The hairless rat: a relevant animal model to predict in vivo percutaneous absorption in humans?

A novel murine model for the in vivo study of transdermal drug penetration

Enhancement of the transdermal penetration of different active agents is an important research goal. Our aim was to establish a novel in vivo experimental model which provides a possibility for exact measurement of the quantity of penetrated drug. The experiments were performed on SKH-1 hairless mice. A skin fold in the dorsal region was fixed with two fenestrated titanium plates. A circular wound was made on one side of the skin fold. A metal cylinder with phosphate buffer was fixed into the window of the titanium plate. The concentration of penetrated drug was measured in the buffer. The skin fold was morphologically intact and had a healthy microcirculation. The drug appeared in the acceptor buffer after 30 min, and its concentration exhibited a continuous increase. The presence of ibuprofen was also detected in the plasma. In conclusion, this model allows an exact in vivo study of drug penetration and absorption.

Percutaneous absorption and exposure assessment of pesticides

Dermal exposure to a diverse range of chemicals may result from various uses. In order to assess exposure and estimate potential risks, accurate quantitative data on absorption are required. Various factors will influence the final results and interpretations of studies designed to assess the ability of compounds to penetrate the skin. This overview will discuss skin penetration by pesticides, emphasizing key parameters to be considered from the perspective of exposure assessment.

Discrepancies between different rat models for the assessment of percutaneous penetration of hazardous substances

By regulatory authorities the rat is considered to be a suitable animal model to predict the percutaneous absorption of hazardous substances in humans. In our study, the percutaneous penetration of 2-butoxyethanol (BE) and toluene was compared in different rat models. Intradermal microdialysis and static diffusion cells were used in in vivo and in vitro experiments with haired Wistar and hairless Lewis rats. Microdialysis experiments showed a steady-state penetration for BE and a penetration maximum for toluene in both rat strains at approximately 60 min after beginning of exposure. However, in diffusion cell experiments the penetration of the test compounds in both rat strains increased until the end of exposure (4 h). Additionally, in microdialysis experiments BE penetrated in hairless rats in a higher amount than in haired rats (factor: 1.4; P < 0.01), for toluene it was just the opposite (factor: 1.9; P < 0.001). In diffusion cell experiments, the penetrated amounts of both compounds were higher in hairless rats compared to haired rats. The fluxes for BE were in diffusion cell experiments at a factor of 14.5 (haired rat) and 18.1 (hairless rat) higher than in microdialysis experiments, the difference factor for toluene was 2.6 (haired rat) and 12.9 (hairless rat). The lag times indicate a significantly faster penetration in microdialysis experiments compared with diffusion cell experiments (P < 0.001). There are great differences in percutaneous penetration behaviour between the techniques and the rat strains. The diffusion cell method has difficulties to describe the percutaneous penetration kinetics, whereas microdialysis describes it more reliable. Due to these differences the reliability of a conversion factor for the transfer of percutaneous absorption data from rat to human skin, as proposed in the literature, is questionable.

The correlation between transepidermal water loss and percutaneous absorption: an overview

Independently, both transepidermal water loss (TEWL) and percutaneous absorption measurements accurately gauge stratum corneum skin water barrier integrity. Both TEWL and percutaneous absorption rates increase when the integrity of the SC barrier is compromised. Experiments to discern a quantitative and/or qualitative correlation between the two indicators have resulted in controversy. This paper reviews some major studies investigating this correlation.

[Methods to measure the cutaneous transfer of radionuclides across the intact or lesioned epidermis, application to radiotoxicology]

Although skin contamination by radionuclides is the most common cause of nuclear workers accidents, few studies dealing with the penetration of radioactive contamination through the skin are available. This work is a review of experimental methods that allow to assess transfer of radionuclides through the skin in occupational conditions, with or without skin trauma. The first section describes the different methods applied for skin transfer assessment of chemicals used in pharmacology. Major radionuclide contamination accidents can be associated with skin traumas. Thus, the second section describes the adaptation of these methods to radiotoxicology. Finally, the third section is an in vivo investigation of cobalt transfer (57CoCl2) through undamaged and damaged skin which simulates different industrial accident conditions (excoriation, acid or alcalin burn, scalding, branding).

Skin absorption and human risk assessment

A common practice is to assume that percutaneous absorption does not significantly contribute to total bioavailability and therefore, absorption through other routes is more important to human risk assessment. The skin can represent a significant barrier to absorption, but some substances are absorbed to a significant extent. Since there is a potential for percutaneous penetration that is not consistent between species or substances, the assessment of the potential contribution of total body burden from dermal exposures should be considered. This review briefly discusses some theories, practices, and factors that affect percutaneous absorption with an emphasis on how percutaneous absorption evaluations apply to human risk assessment.

Transdermal extraction of interstitial fluid by low-frequency ultrasound quantified with 3H2O as a tracer molecule

The transdermal extraction of interstitial fluid by low-frequency ultrasound offers a potential minimally invasive method of obtaining a fluid sample for at-home blood glucose monitoring. Here we show that the application of low-frequency ultrasound (20 kHz) enhances the transdermal transport of interstitial fluid across hairless rat skin. Using 3H2O as a tracer injected intravenously, a measurable amount of water (>1 microL) was extracted without producing any histologic evidence of injury, even after repeated exposures. Piezoelectric transducers were imbedded in the extraction chamber and used to correlate ultrasound spectral properties to the amount of fluid extracted. Results indicate that the highest amount of water extracted occurs when the acoustic coupling media on the surface of the skin is cavitating, resulting in mild ablation of the stratum corneum and a reduction in its resistance to water mass transfer.

Transdermal delivery and accumulation of indomethacin in subcutaneous tissues in rats

Oral non-steroidal anti-inflammatory drugs (NSAIDs) are effective pharmacotherapy for a wide variety of painful, inflammatory disorders. Development of an efficient means of topical administration of NSAIDs could increase local soft-tissue and joint concentrations while reducing systemic distribution of the drug, thereby reducing side-effects. With this in mind we studied the effects of a novel topical penetration enhancer for lipophilic compounds, a trans-phase delivery system (TPDS), a solution of benzyl alcohol, isopropanol and acetone, on the distribution of indomethacin in various tissues locally and remote from the site of application. We compared the TPDS with a 50:50 (v/v) mixture of propylene glycol and ethanol, a commonly used penetration enhancer, and with oral administration. We found that the TPDS was significantly superior to the other approaches at achieving high local-tissue concentrations in the vicinity of the site of application. In addition, comparison of these two carrier systems seems to clarify the different aqueous and hydrophobic pathways of drug penetration which emerge from various experimental findings and theoretical considerations. Our results suggest that this non-aqueous solvent system, and benzyl alcohol in particular, because of its unique physicochemical and solvating characteristics, might be able to deliver therapeutic levels of indomethacin to tissues close to the site of application in a safer and more effective manner than presently accepted forms of delivery.

Pharmacokinetics of cutaneous Sulconazole nitrate in the hairless rat: absorption, excretion, tissue concentrations

After cutaneous application of radioactive solutions of Sulconazole nitrate in the hairless rat, the total absorption of the substance by the skin, estimated from the sum of the cumulative urinary and fecal excretions over 96 h, was 2.4% of the dose administered. The elimination reached a maximum between 6 and 24 h and was virtually complete after 96 h. The excretion was almost equally distributed between the urine and the feces, which corresponds to an intense elimination via the biliary tract. The quantities present in the stratum corneum, epidermis and dermis at the end of the period of contact constituted another estimation of the total absorption of the substance which confirmed the previous estimation (3.6% of the dose). The measurement of the concentrations of Sulconazole and its metabolites in the various layers of the skin revealed a high affinity of the substance for the stratum corneum, where it remained present in large quantities for more than 48 h. This affinity is due to the very intense lipophilia of the molecule. The concentrations in the other tissues were inversely proportional to the distance from the surface of the skin and were virtually nil in the circulating blood. These results suggest the absence of risk of systemic effects after cutaneous administration of Sulconazole and support the recommended therapeutic protocol in man (one administration per day).

The influence of different strains and age on in vitro rat skin permeability to water and mannitol

Water and mannitol were used as test penetrants to study the effect of age on the skin permeability of the Wistar-derived Alderley Park (AP) rat and Sprague-Dawley (SD) rat. Whole-skin membranes were prepared from rats aged 10 to 120 days, while epidermal membranes were prepared from rats aged 24 to 32 days. The results indicated that the skin permeabilities of the two strains were very similar for either whole-skin or epidermal membranes. The influence of age on skin permeability was found to be negligible for the AP rat, and a small decrease in whole-skin permeability was observed for SD rats above 80 days of age. A statistically derived expression ("the separation efficiency factor") was used to determine the optimum age for preparing intact epidermal membranes; these were 26 days for AP rats and 28 days for SD rats. Histological examination of whole-skin membranes for both strains revealed that the stratum corneum and epidermal thickness did not alter significantly with age (10 to 120 days old). Dermal thickness, hair follicle depth, and, to a lesser extent, the surface area occupied by hair follicles all appeared to be influenced by age, although these changes had no detectable effect on skin permeability.

The isolated blood-perfused pig ear: an inexpensive and animal-saving model for skin penetration studies

To overcome most of the disadvantages of current models to investigate percutaneous penetration of drugs or toxic substances, a model is proposed here based on the isolated pig ear, which is obtained at the slaughterhouse, and perfused with oxygenated blood from the same pig. To determine the viability of the preparations, we measured glucose consumption and lactate production as metabolic parameters, Na+ and K+ ions, as well as lactate dehydrogenase activity in blood as markers for cell damage, whereas vasomotor reactivity was assessed by administering noradrenaline and isoxsuprine. After 60 min of equilibration, only insignificant changes in these parameters were observed during the subsequent 3-hr test period (longer periods were not tested). A slight weight increase was noted during the total period 4 hr, presumably due to slight edema formation. On the basis of several types of measurements, such as in vivo blood flow and ear temperature and in vitro glucose metabolism, standard procedures were developed. It is concluded that this technique offers an easy to handle, cost-efficient, and animal-saving model for skin penetration studies that lacks most of the disadvantages of existing models.

Species differences in percutaneous absorption of nicorandil

The species difference in skin permeability, Kp, of nicorandil was determined by using excised skin samples from hairless mouse, hairless rat, guinea-pig, dog, pig, and human. The Kp value of nicorandil in hairless mice was the greatest among the six species, and those in pigs and humans were in good agreement. To clarify the reasons for the species difference, various skin characteristics in each species were measured. It was suggested that the difference of skin surface lipids in each species affected the partitioning of nicorandil from vehicle to stratum corneum, and that such a difference would be a main factor for the species difference in nicorandil permeability. Since pig and human skins had similar surface lipids, barrier thickness, and morphological aspects, percutaneous absorption studies using excised pig skin samples would be useful for the estimation of in vitro human skin permeation behavior.

Distribution of griseofulvin in the rat: comparison of the oral and topical route of administration

Effective penetration of griseofulvin across the dermal barrier has been achieved using an anhydrous solvent system of benzyl alcohol (10%), acetone (40%), and isopropanol (50%). There were quantitative differences in the relative accumulation of griseofulvin in skin compared with internal organs, when the topical and oral routes of administration were compared. The topical route enhanced localized concentrations of griseofulvin at the site of application, and these persisted for several days. After daily topical application a steady state was reached at day 3, when the diffusion across the skin barrier and epidermal loss seemed to equal the total amount applied to the skin surface. The application of griseofulvin topically, required a much smaller amount of drug to achieve similar integumentary levels compared with the amount required orally.