Competitive inhibition of drug-protein interaction in eye fluids and tissues

Albumin Levels in Tear Film Modulate the Bioavailability of Medically-Relevant Topical Drugs

The breakdown of blood-tear barrier that occurs with ocular pathology allows for large amounts of albumin to leak into the tear fluid. This process likely represents an important restriction to drug absorption in ophthalmology, as only the unbound drug is transported across the ocular tissue barriers to exert its pharmacologic effect. We aimed to investigate the effects of albumin levels in tears on the bioavailability of two commonly used ophthalmic drugs: tropicamide, an antimuscarinic that produces mydriasis and cycloplegia, and latanoprost, a PGF2α analog used for the treatment of glaucoma. Eight female beagle dogs underwent a randomized, vehicle-controlled crossover trial. For each dog, one eye received 30 µl of artificial tears (control) or canine albumin (0.4 or 1.5%) at random, immediately followed by 30 µl of 1% tropicamide (2 days, 24 h washout) or 0.005% latanoprost (2 days, 72 h washout) in both eyes. Pupil diameter (digital caliper) and intraocular pressure (IOP; rebound tonometry) were recorded at various times following drug administration (0 to 480 min) and compared between both groups with a mixed model for repeated measures. Albumin in tears had a significant impact on pupillary diameter for both tropicamide (P ≤ 0.001) and latanoprost (P ≤ 0.047), with no differences noted between 0.4% and 1.5% concentrations. Reduction in the maximal effect (pupil size) and overall drug exposure (area under the effect time-curve of pupil size over time) were significant for tropicamide (6.2–8.5% on average, P ≤ 0.006) but not for latanoprost (P ≥ 0.663). The IOP, only measured in eyes receiving latanoprost, was not significantly impacted by the addition of either 0.4% (P = 0.242) or 1.5% albumin (P = 0.879). Albumin in tear film, previously shown to leak from the conjunctival vasculature in diseased eyes, may bind to topically administered drugs and reduces their intraocular penetration and bioavailability. Further investigations in clinical patients and other commonly used ophthalmic medications are warranted.

Applications of microneedles in delivering drugs for various ocular diseases

Treatment of majority of eye diseases involve the use of eye drops or eye ointments, which have major drawbacks of needing frequent administration, lower bioavailability and inability to cross the various eye barriers. This necessitates the use of novel delivery systems. Microneedles (MNs) as an alternate novel delivery system facilitate drug delivery to various ocular diseases with promising approaches in healthcare. Advances in pharmaceutical technology have made MNs provide localized, effective, less invasive and targeted drug delivery in the eye. The purpose of this review is to provide an insight to efficacious therapeutic applications the MNs can bring in various ocular diseases. Out of which, glaucoma, age-related macular degeneration, uveitis, retinal vascular occlusion and retinitis pigmentosa are majorly discussed. Among the various types of MNs; solid coated, hollow and dissolving polymeric MNs are specifically focused for their applications in ocular diseases. In addition, MNs shows improvement in the visual acuity and decreases the progression of the different ocular diseases.

Ocular preparations: the formulation approach

The main aim of pharmacotherapeutics is the attainment of an effective drug concentration at the intended site of action for a sufficient period of time to elicit the response. A major problem being faced in ocular therapeutics is the attainment of an optimal concentration at the site of action. Poor bioavailability of drugs from ocular dosage forms is mainly due to the tear production, non-productive absorption, transient residence time, and impermeability of corneal epithelium. This article reviews: (1) the barriers that decrease the bioavailability of an ophthalmic drug; (2) the objectives to be considered in producing optimal formulations; and (3) the approaches being used to improve the corneal penetration of a drug molecule and delay its elimination from the eye. The focus of this review is on the recent developments in topical ocular drug delivery systems, the rationale for their use, their drug release mechanism, and the characteristic advantages and limitations of each system. In addition, the review attempts to give various analytical procedures including the animal models and other models required for bioavailability and pharmacokinetic studies. The latter can aid in the design and predictive evaluation of newer delivery systems. The dosage forms are divided into the ones which affect the precorneal parameters, and those that provide a controlled and continuous delivery to the pre- and intraocular tissues. The systems discussed include: (a) the commonly used dosage forms such as gels, viscosity imparting agents, ointments, and aqueous suspensions; (b) the newer concept of penetration enhancers, phase transition systems, use of cyclodextrins to increase solubility of various drugs, vesicular systems, and chemical delivery systems such as the prodrugs; (c) the developed and under-development controlled/continuous drug delivery systems including ocular inserts, collagen shields, ocular films, disposable contact lenses, and other new ophthalmic drug delivery systems; and (d) the newer trends directed towards a combination of drug delivery technologies for improving the therapeutic response of a non-efficacious drug. The fruitful resolution of the above-mentioned technological suggestions can result in a superior dosage form for both topical and intraocular ophthalmic application.

Penetration enhancers and ocular bioadhesives: two new avenues for ophthalmic drug delivery

This review is focused on the two avenues of development that promise a major impact on future ocular drug therapeutics: bioadhesives, including hydrogels and other agents like carbopols, polyacrylic acids, chitosan, etc., and penetration enhancers, including different surfactants, calcium chelators, etc. The capacity of some polymers to adhere to the mucin coat covering the conjunctiva and the corneal surface of the eye forms the basis for ocular mucoadhesion. These systems markedly prolong the residence time of a drug in the conjunctival sac, since clearence is now controlled by the much slower rate of mucus turnover rather than the tear turnover rate. But improving the corneal drug retention alone is inadequate in bringing about a significant improvement of drug bioavailability. Another approach consists of transiently increasing the pentration characteristics of the cornea with appropriate substances, known as penetration enhancers or absorption promoters. The main aim of this article is to give an insight into the potential application of mucoadhesives and corneal penetration enhancers for the conception of innovative opthalmic delivery appraoches, to decrease the systemic side effects, and create a more focused effect, which may be achieved with lower doses of the drug. Ophthalmic formulations based on these mucoadhesives and penetration enhancers are simple to manufacture and exhibit an excellent tolerance when administered into the cornea. The use of the former considerably prolongs the corneal contact time and the use of the latter increases the rate and amount of drug transport. The various corneal epithelial barriers along with the major routes of transport of drugs are discussed. The article includes a list of the various substances in use or under investigation for the aforementioned properties, along with their mechanisms of action. A fair appraisal of the subject with regard to these two therapeutic approaches and any expected ill effects has been made.

Influence of alpha-cyclodextrin and hydroxyalkylated beta-cyclodextrin derivatives on the in vitro corneal uptake and permeation of aqueous pilocarpine-HCl solutions

Interactions in aqueous solution between pilocarpine hydrochloride (P-HCl), a rather hydrophilic drug with good water solubility, and various cyclodextrins (CDs) were described recently. To assess the influence of CDs on the diffusion behavior of pilocarpine, in vitro studies were performed using porcine or bovine corneas as diffusion barriers. The affinity of P-HCl for porcine cornea in the presence of alpha-cyclodextrin (alpha-CD) and (hydroxyethyl)-beta-cyclodextrin (HE-beta-CD) was determined by drug uptake experiments. Additionally, in vitro permeation experiments through bovine corneas were conducted with a modified diffusion device optimized for corneal perfusion studies. The results obtained from the corneal uptake studies indicate that the addition of alpha-CD led to increased tissue drug levels. The increase in permeability of pilocarpine in the presence of alpha-CD was approximately 10-fold (log Papp = -4.87 +/- 0.03) in comparison with plain P-HCl solution (log Papp = -5.89 +/- 0.06). Permeation studies with corneas pretreated with alpha-CD solution revealed enhanced corneal permeability of pilocarpine due to alpha-CD induced membrane effects. The hydroxyalkylated beta-CD derivatives HE-beta-CD (log Papp = -6.27 +/- 0.09) and (hydroxypropyl)-beta-cyclodextrin (HP-beta-CD; log Papp = -6.40 +/- 0.03), however, seemed to cause slightly decreased permeation rates, supporting the concept of an interaction between pilocarpine and the hydroxyalkylated-beta-CD derivatives. Considering physiological compatibility, the addition of CDs seems to be an effective tool to modify and optimize the ocular availability of pilocarpine.

Noninvasive assessment of anesthetic activity of topical lidocaine formulations

The effectiveness of a series of lidocaine formulations in producing anesthesia after topical application was evaluated in human volunteers. The formulations, five suspensions in 20% propylene glycol and one cream, were applied to the forearms for 3 h with occlusion with Hilltop chambers. Testing for anesthesia was performed electrometrically. All lidocaine-containing formulations produced significantly greater anesthesia than the blanks. The formulation containing tetradecyltrimethylammonium bromide produced greater anesthesia than that containing octadecyltrimethylammonium chloride. Changing the pH of the formulation from 7.9 to 10.0 had no significant effect. Other formulations (sodium lauryl sulfate and the cream) were no more effective than the plain formulation without surfactants. The rank order for the suspension formulations was the same as for steady-state permeation in in vitro experiments. However, application of the cream formulation produced greater effect in vivo than was anticipated from in vitro flux values.

Ocular pharmacokinetics of tobramycin. A comparative radioimmunoassay and microbiological assay

The ocular pharmacokinetics of tobramycin were studied in human and rabbit eyes following topical and subconjunctival administration in humans and following intravitreal administration in rabbit eyes. Each aqueous and vitreous sample was analysed by radioimmunoassay and microbiological assay. Topical instillation of 0.3% eyedrops by a loading dose schedule provided rapid penetration of the drug into the aqueous humour with a peak titre of 3.24 mg/l (RIA) at 2-hour intervals and therapeutic levels up to 6 hours. A subconjunctivally administered standard dose of 20 mg/0.5 ml of the drug also showed rapid penetration in therapeutically effective concentration with a peak titre of 22.34 mg/l (RIA) and 16.80 mg/l (MBA) at 2-hour intervals and therapeutic levels of the drug remained up to 20 hours later. A study of vitreous clearance following the administration of a single intravitreal injection in rabbit eyes showed bioavailability of the drug at therapeutically effective levels for more than 72 hours. Mostly two, but sometimes only one sample at each time interval, were studied. Radioimmunoassay and microbiological assay results showed excellent correlation, with correlation coefficient > 0.90 at 1.0% level of probability.

Correlation of water and lidocaine flux enhancement by cationic surfactants in vitro

The penetration of radiolabeled water and lidocaine through human epidermis was studied simultaneously using a flow-through apparatus under infinite-dose conditions. The donors were suspensions of lidocaine in propylene glycol:water mixtures containing cationic surfactants of varying alkyl chain length from three classes: alkyl dimethylbenzyl ammonium halides, alkyl trimethyl ammonium halides, and alkyl pyridinium halides. Each skin sample served as its own control; each was subjected to sequential treatments of control formulation with no surfactant, test formulation, and then a repeat control. Higher surfactant concentration resulted in greater enhancement ratios for both permeants. Peak surfactant enhancement effects were seen at alkyl chain lengths of 12 or 14 carbons. Strong correlation was noted between the enhancement ratios of water and lidocaine. Water permeation can serve as a predictor of the effects of surface-active compounds on the permeation of drugs. The data suggest that water and lidocaine utilize the same pathway through the horny layer.

Mechanism of transcorneal permeation of pilocarpine

The mechanism of transcorneal permeation of pilocarpine has been investigated in relation to the physicochemical properties of the permeating species and its interaction with the membrane biophase. In vitro corneal transport experiments suggested the transport of un-ionized as well as ionized pilocarpine species across the corneal membrane. However, the permeability of the ionized pilocarpine species was 4.818 x 10(-6) cm s-1, a value only one-half of that obtained for the un-ionized pilocarpine species (9.744 x 10(-6) cm s-1). Further evidence of ion transport across the cornea was obtained by examining the transport of the quaternized pilocarpine compound (i.e., pilocarpinium methyl iodide). The quaternized compound had a corneal permeability of 4.66 x 10(-6) cm s-1, similar to that obtained for the ionized pilocarpine species. The lipoidal epithelial layer of the corneal membrane appears to be the predominant barrier to the transport of polar species. Therefore, the transport of pilocarpinium cations across the lipoidal epithelium might have occurred as tightly bound ion pairs with dihydrogen phosphate and/or nitrate counter ions. Excellent linear correlation has been obtained between pilocarpine corneal permeability and the 1-octanol-water partition coefficient as a function of the state of ionization of pilocarpine. The ratio of un-ionized to ionized drug permeability across the cornea is expected to be much higher for drugs with higher 1-octanol-water partition coefficients.

Topical ocular drug delivery: recent developments and future challenges

Existing ocular drug delivery systems are fairly primitive and inefficient, but the stage is set for the rational design of newer and significantly improved systems. The focus of this review is on recent developments in topical ocular drug delivery systems relative to their success in overcoming the constraints imposed by the eye and to the improvements that have yet to be made. In addition, this review attempts to place in perspective the importance of pharmacokinetic modeling, ocular drug pharmacokinetic and bioavailability studies, and choice of animal models in the design and evaluation of these delivery systems. Five future challenges are perceived to confront the field. These are: (a) The extent to which the protective mechanisms of the eye can be safely altered to facilitate drug absorption, (b) Delivery of drugs to the posterior portion of the eye from topical dosing, (c) Topical delivery of macromolecular drugs including those derived from biotechnology, (d) Improved technology which will permit non-invasive monitoring of ocular drug movement, and (e) Predictive animal models in all phases of ocular drug evaluation.

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.

Pharmacokinetics of topically applied ophthalmic drugs

Pharmacokinetic applications in recent years have led to significant advances in systemic drug therapy, but applications to topical, ophthalmic therapy have been far less numerous. Pharmacokinetics is the study of the time course of the absorption, distribution, metabolism, and elimination of an administered drug. Thus, it includes the quantitation of such elements of drug bioavailability as tear dilution, drug binding, and vehicle effects; factors affecting corneal absorption and subsequent tissue distribution; dosage form effects; and pathways of elimination. These factors are important because they significantly influence the clinical responses to administered drugs, in some instances to a profound degree. These elements and their clinical implications are discussed along with some recent applications, exemplified with data from studies on frequently prescribed drugs.

Corneal cytotoxicity of topically applied drugs, vehicles and preservatives

The toxicity and side effects of drugs and their adjuncts are reviewed according to effect on corneal epithelium, stroma, and endothelium, when topically applied to healthy or wounded surfaces, or introduced into the anterior chamber of the eye. Corneal research techniques are reviewed, including scanning and transmission electron microscopy, electrophysiological techniques, and specular microscope observation. Clinical interpretation of these results is discussed. Reviews of specific toxicologic techniques and of medications are cited. Present research makes it increasingly apparent that not only medications but also their preservatives cannot be considered innocuous. Preservatives may be effective yet less damaging to the ocular integument when used at lower concentrations than those presently available commercially. Complete avoidance of preservatives by the use of special dispensing systems is found increasingly desirable.

Mechanistic and quantitative evaluation of precorneal pilocarpine disposition in albino rabbits

The low ocular bioavailability of topically applied pilocarpine is attributed to extensive precorneal drug loss in conjection with the resistance to corneal penetration. Several elements of precorneal loss were reported earlier, but a complete mechanistic understanding has not been available. The present study was designed to gain a better understanding of the mechanisms governing pilocarpine disposition in the precorneal areas as well as the relative influence of these parameters on ocular drug bioavailability. Radioactive pilocarpine and glycerin solutions were instilled into the precorneal area of the albino rabbit eye under various experimental conditions, and the drug concentration in the lacrimal lake was monitored as a function of time. The results demonstrated that nonconjunctival loss of pilocarpine, vasodilation due to the drug, and lacrimation due to vehicle formulation are additional aspects of precorneal drug disposition. The individual influence of all precorneal loss parameters on drug bioavailability was then assessed using a mathematical model formulated from experimental findings on both precorneal and intraocular drug disposition. Drainage and vasodilation, as well as nonconjunctival pilocarpine loss, exerted major influences on drug loss at the absorption site.

Comparing radioactive and trephine-disk bioassays of dicloxacillin and gentamicin in ocular tissues in vitro

Trephined disks of tissue (6 mm diameter) from fresh rabbit eyes were immersed overnight in solutions of radioactive carbon (14C)-dicloxacillin or gentamicin, then assayed by one of two methods: agar-diffusion bioassay, using filter-paper disks immersed in known concentrations of antibiotic as standards, or radioactive assay. The bioassay underestimated the concentrations of antibiotic in sclera and iris by 38 to 72%, and slightly overestimated those in the cornea. A corrected bioassay was calculated, based on the differences in fluid mass between tissue specimens and filter-paper disks; this value was within 20% of the radioactive assay in 18 of 24 experiments with dicloxacillin, and in 14 of 24 with gentamicin. There was evidence of pronounced binding of gentamicin to ocular tissues (iris greater than sclera greater than cornea); this appeared to be a saturable and reversible phenomenon. Less marked binding of dicloxacillin to iris and sclera was noted. The results indicated that, depending on the degree of accuracy required, the corrected trephine-disk agar-diffusion bioassay provides a reasonable estimate of the antibiotic content of ocular tissues.

Electrophysiological and anatomical effects of cetylpyridinium chloride on the rabbit cornea

The effects of cetylpyridinium chloride on the trans-corneal potential difference and the surface anatomy of the cornea have been examined. Concentrations of cetylpyridinium chloride from 0.21 mM to 2 mM were used for either 1 or 2 minute exposure times on the in vitro and in vivo cornea for the electrophysiology studies. The potential difference of the in vitro cornea showed a concentration and exposure-time dependent decrease, the in vivo cornea shows a qualitatively similar behaviour although quantitatively less. The fall in potential difference is preceeded by a hyperpolarization. The scanning electromicroscopy reveals a loss of microvilli and microplicae as well as surface pitting, with some exposure of cells underlying the superficial epithelium. These changes occur in a dose-dependent manner. The effect of cetylpyridinium chloride on the cornea is to enhance the permeability of the superficial cells by destroying the cell membranes and causing lysis of the cells.