Mechanisms of action of some penetration enhancers in the cornea: Laser scanning confocal microscopic and electrophysiology studies

Design and validation of a custom-made system to measure transepithelial electrical impedance in human corneas preserved in active storage machine

Corneal epithelial barrier represents one of the major limitations to ocular drug delivery and can be explored non-invasively through the evaluation of its electrical properties. Human corneas stored in active storage machine (ASM) could represent an interesting physiological model to explore transcorneal drug penetration. We designed a new system adapted to human corneas preserved in ASM to explore corneal epithelial barrier function ex-vivo. A bipolar set-up including Ag/AgCl electrodes adaptors to fit the corneal ASM and a dedicated software was designed and tested on freshly excised porcine corneas (n = 59) and human corneas stored 14 days in ASM (n = 6). Porcine corneas presented significant and proportional decrease in corneal impedance in response to increasing-size epithelial ulcerations and acute exposure to benzalkonium chloride (BAC) 0.01 and 0.05%. Human corneas stored 14 days in ASM presented a significant increase in corneal impedance associated with the restoration of a multi-layer epithelium and an enhanced expression of tight junctions markers zonula occludens 1, claudin 1 and occludin. These results support the relevance of the developed approach to pursue the exploration and development of human corneas stored in ASM as a physiological pharmacological model.

Promoting effect of the Maillard reaction products produced during the stir-frying process of Hordei Fructus Germinatus on the intestinal absorption of active ingredients in Hordei Fructus Germinatus

This study was designed to evaluate the absorption promoting capacity of Maillard Reaction Products (MRPs) produced during the stir-frying process of Hordei Fructus Germinatus on catechin, ferulic acid, quercetin and kaempferol by the ex vivo rat everted gut sac model, in situ single-pass intestinal perfusion model and the whole animal model. Moreover, verapamil, EDTA and mannitol were used for determining the transport mechanism of catechin, ferulic acid, quercetin and kaempferol. The tight junction (TJ) proteins including zonula occudens-1(ZO-1) and claudin-1 were chosen to investigate the promoting mechanism of MRPs by quantitative real-time PCR (qRT-PCR) and western blot analyses. The results showed that the MRPs produced during the stir-frying process of Hordei Fructus Germinatus could improve the intestinal absorption of catechin, ferulic acid, quercetin and kaempferol. And the absorption-promoting effect of MRPs was related to chelating effect and the reduced expression of claudin-1 and ZO-1. Our results suggested that MRPs could be promising oral absorption promoters, which might be another processing mechanism of Hordei Fructus Germinatus.

Effect of pH and penetration enhancers on cysteamine stability and trans-corneal transport

Ocular cystinosis is a rare metabolic disorder characterized by the presence of insoluble cystine crystals inside the corneal stroma, with consequent photophobia, keratopathies and frequent corneal erosions. The current therapy consists in the lifetime ophthalmic administration of cysteamine, drug characterized by extremely high hydrophilicity, low molecular weight (77g/mol), and easy oxidization to disulfide. Ocular delivery of cysteamine is very challenging, for its poor permeability and stability in solution. The purpose of the present paper was to study the impact of formulation pH and composition on (1) the trans-corneal delivery and (2) the stability in solution of cysteamine, with particular focus on the use of alpha-cyclodextrin (α-CD), benzalkonium chloride (BAC) and disodium edetate (EDTA). Permeation experiments were performed ex vivo through freshly excised porcine cornea; stability was evaluated for six months at -20°, +4° and +25°C; irritation potential was evaluated using HET-CAM assay. The results showed that cysteamine trans-corneal diffusion is strictly dependent on both pH (7.4 preferred to 4.2) and buffering capacity, that negatively impact on the permeation; EDTA did not enhance the trans-corneal diffusion of cysteamine neither at pH 7.4 nor at pH 4.2, while benzalkonium chloride (BAC), antimicrobial agent present within commercial eye-drops, significantly enhanced it. Notably, α-CD was able to promote the trans-corneal diffusion of cysteamine and, at a 5.5%, a 4-fold higher penetration compared to the BAC-containing formulation was obtained. EDTA and acidic pH demonstrated to be essential for cysteamine stability. The formulation obtained by combining α-CD and EDTA was characterized by significant permeation, good stability profile, and no irritation potential, even if the tolerability should be further confirmed by in vivo test.

A novel in vivo corneal trans-epithelial electrical resistance measurement device

PURPOSE

To develop a device that is capable of easily measuring corneal transepithelial electrical resistance (TER) and changes in the corneal barrier function.

METHODS

We had previously developed an in vivo method for measuring corneal TER using intraocular electrode. This method can be used to precisely measure the decline of the corneal barrier function after instillation of benzalkonium chloride (BAC). In order to lessen the invasiveness of that procedure, we further refined the method for measuring the corneal TER by developing electrodes that could be placed on the cornea and in the conjunctival sac instead of inserting them into the anterior chamber. TER was then calculated by subtracting the electrical resistance, which lacked the corneal epithelial input, from the whole electrical resistance that was measured between the electrodes. Slit lamp examination and scanning electron microscopy (SEM) were used to determine safety of the new device. Corneal TER changes after exposure to 0.02% BAC were determined using the new device as well as SEM and transmission electron microscopy (TEM).

RESULTS

Slit lamp examination before and after exposure of rabbits' corneas to the sensor confirmed safety of the device. SEM examination revealed no difference of the corneal epithelium which exposed to the new device with normal corneas. SEM and TEM pictures revealed damaged microvilli and tight junctions after instillation of 0.02% BAC. TER change after treatment with 0.02%BAC was similar to those determined by the established anterior chamber method.

CONCLUSION

We succeeded to develop a less invasive device for corneal TER measurement in vivo in animals. This new device may be applicable in the future for clinical use in humans.

Effect of deoxycholate conjugation on stability of pDNA/polyamidoamine-diethylentriamine (PAM-DET) polyplex against ionic strength

Polyplexes formed from cationic polymer/pDNA have been known to be vulnerable to external ionic strength. To improve polyplex stability against ionic strength, we attempted the chemical conjugation of the hydrophobic deoxycholate (DC) moiety to the polyamidoamine-diethylenetriamine (PAM-DET) dendrimer. Dynamic light scattering studies showed that the tolerance of the resulting PAM-DET-DC against ionic strength is higher than that of PAM-DET. In addition, we confirmed that the stability of polyplex has a strong relationship with the degree of conjugation of the DC moiety to the PAM-DET dendrimer and the charge ratio of PAM-DET-DC. Furthermore, the transfection efficiency of the PAM-DET-DC polyplex is higher than that of PAM-DET but its cytotoxicity remains the same. Therefore, the chemical conjugation of DC is a safe and effective method for increasing the stability of supramolecules formed from electrostatic interaction.

Polyoxyethylated nonionic surfactants and their applications in topical ocular drug delivery

Topical dosing of ophthalmic drugs to the eye is a widely accepted route of administration because of convenience, ease of use, and non-invasiveness. However, it has been well recognized that topical ocular delivery endures a low bioavailability due to the anatomical and physiological constraints of the eye which limit drug absorption from the pre-corneal surface. Nonionic surfactants as versatile functional agents in topical ocular drug delivery systems are uniquely suited to meet the challenges through their potential ability to increase bioavailability by increasing drug solubility, prolonging pre-corneal retention, and enhancing permeability. This review attempts to place in perspective the importance of polyoxyethylated nonionic surfactants in the design and development of topical ocular drug delivery systems by assessing their compatibility with common ophthalmic inactive ingredients, their impact on product stability, and their roles in facilitating ocular drugs to reach the target sites.

Eye drop delivery of nano-polymeric micelle formulated genes with cornea-specific promoters

BACKGROUND

This study evaluates the eye drop delivery of genes with cornea-specific promoters, i.e., keratin 12 (K12) and keratocan (Kera3.2) promoters, by non-ionic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) polymeric micelles (PM) to mouse and rabbit eyes, and investigates the underlying mechanisms.

METHODS

Three PM-formulated plasmids (pCMV-Lac Z, pK12-Lac Z and pKera3.2-Lac Z) containing the Lac Z gene for beta-galactosidase (beta-Gal) whose expression was driven by the promoter of either the cytomegalovirus early gene, the keratin 12 gene or the keratocan gene, were characterized by critical micelle concentration (CMC), dynamic light scattering (DLS), and atomic force microscopy (AFM). Transgene expression in ocular tissue after gene delivery was analyzed by 5-bromo-4-chloro-3-indolyl-beta-D-galactoside (X-Gal) color staining, 1,2-dioxetane beta-Gal enzymatic activity measurement, and real-time polymerase chain reaction (PCR) analysis. The delivery mechanisms of plasmid-PM on mouse and rabbit corneas were evaluated by EDTA and RGD (arginine-glycine-aspartic acid) peptide.

RESULTS

The sizes of the three plasmid-PM complexes were around 150-200 nm with unimodal distribution. Enhanced stability was found for three plasmid-PM formulations after DNase I treatment. After six doses of eye drop delivery of pK12-Lac Z-PM three times a day, beta-Gal activity was significantly increased in both mouse and rabbit corneas. Stroma-specific Lac Z expression was only found in pKera3.2-Lac Z-PM-treated animals with pretreatment by 5 mM EDTA, an opener of junctions. Lac Z gene expression in both pK12-Lac Z-PM and pKera3.2-Lac Z-PM delivery groups was decreased by RGD peptide pretreatment.

CONCLUSIONS

Cornea epithelium- and stroma-specific gene expression could be achieved using cornea-specific promoters of keratin 12 and keratocan genes, and the gene was delivered with PM formulation through non-invasive, eye drop in mice and rabbits. The transfection mechanism of plasmid-PM may involve endocytosis and particle size dependent paracellular transport.

Pharmaceutical applications of confocal laser scanning microscopy: the physical characterisation of pharmaceutical systems

The application of confocal laser scanning microscopy (CLSM) to the physicochemical characterisation of pharmaceutical systems is not as widespread as its application within the field of cell biology. However, methods have been developed to exploit the imaging capabilities of CLSM to study a wide range of pharmaceutical systems, including phase-separated polymers, colloidal systems, microspheres, pellets, tablets, film coatings, hydrophilic matrices, and chromatographic stationary phases. Additionally, methods to measure diffusion in gels, bioadhesives, and for monitoring microenvironmental pH change within dosage forms have been utilised. CLSM has also been used in the study of the physical interaction of dosage forms with biological barriers such as the eye, skin and intestinal epithelia, and in particular, to determine the effectiveness of a plethora of pharmaceutical systems to deliver drugs through these barriers. In the future, there is continuing scope for wider exploitation of existing techniques, and continuing advancements in instrumentation.

EDTA-induced membrane fluidization and destabilization: biophysical studies on artificial lipid membranes

The molecular mechanism of ethylenediaminetetraacetic acid (EDTA)-induced membrane destabilization has been studied using a combination of four biophysical techniques on artificial lipid membranes. Data from Langmuir film balance and epifluorescence microscopy revealed the fluidization and expansion effect of EDTA on phase behavior of monolayers of either 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or mixtures of DPPC and metal-chelating lipids, such as N(alpha),N(alpha)-Bis[carboxymethyl]-N(epsilon)-[(dioctadecylamino)succinyl]-L-lysine or 1,2-dioleoyl-sn-glycero-3-[N-(5-amino-1-carboxypentyl iminodiacetic acid) succinyl]. A plausible explanation could be drawn from the electrostatic interaction between negatively charged groups of EDTA and the positively charged choline head group of DPPC. Intercalation of EDTA into the lipid membrane induced membrane curvature as elucidated by atomic force microscopy. Growth in size and shape of the membrane protrusion was found to be time-dependent upon exposure to EDTA. Further loss of material from the lipid membrane surface was monitored in real time using a quartz crystal microbalance. This indicates membrane restabilization by exclusion of the protrusions from the surface. Loss of lipid components facilitates membrane instability, leading to membrane permeabilization and lysis.

Acute corneal epithelial change after instillation of benzalkonium chloride evaluated using a newly developed in vivo corneal transepithelial electric resistance measurement method

OBJECTIVE

Acute corneal permeability change after instillation of benzalkonium chloride (BAC) was evaluated using a newly developed in vivo corneal transepithelial electric resistance (TER) measurement method.

METHOD

Corneal TER was measured by Ag/AgCl electrodes placed in the anterior aqueous chamber and on the cornea of live rabbit eyes. TER was measured and TER change after instillation of 0.05% BAC solution was monitored. After TER measurement, cornea was excised and fixed for transmission and scanning electron microscopy. For the control study, physiologic saline was used instead of BAC.

RESULTS

The TER of normal rabbit cornea was 602.3 +/- 195.0 Omega cm(2). TER decreased instantly after instillation of 0.05% BAC. In 5 s, TER decreased to 58.3 +/- 5.2%. In 60 s, TER decreased to 18.5 +/- 3.2%. At all time points, TER after instillation of 0.05% BAC was significantly lower than that of the control (p < 0.0001). Dissociation of tight junctions and the destruction of superficial cell membranes were observed under electron microscopy.

CONCLUSION

Corneal epithelial change with increased permeability is rapid and intense after the instillation of highly concentrated BAC solution, accompanied by disorder of tight junctions and cell membranes of superficial cells. The newly developed in vivo corneal TER measurement method is suitable for assessing acute corneal change after drug instillation.

Effect of formulation factors on in vitro permeation of moxifloxacin from aqueous drops through excised goat, sheep, and buffalo corneas

The purpose of this investigation was to evaluate the effect of formulation factors on in vitro permeation of moxifloxacin from aqueous drop through freshly excised goat, sheep, and buffalo corneas. Aqueous isotonic ophthalmic solutions of moxifloxacin hydrochloride of different concentrations (pH 7.2) or 0.5% (wt/vol) solutions of different pH or 0.5% solutions (pH 7.2) containing different preservatives were made. Permeation characteristics of drug were evaluated by putting 1 mL formulation on freshly excised cornea (0.50 cm(2)) fixed between donor and receptor compartments of an all-glass modified Franz diffusion cell and measuring the drug permeated in the receptor (containing 10 mL bicarbonate ringer at 37 degrees C under stirring) by spectrophotometry at 291 nm, after 120 minutes. Statistical analysis was done by one-way analysis of variance (ANOVA) followed by Dunnett's test. Increase in drug concentration in the formulation resulted in an increase in the quantity permeated but a decrease in percentage permeation. Increase in pH of the solution from 5.5 to 7.2 increased drug permeation, indicating pH-dependent transport. Compared with control formulation, moxifloxacin 0.5% (wt/vol) solution (pH 7.2) containing disodium edetate (EDTA) (0.01% wt/vol) produced significantly (P < .05) higher permeation with all the corneas. Formulation with benzyl alcohol significantly (P < .05) increased permeation with buffalo cornea compared with its control. Presence of benzalkonium chloride (BAK) (0.01% wt/vol) and EDTA (0.01% wt/vol) in the formulation increased permeation to the maximum with all the corneas. The results suggest that moxifloxacin 0.5% ophthalmic solution (pH 7.2) containing BAK (0.01%) and EDTA (0.01%) provides increased in vitro ocular availability through goat, sheep, and buffalo corneas.

Mechanism of permeability-enhancing effect of EDTA and boric acid on the corneal penetration of 4-[1-hydroxy-1-methylethyl]-2-propyl-1-[4-[2-[tetrazole-5-yl]phenyl]phenyl] methylimidazole-5-carboxylic acid monohydrate (CS-088)

This study was conducted to clarify the penetration properties of 4-[1-hydroxy-1-methylethyl]-2-propyl-1-[4-[2-[tetrazole-5-yl]phenyl]phenyl]methylimidazole-5-carboxylic acid monohydrate (CS-088), an ophthalmic agent, and the mechanism of the permeability-enhancing effect of EDTA and boric acid (EDTA/boric acid) on the corneal penetration of CS-088. In the absence of additives, corneal permeability decreased with increasing concentration of CS-088 as CS-088 monomers self-associate to form dimers. Presence of EDTA/boric acid caused no significant changes in the physicochemical properties of CS-088, the apparent partition coefficient or the mean particle size of CS-088. EDTA/boric acid induced only a slight change in the zeta potential of liposomes used as a model of the biological membrane. On the other hand, EDTA/boric acid significantly increased membrane fluidity of liposomes, whereas other buffering agents tested did not. This effect was synergistic and concentration-dependent for both EDTA and boric acid as was observed in in vitro corneal penetration of CS-088. In accordance with the result, the rate of CS-088 permeation into the liposomes significantly increased by the addition of EDTA/boric acid. Therefore, it was demonstrated that EDTA/boric acid promotes corneal penetration of CS-088 through the transcellular pathway by increasing membrane fluidity. Conversely, other buffering agents decreased corneal permeability of CS-088 by inducing further self-association of CS-088 aggregates.

Synergistic effect of EDTA and boric acid on corneal penetration of CS-088

In order to investigate the effects of EDTA and boric acid (EDTA/boric acid) on the corneal penetration of CS-088, an ophthalmic agent, the apparent permeability coefficient of CS-088 in the presence of EDTA/boric acid across the isolated corneal membranes of rabbits was measured using an in vitro penetration chamber system. FITC-dextran (M.W. 4400) and an electrical method based on membrane resistance were used to provide a quantitative assessment of the enhancing effect of EDTA/boric acid. The corneal penetration of CS-088 was significantly enhanced in the presence of EDTA/boric acid by approximately 1.6-fold. The permeability-enhancing effect of EDTA/boric acid was apparently synergistic and concentration-dependent on both EDTA and boric acid. The penetration of FITC-dextran, a paracellular marker, and electrical resistance of corneal membranes were not affected in the presence of EDTA/boric acid. Furthermore, no enhancing effect of EDTA/boric acid was observed in de-epithelialized corneas, although de-epithelialized corneas exhibited a markedly higher permeability of CS-088 that was 24-fold greater than that for intact corneas. In conclusion, EDTA/boric acid synergistically enhances the transcellular permeability of CS-088 in the outer layer but not in the inner layers of the corneal membrane.

Nonionic Polymeric Micelles for Oral Gene Delivery In Vivo

The main aim of this study was to investigate the feasibility of using nonionic polymeric micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) as a carrier for oral DNA delivery in vivo. The size and appearance of DNA/PEO-PPO-PEO polymeric micelles were examined, respectively, by dynamic light scattering and atomic force microscopy, and their zeta potential was measured. Expression of the delivered lacZ gene in various tissues of nude mice was assessed qualitatively by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining of sections and quantitatively by measuring enzyme activity in tissue extracts, using the substrate of beta-galactosidase, chlorophenol red-beta-D-galactopyranoside. In addition, the types of cells expressing the lacZ gene in the duodenum were identified by histological analysis. DNA/PEO-PPO-PEO polymeric micelles are a single population of rounded micelles with a mean diameter of 170 nm and a zeta potential of -4.3 mV. Duodenal penetration of DNA/PEO-PPO-PEO polymeric micelles was evaluated in vitro by calculating the apparent permeability coefficient. The results showed a dose-independent penetration rate of (5.75 +/- 0.37) x 10(-5) cm/sec at low DNA concentrations (0.026-0.26 microg/microl), but a decrease to (2.89 +/- 0.37) x 10(-5) cm/sec at a concentration of 1.3 microg/microl. Furthermore, when 10 mM RGD peptide or 10 mM EDTA was administered before and concurrent with the administration of DNA/PEO-PPO-PEO polymeric micelles, transport was inhibited ([0.95 +/- 0.57] x 10(-5) cm/sec) by blocking endocytosis or enhanced ([29.8 +/- 5.7] x 10(-5) cm/sec) by opening tight junctions, respectively. After oral administration of six doses at 8-hr intervals, the highest expression of transferred gene lacZ was seen 48 hr after administration of the first dose, with gene expression detected in the villi, crypts, and goblet cells of the duodenum and in the crypt cells of the stomach. Reporter gene activity was seen in the duodenum, stomach, and liver. Activity was also seen in the brain and testis when mice were administered 10 mM EDTA before and concurrent with DNA/PEO-PPO-PEO polymeric micelle administration. lacZ mRNA was detected in these five organs and in the blood by reverse transcription-polymerase chain reaction. Taken together, these results show efficient, stable gene transfer can be achieved in mice by oral delivery of PEO-PPO-PEO polymeric micelles.

Ocular toxicity of some corneal penetration enhancers evaluated by electrophysiology measurements on isolated rabbit corneas

The influence on electrical resistance and membrane potential of rabbit corneas in vitro of some chemicals used as adjuvants in ophthalmic formulations was investigated, in the attempt to correlate changes in electrophysiological properties of the corneal tissue (possibly indicative of toxic/damaging effects to the corneal epithelium), with the promoting effect of the substances on transcorneal permeation in vitro of timolol maleate (TM). The chemicals, tested at different concentrations, were benzalkonium chloride (BAC), sodium ethylenediaminetetraacetate (EDTA), polyoxyethylene-20-stearyl ether (PSE), polyethoxylated castor oil (PCO), deoxycholic acid sodium salt (DC) and cetylpyridinium chloride (CPC). For these substances, definite correlations were found between promoting activity for permeation of TM and modification of electrophysiological parameters. These parameters were in all cases significantly altered by all agents at all concentrations after a 5-h contact. However, after a 1-h contact, 0.001% PSE and CPC did not significantly modify the corneal resistance, while PCO and PSE did not significantly modify the transcorneal potential at the tested concentrations. Only 0.001% PSE, a nonionic surfactant used as solubilizer and emulsifier, active as promoter for TM, did not modify both electrophysiological parameters to a significant extent after 1 h. The results of this study indicate correlations between ocular toxicity, promoting activity for transcorneal permeation of timolol and modification of the electrophysiological parameters.

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.

Increased corneal hydration induced by potential ocular penetration enhancers: assessment by differential scanning calorimetry (DSC) and by desiccation

The corneal toxicity of some surfactants of possible use as ocular penetration enhancers was investigated by measuring their effect on hydration of rabbit corneas 'in vitro'. The tested substances were benzalkonium chloride (BAC), cetylpyridinium chloride (CPC), ethylenediaminetetraacetic acid disodium salt (EDTA), polyoxyethylene-20-stearyl ether (Brij 78, PSE), polyethoxylated castor oil (Cremophor EL, PCO) and sodium deoxycholate (DC). Freshly excised corneas, mounted in perfusion cells, were kept in contact for 1 h with solutions of these agents; corneal hydration was then evaluated by measuring: (a) their total (free+bound) water content by desiccation (gravimetric analysis); and (b) their free water content by differential scanning calorimetry (DSC). The DSC measurements also provided a rough quantitative estimate of corneal solutes. All tested agents significantly influenced corneal hydration, evidently as a consequence of alteration of the corneal epithelium. Although a brief contact with the precorneal tissues 'in vivo' may not prove harmful, the use of these compounds as potential ocular permeation enhancers or otherwise as ingredients of topical ocular formulations for long-term use should be considered with caution.

Ocular tolerance of absorption enhancers in ophthalmic preparations

The use of absorption promoters is a way to improve the bioavailability and therapeutic response of topically applied ophthalmic drugs. The ocular tolerance of 9 potential absorption promoters was investigated as well as the influence of the enhancers' concentration on the ocular tolerance. The substances tested were instillated repetitively (4 times per day, during 3 days, and once just before examination) as aqueous solutions onto rabbit corneas. Fluorescein dyeing enabled us to specifically mark corneal damage that was observed by confocal microscopy. The degree of corneal injury was assessed with an image-processing system that calculated the total fluorescent areas. Confocal microscopy results showed the relatively good tolerance of permeation enhancers like dimethyl sulfoxide (DMSO), decamethonium, edetate, glycocholate, and cholate in contrast to the poorly tolerated saponin and fusidate. Increasing the promoters' concentration led generally to an increase in corneal lesions.

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.

Confocal microscopy as a tool for the investigation of the anterior part of the eye

In recent years, confocal microscopy has become a powerful tool for examining microscopic structures in the living eye. The decisive advantage of this technique is that it permits the investigation of optical sections of relatively thick (> 10 microns) specimens. Because confocal microscopy suppresses the out-of-focus blur, sharp three-dimensional images with excellent resolution can be obtained. Confocal microscopy is therefore able to provide more information than the classic methods--i.e., specular microscopy and slit-lamp biomicroscopy. This paper reviews recent applications of confocal microscopy in three fields of ophthalmology: the observation of the anatomy of the anterior parts of the eye, the investigation of these structures after local administration of drugs and, finally, the use of this technique for the diagnosis of infectious ocular diseases.

Effect of benzalkonium chloride/EDTA on the ocular bioavailability of ketorolac tromethamine following ocular instillation to normal and de-epithelialized corneas of rabbits

This study was designed to examine the effect of benzalkonium chloride/ethylenediaminetetraacetic acid (BAK/EDTA) on the ocular bioavailability (Focular) of ketorolac tromethamine after ocular instillation to normal and de-epithelialized corneas of rabbits both in vitro and in vivo. The in vitro Focular of the formulations was measured in flow-through perfusion chambers. For in vivo studies, a 35 microL dose of 0.5% ketorolac tromethamine with or without BAK/EDTA was instilled into rabbit eyes with intact or de-epithelialized corneas. At 0.5, 1, 2, 4, 6, and 8 h postdose, rabbits were euthanized, and the corneas and aqueous humor were collected from both eyes. The ketorolac concentrations from both in vivo and in vitro samples were quantified by reversed-phase high-performance liquid chromatography. The in vitro study results indicated that BAK/EDTA statistically significantly increased the Focular of ketorolac through de-epithelialized corneas but not through intact corneas. The in vivo study results showed that BAK/EDTA had no effect on the Focular of ketorolac in rabbits with intact corneas, based on the values of the area under the aqueous humor concentration versus time curves (AUC0-6h) of ketorolac. As expected, de-epithelialization of the corneas produced a faster and greater ocular absorption of ketorolac as evidenced by the smaller Tmax and larger AUC values compared to those for the intact corneas in vivo. However, BAK/EDTA decreased the ocular absorption of ketorolac in rabbits with de-epithelialized corneas. The half-lives (t 1/2) of ketorolac in corneal tissue and aqueous humor were longer in rabbits with intact corneas than those in rabbits with de-epithelialized corneas. In conclusion, the in vivo Focular of ketorolac was not altered by BAK/EDTA in rabbits with intact corneas, but it was decreased by BAK/EDTA in rabbits with de-epithelialized corneas. Therefore, the formulation with ketorolac alone may be better as a post-operative ocular analgesic.