Kenji Sugibayashi

Prediction of Skin Permeability of Drugs: Comparison of Human and Hairless Rat Skin

Abstract— Relationships between skin permeability and physicochemical properties of drugs were examined to establish a predictive method for the steady‐state permeation rate of drugs through human skin. Human skin permeation properties fell into two categories: one in which the permeability coefficient is correlated to the partition coefficient, revealed with lipophilic drugs; and the other in which the permeability coefficients are almost constant, shown with hydrophilic drugs. The stratum corneum, the main barrier in skin, could be considered as a membrane with two parallel permeation pathways: lipid and pore pathways, and an equation for predicting the steady‐state permeation rate of drugs was derived. The skin permeabilities of drugs for man were compared with those for hairless rat. The species difference in skin permeability found was suggested to be due to the difference in skin permeation pathways, since lipid content and water uptake of the stratum corneum varied between human and hairless rat skin.

Effect and mode of action of aliphatic esters on the in vitro skin permeation of nicorandil

Abstract The effect and mode of action of aliphatic esters such as isopropyl myristate (IPM), which were known as percutaneous absorption enhancers, on the in vitro permeation of a drug across the excised hairless rat skin were investigated. Nicorandil and propylene glycol (PG) or water were used as a model drug and solvent in a drug-donor compartment. It was found that IPM markedly enhanced nicorandil permeation across the full-thickness skin, especially when PG was used as a solvent. The skin permeation profiles e.g., flux and lag time, of nicorandil widely differed with the change of IPM content in PG. The large differences in flux and lag time were also affected by the alteration of kinds of aliphatic esters. That is, the flux increased inversely proportional to a lipophilic index of aliphatic esters derived from partition coefficient between n -hexane and water, and the lag time period was almost proportional to the lipophilic index. Fluxes of an IPM-PG treatment and PG treatment across the stratum corneum-stripped skin, however, were almost the same. The permeation-enhancing effect of aliphatic esters could not be explained only by the effects of the aliphatic esters on the solubility of nicorandil in vehicle and the release of the drug from vehicle. On the other hand, the skin permeation profiles of solvents (PG and water) were almost the same as the nicorandil permeation. Therefore, aliphatic esters such as IPM would mainly act on the stratum corneum, and would increase the diffusivity in the stratum corneum and/or partition coefficient between the stratum corneum and vehicle both of the drug and solvent.

Screening of cationic compounds as an absorption enhancer for nasal drug delivery

Several cationic compounds were screened as potential nasal absorption enhancers to increase intranasal absorption of a model drug, fluorescein isothiocyanate labeled dextran (MW 4.4 kDa, FD-4), without nasal membrane damage in rats. Their effects were compared with those of classical enhancers. Various cationic compounds (poly-L-arginines with different molecular weights (MW 8.9, 45.5 and 92.0 kDa, poly-L-Arg (10), (50) and (100), respectively), L-arginine (L-Arg), L-lysine (L-Lys), and cetylpyridinium chloride (CPCL) were evaluated. Of the cationic compounds, poly-L-Arg and CPCL greatly enhanced the intranasal absorption of FD-4, as did chitosan, a cationic polysaccharide which has been reported to show a great effect on the transnasal delivery of peptide and protein drugs. The enhancing intensity by poly-L-Arg was dependent on its molecular weight. Rank order of the enhancing ratio, calculated from the AUC ratio for the enhancer treatment against the untreatment, was 0.5% poly-L-Arg (100) congruent with0.5% sodium dodecylsulfate congruent with0.5% CPCL?0.5% poly-L-Arg (50)?0.5% sodium deoxycholate congruent with0.5% sodium taurodihydrofusidate?0.5% polyoxyethylene-9-lauryl ether congruent with0.5% lysophosphatidylcholine?0.5% chitosan congruent with0.5% poly-L-Arg (10)>/=10% L-Arg congruent with10% L-Lys?0.5% sodium glycocholate congruent with0.5% sodium taurocholate congruent with0.5% EDTA. Only the poly-L-Args represented almost the same degree of hemolysis of cationic compounds compared with pH 7.0 phosphate buffered saline in the rat erythrocyte lysis experiment. The enhancing ratio by classical enhancers correlated with leaching of protein, phospholipids and LDH from isolated rabbit nasal mucosa. CPCL also fell on the regression lines between the enhancing ratio and their degree of leaching from classical enhancers. In contrast, the enhancing intensities by poly-L-Arg (10), (50) and (100) were greatly shifted from the regression line: the amount of leaching was markedly low in spite of a great enhancement of FD-4 absorption. These findings suggest that of the assessed enhancers only the poly-L-Args enhance the transnasal delivery of high molecular substances without severe damage to the nasal mucosal membrane. Poly-L-Arg is therefore a promising candidate having a good balance between enhancing activity and safety for nasal peptide and protein delivery.

Drug Permeation through the Three Layers of the Human Nail Plate

The in‐vitro permeation characteristics of a water soluble model drug, 5‐fluorouracil, and a poorly water soluble model drug, flurbiprofen, were investigated through three layers of the human nail plate (namely, the dorsal, intermediate and ventral nail plates), using a modified side‐by‐side diffusion cell. The dorsal‐filed nail plate, the ventral‐filed nail plate and the dorsal‐and‐ventral‐filed nail plate were prepared to known thicknesses and then used with the full‐thickness nail plate to investigate the permeation characteristics of each single layer.

Analysis of the Combined Effect of 1-Menthol and Ethanol as Skin Permeation Enhancers Based on a Two-Layer Skin Model

The combined effects of 1-menthol and ethanol as a skin permeation enhancer were evaluated with two equations describing the permeability coefficient through full-thickness skin (PFT) and the full-thickness skin/vehicle concentration ratio (CFT/CV) of drugs as a function of their octanol/vehicle partition coefficient (KOV). A two-layer model was applied for skin, which consists of a stratum corneum (SC) with lipid and porous pathways and a viable epidermis and dermis (ED). The two equations contain one variable (KOV) and nine coefficients, six of which (three diffusion coefficients, the porosity of the SC, and two terms of the linear free energy relationship) were considered different, dependent on the drug vehicle. In vitro permeation of four drugs (morphine hydrochloride, atenolol, nifedipine, and vinpocetine) was determined using excised hairless rat skin and four aqueous vehicles (water, 5% 1-menthol, 40% ethanol, and 5% l-menthol–40% ethanol) to measure each PFT. Drug concentrations in full-thickness skin were also measured to obtain CFT/CV. A nonlinear least-squares method was employed to determine six coefficients using the two equations and experimentally obtained PFT and CFT/CV. The addition of 1-menthol to water and 40% ethanol increased the diffusion coefficient of drugs in lipid and pore pathways of SC, whereas the addition of ethanol to water and 5% 1-menthol increased the drug solubility in the vehicle, decreased the skin polarity, and increased the contribution of the pore pathway to whole-skin permeation.

Effect of the absorption enhancer, Azone, on the transport of 5‐fluorouracil across hairless rat skin

The effect of the percutaneous absorption enhancer, Azone, on the transport of 5‐fluorouracil across hairless rat skin has been investigated by an in‐vitro permeation technique using 2‐chamber diffusion cells. Azone (3% w/v) emulsions were used. Azone enhanced the permeability of drug 10–100 times across the full‐thickness skin although there was a lag time about 10 h. The long lag time, however, disappeared with Azone pretreatment. Azone also affected the transport across stripped skin. These results suggest that Azone mainly affects the stratum corneum. It seems to change the diffusivity of drug in that layer and is not so effective against diffusivities in the epidermis and dermis.

Improved nasal absorption of drugs using poly-l-arginine: effects of concentration and molecular weight of poly-l-arginine on the nasal absorption of fluorescein isothiocyanate–dextran in rats

The effects of the concentration and molecular weight of poly-L-arginine (poly-L-Arg) on the in vivo nasal absorption of fluorescein isothiocyanate-labeled dextran (MW, 4 kDa, FD-4) in rats were studied. When poly-L-Arg with a range of different molecular weights (MW, 8.9, 45.5 and 92.0 kDa) was applied intranasally at various concentrations, the bioavailability (F(0-9 h)) of FD-4 increased with the increasing concentration of poly-L-Arg. The enhanced absorption was also dependent on the molar concentration, in that the poly-L-Arg with a higher molecular weight increased F(0-9 h) at a lower molar concentration. In addition, for each applied concentration, the poly-L-Arg exhibited a molecular weight-dependence as far as the enhancement of FD-4 absorption was concerned. On the other hand, the maximum absorption rate (MAR) of FD-4, calculated by means of a deconvolution method, tended to reach a maximum plateau level at a lower applied concentration for the poly-L-Arg with the highest molecular weight, but this plateau level was almost the same for poly-L-Arg with molecular weights of 45.5 and 92.0 kDa. Moreover, the simulated absorption profiles of FD-4 indicate that the degree of enhancement (the level of MAR and the subsequent reduction in the absorption rate) was dependent on the molecular weight of poly-L-Arg, while the effect of poly-L-Arg was maintained for a longer period, depending on the applied concentration, although the MAR was relatively similar. These results indicate that the molecular weight of poly-L-Arg appears to affect both the enhancing efficiency (absorption rate) and the time-frame of this enhancing effect, whereas the concentrations of each poly-L-Arg system applied only have an effect on the time-frame. These effects may also be associated with the charge density of a poly-L-Arg molecule.

Skin penetration-enhancing effect of drugs by phonophoresis

The skin penetration enhancement of nine drugs by phonophoresis was analyzed using ultrasonic irradiation at 150 kHz in in vitro skin permeation experiments conducted to elucidate the flux and permeability coefficient of drugs, and hydrodynamic parameters of skin. The flux of lipophilic drugs after sonication was similar to that before sonication, whereas that for hydrophilic drugs after sonication was increased 6.88-7.43-fold. Permeability coefficients of hydrophilic drugs through full thickness skin with ultrasound were closer to that through stripped skin without ultrasound, while that of lipophilic drugs was only slightly changed. A comparison of hydrodynamic parameters of skin with and without ultrasonication indicated an increase in the aqueous region of the stratum corneum and a constant pore size. Thus, ultrasonication has a great effect on the skin permeation of hydrophilic drugs which usually have low permeability.

Effect of vehicle on the skin permeability of drugs: polyethylene glycol 400-water and ethanol-water binary solvents

Abstract A predictive method for skin permeability of drugs from polyethylene glycol 400 (PEG)-water and ethanol (EtOH)-water binary solvents is proposed. The method is based on a permeation model, in which the stratum corneum is assumed to be a membrane having two parallel permeation pathways: lipid and pore pathways. Skin permeability of several drugs with a variety of physicochemical properties from various PEG-water and EtOH-water solvents was tested. In PEG-water solvents, almost the same skin permeation rate of lipophilic drugs was obtained independent of the solvent, whereas the permeability coefficient of hydrophilic drugs decreased with increasing PEG fraction. The solvent viscosity was increased and the skin permeability of solvents was decreased by addition of PEG, suggesting the decreased diffusion of the drugs in the pore pathway. EtOH, in contrast, markedly enhanced the skin permeation rate of lipophilic drugs, though no such effect was found on hydrophilic drugs. The enhancing ratio in permeation of lipophilic drugs was dependent not on the lipophilicity of the drug but on the EtOH fraction in the solvent. EtOH-water solvents increased the fluidity of skin lipids, although the extraction of lipids by EtOH-water solvents was relatively low. These results suggest that EtOH enhances diffusion in the lipid pathway. Equations for predicting the skin permeability of a drug from various PEG-water and EtOH-water binary solvents were then derived. The skin permeation behavior of a drug can be predicted by its solubility in octanol and vehicle based on the parallel skin permeation pathway model.

Simultaneous transport and metabolism of ethyl nicotinate in hairless rat skin after its topical application: the effect of enzyme distribution in skin.

An in vitro permeation study of ethyl nicotinate (EN) was carried out using excised hairless rat skin, and simultaneous skin transport and metabolism of the drug were kinetically followed. Fairly good steady-state fluxes of EN and its metabolite nicotinic acid (NA) through the skin were obtained after a short lag time for all the concentrations of EN applied. These steady-state fluxes were not proportional to the initial donor concentration of EN: EN and NA curves were concave and convex, respectively, which suggests that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. Further permeation studies of EN or NA were then carried out on full-thickness skin or stripped skin with an esterase inhibitor to measure their permeation parameters, such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis. Separately, enzymatic parameters (Michaelis constant K(m) and maximum metabolism rate V(max)) were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. The obtained permeation and enzymatic parameters were then introduced to differential equations showing Ficks second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis. The calculated steady-state fluxes of EN and NA by the equations were very close to the obtained data. We then measured the esterase distribution in skin microphotographically using fluorescein-5-isothiocyanate diacetate. A higher enzyme concentration was observed in the epidermal cells and near hair follicles than in the dermis. Simulation studies using the even and the partial enzyme distribution models suggested that no significant difference between the models was observed in the skin permeations of EN and NA, whereas concentration-distance profiles of EN and NA were very different. This finding suggests that the total amount of enzyme in skin which converts EN to NA is a determinant of the metabolic rate of EN in skin. The present approach is a useful tool for analyzing simultaneous transport and metabolism of many drugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.