Relationship between steroid permeability across excised rabbit cornea and octanol-water partition coefficients

Ocular Permeability Screening of Dexamethasone Esters Through Combined Cellular and Tissue Systems

Dexamethasone transport across ocular epithelium was evaluated by means of permeability studies on a series of ester prodrugs with the aim of identifying the most promising candidates for the treatment of the ocular surface. Organotypic conjunctival bovine epithelial cell cultures were assumed representative of an average ocular epithelium and used to describe the mechanism of permeation. Permeability coefficients were also determined in excised rabbit corneas set up and in vivo pharmacokinetic experiments. All dexamethasone esters permeated through the transcellular route and their permeability coefficient rose with the increase of the molecules lipophilicity until a maximum was reached in correspondence of dexamethasone butyrate (Log P = 3.95). It was found that esters hydrolysis occurring in various extent along the transport process, affected the overall permeability rate. There was evidence that the permeation process can be confined at the ocular epithelium layer if the ester is highly hydrophobic and not susceptible of fast hydrolysis.

Intraocular availability of topically applied mycophenolate mofetil in rabbits

The efficacy of mycophenolate mofetil (MMF), a prodrug of mycophenolic acid (MPA), in high risk keratoplasties and ocular immune-mediated diseases has been shown in recent years. As peroral administration of MMF can cause various side effects, topical application should be considered. This study investigates the intraocular availability of MMF and MPA in the rabbit eye. An eye drop solution (MMF-CD; 1% MMF/10% hydroxypropyl-beta-cyclodextrin (HP-beta-CD)) or a 1% aqueous suspension (MMF-SP) was instilled into the lower cul-de-sac of the right eye of each rabbit. Rabbits (each group: n = 4) were put down after 30, 60 and 240 min. Aqueous humor, vitreous, cornea, sclera, conjunctiva, iris-ciliary-body, and plasma were isolated. Several extraction procedures were performed in order to quantify the drug by HPLC. The aqueous humor concentration of the active metabolite MPA was 24 microg/mL after 30 min for both preparations. The ratio of the MPA concentrations after 30, 60, and 240 min was 1 : 2 : 0.07 for MMF-CD and 1 : 0.6 : 0.04 for MMF-SP, respectively. MPA levels in the cornea were 90.78 / 56.90 / 4.08 x 10(-6) microg/microg for MMF-CD, whereas MMF-SP resulted in MPA levels of 102.65 / 31.18 / 2.59 x 10(-6) microg/microg at the three time points. As a high concentration of the active drug MPA in cornea and aqueous humor is desired, e.g. following corneal transplantation, the MMF/HP-beta-CD formulation could be an useful topical treatment. Furthermore, the present study shows that MMF-CD is superior to MMF-SP by increasing intraocular availability.

Pilocarpine permeability across ocular tissues and cell cultures: influence of formulation parameters

In vitro permeation studies of drugs across biological barriers are promising tools for estimating the quality and quantity of drug transport in vivo. The objective of this work was to compare the permeability of the hydrophilic model drug pilocarpine-HCl (P-HCl) through different ocular tissues and cell cultures: isolated pig cornea (PCr) and sclera (PSc), rabbit conjunctiva (RCo), and rabbit conjunctival (RCoEC) or corneal epithelial cell culture (RCrEC). Furthermore, the study included investigations about the influence of the excipients benzalkonium chloride (BAC) and ethylene diamine tetra acetic acid disodium salt (EDTA) on the permeability of the small drug. In general, BAC caused a facilitated drug transport, while EDTA hardly influenced the P-HCl concentration on the acceptor side, except for RCoEC. Additionally, the impact of variation in buffer solution pH and tonicity on drug transport in both cell cultures was tested. The higher the tonicity of the buffer solution (80, 300, and 600 mOsm/kg) the lower the permeability coefficient (P(eff)). At different pH values (6.4, 7.4, and 8.4) the P(eff) showed a directly proportional demeanor. In summary, a good correlation between the isolated tissues and cell cultures with regard to P-HCl transport could be observed.

Cannabinoids in the treatment of glaucoma

The leading cause of irreversible blindness is glaucoma, a disease normally characterized by the development of ocular hypertension and consequent damage to the optic nerve at its point of retinal attachment. This results in a narrowing of the visual field, and eventually results in blindness. A number of drugs are available to lower intraocular pressure (IOP), but, occasionally, they are ineffective or have intolerable side-effects for some patients and can lose efficacy with chronic administration. The smoking of marijuana has decreased IOP in glaucoma patients. Cannabinoid drugs, therefore, are thought to have significant potential for pharmaceutical development. However, as the mechanism surrounding their effect on IOP initially was thought to involve the CNS, issues of psychoactivity hindered progress. The discovery of ocular cannabinoid receptors implied an explanation for the induction of hypotension by topical cannabinoid applications, and has stimulated a new phase of ophthalmic cannabinoid research. Featured within these investigations is the possibility that at least some cannabinoids may ameliorate optic neuronal damage through suppression of N-methyl-D-aspartate receptor hyperexcitability, stimulation of neural microcirculation, and the suppression of both apoptosis and damaging free radical reactions, among other mechanisms. Separation of therapeutic actions from side-effects now seems possible through a diverse array of novel chemical, pharmacological, and formulation strategies.

Ocular drug permeation following experimental excimer laser treatment on the isolated pig eye

Excimer laser photorefractive keratectomy (PRK) is a well-established procedure which is frequently applied to correct myopia. Since structural alterations of the corneal epithelium occur after the treatment, a different drug permeation can be assumed. To investigate the effects of PRK on drug permeation, excimer laser ablations with varying depths were performed on isolated pig eyes. The permeation of lipophilic (diclofenac-sodium; D-Na) and hydrophilic (pilocarpine-hydrochloride; P-HCl model drugs were studied in vitro. Under these experimental conditions, P-HCl demonstrated a significant (p < 0.05) enhancement of permeation in relation to the ablation depth. In contrast, corneal epithelial thickness scarcely influenced the permeation rate of D-Na. Not until removing the entire epithelium did a significantly increased permeability occur, when compared to untreated cornea. These results suggest that PRK may significantly reduce the corneal barrier function and alter pharmacokinetics of topical medication.

A simple model for the prediction of corneal permeability

We derive a simple model for the prediction of corneal permeability of 30 miscellaneous compounds using three descriptors: logP=-5.566Q(H)2+3.027Q(H)-0.155Q(O,N)-9.413x10(-4) V-4.278 (n=30,r=0.921). P is the permeability coefficient across excised rabbit cornea (cm/s), Q(O,N) is the sum of the absolute values of the net atomic charges of oxygen and nitrogen atoms, Q(H) is the sum of net atomic charges of hydrogen atoms attached to these heteroatoms, and V is the molecular volume. These descriptors can be easily obtained from quantum chemical calculation. The model is suitable for the rapid prediction of the corneal permeability of drugs.

In vitro corneal permeation of unoprostone isopropyl (UI) and its metabolism in the isolated pig eye

The corneal permeability, hydrolysis, and metabolism of unoprostone isopropyl (UI), a docosanoid, were examined in isolated porcine ocular tissues. The apparent permeability coefficient (Papp) of the esterified prodrug and of the acid metabolite were determined in a modified Valia-Chien permeation chamber and quantified by high performance liquid chromatography. Enzymatic hydrolysis and subsequent metabolism were examined in isolated tissue homogenates. The prodrug (ester form) was found to permeate the isolated intact porcine cornea with a Papp of 9.47 x 10(-7) cm/sec. Only the acid metabolite could be detected in the receiver chamber, indicating the requirement of hydrolysis for permeation. The acid metabolite could not permeate the intact cornea but was able to cross an epithelium-denuded cornea with a Papp of 1.22 x 10(-6) cm/sec. Enzymatic hydrolysis of UI was confined to the isolated intact cornea and epithelium, indicating that the esterase activity was localized in the corneal epithelium. Incubations with different porcine ocular tissues, conjunctiva, iris-ciliary body, trabeculum, as well as aqueous humor, did not reveal other metabolites. These findings demonstrate that the ocular penetration of UI is dependent on its uptake into the epithelium and subsequent hydrolysis prior to its penetration into the anterior chamber, a very common pathway for ophthalmic drugs. In the pig eye, unoprostone does not appear to be further metabolized.

Permeability of chemical delivery systems across rabbit corneal (SIRC) cell line and isolated corneas: a comparative study

The purpose of this study was to investigate the corneal permeability of phenylephrone chemical delivery systems (CDS) across isolated cornea and to evaluate the utility of the SIRC cell line (epithelial cells originating from rabbit cornea) as an in vitro model for predicting the ocular permeability. The effect of benzalkonium chloride (BAC) on the drug permeability through SIRC cell layers was also studied. The transport of phenylephrone CDS across the isolated cornea of the albino rabbit was measured at various pH values using a two-chamber glass diffusion cell, and the results were compared with the reported permeability values across SIRC cells of rabbit origin. Corneal membranes showed lower flux values for compounds, especially for hydrophilic compounds, than the SIRC cell line. A significant correlation was observed between the permeability coefficients through corneal membranes and SIRC cells. When the pH of the transport medium was increased, the permeability coefficients increased and lag times decreased in both in vitro models. Furthermore, both in vitro models showed significant correlation between permeability coefficients and lipophilicities of the drugs. The three esters, having higher lipophilic characteristics, showed higher permeability than phenylephrine HCl. The phenylacetyl ester of phenylephrone showed a three-fold increase in penetration across SIRC cell layers in the presence of 0.01% BAC. These results suggest that the use of SIRC cell layers can reasonably predict the permeability of ophthalmic drugs across corneal membranes.

In-vitro hydrolysis, permeability, and ocular uptake of prodrugs of N-[4-(benzoylamino)phenylsulfonyl]glycine, a novel aldose reductase inhibitor

To enhance the ocular uptake of N-[4-(benzoylamino)phenylsulfonyl]glycine (BAPSG), two ester (methyl and isopropyl) prodrugs were synthesized and evaluated for their stability in various buffers (pH 1-9), hydrolysis in rabbit ocular tissues (cornea, conjunctiva, iris-ciliary body, lens, aqueous humor, and vitreous humor), transport across cornea and conjunctiva, and in-vivo uptake following topical administration. Over the pH range of 1-9, the rate constants for degradation ranged from 5.67 to 218.9 x 10(-3) h(-1) for the methyl ester and from 3.14 to 4.45 x 10(-3) h(-1) for the isopropyl ester. At all pH conditions, the isopropyl ester was more stable when compared with the methyl ester. A change in buffer concentration at pH 7.4 did not influence the stability of the prodrugs. The prodrugs were rapidly hydrolysed in the tissue homogenates, with the rate constants for hydrolysis ranging from 1.98 to 7.2x 10(-3) min(-1) for the methyl ester and 3.32 to 6.53 x 10(-3) min(-1) for the isopropyl ester. The in-vitro permeability of the methyl ester was less than the parent drug across cornea and conjunctiva. Isopropyl ester levels were not detectable in the receiver chamber even at the end of the 4-h transport study. Following topical administration of BAPSG and the two prodrugs at a dose of 60 microg/eye, the lowest levels were seen in vitreous humor for parent compound and its methyl ester. In general, the tissue uptake of methyl ester was less than BAPSG. Isopropyl ester levels were below detection limits in all the ocular tissues. Lipophilic ester prodrugs of BAPSG showed good aqueous solution stability in tissue homogenates. However, these prodrugs lacking the free carboxylate anion exhibited reduced in-vitro permeability and in-vivo uptake, suggesting the importance of free carboxylate anion in the delivery of BAPSG.

Biopharmaceutical considerations in topical ocular drug delivery

1. Despite the accessibility of the front of the eye, efficient delivery of drug to treat various ocular disorders is a challenge to the formulation scientist. The majority of ophthalmic medications are formulated as eye drops. Due to anatomical constraints, the volume that can be administered is limited to approximately 30 microL. This, together with the efficient clearance system that exists in the front of the eye, makes it difficult to maintain an effective pre-ocular drug concentration for a desired length of time. Various formulation strategies have been used to increase pre-ocular retention of eye drops. The most successful of these has been the inclusion of viscosity enhancing polymers, particularly those able to interact with the mucous layer on the eye surface or those that can undergo a transition from a solution to a gel under the conditions of the pre-ocular area. 2. When the target site is intra-ocular, drug must be absorbed from the pre-ocular region into the eye. The main route for absorption is across the cornea. However, absorption of drug across the cornea is inefficient due to its impermeable nature and small surface area. Thus, the intra-ocular bioavailability of topically administered medications is typically less than 10%. 3. Corneal permeability favours moderately lipophilic compounds. These compounds often have a low aqueous solubility. Problems in ocular drug delivery and formulation are compounded for poorly soluble drugs that must be formulated as suspensions. 4. Reformulation of ophthalmic suspensions as solutions has many advantages. This may be achieved by complexation using cyclodextrins. Solubilization using cyclodextrins can overcome many of the formulation problems. However, it is unclear as to their potential for improving ocular bioavailability, which is seemingly drug dependent and may be influenced by both the physicochemical properties of the drug and the complex formed.

Permeability of cornea, sclera, and conjunctiva: a literature analysis for drug delivery to the eye

The objective of this study was to collect a comprehensive database of ocular tissue permeability measurements found in a review of the literature to guide models for drug transport in the eye. Well over 300 permeability measurements of cornea, sclera, and conjunctiva, as well as corneal epithelium, stroma, and endothelium, were obtained for almost 150 different compounds from more than 40 different studies. In agreement with previous work, the corneal epithelium was shown generally to control transcorneal transport, where corneal stroma and endothelium contribute significantly only to the barrier for small, lipophilic compounds. In addition, other quantitative comparisons between ocular tissues are presented. This study provides an extensive database of ocular tissue permeabilities, which should be useful for future development and validation of models to predict rates of drug delivery to the eye.

Penetration of natural prostaglandins and their ester prodrugs and analogs across human ocular tissues in vitro

The objective of this study was to assess the corneal and scleral permeabilities of natural prostaglandins as well as their prodrugs and analogs through human cornea and sclera in vitro. The "apparent permeability coefficients" (Papp) of natural prostaglandins (PGF2alpha, PGD2 and PGE2), ester prodrugs of PGF2alpha (1-isopropyl PGF2alpha, 11-pivaloyl PGF2alpha and 11,15-dipivaloyl PGF2alpha) and four analogs (16-m-chlorophenoxy tetranor PGF2alpha, 17-phenyl trinor PGF2alpha, 17-phenyl trinor PGE2 and AH 13205) were measured using modified Ussing perfusion chambers and quantitative high performance liquid chromatography. Our results indicate that the corneal penetration of natural prostaglandins (PGs) is poor (the Papp values ranged from 1.65 x 10(-6) to 2.38 x 10(-6) cm/sec), while the PGF2alpha prodrugs showed higher corneal penetration than PGF2alpha. The 11-pivaloyl ester of PGF2alpha penetrated the cornea faster than both 1-isopropyl ester and the lipophilic 11,15-dipivaloyl ester. The PG analogs also showed poor corneal penetration (Papp values ranged from 0.696 x 10(-6) to 1.49 x 10(-6) cm/sec) except for AH 13205. All compounds tested showed good scleral penetration (Papp values ranged from 6.90 x 10(-6) to 17.1 x 10(-6) cm/sec) except PGF2alpha 11,15-dipivaloyl (Papp = 1.22 x 10(-6) cm/sec). The penetration profiles correlated well with tissue uptake ratios (ratio of final tissue concentration to initial dose) for all compounds except 11,15-dipivalate PGF2alpha. All ester prodrugs (but not the PGs and analogs) underwent corneal first-pass metabolism. The study results demonstrate that transcleral absorption may play a significant role in the ocular absorption of these compounds.

Ophthalmic drug delivery systems--recent advances

Eye-drops are the conventional dosage forms that account for 90% of currently accessible ophthalmic formulations. Despite the excellent acceptance by patients, one of the major problems encountered is rapid precorneal drug loss. To improve ocular drug bioavailability, there is a significant effort directed towards new drug delivery systems for ophthalmic administration. This chapter will focus on three representative areas of ophthalmic drug delivery systems: polymeric gels, colloidal systems, cyclodextrins and collagen shields. Hydrogels generally offer a moderate improvement of ocular drug bioavailability with the disadvantage of blurring of vision. In situ activated gel-forming systems are preferred as they can be delivered in drop form with sustained release properties. Colloidal systems including liposomes and nanoparticles have the convenience of a drop, which is able to maintain drug activity at its site of action and is suitable for poorly water-soluble drugs. Among the new therapeutic approaches in ophthalmology, cyclodextrins represent an alternative approach to increase the solubility of the drug in solution and to increase corneal permeability. Finally, collagen shields have been developed as a new continuous-delivery system for drugs that provide high and sustained levels of drugs to the cornea, despite a problem of tolerance. It seems that new tendency of research in ophthalmic drug delivery systems is directed towards a combination of several drug delivery technologies. There is a tendency to develop systems which not only prolong the contact time of the vehicle at the ocular surface, but which at the same time slow down the elimination of the drug. Combination of drug delivery systems could open a new directive for improving results and the therapeutic response of non-efficacious systems.

Unified model for the corneal permeability of related and diverse compounds with respect to their physicochemical properties

Corneal permeability data taken from the literature were analyzed for possible quantitative relationships with physicochemical properties. Although a parabolic relationship was obtained with good correlation between lipophilicity, as expressed by the 1-octanol-water partition coefficients, log Poctanol (or the distribution coefficients, log D for ionizable compounds), and the permeability in individual analyses of compound classes such as beta-adrenoceptor blockers and steroids, the correlation was reduced when taken together. However, delta log P (i.e., log Poctanol-log Palkane) correlated inversely with the combined permeability data for beta-blockers and steroids and played a key role as a unifying variable. To a lesser extent, lipophilicity itself also contributes positively to corneal permeation. Even with the addition of miscellaneous compounds such as methanol and ibuprofen, the delta log P and lipophilicity terms were still significant. However, small molecules were likely to be underestimated, which is consistent with penetration via another pathway besides that governed by delta log P and lipophilicity.

Effects of four penetration enhancers on corneal permeability of drugs in vitro

The usefulness of penetration enhancers in promoting drug permeation across the cornea was investigated for drugs varying from hydrophilic to lipophilic. Four purported penetration enhancers [Azone (laurocapram), hexamethylenelauramide, hexamethyleneoctanamide, and decylmethylsulfoxide] were employed. Corneal permeability coefficients of drugs that were either hydrophilic (acetazolamide, cimetidine, guanethidine, and sulfacetamide), moderately lipophilic (bunolol and prednisolone), or lipophilic (flurbiprofen and its amide analogue) were measured in the absence or in the presence of various Azone concentrations. The effects of penetration enhancers on the corneal penetration of cimetidine were also compared. The corneal penetration of hydrophilic compounds was enhanced by at least 20-fold at 0.1% Azone. For prednisolone and bunolol, the maximal enhancement was at 0.025-0.1% Azone and was marginal (two- to 5-fold), whereas Azone inhibited rather than enhanced the corneal penetration of the lipophilic flurbiprofen and its amide analogue. All four enhancers behaved similarly in enhancing corneal penetration of cimetidine and corneal hydration after incubation in vitro. Possible mechanisms of penetration enhancers on corneal drug penetration were discussed. Penetration enhancers may have clinical benefits in improving ocular drug delivery of hydrophilic compounds, however, their utility may depend on the toxicological profiles.

Minimizing systemic absorption of topically administered ophthalmic drugs

Due to absorption several ocularly applied medications give rise to systemic side-effects. The problem of systemic drug absorption should be taken into account in designing ocular drug and dosage forms so that oculospecificity of the medications is optimized. In this review we summarize the current knowledge about the systemic absorption of ocularly applied topical drugs. Special emphasis is directed to the methods that can be used to minimize systemic absorption and increase the oculospecificity of drugs, e.g., reducing volume and increasing viscosity of eyedrops, controlling drug release from depot preparations, prodrug-derivatization, and addition of vasoconstrictive agents.

An in vitro comparison of the permeability of prednisolone, prednisolone sodium phosphate, and prednisolone acetate across the NZW rabbit cornea

Controversy and ambiguity in the literature concerning the corneal penetration of prednisolone acetate over Prednisolone Sodium phosphate in NZW rabbits has recently prompted comparative studies using specific chromatographic assays. In vitro, corneal penetration studies were performed using the Ussing Chambers to compare the permeability and flux of both esters and prednisolone at 0.5% using a reversed phase HPLC-UV assay. Chromatograms of samples from the receiver chambers show primarily the presence of prednisolone from both esters; only prednisolone phosphate penetrated the cornea intact. Flux measurements were similar for prednisolone and both salt forms in terms of the metabolite prednisolone. Permeability coefficient calculations give the relative comparison: prednisolone acetate greater than prednisolone greater than prednisolone sodium phosphate.

Lenticular uptake and distribution of xenobiotics and amino acids

The objective of this study was to explore the relationship between lipophilicity and the lenticular uptake of radiolabeled xenobiotics. The lenticular uptake of amino acids was also investigated. An organ-culture technique was employed and the partitioning of compounds into the rabbit lens was measured for compounds with log octanol/water partition coefficients (log P o/w) ranging from -1 to 7.3. Drug distribution in the lens was expressed as the concentration ratio of that in the lenticular section (capsule/epithelium, cortex, and nucleus) to that in the incubation medium, (C(lens) section/Cm). The drug partitioning into the lens capsule was facile for all of the compounds tested. The drug concentrations in the lens capsule were higher than those in the cortical and nuclear regions. For compounds with low partition coefficients (such as sulfacetamide and cimetidine), an apparent distribution equilibrium was achieved during a 5-hr period. The C(lens)/Cm value of polar compounds, being less than one, did not further increase by prolonging the incubation period. Only compounds with log Po/w values between 3 and 6 had Cnucleus/Cm values exceeding unity. The maximal values of Ccortex/Cm and Cnucleus/Cm, approximately 15 and 4, respectively, for anthracene (log Po/w = 4.5) and diethylstilbestrol (log Po/w = 5.1), were observed in this study. A bell-shaped relationship was observed between the lenticular uptake rate and drug lipophilicity, of which the maximum occurred around log Po/w of 4. These results indicate the existence of a lipophilicity window for favorable drug distribution into the deeper region of the lens. For several lipophilic compounds, such as padimate-O, values of Cnucleus/Cm increased steadily over a 24-hour incubation period. This suggests that the nucleus behaved as a deep compartment for these compounds. L-Cysteine and L-serine were actively taken up by the lens and the lenticular absorption of L-cysteine was concentration-dependent with Vmax and Km values of 18.3 mumol/gm/hr and 49.8 mM, respectively. In summary, a relationship between lenticular uptake and drug lipophilicity was demonstrated. The optimal log Po/w value for drug uptake into the lens was between 4 and 5. The slowness of reaching significant drug concentrations in the nucleus necessitates a chronic dosing regimen to deliver therapeutic drug levels inside the lens.

Effect of 2-hydroxypropyl-beta-cyclodextrin on the ocular absorption of dexamethasone and dexamethasone acetate

Complexation of dexamethasone (DX) and dexamethasone acetate (DXA) with 2-hydroxypropyl-beta-cyclodextrin (HPCD) was investigated with an ultimate goal of formulating a topical ophthalmic solution of DXA. Aqueous solubility of DX and DXA was markedly increased due to formation of soluble inclusion complexes with HPCD. Based on characterization of complex formation by phase solubility and UV-spectroscopy methods, a stoichiometry of 1:1 and 1:1, 1:2 was assumed for DX-HPCD and DXA-HPCD complexes, respectively. The stability constants for complex formation estimated by phase solubility and UV-spectroscopy methods, respectively, were as follows: for DX-HPCD complex, K1:1 = 2193 and 2221 M-1; and for DXA-HPCD complex, K1:1 = 2240 and 2445 M-1 and K1:2 = 3 and 17 M-1. K1:1 of 2266 M-1 and K1:2 of 20 M-1 were also estimated for the DXA-HPCD complex by kinetics. The kinetics of DXA degradation in pH 7 phosphate buffer at 25 degrees C followed pseudo first order. The addition of HPCD decreased the rate but the order of reaction remained unchanged. Free DXA degraded at a faster rate than complexed DXA. Ocular bioavailability in conjunctiva, cornea, iris, and aqueous humor postadministration of a 25-microliters dose of formulations containing an equivalent of 0.1% (w/v) DX followed a rank-order of DXA-HPCD solution greater than DXA suspension greater than DX-HPCD solution greater than DX suspension.

Lipophilicity influence on conjunctival drug penetration in the pigmented rabbit: a comparison with corneal penetration

The influence of lipophilicity on the conjunctival penetration of beta blockers in the pigmented rabbit was investigated and compared with that on corneal penetration. The beta blockers were hydrophilic sotalol, atenolol, nadolol, pindolol, and acebutolol; lipophilic metoprolol, timolol, oxprenolol, levobunolol, labetalol, and alprenolol; and the very lipophilic propranolol and betaxolol. Drug penetration was evaluated by using the isolated pigmented rabbit conjunctiva and cornea in the modified Ussing chamber and was monitored by reversed phase HPLC. The conjunctiva was more permeable to all the beta blockers than was the cornea. A sigmoidal relationship, rather than the familiar parabolic relationship, best described the influence of lipophilicity on both conjunctival and corneal drug penetration. The ratio of corneal to conjunctival permeability coefficients was most sensitive to changes in log PC within the region of 1.5 and 2.5. Outside of this region, the ratio was relatively independent of changes in lipophilicity. For several beta blockers, their intrinsic sympathomimetic activity may play a minor role in influencing their conjunctival and corneal penetration.

Role of enzymatic lability in the corneal and conjunctival penetration of timolol ester prodrugs in the pigmented rabbit

The main objective of this study was to investigate how enzymatic lability would affect the extent of corneal and conjunctival penetration of a series of alkyl, cycloalkyl, and aryl ester prodrugs of timolol in the pigmented rabbit. Enzymatic lability of the prodrugs was studied in corneal epithelial and conjunctival homogenates, while their corneal and conjunctival penetration was determined using the isolated tissues in the modified Ussing chamber. The straight-chain alkyl and the unsubstituted cycloalkyl esters were hydrolyzed more rapidly than their corresponding branched-chain and substituted analogues as well as the aryl esters. The corneal and conjunctival penetration of all prodrugs, regardless of enzymatic lability, varied parabolically with lipophilicity. Moreover, the enzymatically more labile straight-chain alkyl esters penetrated the cornea and the conjunctiva more readily than the more stable branched-chain esters of comparable lipophilicity. Enzymatic lability is, therefore, an additional factor that should be considered in designing alkyl ester prodrugs with improved ocular drug delivery characteristics. Enzymatic lability does not, however, play as important a role as lipophilicity in the corneal and conjunctival penetration of cycloalkyl and aryl ester prodrugs.

Determination of physicochemical properties, stability in aqueous solutions and serum hydrolysis of pilocarpic acid diesters

New alkyl and aralkyl pilocarpic acid diesters, prodrugs of pilocarpine, were synthesized with the aim of improving the bioavailability of pilocarpine by increasing its corneal permeability. These esters were several orders of magnitude more lipophilic than pilocarpine as determined by their apparent partition coefficients between 1-octanol and phosphate buffer (pH 7.40) (log P). Good correlation between log P and HPLC capacity factors of the compounds was observed. All the compounds are stable in acidic aqueous solution; in serum, however, pilocarpic acid diesters are hydrolysed enzymatically to pilocarpic acid monoester, which undergoes spontaneous cyclization to active pilocarpine and inactive isopilocarpine. The half-lives of the diesters in serum varied from 6-232 min. In addition to the direct effects of the R2, R1 moiety had a remarkable effect on the rate of enzyme-catalysed hydrolysis taking place in moiety R2. The formed pilocarpine was analysed with a new HPLC method which allowed good resolution of pilocarpine, isopilocarpine, pilocarpic acid and isopilocarpic acid. Rates for pilocarpine formation were both determined by experiment and calculated using the STELLA simulation programme with known degradation rate constants of pilocarpic acid diesters and monoesters. Since the simulations were in good agreement with the experimental results, it is concluded that STELLA simulation programme is useful in predicting pilocarpine formation.

A corneal perfusion device for estimating ocular bioavailability in vitro

An in vitro method for investigating drug penetration across the cornea from acute doses has been developed. The donor (epithelial or tear) side of a corneal chamber similar to those used by Edelhauser and co-workers was modified with a reduced-volume insert so that the donor side volume approximated 6% of the receiver (endothelial or aqueous humor) chamber volume. Fluid was pumped through the anterior chamber to simulate the physiologic tear turnover in vivo. The receiver chamber was bubbled with oxygen:carbon dioxide (95%:5%) to aerate and circulate the fluid. We investigated the in vitro ocular bioavailability of several marketed ophthalmic drug formulations using this model (0.03% flurbiprofen, 0.5% levobunolol, 0.1 and 0.25% fluorometholone, and 1% prednisolone acetate). At 1 min after the dose administration, drug was eluted from the donor chamber at a dose turnover rate of 12%/min. In all experiments, a small percentage of the applied dose penetrated the cornea. Using this chamber device, the ocular bioavailability via corneal absorption ranged between 5 and 16% for solution formulations, whereas the ocular bioavailability for the suspension formulations was less than 0.3%. The major portion of the administered dose was recovered from the donor side effluent. This observation was consistent with findings showing low ocular bioavailability of ophthalmic preparations due to the rapid precorneal washout in vivo. The extent of drug penetrating as its corneal metabolite in the model correlated well to in vitro corneal metabolism rate constants. The new model opens several potentially useful areas of research into ocular absorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of corneal drug penetration. I: In vivo and in vitro kinetics

Corneal penetration studies were conducted in unanesthetized albino rabbits using various organic compounds representing both polar and nonpolar species. Very low molecular weight compounds demonstrate rapid uptake into the aqueous humor despite the lipid-like barrier imposed by the corneal epithelium. Evidence that these compounds may have access to a diffusional channel for corneal transport is presented. In vitro permeability studies were also conducted in an effort to quantitate the corneal diffusion of compounds covering a range of molecular weights and partition coefficients; the results corresponded well with the results of in vivo experiments. Calculations of energies of activation, taken from Arrhenius plots, indicate that the diffusion of drug across the cornea may be by two different mechanisms that depend on the physical-chemical characteristics of the perfusant. One mechanism appears similar to drug movement in an aqueous environment and is characterized by an activation energy similar to that for diffusion in water. The other relates to the expected partitioning of a compound across cellular membranes represented by a relatively high activation energy for diffusion. For hyrdophilic compounds, the epithelium appears to be rate limiting to drug movement, whereas for hydrophobic compounds, the stroma is rate limiting. In the presence of calcium-chelating agents, glycerol demonstrated an increase in corneal penetration in vivo. This effect appears to be reversible at specific concentrations of chelator. In contrast, divalent cations reduced corneal penetration of glycerol. The known calcium chelator EDTA was shown to penetrate the cornea, conjunctiva, and iris/ciliary body from a topically applied dose. The implications of this observation pertain to toxicity effects when EDTA is incorporated into ocular drug products for stability purposes, or novel stratagems for improving ocular bioavailability of topically applied drugs are employed. The addition of calcium-chelating agents to in vivo mounted corneas demonstrated increases in permeability of the cornea to glycerol which were directly related to the concentration of chelating agent used. These results paralleled the findings of similar in vivo studies. The results of these studies are consistent with a currently proposed 'pore' model for the penetration of drugs through the cornea which demonstrates both a partition coefficient and molecular weight dependency on the permeability of the cornea to transported compounds.

Application site dependent ocular absorption of timolol

Ocular absorption of timolol in rabbits was studied after topical ocular administration of 3H-timolol in an eyedrop or in silicone cylindrical devices that released timolol at 7.2 micrograms/h. The devices were applied in either the inferior or superior conjunctival sac. Timolol concentrations were nearly equal in the inferior and superior portions of ocular tissues when the drug was administered in an eyedrop. Administration in the devices resulted in unequal timolol distribution in the cornea, conjunctiva, sclera, and iris-ciliary body. Timolol concentrations were higher in the part of each tissue that was closer to the site of the device application. Unequal concentrations of timolol in the superior and inferior part of the eye and very low timolol concentrations in the aqueous humor indicated that timolol was absorbed mainly via a noncorneal route from the device placed in the inferior conjunctival sac. Induced blinking at one minute intervals did not change ocular absorption of timolol. Compared with inferior conjunctival sac applications, placement of the devices in the superior conjunctival sac resulted in increased corneal and total ocular absorption of timolol as indicated by higher timolol concentrations in the aqueous humor and by a smaller difference between concentrations in the superior and inferior portions of the examined tissues. The application site dependent ocular absorption indicated that controlled release of timolol in the tear fluid did not result in a uniform timolol distribution in the preocular tear fluid of rabbit eyes.

Mechanisms of corneal drug penetration. III: Modeling of molecular transport

A model relating the parameters of permeability coefficient in the cornea with partition coefficient and molecular weight of the penetrating species is presented. The development of the model is unique in that it includes the availability of a "pore" pathway with the corresponding kinetic data to substantiate this premise. The "pore" pathway is applied to small hydrophilic compounds and assumes that an aqueous diffusional space is available for transport of these compounds. This is in contrast to an alternate "partitioning" mechanism which is the most probable route of transport for larger or more lipophilic entities. The model is consistent with published data from this and other laboratories.

Ophthalmic beta-blockers since timolol

In the twenty years since beta-blockers were proposed for treatment of glaucoma, use of topical timolol has increased to account for 70% of all glaucoma medications used. The objective of this article is to review the "newer" beta-blockers, and to address the generalization that "all ophthalmic beta-blockers are the same." The review concentrates on agents that have been studied as topical treatments for patients with elevated intraocular pressure. Sections on pharmacology and design of clinical trials are included to aid the ophthalmologist in evaluating the new drugs and published clinical reports. The major questions to consider in evaluating the therapeutic potential of a new beta-blocker for the treatment of glaucoma involve efficacy and safety: Is the drug as effective as timolol? Does it have a duration of action at least as long as timolol? Does it have ocular toxicity? Is it comfortable? What are its systemic effects?

Pharmacokinetic transfer functions and generalized clearances

A generalized steady-state clearance coefficient is defined to relate drug or metabolite mass transfer rates into one area of the body (area II) to the constant drug concentration in another area (area I). It is demonstrated that this clearance coefficient may be used in the transient case to calculate total mass transfer into area II when the drug AUC in area I is known. This generalizes the well-known "clearance formula" CL = Ae/AUC to cases in which several compartments separate areas I and II, as well as to cases where distributed models are preferred. Several applications are discussed, including a noninvasive means of determining the fraction absorbed of topically administered drugs.

Skin permeability of various non-steroidal anti-inflammatory drugs in man

The skin permeabilities of a series of eight salicylates and ten non-steroidal anti-inflammatory drugs were investigated in human subjects. The logarithms of % absorption through intact skin and of n-octanol/water partition coefficients (log P) of the test compounds were plotted against one another, and a parabolic relationship was obtained in both compound series.

Ocular pharmacokinetic models of clonidine-3H hydrochloride

A single topical instillation of clonidine-3H HCl solution (0.2%) was administered to the rabbit eye (30 microliter) in order to study the drug's ocular pharmacokinetics. Seven different tissues and plasma were excised and assayed for drug over 180 min. By 45-60 min pseudoequilibrium is reached for the cornea, iris/ciliary body, and aqueous humor. Thereafter, drug levels in these tissues decline in parallel. The data are fit separately to a physiological model and a classical diffusion model for which seven ocular tissue compartments and a plasma reservoir are constructed for each model. Clearance terms and distribution equilibrium coefficients are determined from the tissue level data and used as parameters in fitting the mass balance differential equations representing the physiological model. The model parameters can also be fit to a 0.4% single dose. In a separate experiment, a topical infusion technique was designed to provide a constant rate input to the cornea until an apparent steady state was reached in aqueous humor at 55 min. Aqueous humor levels were assayed for clonidine over the infusion and postinfusion periods. The physiological model parameters are fit to the topical infusion data and show good agreement between the predicted and experimental data. The classical model is too complex to fit the data to integrated exponential equations primarily because the method of residuals is inadequate in determining a sufficient set of initial estimates. This is overcome by dividing the eight-compartment model into seven fragmental models, each representing one to five compartments. A stepwise procedure is developed in which initial estimates are obtained for each separate fragmental model and refined. The refined parameter values can then be used as initial estimates for the complex model. Differential equations for the complex model are fit simultaneously to tissue levels representing each compartment. By observation, the classical model fit the data more closely than the physiological model. Statistical moment theory is also applied to the topical infusion data to determine ocular pharmacokinetic parameters for clonidine. The calculated values are: corneal absorption rate constant ka, 0.00139 min-1, aqueous humor elimination rate constant k10, 0.0658 min-1; mean residence time MRTd, 35.6 min; apparent steady-state volume of distribution Vss, 0.530 ml; and ocular clearance Qe, 14.9 microliter/min. The fraction absorbed from the single instillation is estimated as 0.0163.

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.

Topical carbonic anhydrase inhibitors. III: Optimization model for corneal penetration of ethoxzolamide analogues

An analogue series representing modification to the benzene ring of ethoxzolamide has been evaluated for solubility, pKa, partitioning, and permeability across excised rabbit corneas. These physical parameters were correlated to Hammett sigma (para) and/or Hansch pi parameter values for each compound. From these correlations, a mathematical model was developed relating corneal permeability to molecular modifications of ethoxzolamide. A three-dimensional plot of maximum attainable penetration rate versus sigma (para) and pi yielded an optimal range of pi and sigma values from which an optimally penetrating analogue could be designed.