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

Revisiting the Effect of Aging on the Transport of Molecules through the Skin

Both intrinsic and extrinsic aging lead to a series of morphological changes in the skin including the flattening of the dermal-epidermal junction, increased stratum corneum dryness, reduction in sebaceous gland activity and enzyme activity as well as atrophy of blood vessels. In this study, the impact of these changes on the transport of molecules through the skin was revised. The increase in the number of transdermal formulations on the market in recent decades and life expectancy represent the main reasons for an in-depth discussion of this topic. Furthermore, elderly subjects have often been excluded from clinical trials due to polypharmacy, raising concerns in terms of efficacy and safety. In this way, ex vivo and in vivo studies comparing the transport of molecules through the mature and young skin were analyzed in detail. The reduced water content in mature skin had a significant impact on the transport rate of hydrophilic molecules. The lower enzymatic activity in aged skin, in turn, would explain changes in the activation of prodrugs. Interestingly, greater deposition of nanoparticles was also found in mature skin. In vivo models should be prioritized in future experimental studies as they allow to evaluate both absorption and metabolism simultaneously, providing more realistic information.

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

The application and limitations of mathematical modelling in the prediction of permeability across mammalian skin and polydimethylsiloxane membranes

OBJECTIVES

Predicting the rate of percutaneous absorption of a drug is an important issue with the increasing use of the skin as a means of moderating and controlling drug delivery. One key feature of this problem domain is that human skin permeability (as K(p)) has been shown to be inherently non-linear when mathematically related to the physicochemical parameters of penetrants. As such, the aims of this study were to apply and evaluate Gaussian process (GP) regression methods to datasets for membranes other than human skin, and to explore how the nature of the dataset may influence its analysis.

METHODS

Permeability data for absorption across rodent and pig skin, and artificial membranes (polydimethylsiloxane, PDMS, i.e. Silastic) membranes was collected from the literature. Two quantitative structure-permeability relationship (QSPR) models were used to compare with the GP models. Further performance metrics were computed in terms of all predictions, and a range of covariance functions were examined: the squared exponential (SE), neural network (NNone) and rational quadratic (QR) covariance functions, along with two simple cases of Matern covariance function (Matern3 and Matern5) where the polynomial order is set to 1 and 2, respectively. As measures of performance, the correlation coefficient (CORR), negative log estimated predictive density (NLL, or negative log loss) and mean squared error (MSE) were employed.

KEY FINDINGS

The results demonstrated that GP models with different covariance functions outperform QSPR models for human, pig and rodent datasets. For the artificial membranes, GPs perform better in one instance, and give similar results in other experiments (where different covariance parameters produce similar results). In some cases, the GP predictions for some of the artificial membrane dataset are poorly correlated, suggesting that the physicochemical parameters employed in this study might not be appropriate for developing models that represent this membrane.

CONCLUSIONS

While the results of this study indicate that permeation across rodent (mouse and rat) and pig skin is, in a statistical sense, similar, and that the artificial membranes are poor replacements of human or animal skin, the overriding issue raised in this study is the nature of the dataset and how it can influence the results, and subsequent interpretation, of any model produced for particular membranes. The size of the datasets, in both absolute and comparative senses, appears to influence model quality. Ideally, to generate viable cross-comparisons the datasets for different mammalian membranes should, wherever possible, exhibit as much commonality as possible.

Evaluation of simultaneous permeation and metabolism of methyl nicotinate in human, snake, and shed snake skin

The transdermal permeation and metabolic characteristics of methyl nicotinate (MN) in stratum corneum and split-thickness human skin and three species of shed snake and snake skin (Elaphae obsoleta, Naja kaouthia, and Python molurus bivittatus) were evaluated. In vitro skin transport using excised skin and hydrolysis experiments using skin homogenate were carried out. The flux of MN, a metabolite, nicotinic acid (NA), and the total (MN+NA), as well as kinetic parameters (V(max) and K(m)) for hydrolysis of MN were determined and compared among various skin types. The total flux from MN-saturated solution through human skin was not significantly different from that through snake and shed snake skin of Elaphae obsoleta, Naja kaouthia but was significantly higher than that through snake and shed snake skin of Naja kaouthia (p < 0.05). A great difference in skin esterase activity was observed between human and snake in both snake skin and shed snake skin of all species. In all skins except the stratum corneum of human skin, NA flux increased with an increase in MN donor concentration and reached a plateau, suggesting that metabolic saturation was taking place in the skin. NA flux at the plateau and MN donor concentrations at which the NA flux reached a plateau also varied by species. These findings indicated that the discrepancy in transdermal profiles of MN among skins tested was predominantly due to the difference in the esterase activity in the skin.

Comparison of skin transport and metabolism of ethyl nicotinate in various species

The skin transport and metabolism characteristics of ethyl nicotinate (EN) in rabbit, rat, guinea-pig, pig, shed snake skin and human were compared. In vitro skin transport using excised skin and hydrolysis experiments using skin homogenate were carried out. Flux of EN, a metabolite, nicotinic acid (NA), and the total (EN + NA), as well as kinetic parameters (V(max) and K(m)) for hydrolysis of EN were determined and compared among various species. The enzymatic conversion of EN to NA was observed for all skin permeation experiments. Total flux from EN-saturated solution between rabbit, rat, guinea-pig and human was significantly different (P < 0.05). A great difference between species was observed in skin esterase activity. The NA/total flux ratio of human was significantly lower than that of rabbit, rat or guinea-pig but lower than that of shed snake skin (P < 0.05). There is no significant difference in skin permeation and metabolism between human and pig (P > 0.05). Total flux increased linearly with an increase in EN donor concentration for all species. For pig, shed snake skin and human, NA flux increased with an increase in EN donor concentration and reached a plateau, suggesting the metabolic saturation was taking place in the skin. NA flux at plateau and EN donor concentration in which the NA flux reached a plateau were also affected by species difference. These findings indicated that the discrepancy in transdermal profiles of EN among species tested was predominantly due to the difference in the esterase activity in the skin.

Acyclovir permeation through rat skin: mathematical modelling and in vitro experiments

The aim of this work is to characterise the skin permeation properties of a male rat by means of a purely diffusive mathematical model based on Fick's second law. Additionally, in the attempt of proposing a reliable tool allowing the skin permeability (or resistance) determination on the basis of experimental data, the model automatically accounts also for two typical experimental conditions. In particular, drug dissolution in the donor environment and receiver sampling technique (part of the receiver volume is withdrawn and immediately replaced by fresh solvent) are considered. The results of this characterisation are then compared with those coming from a common simplified approach. Acyclovir is chosen as model drug and a thermostatic (37 degrees C) Franz cell apparatus is used to perform permeation experiments. This study suggests that Acyclovir permeation through the rat skin can be well described by the proposed model and that some differences arise in the evaluation of the full-skin resistance performed by means of our model or the usual simpler approach.

Rapid integrity assessment of rat and human epidermal membranes for in vitro dermal regulatory testing: correlation of electrical resistance with tritiated water permeability

An approach is presented that allows for rapid selection of robust rat and human epidermal membranes for use on in vitro dermal regulatory studies. Tritiated water (THO) permeability was correlated with electrical resistance (ER) and the results used to propose ER values to judge membrane integrity. Rat and human epidermal membranes were prepared and mounted onto in vitro glass static diffusion cells (0.64 cm(2)) maintained at 32 degrees C. THO permeability coefficients (Kp) were determined and compared with ER measurements. Electrical resistance was also determined for various in vitro cell exposure areas from 0.64 cm(2) to 2.54 cm(2). Our results show that rat epidermal membrane THO Kp values exhibited a lognormal distribution with a median value of 2.76 x 10(-3) cm/h. Human epidermal membrane THO Kp values were best described by a Weibull distribution with a median value of 1.13 x 10(-3) cm/h. The corresponding median electrical resistance measurements were 5.59 kOmega for rat and 23 kOmega for human epidermal membranes. Based on the widely used and accepted single point THO Kp thresholds of </=2.5 x 10(-3) cm/h (rat) and </=1.5 x 10(-3) cm/h (human), the corresponding ER values of >/=5.87 kOmega and >/=17.1 kOmega were calculated and proposed as acceptable benchmarks for pre-qualifying membranes. In our research exploring the relationship between ER and exposure area we report that an inverse relationship exists between ER and in vitro cell exposure area; as cell area increased, ER decreased. The use of electrical resistance provides a rapid and reliable method for evaluating the integrity of rat and human epidermal membranes for in vitro dermal kinetic testing.

Changes in electrophysiological properties of rat skin with age

The age-related changes in the electrical and physiological properties of the skin were examined in rats at the ages of 5, 10, 21, 90, and 180 d. The resistance of the stratum corneum, the resistance of the viable skin (epidermis and dermis), and the capacitance of the stratum corneum were analyzed from skin impedance data using an equivalent circuit. With development and aging, the resistance of the stratum corneum and the viable skin increased, whereas the capacitance of the stratum corneum decreased. Physiological characteristics such as the thickness of skin strata and the content of lipid and water in the stratum corneum were also measured. The lipid content in the stratum corneum was constant at all ages. The water content in the stratum corneum decreased, and the thickness of skin strata increased with age. Comparison between electrical data and physiological properties suggested that the increase in the resistance of the stratum corneum with aging is primarily caused by the decrease in the water content and that the capacitance of the stratum corneum and the resistance of the viable skin depend on age-related increases in the thickness of skin strata. In conclusion, the age dependency of cutaneous electrical properties may affect the permeation profile of drugs through the skin, and impedance analysis can be used to estimate age-related changes in transdermal drug delivery.

Age difference in simultaneous permeation and metabolism of ethyl nicotinate in rat skin

The age-dependent characteristics of transdermal permeation of ethyl nicotinate (EN) and its metabolism to nicotinic acid (NA) were examined in rats at the ages of a fetus at 21 d, 3, 10, 50, 270 and 360 d. Skin transport in vitro was investigated using mounted skin in side-by-side diffusion chambers, and flux of EN and NA was determined. With developing and aging in rats, EN flux from EN-saturated solution declined gradually, however, NA flux increased drastically at 10 and 50 d. To elucidate the mechanism of this age-dependent escalation of NA flux, a hydrolysis study was performed using skin homogenate, and the Michaelis-Menten parameters (Vmax and Km) of EN were evaluated. Vmax and Vmax/Km ratio showed the same tendency with NA flux/total (EN+NA) flux ratio, suggesting that skin esterases in rats are developed gradually after birth, then increase markedly and become steady in the adult period. On the other hand, the affinity parameter, Km, was almost the same among all ages. Moreover, metabolic saturation of esterase during the transdermal process occurred in all ages, and maximal NA flux and EN concentration in the donor compartment for the maximal NA flux were also affected by age. These findings indicated that the discrepancy in transdermal profiles of EN among the ages tested was dominantly due to the difference in the development of esterase in the growth process.

Evaluation of Göttingen minipig skin for transdermal in vitro permeation studies

The optimal skin type for in vitro permeability studies depends on the purpose of the specific transdermal study. In a number of cases, it may be advantageous to use animal skin as an alternative to human skin although they have different characteristics. Recently, Göttingen minipigs have been reported as good models in toxicological and pharmacokinetic studies of drug substances. In this paper, the potential use of skin from the Göttingen minipig is evaluated by studying three model drug substances (nicotine, salicylic acid and testosterone) through skin from humans, domestic pigs and three ages of the Göttingen minipig. An analysis of variance and a Student's t-test showed that both the skin from the Göttingen minipig and the domestic pig possessed transdermal permeabilities, which correlated with human skin and exhibited a lower intra- and intervariation. Furthermore, it was shown that permeability and variation of fluxes through skin from Göttingen minipigs were dependent on the age of the minipig and of the drug substance. It is concluded that the Göttingen minipig, like the domestic pig, is a good skin model for in vitro permeation through human skin.

Pore charge distribution considerations in human epidermal membrane electroosmosis

The aim of this study was to assess the extent to which a model with pores having only net negative charges would adequately describe transdermal electroosmosis in human epidermal membrane (HEM) at neutral pH. Such information would enhance the predictive value of the modified Nernst-Planck model for transdermal iontophoresis, in addition to providing insights regarding the likelihood of significant pore charge distribution in HEM. Baseline results (the control) obtained from 0.1 to 0.4 V anodal and cathodal electroosmosis experiments with synthetic polycarbonate membranes (Nuclepore membranes), using radiolabeled urea and mannitol as the model permeants, demonstrated that such a membrane system can be modeled by the electrokinetic (electroosmosis) theory with the assumption of the pores possessing only negative charges. The studies with HEM were carried out at low voltage (</=0.5 V) where alterations in the barrier properties of HEM were minimal and at higher voltages (>/approximately = 1.0 V) where significant field-induced pore formation in HEM occurred. In both the low and high voltage studies, radiolabeled urea, mannitol, and water were employed as permeants in cathodal and anodal iontophoresis experiments. The results of the low voltage iontophoresis experiments suggest significant pore charge distribution in HEM (a significant deviation between the predictions from the single pore charge type assumption and the experimental data). Under the higher applied voltage conditions (>/approximately = 1.0 V), results from anodal and cathodal electroosmosis studies were consistent with the model in which the HEM has only pores that are net negatively charged.

Effect of transdermal iontophoresis codelivery of hydrocortisone on metoclopramide pharmacokinetics and skin-induced reactions in human subjects

The effects of transdermal iontophoresis (IP) codelivery of hydrocortisone (HC) on metoclopramide hydrochloride (MCP) pharmacokinetics and on skin-induced reactions were evaluated in a randomized, crossover clinical study. MCP, an antiemetic, low molecular weight, cationic drug intended for systemic delivery, was delivered from the anode of IP systems at a constant current of 100 microA/cm(2). HC, a neutral endogenous antiinflammatory agent, was codelivered from the same electrode, primarily by electroosmotic processes. Each subject (n = 7) wore two identical IP systems (MCP alone or MCP plus HC), each supplying 500 microA, one on each upper arm for 4 h. One week later, each subject repeated the procedure with the alternate type of MCP system. HC did not change the pharmacokinetics of MCP: There were no statistically significant differences in MCP plasma concentrations, half-life, area under the curve (AUC), or rate of absorption between the two treatment groups. However, HC significantly decreased erythema and edema scores produced by the IP of MCP. In both groups, a steady-state MCP flux of about 100 microg/(cm(2) x h) was achieved after only 1 h of transport, and input rate dropped dramatically immediately after removal of the system. In vitro, HC flux through human epidermis from an MCP plus HC formulation was 2.8 +/- 1.1 microg/(cm(2) x h) after 4 h transport at 100 microA/cm(2), suggesting negligible systemic exposure to hydrocortisone. These data indicate that MCP input rate and its clearance from the skin are unaltered by HC and that the codelivery of HC by IP is an effective strategy for inhibition of local reactions resulting from the transdermal delivery of drugs.

Prediction of the percutaneous penetration and metabolism of dodecyl decaethoxylate in rats using in vitro models

Percutaneous absorption of a lipophilic surfactant, dodecyl decaethoxylate, can be predicted using in vitro models. In vivo, dermal penetration of dodecyl decaethoxylate was found to be 22.9% in 48 h. All of the absorbed dodecyl decaethoxylate in the rat was metabolised and excreted in expired air as carbon dioxide, or in the urine and faeces. Using rat skin mounted in the unoccluded flow-through diffusion cell with MEM as receptor fluid, in vivo absorption was predicted by the percentage of the applied dose recovered in the stratum corneum, epidermis, dermis and receptor fluid at 24 h (25%). Conversely, the penetration of dodecyl decaethoxylate was over-predicted in the unoccluded static diffusion cell using aqueous ethanol (50% v/v) as the receptor fluid where 49.4% recovered in the receptor fluid at 24 h. In vitro models may be used to predict percutaneous absorption and reduce animal use, provided a suitable receptor fluid is used in which the penetrant is soluble. Dermal metabolism of dodecyl decaethoxylate was low and not considered to influence dermal absorption.