In vivo methods for the assessment of topical drug bioavailability

Transdermal iontophoretic delivery of terbinafine hydrochloride: quantitation of drug levels in stratum corneum and underlying skin

The objective of this study was to determine the effect of iontophoresis on the delivery of terbinafine hydrochloride (4%, w/w) into and across hairless rat skin. In vitro skin uptake and permeation studies were performed using Franz diffusion cells. Anodal iontophoresis was applied for 1h at current densities of 0.2, 0.3 and 0.4mA/cm(2). In addition, iontophoresis was applied for 15, 30, 45 and 60min. Studies were conducted in which the formulation was either removed or left in contact with the skin following iontophoresis and then passive delivery was assessed 23h later. Tape stripping and skin extraction were performed to quantify drug levels in the stratum corneum and the underlying skin, respectively. The samples were analyzed using HPLC. The amount of drug delivered into the stratum corneum following iontophoresis was not significantly different from the amount delivered passively (p>0.05). However, drug levels in the underlying skin were significantly higher for the iontophoretic group. The amount of terbinafine delivered into the skin layers was influenced by current density and duration of current application. Leaving the drug formulation in contact with the skin during the post-iontophoretic period had a significant effect on drug levels delivered into skin layers. Iontophoresis enhanced the delivery of terbinafine hydrochloride into the skin layers and, therefore, may be used to improve the treatment of skin fungal infections.

Use of excised human skin to assess the bioequivalence of topical products

BACKGROUND

Establishing the bioequivalence of topical drug products is a costly and time-consuming process since, with few exceptions, clinical efficacy trials are required.

OBJECTIVE

To develop a surrogate for clinical bioequivalence testing through evaluation of the kinetics of drug absorption in vitro through excised human skin.

METHODS

The percutaneous absorption of seven approved generic topical drug products was compared with their corresponding reference products during preclinical development using the Franz diffusion cell. Thereafter, following the conduct of bioequivalence trials and regulatory approval of these products in the United States, clinical data became available to which the in vitro data were compared.

RESULTS

In six of the seven cases the in vitro test:reference ratio for total absorption was close to one and indicated that the products were equivalent, in agreement with the clinical data. Results from the seventh case, in which the test:reference ratio was only 0.63, indicated that the in vitro model actually had greater sensitivity than the clinical method to detect small differences between products.

CONCLUSION

These data demonstrate the relevance and predictive power of the in vitro human skin model and strongly support its use as a surrogate for in vivo bioequivalence studies.

Absorption of chemicals through compromised skin

Skin is an important route of entry for many chemicals in the work place. To assess systemic uptake of a chemical in contact with the skin, quantitative information on dermal absorption rates of chemicals is needed. Absorption rates are mainly obtained from studies performed with intact, healthy skin. At the work place, however, a compromised skin barrier, although not necessarily visible is common, e.g. due to physical and chemical damage. As reviewed in this article, there are several lines of evidence that reduced integrity of the skin barrier may increase dermal absorption of chemicals in the occupational setting. An impaired skin barrier might lead not only to enhanced absorption of a specific chemical, but also to entrance of larger molecules such as proteins and nanoparticles which normally are not able to penetrate intact skin. In addition to environmental influences, there is increasing evidence that some individuals have an intrinsically affected skin barrier which will facilitate entrance of chemicals into and through the skin making these persons more susceptible for local as well for systemic toxicity. This review addresses mechanisms of barrier alteration caused by the most common skin-damaging factors in the occupational settings and the consequences for dermal absorption of chemicals. Furthermore, this review emphasizes the importance of maintained barrier properties of the skin.

Application of attenuated total reflection Fourier transform infrared imaging and tape-stripping to investigate the three-dimensional distribution of exogenous chemicals and the molecular organization in Stratum corneum

Attenuated total reflection Fourier transform infrared spectroscopic imaging combined with tape-stripping is an advantageous approach to map the depth penetration and lateral distribution of topically applied chemicals in Stratum corneum (SC) and the conformational order of SC lipids. Tape-stripping progressively removes layers of SC, and chemical imaging provides spatially resolved information on the chemical composition of both the newly exposed SC surface and of the tapes used for stripping. The procedure is rapid, minimally invasive, and does not necessitate cross-sectioning of the skin. This approach offers a simple and direct way to determine the distribution of exogenous volatile and non-volatile chemicals in SC as a function of the chemical composition of the formulation and time, and the conformational order of SC lipids in native and topically treated skin. The procedure described here is well suited to address questions of relevance for the areas of drug delivery, dermatology, and skin care.

The determination of stratum corneum thickness: an alternative approach

The individual thickness of the stratum corneum is required to normalise drug permeation profiles in dermato-pharmacokinetic studies. The thickness is often estimated using tape-stripping combined with transepidermal water loss measurements. A linear transformation of Fick's first law is used to relate the progressively thinner barrier with the corresponding increase in transepidermal water loss and to estimate the thickness by linear regression. However, the data from an important subset of subjects are poorly fitted to this linear model. This is typically due to the removal of loose outer layers of stratum corneum, which do not contribute significantly to barrier function. This work proposes two alternative non-linear models. All three models were used to fit data from 31 in vivo tape-stripping experiments and their outcomes and goodness-of-fit compared. The results suggest that the linear model may overestimate the stratum corneum thickness and is open to subjectivity regarding the selection of data points to be fitted. The non-linear models satisfactorily fitted all the data, including all data points. No significant differences were found between the thicknesses derived from the two non-linear models. However, the analysis of the goodness-of-fit of the models to the data suggests a preference for a baseline-corrected approach.