Development of a Dynamic Skin Permeation System for Long-Term Permeation Studies

Bio-Functional Textiles: Combining Pharmaceutical Nanocarriers with Fibrous Materials for Innovative Dermatological Therapies

In the field of pharmaceutical technology, significant attention has been paid on exploiting skin as a drug administration route. Considering the structural and chemical complexity of the skin barrier, many research works focused on developing an innovative way to enhance skin drug permeation. In this context, a new class of materials called bio-functional textiles has been developed. Such materials consist of the combination of advanced pharmaceutical carriers with textile materials. Therefore, they own the possibility of providing a wearable platform for continuous and controlled drug release. Notwithstanding the great potential of these materials, their large-scale application still faces some challenges. The present review provides a state-of-the-art perspective on the bio-functional textile technology analyzing the several issues involved. Firstly, the skin physiology, together with the dermatological delivery strategy, is keenly described in order to provide an overview of the problems tackled by bio-functional textiles technology. Secondly, an overview of the main dermatological nanocarriers is provided; thereafter the application of these nanomaterial to textiles is presented. Finally, the bio-functional textile technology is framed in the context of the different dermatological administration strategies; a comparative analysis that also considers how pharmaceutical regulation is conducted.

Small molecule delivery across a perforated artificial membrane by thermoreversible hydrogel poloxamer 407

Microperforations in the round window membrane have been suggested for enhancing the rate and reliability of drug delivery into the cochlea. Intratympanic injection, the most common delivery method, involves injecting therapy into the middle ear to establish a reservoir from which drug diffuses across the round window membrane into the cochlea. This process is highly variable because (i) the reservoir, if liquid, can lose contact with the membrane and (ii) diffusion across the membrane is intrinsically variable even with a stable reservoir. To address these respective sources of variability, we compared the thermoreversible hydrogel poloxamer 407 (P407) to saline as a drug carrier and studied the effect of membrane microperforations on drug diffusion rate. We used Rhodamine B as a drug proxy to measure permeance across an artificial membrane in a horizontal diffusion cell. We found that permeance of Rhodamine B from a saline reservoir was an order of magnitude higher than that from a P407 reservoir across unperforated membranes. Moreover, permeance increased with total perforation cross-sectional area regardless of number of perforations (p < 0.05 for all saline-based experiments), but the same association was not found with P407. Rather, for a P407 reservoir, only a large perforation increased permeance (p < 0.001), while multiple small perforations did not (p = 0.749). These results confirm that for drug dissolved in saline, multiple small perforations can effectively enhance diffusion. However, for drug dissolved in P407, larger perforations are necessary.

Formulation and In Vitro Characterization of Thiolated Buccoadhesive Film of Fluconazole

The present work is focused on the development of thiolated film for fluconazole buccal delivery. To this end, unmodified polymers chitosan and sodium carboxymethylcellulose (NaCMC) backbone was covalently modified by thioglycolic acid (TGA) and cysteine, respectively. The thiolated buccoadhesive film was evaluated in terms of thickness, weight uniformity, water-uptake capacity, drug content, and release patterns. Moreover, mucoadhesion profile was investigated on buccal mucosa. The resulting chitosan-TGA and NaCMC-cysteine conjugates displayed 171 ± 13 and 380 ± 19 μmol thiol groups per gram of polymer (mean ± SD; n = 3), respectively. The water binding capacity of the thiolated film was significantly ∼2-fold higher (p < 0.05) as compared to unmodified film. The obtained thiolated film displayed 5.8-fold higher mucoadhesive properties compared with corresponding film. Controlled release of drugs from film was observed over 8 h. The transport of fluconazole across excised buccal mucosa was enhanced up to 17-fold in comparison with fluconazole applied in buffer. Based on these findings, thiolated film seems to be promising for fluconazole buccal delivery.

Equipment and method for in vitro release measurements on topical dosage forms

CONTEXT

In countries where research budgets are meager as Argentina, the tendency to innovation and improvements in the designs prototypes "made in Argentina" marks a growing trend adopted by researchers. This article presents a diffusion cell of original design, for release studies of Active Pharmaceutical Ingredient (API) from classical topical dosage forms, also includes the methodology for its optimization and validation. The objective was to evaluate and validate a system designed and to compare it to the Franz cells system.

METHODS

Parameters, reproducibility and robustness were performed included factors as, stirring conditions, membrane stabilization treatment and temperature variation. Release and retention on membrane assay were performed using two different API and formulations.

RESULTS

The method is reproducible and robust for the parameters tested. Release assays show that no significative difference with the Franz Cells system. Our system allows the simultaneous measurement of different parameters, representing an innovation on these methodologies. The LMC was used for assays of in vitro retention on membrane and the values obtained were reproducible and coincident whit values obtained for other authors.

CONCLUSIONS

The system designed and the methodology employed, are acceptable for in vitro release studies. The device and method has the characteristics required.

Preliminary investigation of the nasal delivery of liposomal leuprorelin acetate for contraception in rats

The purpose of the study was to investigate the nasal route as a non-invasive alternative for delivery of leuprorelin acetate (leuprolide acetate, LEU) and to achieve an effective concentration of leuprorelin acetate in blood after nasal administration for contraception in rats. The plain drug solution, physical mixture (plain drug along with constituents of liposomes), or drug encapsulated in either neutral or charged liposomes containing 5 μg leuprorelin acetate were administered to rats through the nasal route. The plain drug solution was administered subcutaneously (s.c.). Simultaneous evaluation was performed on the influence of a mucoadhesive agent (chitosan) on nasal absorption of the plain drug and the liposome-encapsulated drug. Blood samples were taken at regular time intervals and subjected to luteinising hormone (LH) analysis using a specific immunoassay kit. The plasma luteinising hormone concentration vs time data of nasal and subcutaneous treatments were plotted and compared with that of subcutaneous administration. Relative percentage of bioavailability (F) for nasal treatments was calculated from plasma concentration vs time plots. Sperm count and fertility performance studies were carried out for selected formulations in rats. Neutral liposomes (LLEU) and negatively-charged liposomes (LLEUn) showed higher relative percentage of bioavailability (F 27.83 and 21.30%, respectively) as compared with the plain drug and the physical mixture (F 10.89 and 10.96%, respectively) after nasal administration. Hence, work on neutral liposomes was continued. F was further improved after incorporation of chitosan i.e. 10.89 to 49.13% for plain leuprorelin acetate and 27.83 to 88.90% for liposomal leuprorelin acetate formulations. Liposomal chitosan formulation administered nasally and leuprorelin acetate solution subcutaneously achieved complete azoospermia. No implantation sites were observed after the mating of female rats with treated males. It was observed that in the treated female rats, the estrous cycles ceased with the same formulations from the first treatment cycle. The findings of these investigations demonstrated that the bioavailability of the nasally-administered liposomal leuprorelin acetate with chitosan formulation was comparable with that of the subcutaneously administered drug. Complete contraception was obtained in male and female rats that had been treated with either the nasally administered liposomal leuprorelin acetate with chitosan or the subcutaneously administered drug.

Development and in-vitro evaluation of transdermal matrix films of metoprolol tartrate

The transdermal matrix films of metoprolol tartrate (MT) were prepared by casting on mercury substrate employing different ratios of polymers, ethyl cellulose (EC) and polyvinyl pyrrolidone (PVP), using dibutyl phthalate (DBT) as a plasticizer. Four formulations were prepared. Formulations MF-1, MF-2, MF-3 and MF-4 were composed of EC and PVP in the following ratios: 4.5:0.5, 4:1, 3:2 and 2:3 respectively. The formulations were subjected to various physical characterization studies namely, thickness, weight variation, drug content, moisture uptake, in vitro drug release and in vitro skin permeation. The in vitro permeation studies were carried out across excised porcine ear skin using Franz diffusion cell. Cumulative amounts of the drug released in 24 hours from the four formulations were 69.58%, 96.13%, 98.85% and 99.60%, respectively. Corresponding values for the cumulated amounts of drug permeated across the porcine skin for the above matrix films were 124.38, 153.22, 156.46 and 163.25 mug/cm(2) respectively. By fitting the data into zero order, first order and Higuchi model, it was concluded that drug release from matrix films followed Higuchi model (r(2)=0.9147-0.9823), and the mechanism of release was diffusion mediated. Based on the physical evaluation, in vitro drug release & permeation characteristics, it was concluded that for potential therapeutic use, monolithic drug matrix films MF-3, composed of EC: PVP (3:2), may be suitable for the development of a transdermal drug delivery system of MT.

A potent and selective tacrine analog--biomembrane permeation and physicochemical characterization

Cholinesterase inhibitors have been used for years in treatment of Alzheimer's disease (AD). Tacrine is the first acetylcholinesterase inhibitor approved for treating AD by the regulatory agencies around the world. Unfortunately, a number of studies have shown tacrine to be associated with some severe side effects, including hepatotoxicity. These adverse effects may be attributed to its poor selectivity for acetylcholinesterase and have thus necessitated the research and development of more selective cholinesterase inhibitors with a greater specificity and higher potency. The heptylene-linked bis-tacrine analog (bis-THA) of Tacrine is a second-generation inhibitor of acetylcholinesterase, which has a potency that is 1000 times more potent than Tacrine in inhibition of the rat brain acetylcholinesterase and 10,000 times more selective for acetylcholinesterase over butyrylcholinesterase. A series of investigations have thus been initiated to characterize the physicochemical properties (e.g., pKa, partition coefficient, and stability) of this bis-THA analog as compared to its parent molecule (Tacrine). For AD treatment, the cholinesterase inhibitors need to be taken daily for long periods of time. Use of controlled-release dosage forms to deliver drugs for chronic administration, by taking advantage of their rate-controlling drug delivery features, has gained increasing popularity in recent years. On the other hand, the nasal route, which has been used to deliver drugs for achieving a direct delivery to the brain (via the olfactory pathway), could offer the benefits of brain targeting to the delivery of Tacrine and bis-THA. To investigate this feasibility, the permeation of Tacrine and bis-THA across the nasal mucosa was evaluated (in comparison with other absorptive mucosae). Studies of their permeation kinetics across the various absorptive mucosae, which were freshly excised from the domestic pig, indicated that the nasal mucosa could present a viable pathway for the systemic delivery of bis-THA. Delipidization studies suggested that the lipophilic components in the absorptive mucosae could play an important role in the permeation of bis-THA. The bis-THA has a pKa of approximately 8 and its partition coefficient showed a sigmoidal pattern with solution pHs. It was found to be relatively stable at acidic pHs but subjected to a base-catalyzed degradation at the alkaline pHs (> or = 8) and at higher temperatures (> or = 50 degrees C).

Development and in vitro evaluation of diltiazem hydrochloride transdermal patches based on povidone-ethylcellulose matrices

To select a suitable formulation for the development of transdermal drug-delivery system of diltiazem hydrochloride. Transdermal patches of the drug, employing different ratios of polymers, ethylcellulose (EC), and povidone (PVP) were developed and evaluated for the potential drug delivery using depilated freshly excised abdominal mouse skin. The influence of different film compositions on in vitro drug permeation into receptor fluid were studied using a modified Franz diffusion cell. The cumulative amount of drug was found to be proportional to the square root of time, i.e., Higuchi kinetics. From this study, it was concluded that the films composed of povidone:ethylcellulose (1:2) should be selected for the development of transdermal drug-delivery system of diltiazem hydrochloride, using a suitable adhesive layer and backing membrane, for potential therapeutic use.

Iontophoretic delivery of apomorphine: from in-vitro modelling to the Parkinson patient

Apomorphine is a mixed dopamine D1/D2 receptor agonist which is potentially useful in the treatment of Parkinson's disease. The delivery of apomorphine is however complicated because it is not absorbed orally and other delivery routes with the exception of the intravenous route seem to fail. The most interesting route for controlled delivery of apomorphine is transdermal iontophoresis because this could enable the Parkinson patient to directly control the needed amount of apomorphine by increasing or decreasing the drug input in order to achieve optimal drug therapy ('on-demand') with a minimum of toxic side effects. The typical features of Parkinson's disease could be used to monitor the needed drug input and even more elegantly by means of suitable chip sensors which are able to directly measure bradykinesia, akinesia and/or tremor and to regulate in such a way the drug input. Such a chip-controlled iontophoretic system would be the first closed-loop system monitoring not pharmacokinetic data (blood levels) but more importantly externally measurable pharmacodynamic effects of Parkinson's disease. This scenario is more feasible as skin irritation and toxicity studies have proven that iontophoresis is a safe route of treatment. This review describes the basics of iontophoresis and the development of a transdermal iontophoretic delivery system on the basis of integrated pharmacokinetic/pharmacodynamic (PK/PD) investigations in patients with idiopathic Parkinson's disease. Transdermal iontophoretic transport of apomorphine was studied both in vitro with human stratum corneum using a newly developed iontophoretic continuous flow-through transport cell and in vivo in a first exploratory study in patients with Parkinson's disease. These studies showed that the delivery of apomorphine is feasible and furthermore the rate of delivery can be controlled by variation of the current densities. Additionally the pretreatment of the skin either with a mono-surfactant or a vesicular suspension of elastic liquid-state vesicles may be useful to further increase the apomorphine flux across the skin in combination with iontophoresis.

Effect of different terpene-containing essential oils on permeation of estradiol through hairless mouse skin

Purpose of the present investigation was to evaluate six terpene-containing essential oils for their capacity to promote permeation of estradiol (ES) through hairless mouse skin in vitro. Tests on cajuput, cardamom, melissa, myrtle, niaouli and orange oil, all used at the 10% w/w concentration in propylene glycol (PG), evidenced niaouli oil (NIA) as the best permeation promoter for ES. Tests on the main terpene components of NIA (1,8 cineole, alpha-pinene, alpha-terpineol and D-limonene), evaluated neat (10% w/w in PG) or in admixture, confirmed the better promoting activity of whole NIA. The present data point to the validity of complex terpene mixtures, such as that composing NIA, as transdermal penetration enhancers for moderately lipophilic drugs like ES.

Characterization of captopril sublingual permeation: determination of preferred routes and mechanisms

Although sublingual captopril has been used clinically to treat hypertensive emergencies, a mechanistic understanding of sublingual permeation will facilitate the optimization of drug delivery. A correlation of sublingual steady-state flux with donor captopril concentration in a porcine model showed the absence of saturability and suggested a passive diffusion permeation mechanism. A simultaneous evaluation of permeability and partition coefficient demonstrated that the paracellular route is the predominant pathway for sublingual permeation. The enhancement factors of specific ion permeabilities in the presence of tight junction perturbants indicated that although the paracellular pathway is preferred by the ionized species of captopril, the lipophilic transcellular pathway is preferred by the neutral, un-ionized species.

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.

Transdermal testosterone delivery: comparison between scrotal and nonscrotal delivery systems

The purpose of this investigation was to study the bioequivalence of two testosterone transdermal delivery systems (T-TDSs). Testoderm, designed to deliver testosterone through scrotal skin, and Androderm, designed for nonscrotal permeation. In vitro permeation and release kinetics as well as in vivo pharmacokinetics in the castrated Yucatan miniature swine (minipigs) model of both T-TDSs were studied side by side under the same experimental conditions. In vitro skin permeation kinetics studies demonstrated that testosterone permeates through minipig dorsal skin at zero-order kinetics from both T-TDSs. The nonscrotal T-TDS, however, has a permeation rate which is approximately 13 times higher than that for the scrotal T-TDS. The release of testosterone from the nonscrotal T-TDS showed a biphasic release profile between cumulative amount released and time, whereas a monophasic release profile between cumulative amount released and square root of time was observed for the scrotal T-TDS. Pharmacokinetic analysis of plasma testosterone profiles in minipigs indicated a significant difference (p < 0.001) in daily dose of testosterone delivered (1.20 versus 4.83 mg/day), maximum concentration (Cmax) (54.2 versus 218.0 ng/dl), and area under concentration-time curve (AUC0-28)[665 versus 3208 (ng/dl) x hr] between these T-TDSs. However, there is no difference in time to reach Cmax mean residence time, and daily-delivered-dose-normalized Cmax and AUC0-28. The difference in pharmacokinetic profiles resulted from the difference in daily doses delivered, which could be attributed remarkably to the difference in permeation rate (approximately 13-fold) between the nonscrotal and scrotal T-TDSs.

Permeation of piroxicam from the poloxamer gels

Topical formulations of piroxicam were prepared using poloxamer 407 or poloxamer 188 by a cold method, and the permeation characteristics of piroxicam were evaluated. The permeation rate of piroxicam across the synthetic cellulose membrane and the rat skin decreased as the concentration of poloxamer increased. Though poloxamer gel exhibits reversed thermal behavior, the permeation rate of piroxicam increased with increasing temperature, indicating that the diffusional pathway of piroxicam is a water channel within the gel formulation. The pH of the gel did not affect the permeation rate of piroxicam significantly. As the concentration of piroxicam in the gel formulation increased, the permeation rate of piroxicam increased up to 1% and reached a plateau above 1%. Among various enhancers tested, polyoxyethylene-2-oleyl ether showed the highest enhancing effect, with an enhancement ratio of 2.84. Based on experimental results, the permeation rate of piroxicam can be controlled by changing the poloxamer concentration or drug concentration and by the addition of an appropriate enhancer.

Hemoglobin diffusion across a venous wall: an experimental study

The aim of this study was to investigate the permeation behavior of a large molecule through a venous wall; hemoglobin was chosen as a model substance. In vitro experiments were performed using a Chien-Valia diffusion chamber. Postmortem, hemolyzed, and fresh nonhemolyzed blood samples were investigated as permeants. Vein patches from vena cava inferior and vena jugularis interna were used as diffusion barriers. Applying this technique, extravasation of hemoglobin was detectable. The portion of hemoglobin molecules passing through the vascular wall depended on time, vein type, and graduation of hemolysis. The passage of hemoglobin across the wall of a large vein suggests intravascular changes in drug concentrations from postmortem blood samples not to be restricted on the unbound portion of the particular drug.

Comparative biomembrane permeation of tacrine using Yucatan minipigs and domestic pigs as the animal model

Tacrine (THA), a centrally acting acetylcholine-esterase inhibitor, is presently administered perorally for the treatment of Alzheimer's disease (AD). However, its low bioavailablity (i.e., 17%) and short half-life (2-4 h) demand the search for alternative routes of administration. The primary objective of this study was to assess the potential of absorptive mucosae and skin as routes for improving the systemic delivery of THA. The Yucatan minipig, which has been used increasingly in biomedical research as a useful model for humans, and the domestic pig, which is available at low cost, were evaluated for their suitability as animal model. Permeation kinetics of THA across various absorptive mucosae (nasal, buccal, sublingual, and rectal) of both species of swine were studied in the hydrodynamically well-calibrated Valia-Chien permeation cells. For comparison, permeation through various intestinal segments (duodenum, jejunum, and ileum) was also measured. Results indicated that both species display similar permeation characteristics. However, the data obtained for the domestic pigs shows lower intra- and inter-animal variabilities than that of the Yucatan minipigs. The nasal mucosa was found to have the highest permeability, while the buccal mucosa had the lowest among the absorptive mucosae. The intrinsic permeabilities and diffusivity of THA across the four absorptive mucosae were not significantly different between species but lower than that for the intestinal segments for both species. Using dorsal skin as the model, the skin permeation of THA was also investigated and the results indicated that the domestic swine has a significantly higher skin permeability than the Yucatan minipig, with more than a 2-fold difference in intrinsic permeabilities. The intrinsic permeability, partition coefficient, and diffusivity for domestic pig skin are very similar to that for human cadaver skin. Considering the potential of bypassing the hepatic "first-pass" elimination, the absorptive mucosae may be useful routes for systemic delivery of THA to achieve improved bioavailability. With additional advantages of lower variability, ease of membrane excision, good accessibility, and lower cost, it is concluded that the domestic swine is a better animal model than the Yucatan minipig for preclinical studies on the systemic delivery of tacrine.

Validation and testing of a new iontophoretic continuous flow through transport cell

A new continuous flow through transport cell is presented. The design is based on a minimisation of contributions of the cell conformation and experimental protocol to the overall transport kinetics. The system is validated by measuring the washout period of methylene blue and apomorphine. Preliminary results on the iontophoretic transport of apomorphine across human stratum corneum are presented. These data are used to calculate the deviation of the measured apparent flux from the calculated intrinsic flux across the membrane. The steady state iontophoretic flux was 90 +/- 6 nmol cm-2 h-1 when a 15 mM apomorphine solution was applied in the anodal chamber at a current density of 500 microA cm2. At any point in time the contribution of the cell to the transport rate was < or = 3%. The supporting membrane does not contribute significantly to the overall transport rates. Sink conditions are guaranteed at a flow rate > or = 6.5 ml h-1. For this new transport cell it can be concluded that the apparent flux that was measured, at validated experimental conditions, is a reliable estimate for the intrinsic flux.

In vitro corneal permeability of diclofenac sodium in formulations containing cyclodextrins compared to the commercial product voltaren ophtha

The influence of different cyclodextrin derivatives on the in vitro permeability of diclofenac sodium through pig cornea was investigated and compared to the commercial product Voltaren ophtha. (Hydroxypropyl)-beta-cyclodextrin (HP beta CD) and two amorphous methylated cyclodextrins with different degrees of substitution were used. In hemolysis studies on human erythrocytes, the hemolytic activity of the different cyclodextrins and the drug was assessed. It was shown that HP beta CD reveals the most favorable toxicological properties. A decrease in the hemolytic activity of diclofenac was yielded by adding HP beta CD. In the permeability experiments the dependency of the permeability coefficients and lag times on the type of cyclodextrin and pH of the solutions were examined. A solution containing HP beta CD buffered in the pH range 6.5 to 7 is proposed as a useful eye drop formulation. All cyclodextrin formulations showed advantages as compared to Voltaren ophtha.

Transmucosal delivery of methionine enkephalin. I: Solution stability and kinetics of degradation in various rabbit mucosa extracts

To evaluate the feasibility of transmucosal delivery of methionine enkephalin (Tyr-Gly-Gly-Phe-Met; Met-Enk), it is important to first investigate its physicochemical and enzymatic stability. The kinetics of degradation of Met-Enk in aqueous solution was determined at pH 2.01-9.84 and 37-45 degrees C by high-performance liquid chromatography. The first-order rate constant (k) was calculated, and the log k-pH profile showed that Met-Enk is most stable at pH approximately 5.0. Various mucosae excised from rabbit were mounted on Valia-Chien permeation cells and exposed to isotonic phosphate buffer at physiologic pHs. Mucosal and serosal extracts were collected from the donor and receptor solutions, respectively. The degradation of Met-Enk in the extracts followed first-order kinetics, but no significant difference in the degradation rates was observed between mucosal and serosal extracts, regardless of the type of mucosa used. Degradation was most rapid in the extracts of rectal mucosa, followed by vaginal and nasal mucosae. The major metabolites were Des-Tyr-Met-Enk and Tyrosine (Tyr), indicating the enzymatic hydrolysis by aminopeptidases. However, the data also suggested that dipeptidyl peptidase and dipeptidyl carboxypeptidase could play some roles in the degradation of Met-Enk. The degradation pathways of Met-Enk were further explored by concomitantly determining the formation of smaller metabolites of primary hydrolytic fragments of Met-Enk in the mucosal extracts.

In-vitro permeation of some beta-blockers across the hairless mouse skin

To support the development of a suitable transdermal dosage form for beta-blockers, in-vitro, skin permeation studies of nine beta-blockers were conducted at 37 degrees C across the excised abdominal skin of hairless mouse mounted on the receptor compartment of a two-chambered Valia-Chien glass diffusion cell. The drugs varied in lipophilicity, whereas pKa values were comparable. Permeability coefficients were calculated from the steady-state flux values. Agreement was found between the permeability coefficient and the drug lipophilicity, expressed as the octanol-buffer distribution coefficient.