In vivo microdialysis for the transdermal absorption of valproate in rats

Transdermal permeation of drugs with differing lipophilicity: Effect of penetration enhancer camphor

The aim of the present study was to investigate the potential application of (+)-camphor as a penetration enhancer for the transdermal delivery of drugs with differing lipophilicity. The skin irritation of camphor was evaluated by in vitro cytotoxicity assays and in vivo transdermal water loss (TEWL) measurements. A series of model drugs with a wide span of lipophilicity (logP value ranging from 3.80 to -0.95), namely indometacin, lidocaine, aspirin, antipyrine, tegafur and 5-fluorouracil, were tested using in vitro transdermal permeation experiments to assess the penetration-enhancing profile of camphor. Meanwhile, the in vivo skin microdialysis was carried out to further investigate the enhancing effect of camphor on the lipophilic and hydrophilic model drugs (i.e. lidocaine and tegafur). SC (stratum corneum)/vehicle partition coefficient and Fourier transform infrared spectroscopy (FTIR) were performed to probe the regulation action of camphor in the skin permeability barrier. It was found that camphor produced a relatively low skin irritation, compared with the frequently-used and standard penetration enhancer laurocapram. In vitro skin permeation studies showed that camphor could significantly facilitate the transdermal absorption of model drugs with differing lipophilicity, and the penetration-enhancing activities were in a parabola curve going downwards with the drug logP values, which displayed the optimal penetration-enhancing efficiency for the weak lipophilic or hydrophilic drugs (an estimated logP value of 0). In vivo skin microdialysis showed that camphor had a similar penetration behavior on transdermal absorption of model drugs. Meanwhile, the partition of lipophilic drugs into SC was increased after treatment with camphor, and camphor also produced a shift of CH2 vibration of SC lipid to higher wavenumbers and decreased the peak area of the CH2 vibration, probably resulting in the alteration of the skin permeability barrier. This suggests that camphor might be a safe and effective penetration enhancer for transdermal drug delivery.

In vivo methods for the assessment of topical drug bioavailability

This paper reviews some current methods for the in vivo assessment of local cutaneous bioavailability in humans after topical drug application. After an introduction discussing the importance of local drug bioavailability assessment and the limitations of model-based predictions, the focus turns to the relevance of experimental studies. The available techniques are then reviewed in detail, with particular emphasis on the tape stripping and microdialysis methodologies. Other less developed techniques, including the skin biopsy, suction blister, follicle removal and confocal Raman spectroscopy techniques are also described.

Microdialysis

Microdialysis is a probe-based sampling method, which, if linked to analytical devices, allows for the measurement of drug concentration profiles in selected tissues. During the last two decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs, e.g. antimicrobial agents or other drugs mediating their action through surface receptors. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations. Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. The combined use of microdialysis with non-invasive imaging methods such as positron emission tomography and single photon emission tomography opened the window to exactly explore and describe the fate and pharmacokinetics of a drug in the body. Linking pharmacokinetic data from the ISF to pharmacodynamic information appears to be a straightforward approach to predicting drug action and therapeutic success, and may be used for decision making for adequate drug administration and dosing regimens. Hence, microdialysis is nowadays used in clinical studies to test new drug candidates that are in the pharmaceutical industry drug development pipeline.

Effects of HPE-101, a skin penetration enhancer, on human erythrocyte membranes

The primary aim of this study was to investigate the skin permeation-enhancing mechanism of HPE-101 using erythrocyte ghost cells prepared from human whole blood as a biomembrane model. The extent of hemolysis of erythrocytes induced by HPE-101 was measured using a spectrophotometer at 540nm. The effect of HPE-101 on lipid fluidity was examined by observing the change of intramolecular excimer formation and fluorescence polarization using an intramolecular probe (1,3-bis(pyrene) propane) and a lipid probe (1,6-diphenyl 1,3,5-hexatriene), respectively. Hemolysis of erythrocytes was observed at 0.01mM and completed at 1.0mM of HPE-101. The fluorescence polarization of the ghost membrane decreased with the addition of HPE-101, whereas the intramolecular excimer formation increased. HPE-101 thus enhanced the rotational mobility and the lateral diffusion, thereby decreasing the microviscosity of ghost membranes, implying that HPE-101 increases the lipid fluidity of ghost membranes. Therefore, HPE-101 seems to cause an increase in fluidity of the lipid bilayers in the stratum corneum of the skin, resulting in the reduction of diffusion resistance.

Effect of glycerol-induced acute renal failure on the pharmacokinetics of lidocaine after transdermal application in rats

In this study, the effect of glycerol-induced acute renal failure (ARF) on the pharmacokinetics of lidocaine after transdermal application was investigated in rats. Microdialysis method was applied in vitro and in vivo to the abdominal skin of rats. After topical application of 1% lidocaine, the cumulative amount of lidocaine permeated through the excised rat abdominal skin showed parallel effect between normal and ARF rats with no significant difference in the in vitro permeability coefficient of lidocaine between them, while area under the plasma concentration versus time curve of lidocaine in ARF rats increased significantly. The protein binding rate of lidocaine in ARF plasma and the blood vessel permeability to muscle tissues, assessed by beta-D-glucopyranosyl fluorescein isothiocyanate-labeled (FITC) albumin, increased significantly. After intravenous infusion of 5 mg/h/kg lidocaine, both of the total body clearance and the volume of distribution of lidocaine in the ARF rats decreased significantly. These results suggested that renal dysfunction did not have any effect on the skin permeability of lidocaine, but might change the plasma protein binding of drug and blood vessel permeability which led to high plasma concentration of lidocaine.

The prediction of plasma and brain levels of 2,3,5,6-tetramethylpyrazine following transdermal application

The purpose of this study was to construct a pharmacokinetic (PK) model and to determine PK parameters of 2,3,5,6-tetramethylpyrazine (TMP) after application of TMP transdermal delivery system. Data were obtained in Sprague-Dawley (SD) rats following a single dose of TMP transdermal delivery system. Blood samples were obtained at 0, 0.25, 0.5, 1, 2, 4, 6, 16, and 24 hours after the transdermal application. In the brain level study, 18 SD rats were divided into 6 groups. Three SD rats before and after transdermal application were culled and sacrificed at each of the following time intervals: 2, 4, 6, 16, and 24 hours after the TMP-TTS application. TMP concentrations in plasma and brain tissues were determined using high performance liquid chromatography and data were fitted using a zero-order absorption and a first-order-elimination 3-compartment PK model. Fitted parameters included 2 volumes of distribution (V1, V2) and 2 elimination rate constants (k10, k20). The elimination half-life for TMP in plasma and brain was 26.5 and 31.2 minutes, respectively. The proposed PK model fit observed concentrations of TMP very well. This model is useful for predicting drug concentrations in plasma and brain and for assisting in the development of transdermal systems.

Assessment of cutaneous drug delivery using microdialysis

During the last decade microdialysis has been successfully applied to assess cutaneous drug delivery of numerous substances, indicating the large potential for bioequivalence/bioavailability evaluation of topical formulations. The technique has been shown to be minimally invasive and supply pharmacokinetic information directly in the target organ for cutaneous drug delivery with high temporal resolution without further intervention with the tissue after implantation. However, there are a few challenges that need to be addressed before microdialysis can be regarded as a generally applicable routine technique for cutaneous drug delivery assessments. Firstly, the technique is currently not suitable for sampling of highly lipophilic compounds and, secondly, more studies are desirable for elucidation of the variables associated with the technique to increase reproducibility. The present literature indicates that the condition of the skin at the individual assessment sites is the main variable, but also variables associated with relative recovery, differentiation between the pharmacokinetic parameters (i.e., lag time, distribution, absorption and elimination rate) can influences the reproducibility of the technique. Furthermore, it has been indicated that cutaneous microdialysis in rats may be useful for prediction of dermal pharmacokinetic properties of novel drugs/topical formulations in man.

Extracellular glutamate changes in rat striatum during ischemia determined by a novel dialysis electrode and conventional microdialysis

Our newly developed method using a dialysis electrode has made it possible to perform real time monitoring of extracellular glutamate concentration ([Glu]e) utilizing the oxygen-independent reaction with glutamate oxidase and ferrocene. In this study, we therefore, investigated [Glu]e changes during brain ischemia using both the conventional microdialysis method and the dialysis electrode method. A comparison between our newly developed dialysis electrode and conventional microdialysis methods provided the following results. When the conventional microdialysis method was employed: (1) the elevation of [Glu]e during complete global ischemia was delayed; and (2) the elevation of concentration and reuptake of glutamate were delayed during 10-min transient ischemia, and the elevation of [Glu]e reached a maximum later using conventional microdialysis than using our dialysis electrode. (3) The biphasic [Glu]e elevation of glutamate concentration detected using the dialysis electrode method was not observed using the conventional microdialysis method. It was additionally investigated why the conventional microdialysis method provides inferior time resolution. In this study, we also demonstrated with the chromatographic SMART procedure coupled to UV detection that biogenic substances, i.e. low molecular weight proteins and peptides, are released during ischemic injury, and they may cause a delay in the time resolution in the microdialysis method.

Influence of a microemulsion vehicle on cutaneous bioequivalence of a lipophilic model drug assessed by microdialysis and pharmacodynamics

PURPOSE

The aim of the study was to investigate the cutaneous bioequivalence of a lipophilic model drug (lidocaine) applied in a novel topical microemulsion vehicle, compared to a conventional oil-in-water (O/W) emulsion, assessed by a pharmacokinetics microdialysis model and a pharmacodynamic method.

METHODS

Dermal delivery of lidocaine was estimated by microdialysis in 8 volunteers. Absorption coefficients and lag times were determined by pharmacokinetic modelling of the microdialysis data. Subsequently, the anaesthetic effect of the treatments was assessed by mechanical stimuli using von Frey hairs in 12 volunteers.

RESULTS

The microemulsion formulation increased the cutaneous absorption coefficient of lidocaine 2.9 times (95% confidence interval: 1.9/4.6) compared with the O/W emulsion-based cream. Also, lag time decreased from 110 +/- 43 min to 87 +/- 32 min (P = 0.02). The compartmental pharmacokinetic model provided an excellent fit of the concentration-time curves with reliable estimation of absorption coefficient and lag time. A significant anaesthetic effect was found for both active treatments compared to placebo (P < 0.02), but the effect did not diverge significantly between the two formulations.

CONCLUSIONS

The microemulsion vehicle can be applied to increase dermal drug delivery of lipophilic drugs in humans. The microdialysis technique combined with an appropriate pharmacokinetic model provides a high sensitivity in bioequivalence studies of topically applied substances.

Microdialysis for the evaluation of penetration through the human skin barrier - a promising tool for future research?

The direct measurement of local drug concentration levels at discreet skin locations with minor trauma has recently become possible with the introduction of cutaneous microdialysis. Cutaneous microdialysis is an in vivo sampling technique for measuring solutes in the extracellular fluid of the dermis. When used in combination with other experimental approaches, for example with a variety of non-invasive techniques to describe the functional status of the skin (bioengineering methods), it may help investigators to gain new insights into the fields of skin diseases, metabolism and drug absorption/penetration. An important parameter to describe the efficacy of microdialysis is the relative recovery. This is the ratio between the concentration of a substance in the dialysate and the true extracellular concentration. Several methods are in common use to describe the relative recovery (no-net-flux method or retrodialysis). Parameters such as probe design, depth of the probe in the dermis, physico-chemical properties of the compound of interest, and analytical aspects are important factors influencing microdialysis. Microdialysis has been used to investigate the influence of penetration enhancers, vehicles or iontophoresis on percutaneous absorption, performed by in vivo studies in rats. In human volunteers, most of the experiments have been performed to study the kinetics of fast penetrating substances, e.g. nicotine, non-steroidal antiinflammatory drugs, local anaesthetics, or solvents. Problems have been encountered in the detection of lipophilic and highly protein-bound substances. Further, dermal metabolism and the influence of barrier perturbation on percutaneous absorption have been analyzed. Investigations suggest that microdialysis, in combination with traditional techniques, might give valuable information regarding the assessment of the penetration of drugs and other exogenous agents through the skin. In spite of the clearly defined and accepted advantages of microdialysis technology for studies of transdermal drug delivery, to date no standardized test procedure exists nor has the reproducibility of the results been evaluated. In the future, these problems have to be solved to enable this method to find its place in standard research.

Chemical enhancers for transdermal drug transport

In its first part, this review paper discusses skin morphology and barrier function of the stratum corneum for drug permeation after its transdermal administration or topical application. Further, the paper presents the main methods for overcoming the skin permeation barrier, which plays an important role for transdermal drug administration. Focus is on the method of chemical permeation enhancement. The chemical enhancers are categorised by their chemical structure. Examples of the most effective enhancers are given for the chemical groups of alcohols, amines and amides, polyalcohols, terpenes, fatty acids and their esters, macro cyclic compounds, sulfoxides, tensides, and others, as e.g. soft enhancers.

Microdialysis sampling coupled to HPLC for transdermal delivery study of ondansetron hydrochloride in rats

The transdermal delivery of ondansetron hydrochloride (ON) solution in propylene glycol (PG) with a widely used penetration enhancer, oleic acid (OA), was studied in rats by a microdialysis sampling technique. Dialysate samples collected from the probe were directly injected into the HPLC system without any pre-treatment and no interference occurred in the blank sample. A good linearity between the standard concentrations and peak areas within the calibration range was achieved. In vivo recovery (32.52 +/- 1.8%) of the probe was assessed with the retrodialysis method, which was used to calculate the ON concentration in the dermis. Oleic acid at the concentrations of 2% and 5% (w/v) increased the steady-state delivery rate from 0.001 to 0.030 and 0.058 microg/h, respectively. OA proved to be an effective enhancer for transdermal delivery of ON in rats.

Evaluation of transdermal iontophoresis of enoxacin from polymer formulations: in vitro skin permeation and in vivo microdialysis using Wistar rat as an animal model

Polymers were used in vehicles to form hydrogel matrices in this study to evaluate the in vitro permeation and in vivo microdialysis of enoxacin. The highest transdermal delivery determined by area under flux-time curve (AUC) and intracutaneous enoxacin concentration were observed in methylcellulose (MC) and polyvinylpyrrolidone (PVP) hydrogels, respectively. To avoid the pH shift in vehicles during iontophoresis, buffer species were added to formulations to increase the buffer capacity. As expected, the permeability of enoxacin of anodal iontophoresis was larger than that of cathodal iontophoresis. Combination of benzalkonium chloride, a cationic surfactant as an enhancer, and iontophoresis exerted an enhancing effect for anionic enoxacin at pH 10.0. However, no effect or a negative effect was detected for cationic enoxacin in deionized water or pH 5.0 buffer, due to the shielding of the negative charge in the skin. The skin residue of enoxacin was slightly increased after the incorporation of Azone in PVP hydrogel. The result of in vivo microdialysis was in accordance with that of in vitro study. The effect of Azone on the intracutaneous enoxacin was more significant for in vivo microdialysis than in the in vitro study indicating the clinical feasibility of Azone for iontophoretic delivery. Microdialysis can be considered as a useful technique to investigate the pharmacokinetics of transdermal iontophoresis in vivo.

Transdermal iontophoretic delivery of diclofenac sodium from various polymer formulations: in vitro and in vivo studies

The objective of this study was to evaluate the in vitro and in vivo transdermal iontophoresis of various diclofenac sodium polymer formulations. The excised rat skin, human skin as well as cellulose membrane were used to examine the in vitro drug permeation whereas the microdialysis technique was used to monitor the drug concentration in vivo. Polymer solutions based on polyvinylpyrrolidone (PVP) and hydroxypropyl methylcellulose (HPMC) binary system showed higher drug permeability than that of single polymer vehicle. The effect of formulations on drug permeation through cellulose membrane was quite different from those through rat skin and human skin, which can be explained by the different permeation pathways between them. It appeared to be a membrane-controlled mechanism but not the vehicle matrix-controlled mechanism for diclofenac hydrogels when using skin as the diffusion barrier. The recovery of diclofenac sodium in the in vivo microdialysis was approximately 80-90%, indicating this technique can be used in the intradermal drug monitoring. For all the polymer formulations tested, there was a good relationship between the in vitro and in vivo drug permeation. A synergistic effect on drug permeation was observed when transdermal iontophoresis combined with the pretreatment of cardamom oil as a permeation enhancer.

Topical delivery of keloid therapeutic drug, tranilast, by combined use of oleic acid and propylene glycol as a penetration enhancer: evaluation by skin microdialysis in rats

Topical delivery of tranilast (N-(3,4-dimethoxycinnamoyl)anthranic acid), an inhibitor of collagen synthesis and a therapeutic drug for keloid and hypertrophic scar, was examined, in rats, with oleic acid alone or a combination of oleic acid and propylene glycol as penetration enhancer. Evaluation was by measurement of the concentration of tranilast in plasma and in the dialysate from skin microdialysis. When tranilast at a dose of 1.5 mg was applied topically as an ethanol solution containing 5% polyvinylpyrrolidone on a dorsal skin surface (2.25 cm2), the maximum concentration of tranilast in skin dialysate was approximately 2 microM. When 10 or 20% oleic acid was added to the same ethanol solution the maximum concentration of tranilast in the dialysate increased to 10-20 microM, and this value was further increased to 60 microM by the addition of a combination of oleic acid (10 or 20%) and propylene glycol (10%) to the solution. With the combination of oleic acid and propylene glycol the area under the plot of the concentration of tranilast in skin dialysate against time between 0 and 4 h (AUC0-4) was more than 400-fold that after intravenous administration. The transdermal bioavailability of tranilast as assessed by the AUC0-4 of tranilast in plasma, was 0.2% of the dose applied in the ethanol solution, 3-5% of that applied in the ethanol solution containing oleic acid, and 14-16% of that applied in the ethanol solution containing both oleic acid and propylene glycol. These results suggest that the topical delivery of tranilast with an absorption enhancer such as a mixture of oleic acid and propylene glycol might be a more effective medication than oral administration of tranilast for the treatment of keloid and hypertrophic scar.

Effect of ointment bases on topical and transdermal delivery of salicylic acid in rats: evaluation by skin microdialysis

Microdialysis has been used to determine the concentration of salicylic acid in skin tissue and plasma periodically for 4 h to evaluate the effect of ointment bases on topical and transdermal delivery of salicylic acid. The ointment bases examined were solbase (water-soluble), poloid and white petrolatum (oleaginous), hydrophilic poloid (water in oil (w/o) type emulsion lacking water) and absorptive ointment (w/o-type emulsion containing water). The ointments (0.1 g) containing 25 micromol salicylic acid were applied for 2 h to the surface of rat skin (1 cm2) with (intact) or without the stratum corneum. For intact skin, the extent of topical delivery from different ointments, evaluated by the area under the concentration-time curve (AUC) of salicylic acid in the skin tissue (AUCskin), increased in the order solbase << white petrolatum, poloid, hydrophilic poloid << absorptive ointment. The ratio of AUCskin (topical delivery) to the AUC of salicylic acid in plasma (AUCplasma, transdermal delivery) varied remarkably among the different bases, the greatest ratio being observed for absorptive ointment. When the ointments were applied to skin surface without stratum corneum, AUCskin for solbase was much higher (about 45 times that for intact skin), whereas only a small (two-fold) increase was observed for poloid and hydrophilic poloid and the increase was negligible for white petrolatum and absorptive ointment. For skin without the stratum corneum, the ratio AUCskin/AUCplasma for the different ointments was comparable, although the magnitudes of AUCskin and AUCplasma still varied substantially. The variance of AUC values arises as a result of the different rates of release of salicylic acid from the bases. These results indicate that: the topical and transdermal delivery of salicylic acid in intact skin varies substantially among different ointment bases, and the greatest topical delivery is observed for absorptive ointment; use of absorptive ointment increases the retention of salicylic acid in the stratum corneum; and the stratum corneum functions strongly as a penetration barrier for solbase, moderately for poloid and hydrophilic poloid, and less for absorptive ointment and white petrolatum.

Diclofenac concentrations in defined tissue layers after topical administration

INTRODUCTION

To date it is unclear whether therapeutic concentrations are attained in target tissues after topical administration of nonsteroidal anti-inflammatory drugs. Therefore this study in healthy volunteers was undertaken to measure diclofenac concentrations attained in defined tissue layers directly underlying the site of topical diclofenac application by in vivo microdialysis.

METHODS

In each experiment two microdialysis probes were inserted, one into a superficial (3.9 +/- 0.3 mm) and one into a deep (9.3 +/- 0.5 mm) tissue layer, in 20 healthy volunteers and calibrated in vivo. The distance between the surface of the skin and the tips of the microdialysis probes was measured by 7.5 MHz ultrasound. Diclofenac was administered topically as a single dose of approximately 300 mg/100 cm2. Concentration versus time profiles in tissue layers were monitored for 5 hours.

RESULTS

Concentration versus time profiles were obtained in 11 of 20 experiments. However, there was no correlation between area under the concentration-time curve (AUC) in a defined layer and the depth of probe insertion. In those experiments where concentration versus time profiles were obtained for both probes mean AUC was 532 +/- 197 microg x min x ml(-1) for superficial layers, and 438 +/- 249 microg x min x ml(-1) for deep layers.

CONCLUSION

We conclude that transdermal penetration of diclofenac, at least after single doses, is not predictable and may strongly be influenced by individual skin properties.

In-vivo microdialysis study of the distribution of cisplatin into brain tumour tissue after intracarotid infusion in rats with 9L malignant glioma

Simultaneous brain microdialysis in tumour and non-tumour tissues has been used for kinetic determination of the local distribution of an anticancer agent, cisplatin, in rats. Rat brain was implanted with 9L malignant glioma and cisplatin (3.5 mg kg-1) was administered as a selective intracarotid infusion for 30 min to rats prepared for brain microdialysis. The amount of platinum in the dialysate collected from tumour and non-tumour brain tissues was determined by atomic absorption spectrophotometry, as representative of cisplatin. Total and free platinum concentrations in plasma were also measured. Free platinum is accumulated preferentially in the tumour tissue and the brain tumour distribution coefficient (the ratio of brain tumour platinum AUC to plasma free platinum AUC, where AUC is the area under the platinum concentration-time curve) was 0.69, although there was little distribution into normal brain tissue. Drug binding to plasma proteins was 65%. It is concluded that simultaneous microdialysis is an easy and available method for assessing in-vivo local pharmacokinetics and distribution of cisplatin in tumour and non-tumour tissues of the brain.

Application of microdialysis in pharmacokinetic studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.

Evaluation of in-vivo transdermal absorption of cyclosporin with absorption enhancer using intradermal microdialysis in rats

The purpose of this study was to evaluate the effect of absorption enhancer on in-vivo transdermal absorption of cyclosporin using intradermal microdialysis in rats. Cyclosporin oily solutions (0.5, 2, 8% w/v) were prepared from Sandimmun (10% w/v oily oral preparation of cyclosporin) by diluting with olive oil. 1-[2-(Decylthio)ethyl] azacyclopentan-2-one (HPE-101) and glycerin were added to the cyclosporin formulation as an absorption enhancer at various concentrations between 1 and 20%. These formulations were applied to the shaved abdomen of rats treated with intradermal microdialysis at a flow rate of 2.5 microL min-1 for 6 h. Cyclosporin was immediately detected and attained a plateau in the dermal dialysate after topical application of cyclosporin oily solution alone. Cyclosporin levels in the dialysate increased with increasing cyclosporin concentrations in the formulation from 0.5 to 8% (w/v). HPE-101 did not influence cyclosporin absorption at concentrations less than 6% (w/v). Addition of 10% (w/v) HPE-101 significantly enhanced an apparent absorption rate of cyclosporin by 4.9 times. However, 20% (w/v) HPE-101 did not show the enhancing activity. On the other hand, addition of glycerin at concentrations of 6, 10, and 20% (v/v) significantly enhanced an apparent absorption rate of cyclosporin by 3.0, 6.4, and 6.9 times, respectively. The time lag for cyclosporin absorption was less than 0.21 h in all tested cases. This microdialysis study shows that glycerin is a suitable enhancer for improving the in-vivo cyclosporin absorption from the skin.