Model analysis of flux enhancement across hairless mouse skin induced by chemical permeation enhancers

Digital Twins for Tissue Culture Techniques—Concepts, Expectations, and State of the Art

Techniques to provide in vitro tissue culture have undergone significant changes during the last decades, and current applications involve interactions of cells and organoids, three-dimensional cell co-cultures, and organ/body-on-chip tools. Efficient computer-aided and mathematical model-based methods are required for efficient and knowledge-driven characterization, optimization, and routine manufacturing of tissue culture systems. As an alternative to purely experimental-driven research, the usage of comprehensive mathematical models as a virtual in silico representation of the tissue culture, namely a digital twin, can be advantageous. Digital twins include the mechanistic of the biological system in the form of diverse mathematical models, which describe the interaction between tissue culture techniques and cell growth, metabolism, and the quality of the tissue. In this review, current concepts, expectations, and the state of the art of digital twins for tissue culture concepts will be highlighted. In general, DT’s can be applied along the full process chain and along the product life cycle. Due to the complexity, the focus of this review will be especially on the design, characterization, and operation of the tissue culture techniques.

Dendrimer-coupled sonophoresis-mediated transdermal drug-delivery system for diclofenac

The purpose of the present study was to develop a novel transdermal drug-delivery system comprising a polyamidoamine dendrimer coupled with sonophoresis to enhance the permeation of diclofenac (DF) through the skin. The novel transdermal drug-delivery system was developed by using a statistical Plackett–Burman design. Hairless male Wistar rat skin was used for the DF-permeation study. Coupling media concentration, ultrasound-application time, duty cycle, distance from probe to skin, and a third-generation polyamidoamine-dendrimer concentration were selected as independent variables, while in vitro drug release was selected as a dependent variable. Independent variables were found to be statistically significant (P<0.05). DF gel without dendrimer and ultrasound treatment to skin (passive delivery, run 13) showed 56.69 µg/cm² cumulative drug permeated through the skin, while the DF-dendrimer gel without sonophoresis treatment (run 14) showed 257.3 µg/cm² cumulative drug permeated through the skin after 24 hours. However, when the same gel was applied to sonophoresis-treated skin, drastic permeation enhancement was observed. In the case of run 3, the cumulative drug that permeated through the skin was 935.21 µg/cm². It was concluded that dendrimer-coupled sonophoresis-mediated transdermal drug delivery system has the potential to enhance the permeation of DF through the skin.

Effects of oxygen-containing terpenes as skin permeation enhancers on the lipoidal pathways of human epidermal membrane

The present study investigated the effects of oxygen-containing terpenes as skin permeation enhancers on the lipoidal pathways of human epidermal membrane (HEM). The enhancement (E(HEM)) effects of menthol, thymol, carvacrol, menthone, and cineole on the transport of a probe permeant, corticosterone, across HEM were determined. It was found that the enhancer potencies of menthol, thymol, carvacrol, and menthone were essentially the same and higher than that of cineole based on their aqueous concentration in the diffusion cell chamber at E(HEM) = 4. Thymol and carvacrol also had the same E(HEM) = 10 concentration further supporting that they had the same enhancer potency based on the aqueous concentration. The uptake amounts of terpene into the HEM stratum corneum (SC) intercellular lipid under the same conditions indicate that the intrinsic potencies of the studied terpenes are the same based on their concentration in the SC and similar to those of n-alkanol and n-alkylphenyl alcohol. Moreover, they are all better enhancers compared to branched-chain alkanol. The approximately same uptake enhancement of beta-estradiol induced by the studied terpenes and alcohols at E(HEM) conditions into the SC intercellular lipids suggests that the mechanism of enhancement action for the terpenes and those of alcohols are essentially the same.

Effects of chemical enhancers on human epidermal membrane: Structure-enhancement relationship based on maximum enhancement (E(max))

Chemical penetration enhancers are widely used in transdermal pharmaceuticals as well as cosmetic products. Selection of suitable enhancers in topical formulations requires an understanding of the mechanism of action of these enhancers. The objective of the present study was to evaluate the enhancement effects of a number of commonly known enhancers and cosmetic ingredients on permeation across human epidermal membrane (HEM). The potencies of these chemical enhancers-maximum enhancement, E(max)-were compared at their highest thermodynamic activity in equilibrium with HEM (i.e., solubility equilibrium). This was achieved by the treatment of HEM with the enhancer or phosphate buffered saline (PBS) saturated with the enhancer. Passive transport experiments were then conducted with a model permeant corticosterone to determine the effects of these enhancers on the lipoidal pathway of HEM. The results suggest that E(max) of an enhancer is related to its octanol/water partition coefficient and its solubility in the HEM lipid domain. A relationship between enhancer E(max) and its solubility in silicone elastomer was also observed, suggesting the use of silicone solubility to predict enhancer potency. Based on the E(max) results, some common topical ingredients were found to be more potent enhancers than a number of well-known chemical enhancers.

Comparison of the effects of chemical permeation enhancers on the lipoidal pathways of human epidermal membrane and hairless mouse skin and the mechanism of enhancer action

Previously, the effects of chemical permeation enhancers upon the permeability of the lipoidal pathway of hairless mouse skin (HMS) were investigated and a quantitative structure enhancement relationship was established. The present study was to study the effects of these enhancers on human epidermal membrane (HEM) using the same experimental method employed in the previous HMS studies. The effects of enhancers on the permeability coefficients of the lipoidal pathways of HEM and HMS for corticosterone were found to be essentially the same. In the equilibrium uptake studies of the enhancers and beta-estradiol, it was found that the amounts of enhancers taken up and the partitioning of beta-estradiol into the HEM stratum corneum (SC) intercellular lipid under the E = 10 conditions were different from those of HMS. Despite these differences, the HEM data show a correlation between the intercellular lipid/PBS partition coefficients of the enhancers and the enhancer n-octanol/PBS partition coefficients. This correlation is consistent with the observed chemical microenvironment of the site of enhancer action in the HMS SC in previous studies. Therefore, provided with proper experimental protocols, HMS can be a reliable model for the evaluation of the effects of skin permeation enhancers on the lipoidal pathway of HEM.

Enhanced Transscleral Iontophoretic Transport with Ion-Exchange Membrane

PURPOSE

Transscleral iontophoresis has been recently re-examined for drug delivery to the back of the eye. In conventional iontophoresis, due to the relatively high electromobility of the endogenous competing ions (counterions) relative to that of the drug ion in the tissue barrier, the efficiency of iontophoretic drug delivery is generally low. The objective of the present study was to examine ion-exchange membrane-enhanced transscleral iontophoretic transport in which the ion-exchange membrane in series with the sclera can hinder the transport of the competing counterions and selectively allow the transport of the permeant across the sclera.

METHODS

The physical properties of the Ionac ion-exchange membrane and excised rabbit sclera were determined in equilibrium uptake experiments and in passive and iontophoretic transport experiments with salicylate, tetraethylammonium, urea, and mannitol. Transscleral experiments with the ion-exchange membrane were conducted with salicylate and excised rabbit sclera in vitro. The contribution of electroosmosis to electrotransport during transscleral iontophoresis was assessed with urea and mannitol.

RESULTS

The ion-exchange membrane is highly positively charged and has a small effective pore size. The sclera is relatively porous with a large effective pore size and low pore tortuosity. The sclera is also net negatively charged but this does not significantly affect the transport of small ions. A three-fold steady-state transscleral flux enhancement of salicylate was observed in ion-exchange membrane-enhanced iontophoresis over conventional transscleral iontophoresis without the membrane. Such enhancement was relatively independent of the applied electric current density and the thickness of the studied ion-exchange membrane assembly. Although the ion-exchange membrane altered transscleral electroosmosis, the contribution of electroosmosis to electrotransport was not significant.

CONCLUSIONS

The present study has demonstrated the potential of ion-exchange membranes for enhancing iontophoretic transport and drug delivery.

Overcoming the Stratum Corneum: The Modulation of Skin Penetration

It is preferred that topically administered drugs act either dermally or transdermally. For that reason they have to penetrate into the deeper skin layers or permeate the skin. The outermost layer of the human skin, the stratum corneum, is responsible for its barrier function. Most topically administered drugs do not have the ability to penetrate the stratum corneum. In these cases modulations of the skin penetration profiles of these drugs and skin barrier manipulations are necessary. A skin penetration enhancement can be achieved either chemically, physically or by use of appropriate formulations. Numerous chemical compounds have been evaluated for penetration-enhancing activity, and different modes of action have been identified for skin penetration enhancement. In addition to chemical methods, skin penetration of drugs can be improved by physical options such as iontophoresis and phonophoresis, as well as by combinations of both chemical and physical methods or by combinations of several physical methods. There are cases where skin penetration of the drug used in the formulation is not the aim of the topical administration. Penetration reducers can be used to prevent chemicals entering the systemic circulation. This article concentrates on the progress made mainly over the last decade by use of chemical penetration enhancers. The different action modes of these substances are explained, including the basic principles of the physical skin penetration enhancement techniques and examples for their application.