium. While a combination of chlorobutanol 0.5% and phenylethylalcohol 0.5% did not result in an increase in corneal damage (when compared to the use of each separately), the associations of thiomersal 0.02% and phenylethylalcohol 0.4% on one hand and of edetate disodium (EDTA) 0.1% and benzalkonium 0.01% on the other, resulted in significant increases in the amount of corneal damage. However, in none of the tested combinations, the increase in the observed damage exceed the limit of ocular intolerance we had defined beforehand: thus, they were all deemed relatively well-tolerated. In the last part of the study, we investigated the effects of combining several preservatives, at usual concentrations, with an anesthetic solution of oxybuprocaine and found no notable increase in ocular damage.

Mucoadhesion of copolymers and mixtures containing polyacrylic acid

Water-soluble polymers were synthesized from dextran and polyacrylic acid and their ocular mucoadhesion was evaluated. One series had polyacrylic acid grafted onto the polysaccharide backbone of dextran, and another series had dextran grafted onto the polyacrylic acid backbone. Mucoadhesion of these copolymers was investigated using a tensile apparatus and compared with that of polyacrylic acid/dextran mixtures prepared in different proportions. Whatever the copolymer structure, no synergistic effects were seen and mucoadhesion was not markedly increased compared to dextran. The adhesion of copolymers was the same as that of mixtures having a similar polyacrylic acid content and was always less than that of polyacrylic acid alone. Formation of an interpolymer complex occurred at concentrations up to 60% polyacrylic acid, and only above this value did bioadhesion increase above that of dextran. When this complex was dissociated by neutralization of the carboxyl groups of polyacrylic acid, the mucoadhesion of the copolymers and the mixtures was improved. These experiments demonstrated that copolymers and mixtures of dextran and polyacrylic acid did not produce polymers with improved ocular mucoadhesion.

The effect of vehicle viscosity on the ocular bioavailability of L-653,328

Abstract The effect of the viscosity of an ophthalmic vehicle on ocular drug penetration has been investigated. Ocular concentrations of L-652,698 have been measured using HPLC and fluorescence detection, in the cornea, aqueous humor and iris + ciliary body of rabbits after instillation of 1% solutions of L-653,328 in 0, 0.2, 0.25, 0.3, 0.35, 0.4 and 0.5% hydroxyethyl cellulose (HEC). Maximum drug concentrations in all three ocular sites increased concomitantly with increase in viscosity. The correlation coefficients between ocular bioavailability, assessed by AUC (0–4 h), and with HEC viscosity were 0.93, 0.96 and 0.83 in cornea, aqueous humor and iris + ciliary body, respectively. When isoviscous solutions containing polyvinyl alcohol were examined, ocular bioavailability was similar in cornea and aqueous humor but reduced by 50% in the iris + ciliary body when compared to the equivalent HEC solution.