Effects of drug vehicles on ocular contact time

Fortified antibiotic ointment in bacterial keratitis

We conducted experiments to determine the effectiveness of fortified antibiotic ointment in the treatment of Pseudomonas keratitis in rabbits. We evaluated gentamicin ointment (3, 10, 20, and 40 mg/g), gentamicin solution (3 and 10 mg/ml), and placebo, each given every 30 minutes. We also examined the effectiveness of fortified ointment given in extended treatment intervals. In short-term trials, commercial-strength gentamicin solution (3 mg/ml) was therapeutically superior (P less than .001) to commercial-strength gentamicin ointment (3 mg/g) in reducing corneal bacterial colony counts. No significant difference in antimicrobial effect was noted between fortified gentamicin ointment and fortified gentamicin solution at 30-minute treatment intervals. Fortified gentamicin ointment reduced colony counts even at extended treatment intervals of up to four hours in a severe keratitis model.

Ophthalmic bioequivalence of steroid/antibiotic combination formulations

This study compared the relative ocular bioavailability in rabbits of the antibiotic/steroid combination of 0.3 per cent tobramycin and 0.1 per cent fluorometholone acetate to each of the two single-entity products. Two separate studies were conducted, one measuring fluorometholone acetate in cornea and aqueous humour through 240 min and the other measuring tobramycin in the cornea through 120 min. The results for fluorometholone acetate show that the combination product is 15 per cent higher in AUC for the cornea than the single-entity product (0.05 less than p less than 0.1). However, the combination product is only 4.4 per cent higher in AUC when aqueous humour levels are compared (N.S.). The single-entity fluorometholone acetate ointment yielded cornea and aqueous humour levels which were 20 per cent (p less than 0.01) and 6 per cent (N.S.) higher in AUC than the same tissues measured after topical instillation of the suspension. Statistical treatment of the data indicated that the suspension and ointment products, were bioequivalent with respect to fluorometholone acetate in aqueous humour, but not with respect to corneal concentrations. Tobramycin was measured in the cornea following instillation of either the single-entity solution or the combination suspension. Per cent bound (tobramycin to rabbit antibodies determined in vitro from the radioimmunoassay results) was used for statistical analyses of all group means and standard errors. The results indicated that the products were bioinequivalent.

Concentration-dependent precorneal loss of pilocarpine in rabbit eyes

Precorneal loss of pilocarpine was studied in pigmented and albino rabbits. We instilled 25 microliter of isotonic pilocarpine solution, pH 6.4, into rabbit eyes and monitored drug concentration in the precorneal tear film. Increased concentration (0.2% - 2.0%) of pilocarpine accelerated precorneal drug loss from the tear film from 0.686 min-1 to 1.064 min-1. This increase was mainly due to induced lacrimation. Polyvinyl alcohol (1.4%) retarded precorneal loss of pilocarpine. The rate of loss was the same in pigmented and albino rabbits. The effects of the changed precorneal loss of pilocarpine on corneal drug absorption are discussed.

Lacrimal scintigraphy. I. Compartmental analysis of data

Linear compartmental analysis was applied to lacrimal scintigraphy data from asymptomatic and obstructed lacrimal systems. Failure to achieve satisfactory results in most of the asymptomatic systems studied suggests that variable tear flow is a feature of normal lacrimal drainage which precludes linear analysis. However, in cases where the analysis was successful the results were consistent with the occurrence of reflux between the conjunctival and lacrimal sacs, and between the lacrimal sac and nasolacrimal duct. Further support for the existence of reflux between compartments was provided by the analysis of obstructed systems, particularly those with obstruction at the upper end of the nasolacrimal duct. In such cases there was rapid equilibrium of the tracer between the conjunctival and lacrimal sacs, which in the absence of tear flow could be explained by reflux between these 2 compartments.

Prolonged pulse-entry of pilocarpine with a soluble drug insert

Apparent biophase availability of pilocarpine was studied in the eyes of albino rabbits. Pilocarpine doses of 0.85 and 2.30 mg in aqueous solutions, 1.00 mg in oil and 0.85 mg in a solid insert, were applied ocularly. The insert was a water soluble polyvinylpyrrolidone (PVP) matrix, which released 80% of its pilocarpine content in 35 min in vitro. In the inferior fornix of the eye this insert gelled in about 5 min and dissolved in 1 h. Pupillary diameters were measured and converted to values for the response parameter (RP). Time delay and magnitude of peak response, apparent biophasic availability (area under the curve of RP vs time), and a constant for the apparent rate of elimination were calculated from RP values. The time delay for the peak response was 16.l3-24.0 min, and the constant for apparent rate of elimination was 0.69-0.81 h-1. Neither time delay nor this constant was affected by the dose or the dosage form. Magnitude of the peak response and apparent biophasic availability were influenced by the vehicle and the dose: insert (0.85 mg) greater than oily solution (1.00 mg) greater than aqueous solution (2.30 mg) greater than aqueous solution (0.85 mg). The insert and oily solution did not show vehicle-controlled drug absorption and can be regarded as prolonged pulse-entry medication.

Tear flow in normal human eyes. Determination by means of radioisotope and gamma camera

Tear flow was measured in normal human eyes by means of a radioisotope (technetium as pertechnetate in a normal saline solution), a gamma camera and a computer. By "region of interest" technique the elimination was shown to have two phases: an initial rapid elimination followed by a slower elimination after 5--7 min. The mean fractional turnover rate in the initial phase was 0.197 min.-1 (n = 35, SEM = 0.013) and in the basal phase 0.083 min.-1 (n = 35, SEM = 0.003). Assuming a constant tear volume these values corresponded to a tear flow of 1.4 microliter min.-1 and 0.6 microliter min.-1, respectively. There was no significant difference between the fractional turnover rate in the upright and supine position. No difference was found in tear flow between males and females. With the eyes closed the fractional turnover rate was low with intermittently rapid outflow of tears. Irritation to the contralateral eye with a filterpaper caused a stimulated tear flow of 4.4 microliter min.-1. A nomogram facilitating background corrections was constructed.

Effect of oily vehicle on ocular pilocarpine concentration

The effect of 1.4% polyvinyl alcohol (PVA) vehicle and of castor oil vehicle on ocular pilocarpine concentration was studied by radioactive method in the rabbit eye. Statistically higher radioactivities were measured from the anterior surface structures of the eyes dropped with oily vehicle when compared to PVA vehicle at 120 min. It is concluded that the conjunctiva and the cornea serve as a drug reservoir for the longlasting drug effect observed in the literature after oily pilocarpine drops.

An in vitro method simulating drug release from viscous eye drops in rabbit and man

A method has been developed using a diffusion cell in an attempt to determine the drug release from viscous solutions under conditions simulating the blinking movements in the rabbit and human eye. Diffusion coefficients were determined at rest and at different velocities. For the solutions at rest, corresponding with the conditions at the surface of the rabbit cornea, the diffusion velocity decreases with increasing viscosity. When the solution is moved at a velocity corresponding to that of lachrymal fluid at the surface of the human eye, the influence of viscosity may be neglected.

An increased effect of pilocarpine on the pupil by application of the drug in oil

In healthy volunteers pilocarpine in an oily solution had a greater degree and duration of effect on the pupil than the same amount of drug given in aqueous solution. Blue- and brown-eyed subjects had similar degrees of miosis, but the time taken to reach peak effects was longer with the latter. It can be concluded that eye drops made with an oily solvent may provide an improvement in the treatment of glaucoma with pilocarpine.

Ophthalmic ointments

The use of ointments as an ocular drug vehicle adds an important dimension to topical therapy. Ointments are well tolerated, fairly safe, and provide an excellent means for enhanced ocular contact time. In the case of certain antibiotics, this improved contact time yields increased ocular drug levels. However, experimental data indicate that corticosteroid ointments do not penetrate into the eye as well as do suspension preparations. This may relate to the binding of the drug to the ointment base and also to the particular steroid compound. Like other ophthalmic preparations, ointments may become contaminated. Ophthalmic ointments should not be instilled into eyes with open wounds. Instillation of ointments into postoperative eyes where wound closure is secure appears to be safe and effective.

Ocular bioavailability and systemic loss of topically applied ophthalmic drugs

We used 20-day-old rabbits as a model to show that the ocular bioavailability of topically applied pilocarpine nitrate increased as the instilled volume of the drug was decreased. Decreasing the instilled volume from 25 to 5 microliter permitted a dosage reduction of greater than 2.5 times without sacrificing overall drug concentrations in the eye. Since only a small fraction of topically applied doses to the eye actually reached the interior of the eye, the remainder of the dose was lost and available for systemic absorption. The reduction in dosage permitted by this approach resulted in less drug appearing in the general circulation, as shown by comparative plasma level-time profiles. The advantages of reducing drop size are improved ocular bioavailability permitting the use of smaller doses; and less systemic drug loss, thus reducing the potential for systemic side effects. These advantages could be especially significant in the pediatric and geriatric age groups.

Methodological aspects of tear flow determination by means of a radioactive tracer

Theoretical considerations on a simplified biological system representing the tear pathways lead to the assumption of an exponential pattern of elimination of tracer substances from the conjunctival sac. The tracer used in the present study was technetium, Tc99m, as sodium pertechnetate. The detection system consisted of a gamma camera coupled to a digital system, a minicomputer and a magnetic tape unit. After instillation of 10 microliter normal saline solution containing Tc99m the decay of activity was followed by means of an activity-time function curve to which exponential curves were approximated by the computer. As a parameter for tear flow the fractional turnover rate was calculated from 7 to 15 min after instillation. An initial faster elimination was found in the first 7 min following instillation. Corrections for background radiation, evaporation of water and transconjunctival transport of Tc99m were estimated.

Vehicle effects on ocular drug bioavailability II: Evaluation of pilocarpine

The influence of vehicle composition on ocular penetration of pilocarpine was studied in the albino rabbit. Increasing the pH of a vehicle promoted increased corneal penetration for pilocarpine, in accordance with the pH-partition hypothesis, but a similar series of experiments with a nonionizable drug, glycerin, gave similar results. The extent of pH-induced lacrimation by the vehicle and its effect on precorneal drug concentration also was determined. Increased pilocarpine absorption at neutral to slightly alkaline pH was due primarily to its peculiar solubility characteristics coupled with less irritation and lacrimation rather than a direct pH effect on the molecule. Incorporation of pilocarpine into a petrolatum-based ointment vehicle resulted in increased aqueous humor pilocarpine levels above those provided by an equivalent dose of aqueous solution. The mechanism of this increase was determined to be a higher effective concentration of pilocarpine in the ointment vehicle coupled with an increase in contact time of the dose. The ointment system also exerted an unusual form of vehicle control in that it promoted the corneal penetration of pilocarpine while impeding uniform mixing of the dose with tears and thereby imposed a restriction on the amount of pilocarpine available to the ocular tissues.

Effects of ophthalmic vehicles on the stability of the precorneal film

The break up time (B.U.T.) of the precorneal film has been studied before and after application of 34 different vehicles. The material examined comprised 646 eyes. Maximum increase of the B.U.T. was obtained with 2% methyl cellulose (four times) and 10% polyvinyl alchohol (seven times). These vehicles in the usually employed concentrations, fat-free ointment (polyethylene glycol), acetyl cysteine and polysaccharide (dextran) affected a less pronounced prolongation. The B.U.T. was reduced four or five times by fatty, anhydrous ointments and by silicone oil, about two or three times by emulsions and oils, and twice by 0.01% benzalkonium chloride. The clinical significance of the B.U.T. alterations is discussed.

Effect of drug vehicle on human ocular retention of topically applied tetracycline

Tetracycline hydrochloride--as a 1% suspension in oil, 1 and 2% suspension in ointment, and 1 and 2% solution in water or balanced salt solution USP--was applied once to the conjunctival cul-de-sacs of volunteers and patients before cataract extraction. The tear film concentrations of drug were maintained above a bacteriostatic level in excess of six hours for 1 and 2% tetracycline in ointment, less than two hours for 1% tetracycline in oil, and less than 30 minutes for 1 and 2% tetracycline in water or balanced salt solution. The 2% tetracycline in ointment produced the highest tear film levels of drug. The 1% tetracycline in oil induced excessive lacrimation and much of the drug was washed from the conjunctival surface. The tear film levels of the drug were mirrored by the maintenance of bacteriostatic levels of tetracycline in the aqueous humor for 1 1/2 hours with 2% tetracycline in ointment. Whereas 1% tetracycline in ointment produced drug levels approaching bacteriostasis in the aqueous humor, 1% tetracycline in oil produced only trace levels of the drug within the anterior chamber. The ointment acted as a depot for the suspended drug and most of the drug in the absence of excessive tearing was lost from the conjunctiva via the lacrimal system.

Effects of drug vehicles on ocular uptake of tetracycline

Varying concentrations of tetracycline hydrochloride were topically applied to the rabbit eye in several ophthalmic vehicles. The antibiotic levels in the corneal epithelium, aqueous humor, and lens were markedly elevated by increasing the drug concentration and the drug contact time with the eye. The ocular levels of tetracycline were increased by the vehicle in this ascending order: isotonic saline, 1.4% polyvinyl alcohol, 1% alpha-methylcellulose, and a 6:4 mixture of white petrolatum-mineral oil ointment. Tetracycline hydrochloride as a 2.0% suspension in ointment produced anterior chamber and corneal concentrations for several hours in the range of bacteriostasis for many gram-negative bacteria that occasionally invade the eye.