Hydrogel Ring for Topical Drug Delivery to the Ocular Posterior Segment.

PURPOSE: To investigate the efficacy of a topical hydrogel ring for drug delivery to the posterior segment of the rabbit eye. MATERIALS AND METHODS: Novel hydrogel corneal lenses (CL), scleral/corneal lenses (S/CL), and rings were prepared using poly(hydroxyethyl methacrylate). The devices were immersed in 0.3% ofloxacin ophthalmic solution (OOS) to homogeneously distribute the drug throughout the hydrogel. The medicated CL, S/CL, Ring 1 (standard ring), or Ring 2 (shape-optimized ring) was applied to the surface of the cornea, cornea/bulbar conjunctiva, or bulbar conjunctiva of albino rabbits, respectively. Medicated rings did not touch the corneal surface. In another group, one OOS drop was administered to the eye. After 0.25-8 hours, the hydrogel devices were removed and ocular tissues were harvested. High-performance liquid chromatography (HPLC) was used to measure the ofloxacin concentration in the devices and tissues. The drug concentrations in the posterior segment tissues were compared among ofloxacin delivery methods. RESULTS: One hour after placement, eyes treated with Ring 1 or S/CL had markedly higher ofloxacin levels in the posterior segment tissues (conjunctiva, sclera, and retina/choroid) than eyes treated with topical OOS or a CL. Lower levels of ofloxacin were found in anterior segment tissues (cornea and aqueous humor) in eyes treated with Ring 1 compared to those treated with S/CL. Ring 2 most effectively delivered ofloxacin to the retina/choroid. The tissue ofloxacin concentration in the fellow eye was markedly lower than the eye treated with Ring 2. CONCLUSIONS: Our results suggest that hydrogel rings are effective in delivering topical ophthalmic drugs to the posterior segment. The drugs are most likely delivered via the transconjunctival/scleral route by lateral diffusion across the bulbar conjunctiva and through the sclera. Systemic drug delivery to the posterior segment is minimal.

Effect of diffusive direction across the skin on the penetration profile of chemicals in vitro.

Skin has various types of transporters and is a biochemically active organ. These aspects of skin influence the distribution of chemicals in skin and their elimination from skin. The biochemical and histological variations of the skin must be taken into account when conducting transdermal penetration research. Here we used hairless mouse skin to investigate the percutaneous absorption of chemicals in vitro from the stratum corneum (SC) side to the viable skin (VS) side (forward direction) and from the VS side to the SC side (backward direction). We examined the effects of molecular weight, lipophilicity (Log Ko/w), electric charge, and the molecular structure of penetrants. The penetration flux of verapamil hydrochloride (VRP) for the backward direction was 3.2 times larger than that for the forward direction. The flux values of benzoic acid (BA) and para-hydroxybenzoic acid (pHBA) for the forward direction were 2.1 and 4.6 times larger than those for the backward direction, respectively. This directional difference was caused by the active transporter for VRP, the histological distribution of BA solubility, and the intermolecular hydrogen bonding between pHBA and skin tissue in the stripped skin. Across intact skin, in contrast, there was no difference in the skin penetration profile between the forward direction and backward directions.

Skin accumulation and penetration of a hydrophilic compound by a novel gemini surfactant, sodium dilauramidoglutamide lysine.

We investigated a novel peptide-based gemini amphiphilic compound, sodium dilauramidoglutamide lysine (DLGL), as a chemical enhancer for the skin penetration of l-ascorbic acid 2-glucoside (AAG). A three-dimensional cultured human skin product, TESTSKIN™ LSE-high (LSE-high), was used as a skin model. The penetration flux of AAG with DLGL and that obtained with sodium lauramidoglutamide (LG) as a conventional surfactant across LSE-high were increased by 12.56 and 69.29 times compared to the control, respectively. The ratio of AAG amount with DLGL in the skin (21.78% total dose) was significantly increased (p<0.05) compared to the control (7.23%) and to the AAG amount with LG (8.13%). The AAG amounts in receptor were 1.06% (control), 3.19% (+DLGL) and 21.00% (+LG). Thus, DLGL preserved AAG in skin, resulting in enhanced AAG penetration flux. However, LG might create the pathways through the skin. We conclude that DLGL is a gemini surfactant that accumulates a hydrophilic compound in skin and enhances the penetration flux. DLGL may therefore be a novel addition agent for skin local therapy.

Prediction of percutaneous absorption in human using three-dimensional human cultured epidermis LabCyte EPI-MODEL.

The objective of this study is to establish a relationship of the skin penetration parameters between the three-dimensional cultured human epidermis LabCyte EPI-MODEL (LabCyte) and hairless mouse (HLM) skin penetration in vitro and to predict the skin penetration and plasma concentration profile in human. The skin penetration experiments through LabCyte and HLM skin were investigated using 19 drugs that have a different molecular weight and lipophilicity. The penetration flux for LabCyte reached 30 times larger at maximum than that for HLM skin. The human data can be estimated from the in silico approach with the diffusion coefficient (D), the partition coefficient (K) and the skin surface concentration (C) of drugs by assuming the bi-layer skin model for both LabCyte and HLM skin. The human skin penetration of ß-estradiol, prednisolone, testosterone and ethynylestradiol was well agreed between the simulated profiles and in vitro experimental data. Plasma concentration profiles of ß-estradiol in human were also simulated and well agreed with the clinical data. The present alternative method may decrease human or animal skin experiment for in vitro skin penetration.

Evaluation of the predicted time-concentration profile of serum tulobuterol in human after transdermal application.

We proposed an in vitro/in vivo/in silico method for evaluating the clinical performance of matrix type transdermal therapeutic systems (TTSs). This method is based on the following four approaches: (1) drug release experiment, (2) in vitro penetration experiment using excised hairless mouse skin, (3) clinical pharmacokinetic study, and (4) mathematical model for evaluating the pharmacokinetic profile. The tulobuterol TTS was used as an example of a matrix type TTS in this study. The drug diffusion coefficient in the matrix device was calculated from the result of the release experiment. The drug diffusion coefficient and the partition coefficient in the skin were calculated from the results of in vitro skin penetration experiments where hairless mice and rats were used. Those parameters were used as substitutes of human. Further, these parameters were used for solving the governing partial differential equation on skin penetration. The time profiles of the serum concentration in human after applying the tulobuterol TTS were predicted and compared with the clinical data. The predicted profiles obtained from the data of hairless mice reproduced the influence of drug depletion adequately and well agreed with the clinical data, while those from the data of rats differed clearly in the initial rise. This method is useful for prediction of pharmacokinetic profiles of TTSs.

Drug penetration of the posterior eye tissues after topical instillation: in vivo and in silico simulation.

The purpose of this study was to analyze drug pharmacokinetics in the posterior eye tissues after topical instillation. For the in vivo study, the concentrations of ofloxacin in rabbit ocular tissues were analyzed by high performance liquid chromatography at 1, 2, and 3 h after instillation. For the in silico simulation, the concentration distribution of ofloxacin in the eye was calculated by the ocular pharmacokinetic model based on the diffusion/partition model. The simulated profiles were then compared with the in vivo experimental findings. In the in vivo study, the drug concentration in the posterior vitreous body initially decreased with time after topical instillation, and thereafter, the concentration increased. The in silico simulation of ocular pharmacokinetics indicated that the drug penetration of the posterior vitreous body was determined by three major pathways: (1) the initial transscleral penetration, (2) the intermediate transcorneal penetration, and (3) the late transretinal penetration. The in vivo findings were well described by a series of contributions by these three pathways. In conclusion, the present in vivo and in silico studies suggest that the instilled drugs initially reached the posterior vitreous body by diffusion through the sclera and then later by corneal penetration and systemic circulation.

Effect of environmental temperature on transdermal drug penetration.

The effect of environmental temperature on the penetration from matrix-type transdermal patch of non-steroidal anti-inflammatory drugs (NSAIDs) as model drugs was investigated using in vitro and in silico experiment. The patch was applied on the stratum corneum (SC) side of the skin. The dermal side of the skin was mounted on a diffusion cell. The donor compartment of the diffusion cell was filled with distilled water. The donor temperature was set at 2, 25, 37, and 47 °C, respectively. The receptor compartment was kept at 37 °C and filled with phosphate buffer solution during the experiment. The permeation of the drugs from patch increased with increasing the donor temperature. The rate of permeation increased exponentially with increasing skin surface temperature. The diffusion coefficient in the skin remained almost constant, while the skin surface concentration was correlated with the skin surface temperature. The plasma concentrations in human were simulated by SKIN-CAD(®) together with the in vitro penetration experiment. The plasma concentration quickly changed with varying the environment temperature.

Effects of pathological conditions on ocular pharmacokinetics of antimicrobial drugs.

A diffusion model of ocular pharmacokinetics was used to estimate the effects of pathological conditions on ocular pharmacokinetics. In vivo rabbit data after topical instillation of ciprofloxacin and ofloxacin were compared with the simulated concentrations in the aqueous and vitreous humors. The barrier capacity of the surrounding membranes such as the retina/choroid/sclera (RCS) membrane and the cornea was characterized by dimensionless Sherwood number derived by the pseudo-steady state approach (PSSA). We assumed the barrier capacity decreased by inflammation; when the barrier capacity of the RCS membrane and the cornea was assumed to be one-tenth for the RCS membrane and a half for the cornea respectively, the in vivo data agreed with the simulated profile without contradiction. The drug concentration gradient simulated in the vitreous body near the RCS membrane was more significant in the inflamed eyes than in the normal eyes, suggesting that the elimination of the drugs from the RCS membrane was enhanced by inflammation. The present diffusion model can better describe the ocular pharmacokinetics in both normal and diseased conditions.

Mucoadhesive properties of chitosan-coated ophthalmic lipid emulsion containing indomethacin in tear fluid.

To evaluate the residence of chitosan-coated emulsion (CE) containing indomethacin in tears, the drug retention of CE in tear fluid was compared with non-coated emulsion (NE) after instillation in rabbit eyes. CE had mean concentrations 3.6-fold and 3.8-fold higher than NE at 0.5 h and 0.75 h after instillation, respectively. Mean residence time and half-life of CE were lengthened to 1.5-fold and 1.8-fold those of NE, respectively. Volume of distribution of CE in tear fluid was also 1.6-fold greater than that of NE. These findings indicated that retention of the drug in tears was appreciably prolonged by chitosan-coated emulsion, and that CE had higher distribution on the ocular surface than NE. The drug levels in cornea, conjunctiva, and aqueous humor at 1 h after instillation were clearly higher than those of NE. In a generalized ocular pharmacokinetic model, the ratio of CE to NE for peak concentration values (C(max)) and the area under the concentration/time curve (AUC) nearly corresponded to aqueous humor levels in vivo. Additionally, tensile testing showed that the force of detachment between CE and mucin was significantly larger than that of emulsion containing hydroxypropylmethyl cellulose (HPMCE) with a viscosity similar to CE; the forces of detachment of CE and HPMCE measured using phosphate-buffered saline (PBS) were almost the same since these formulations have similar viscosity. Mucoadhesive strength of CE was confirmed by measurements of force of detachment between formulations and mucin. The residence time of the emulsion in tear fluid was prolonged by chitosan coating because of its mucoadhesive properties.

Mechanisms of synergistic skin penetration by sonophoresis and iontophoresis.

The mechanism of skin penetration enhancement by ultrasound under sonophoresis (US) or by an electrical field under iontophoresis (IP) was investigated using hairless mouse skin in vitro. The seven model chemicals with different molecular weights (122-1485) were dissolved in a hydrophilic gel. Donor gel with the chemicals was loaded on the skin surface and then the skin was treated with US (300 kHz, 5.2 W/cm(2), 5.4% duty-cycle) and IP (0.32+/-0.03 mA/cm(2)) individually or with US and IP in combination (US+IP). The penetration profiles of the chemicals with a molecular weight of less than 500 were influenced by the presence of an electric charge, the profiles of ionized chemicals for US+IP were the same as profiles for IP, while the penetration flux of a non-ionized chemical synergistically increased with US+IP compared with the individual flux of US and IP. The chemicals with molecular weight of more than 1000 showed synergistic effects with US+IP. The mathematical simulation assuming a bilayer skin model revealed that the synergistic effects were mainly influenced by electroosmosis in the stratum corneum (SC). Therefore the synergistic effects of US+IP was mainly caused by the SC diffusivity of chemicals increased by US and the electroosmotic water flow by IP application.

Bioequivalence of marketed transdermal delivery systems for tulobuterol.

Tulobuterol permeation through skin from various transdermal delivery systems has been compared for the bioequivalence among devices marketed. Both the permeation profiles across the hairless mouse skin and the release profiles from the devices were measured under well-controlled in vitro conditions. The release rate of the drug from the devices was appreciably higher than the penetration rate across the intact skin, indicating the skin-controlled delivery systems. However the deviation between the release rate and the permeation rate differs depending upon the system design. The brand patch showed the least difference between the release and permeation profiles among the brand and three generic devices examined. From the in vitro permeation profiles for both intact and stripped skins, the diffusion coefficient and the partition coefficient were evaluated on the basis of bi-layer skin model. The effect of the stratum corneum thickness was then simulated by SKIN-CAD. The simulated profile has suggested that the clinical performance for transdermal tulobuterol delivery is influenced not only by the thickness of the stratum corneum but by the device design as well. This is particularly the case for the stratum corneum, thinner than about 10 microm or damaged skin with the decreased barrier capacity. For the stratum corneum thicker than 20 microm, on the other hand, the clinical performance may not be significantly influenced by the device designs investigated in this study.

Development of a stick-type transdermal eyelid delivery system of ketotifen fumarate for ophthalmic diseases.

A transdermal eyelid delivery system for treating ocular diseases (eye-stick) has been developed. Ketotifen fumarate (KT) was used as a model drug. An in vivo study using rabbits showed that the eye-stick device maintained a constant conjunctival concentration of the drug for an extended period of time, which was equivalent or higher than the therapeutic level following eye drop administration. Moreover, the conjunctival concentration after eye-stick application was well predicted using the physicochemical parameters, diffusion coefficient and partition coefficient, obtained from in vitro hairless mouse skin permeation experiments.

Skin permeation of ketotifen applied from stick-type formulation.

A stick-typed long lasting device for both transdermal and topical drug delivery has been developed. Ketotifen fumarate (KT) was used as a model drug. The effect of a variety of permeation enhancers was investigated using hairless mouse skin in vitro. Polyoxyethylene oleyl ether (POE), among the enhancers used, most enhanced the skin permeation of KT. The permeation enhancement was mainly due to the increase in the drug solubility in the stratum corneum and the resulting increase in the partition coefficient. The rate of skin permeation of KT was approximately proportional to the loading dose of the drug.

Skin irritation in transdermal drug delivery systems: a strategy for its reduction.

PURPOSE: Active pharmaceutical ingredients (API) in transdermal drug delivery systems (TDS) often causes skin irritation such as erythema and edema. We have studied a possible approach for the reduction of skin irritation by patch formulations that control the rates of skin permeation and elimination of API. METHODS: Loxoprofen (LX-base) was used to induce the skin irritation. The redness value (Deltaa) was evaluated as a measure of erythema by Chroma Meter. The in vitro skin permeation and release profiles were also investigated by using a side-by-side diffusion cell. RESULTS: The redness values were not correlated either with the cumulative amount of API permeated or the concentration of LX-base in the skin, but well correlated with the elimination rate of LX-base from the skin after the removal of the formulation. The formulation with gradual decrease of permeation rate during application accelerated the elimination rate after application, and resulted in the reduction of the skin irritation. CONCLUSIONS: The skin pharmacokinetics of API, not only permeation during application but also release after the patch removal, was found to be a significant factor for skin irritation. To minimize the skin irritation, it's also important to eliminate the residual API in the skin promptly after application.

Effect of microdermabrasion on barrier capacity of stratum corneum.

A new microdermabrasion system is used for peeling the stratum corneum in a controlled manner. The system uses inert corundum powders under various degrees of vacuum. The fine corundum powders ejected by suction power, being quickly in contact with the skin surface, abrade and remove a tiny fragment of stratum corneum. The fraction of the stratum corneum removed by microdermabrasion can be controlled by the operating conditions; the duration of application (L) and the degree of vacuum setting (V). The stratum corneum barrier function with respect to the rate of skin penetration is well correlated by the product of the square of the degree of the vacuum and the duration of the probe setting.

A pharmacokinetic model for ocular drug delivery.

A pharmacokinetic model of ocular drug delivery has been developed for describing the elimination and distribution of ocular drugs in the eye. The model, based on Fick's second law of diffusion, assumes a modified cylindrical eye with three pathways for drug transport across the surface of the eye: the anterior aqueous chamber, the posterior aqueous chamber and the retina/choroids/scleral membrane covering the vitreous body. The model parameters such as the diffusion coefficient and the partition coefficient in various eye tissues can be evaluated from the in vitro membrane penetration experiments using a side-by-side diffusion cell system. The diffusion coefficient for a drug is also predicted by taking account of the effect of the molecular weight of model compounds. The present ocular pharmacokinetic model, which can predict the local concentration distribution in the eye, has well described the in vivo concentration profile in the various eye tissues, the lens, the aqueous humor and the vitreous body, following not only topical eye drop instillation but systemic administration as well. The present model also simulates the effects of binding and metabolism in the eye as well as the individual difference in ocular functions and structure such as cataract surgery and vitreous fluidity on the distribution and elimination of drug molecules in the eye.

Improvement of the ocular bioavailability of timolol by sorbic acid.

The ocular bioavailability of timolol increased in sorbic acid solution due to ion pair formation. Its octanol/water partition coefficient also increased, suggesting the formation of a more lipophilic complex. The concentration of timolol in rabbit aqueous humor was determined after instillation of timolol ophthalmic solution containing sorbic acid. When the molar ratio of sorbic acid to timolol was two or higher, the concentration of timolol in the aqueous humor was higher than with timolol alone. In the presence of sorbic acid the maximal aqueous humor concentration and the area under the curve were more than two-fold higher than those of Timoptol, a timolol maleate ophthalmic solution, and similar in value to TIMOPTIC-XE, a gel-forming ophthalmic solution. To investigate the transcorneal absorption mechanism, in vitro permeation profiles across the intact and de-epithelialyzed cornea were analyzed on the basis of the bilayer diffusion model. The partition coefficient in the epithelium was about twice as high in the presence of sorbic acid than with timolol alone, although the diffusion coefficient in the epithelium did not change. We conclude that the improved ocular bioavailability in the presence of sorbic acid is due to increased partitioning of timolol in the corneal epithelium.

Drug delivery to the eye with a transdermal therapeutic system.

A matrix-type transdermal therapeutic system was developed for treating diseases of the eye where it is difficult for drug molecules to reach with conventional topical instillation. Prednisolone was employed as a model drug. An in vivo study using rats showed that the daily application of the patch maintained a constant plasma concentration of the drug, which was equivalent the therapeutic plasma level following three times daily oral administration (30 mg), for approximately 24 h. Transdermal delivery provided equivalent to or higher bioavailability (drug distribution) to the eyeball of topical administration. Moreover, pharmacokinetic analysis indicated that the present transdermal therapeutic system may be clinically effective as a new treatment for ocular diseases.

Comparison of skin distribution of hydrolytic activity for bioconversion of beta-estradiol 17-acetate between man and several animals in vitro.

We have investigated the distribution of hydrolytic enzymes which metabolize beta-estradiol 17-acetate (EA) to beta-estradiol (E) in man and animal skins in vitro. The distribution of hydrolytic enzymes in human cadaver, hairless dog, rat and hairless mouse skin, was investigated by a skin-slicing technique. We performed histological studies with hematoxylin and eosin stain. The highest amount of metabolite (E) appeared in the layers of 80-120 microm from the skin surface, the basement layer in human skin, while the amount of metabolite was distributed evenly in the hairless dog skin from 0 to 180 microm. In the rat and hairless mouse skin, on the other hand, peak levels of metabolite were observed in the basement layer of dermis, the surrounding area of the cutaneous plexus. The total metabolic activities in the area of epidermis in human, hairless dog and hairless mouse skin were 2.59, 8.03 and 0.33 x 10(-4) microg/ml/microm/h, respectively. The values in whole skin layers in the hairless dog and hairless mouse skin were 3.35 and 1.85 x 10(-4)microg/ml/microm/h, respectively. EA transported across the human and hairless dog skin can be effectively metabolized before entering the capillary. Among animal models investigated, hairless dog skin might be the most facile model in simulating drug metabolism for human skin under the clinical (in vivo) conditions. Hairless mouse skin, on the other hand, was also an excellent model in excised human skin under in vitro conditions.

Prediction of plasma concentration of GTS-21 in hairless rats following monolithic transdermal delivery.

The transdermal therapeutic systems (TTS) usually achieve constant plasma concentration for an extended period of time. This is because a sufficient drug stored in the device can keep the constant concentration on the surface of the stratum corneum during the system application. When the drug molecules are not enough to provide the constant surface concentration, the rate of drug penetration decreases with time because of decreased supply of the drug molecules from the delivery device. This paper has proposed an empirical simple approach to predict the plasma concentration for such a TTS. A novel compound, GTS-21, for Alzheimers' disease currently under development was used as a model drug. In vivo and in vitro experiments were carried out in hairless rats. The in vivo plasma concentration-time profile in hairless rats following the application of TTS well agreed with the predicted profile based on the skin pharmacokinetic model together with the model parameters determined from the in vitro experiment.