Comparative investigations to evaluate the use of organotypic cultures of transformed and native dermal and epidermal cells for permeation studies

Improved in vitro models for preclinical drug and formulation screening focusing on 2D and 3D skin and cornea constructs

The present overview deals with current approaches for the improvement of in vitro models for preclinical drug and formulation screening which were elaborated in a joint project at the Center of Pharmaceutical Engineering of the TU Braunschweig. Within this project a special focus was laid on the enhancement of skin and cornea models. For this reason, first, a computation-based approach for in silico modeling of dermal cell proliferation and differentiation was developed. The simulation should for example enhance the understanding of the performed 2D in vitro tests on the antiproliferative effect of hyperforin. A second approach aimed at establishing in vivo-like dynamic conditions in in vitro drug absorption studies in contrast to the commonly used static conditions. The reported Dynamic Micro Tissue Engineering System (DynaMiTES) combines the advantages of in vitro cell culture models and microfluidic systems for the emulation of dynamic drug absorption at different physiological barriers and, later, for the investigation of dynamic culture conditions. Finally, cryopreserved shipping was investigated for a human hemicornea construct. As the implementation of a tissue-engineering laboratory is time-consuming and cost-intensive, commercial availability of advanced 3D human tissue is preferred from a variety of companies. However, for shipping purposes cryopreservation is a challenge to maintain the same quality and performance of the tissue in the laboratory of both, the provider and the customer.

Anti-proliferative and anti-migratory effects of hyperforin in 2D and 3D artificial constructs of human dermal fibroblasts - A new option for hypertrophic scar treatment?

Hyperforin (HYP), one of the main bioactive compounds in extracts of Hypericum perforatum, is a potential drug candidate for the treatment of skin diseases. Since extracts have proven to support wound healing, in the present study effects of HYP on human dermal fibroblasts (HDF) were evaluated in 2D and 3D in vitro dermal constructs. Viability and cytotoxicity assays as well as a live-dead cell staining were performed to test at which concentration HYP reduces viability and/or shows cytotoxicity. Furthermore a differentiation between cytotoxic, anti-proliferative and anti-migratory effects was done. For the latter purpose a 2D migration assay was performed. HDF-induced contraction of a 3D artificial dermal (AD) construct was determined at given HYP concentration. Induction of apoptosis was examined by determination of caspase 3/7 activities. HYP reduced viability of HDF down to 70% at concentrations of 5-10µM. This decrease was not due to cytotoxicity but to a reduction in proliferation as shown from both the proliferation assay and the cytotoxicity assay as well as from live-dead cell staining. The 2D migration assay showed that HYP reduced migration activity of HDF cells at a concentration of 10µM. At this concentration HYP also reduced the HDF-induced contraction of collagen gels as 3D AD constructs. Apoptotic effects of HYP were excluded performing a caspase 3/7 activity detecting assay. The results show for the first time that HYP may be rather a potential candidate for treatment of hypertrophic scars than promoting effects which are understood as important in wound healing.

Skin penetration and stabilization of formulations containing microfine titanium dioxide as physical UV filter

Microfine titanium dioxide (TiO(2)) has become a frequently used physical UV filter in sunscreen formulations. Penetration of microfine TiO(2) into human skin seems to be possible because of the mean particle size of 20 nm. The small particle size results in a high surface activity of the primary particles and causes a formation of agglomerates in the formulation. The aim of this study was to investigate the in vivo and in vitro penetration behaviour of the physical UV filter into human skin. Furthermore, a stable sunscreen formulation with microfine TiO(2) which does not penetrate into the skin should be developed. According to our experiments, microfine TiO(2) penetrates deeper into human skin from an oily dispersion than from an aqueous one. Therefore, an o/w emulsion containing the dispersed micropigment in the aqueous phase was manufactured. Microfine TiO(2) cannot penetrate into human skin from this emulsion, but the storage stability of the formulation is very low at different temperatures. The encapsulation of the micropigment into liposomes does not result in a better stability but it causes a higher penetration depth of the particles into the skin.

Interaction of lipid nanoparticles with human epidermis and an organotypic cell culture model

Various lipid nanoparticle formulations were investigated with respect to (trans)dermal drug delivery with special regard to the mechanism of their effects on human and an organotypic cell culture epidermis. Potential alterations of stratum corneum lipid domains were studied using fluorescence assays with labeled liposomes and thermal analysis of isolated stratum corneum. Influences on the permeation of corticosterone were investigated and the occlusive properties of the nanoparticles were determined by measurements of the transepidermal water loss (TEWL). The penetration of a fluorescence dye was visualized by fluorescence microscopy of cross sections of human epidermis after incubation with cubic and solid lipid nanoparticles. Corticosterone permeation was limited when applied in matrix-type lipid nanoparticles (fat emulsion, smectic and solid lipid nanoparticles). An adhesion of solid lipid nanoparticles was clearly observed in thermal analysis as reflected by additional phase transitions probably caused by the nanoparticle matrix lipid. However, as for the other matrix-type nanoparticles, no distinct alterations of the phase transitions of the stratum corneum lipids were observed. Cubic nanoparticles led to the most predominant effect on skin permeation where the surface-active matrix lipid may act as penetration enhancer. An alteration of the stratum corneum lipids' thermal behavior as well as an interaction with fluorescence labeled liposomes was observed. Differences observed in permeation studies and thermal analysis of human and cell culture epidermis indicate that surface lipids, which are not present to the same extent in the cell culture model than in human epidermis, seem to play an important role.

3D-resolved investigation of the pH gradient in artificial skin constructs by means of fluorescence lifetime imaging

PURPOSE

The development of substitutes for the human skin, e.g., artificial skin constructs (ASCs), is of particular importance for pharmaceutical and dermatologic research because they represent economical test samples for the validation of new drugs. In this regard, it is essential for the skin substitutes to be reliable models of the genuine skin, i.e., to have similar morphology and functionality. Particularly important is the barrier function, i.e., the selective permeability of the skin, which is strongly related to the epidermal pH gradient. Because the pH significantly influences the permeation profile of ionizable drugs such as nonsteroidal anti-inflammatory drugs, it is of major importance to quantitatively measure the epidermal pH gradient of the ASC and compare it to that of genuine skin.

METHODS

Using three-dimensional fluorescence lifetime imaging combined with two-photon scanning microscopy, we measured with submicron resolution the three-dimensional pH gradient in the epidermis of ASCs stained with 2',7'-bis-(2-carboxyethyl)-5/6-carboxyfluorescein.

RESULTS

Similar to genuine skin, the surface of the artificial epidermis has an acidic character (pH 5.9), whereas in the deeper layers the pH increases up to 7.0. Moreover, the pH gradient differs in the cell interior (maximally 7.2) and in the intercellular matrix (maximally 6.6). Apart from the similitude of the pH distribution, the genuine and the artificial skin prove to have similar morphologies and to be characterized by similar distributions of the refractive index.

CONCLUSIONS

Artificial skin is a reliable model of genuine human skin, e.g., in permeability studies, because it is characterized by a similar pH gradient, a similar morphology, and a similar distribution of the refractive index to that of genuine skin.

Lateral Diffusion of Ibuprofen in Human Skin during Permeation Studies

In order to characterize the quality of dermal preparations, permeation studies using human stratum corneum or artificial skin constructs are carried out. For a better understanding of the diffusion processes a method to measure the lateral diffusion in skin samples was developed allowing an estimation of built-up drug depots. By extracting concentric skin segments surrounding the site of application, lateral drug diffusion was determined. Both, excised human skin and artificial skin constructs, showed comparable results with two phases of lateral diffusion (accumulation/redistribution). The use of permeation enhancers promoted lateral diffusion and thus increased the tendency to create a drug depot within the skin.

The use of a porcine organotypic cornea construct for permeation studies from formulations containing befunolol hydrochloride

The purpose of this study was to develop an organotypic cornea equivalent consisting of three different cell types (epithelial, stromal and endothelial cells) and to investigate its usefulness as in vitro model for permeation studies. The different cell types of a porcine cornea were selectively isolated and a multilayer tissue construct was created step-by-step in Transwell cell culture insert. Histology, basement membrane components (laminin, fibronectin) and surfaces of cornea construct were investigated to evaluate the degree of comparability to porcine cornea from slaughtered animals. The cornea construct exhibited similarities to the original cornea. Ocular permeation of befunolol hydrochloride from different formulations across the cornea construct was tested using modified Franz cells and compared with data obtained from excised cornea. The cornea construct showed a similar permeation behavior for befunolol hydrochloride from different formulations compared with excised porcine cornea. However, permeation coefficients K(p) obtained with the construct were about three to fourfold higher for aqueous formulations and same for the w/o-emulsion. The reconstructed cornea could be an alternative to excised animal tissue for drug permeation studies in vitro.

Comparative permeation studies for delta-aminolevulinic acid and its n-butylester through stratum corneum and artificial skin constructs

The improvement of the permeation properties through excised human stratum corneum and artificial skin constructs (ASC) of delta-aminolevulinic-n-butylester (ABE) compared with delta-aminolevulinic acid (ALA) was investigated. For this purpose the permeated amounts of each substance were determined depending on time in a Franz diffusion cell experiment with stratum corneum and ASC, respectively. Furthermore the barrier properties of ASC were compared with those of stratum corneum. Detection of both substances was performed by high-performance liquid chromatography (HPLC) analysis. For the determination of ABE a new HPLC method was developed. ABE could be determined with the new HPLC method with sufficient sensitivity (detection limit: 0.1 microg/ml) after derivatisation with o-phthalaldehyde (OPA). Stratum corneum and ASC were more permeable for ABE than for ALA. The permeation coefficient P of ABE through stratum corneum was nearly ten-fold higher than that of ALA. Using ASC as permeation barrier the permeation coefficient of ABE was about 22-fold higher than that of ALA. ABE and ALA permeated 142-fold and 64-fold, respectively, faster through ASC than through stratum corneum.

Interrelation of permeation and penetration parameters obtained from in vitro experiments with human skin and skin equivalents

In a comparative study, two different in vitro cutaneous test systems were examined: (1) The Franz diffusion cell (FD-C), a test system to study drug permeation through the skin and to obtain data like steady state flux and lag time as well as permeability and diffusion coefficients. (2) The Saarbruecken penetration model (SB-M), a test system to investigate drug penetration into different skin layers and after varying incubation times to acquire values about the quasi steady state drug amounts in the stratum corneum (SC). Three drug concentrations (0.9, 0.45 and 0.225%) of a lipophilic model drug preparation, flufenamic acid in wool alcohols ointment, were applied on the skin's surface using 'infinite dose' conditions. Trypsin-isolated SC, heat-separated epidermis, full-thickness skin and reconstructed human skin (RHS) served as skin membranes in the FD-C, while the SB-M experiments were only carried out using full-thickness skin. Increasing steady state flux data and m(ss) values (steady state drug amount in the SC) were detectable after the application of rising drug amounts. Concerning the permeability of the used skin membranes in establishing barrier properties, the following rank order was observed: RHS>SC> or =epidermis>full skin. The flux data of the FD-C experiments for isolated SC, separated epidermis and RHS were linearly related with the m(ss) values of the SB-M investigations, allowing a direct comparison of permeation with penetration parameters. Concerning the drug amount in the SC, previous investigations succeeded in the establishment of an in vivo/in vitro correlation. Based on the results presented here, the prediction of drug amounts present in the SC after different incubation times in vivo is now possible after penetration as well as permeation experiments using the lipophilic model drug preparation, flufenamic acid in wool alcohols ointment.

Reconstruction of an in vitro cornea and its use for drug permeation studies from different formulations containing pilocarpine hydrochloride

The aim of the present contribution was to develop a functional three-dimensional tissue construct to study ocular permeation of pilocarpine hydrochloride from different formulations. The in vitro model was compared to excised bovine cornea. Modified Franz cells were used to study the transcorneal permeability. Analysis was performed by reversed-phase high-performance liquid chromatography. Comparisons of the permeation rates through excised bovine cornea and the in vitro model show the same rank order for the different formulations. The permeation coefficient, K(P), obtained with the cornea construct, is about 2-4-fold higher than that from excised bovine cornea. It is possible to reconstruct bovine cornea as an organotypic culture and also to use this construct as a substitute for excised bovine cornea in drug permeation studies in vitro.