In Vitro Skin Permeation Studies

Most publications in the field of skin permeation research are carried out using a large variety of setups and experimental protocols varying from laboratory to laboratory. This raises questions of standardization and regularization. 

Both static and flow-through diffusion cells are approved by the authorities, and data are available on their relevance for predicting the in vivo situation [2, 61-64], Basically, a donor and an acceptor compartment are separated by a membrane of either native skin or bioengineered materials. These materials can be of human, animal, or artificial origin. Sampling from the acceptor compartment is performed either continuously or at predetermined time intervals. 

Dosing is possible in infinite (typically 10 /xl/cm2 or 10 mg/cm2) or finite manner ( 10 /xl/cm2 or 10 mg/cm2). The donor chamber may either be left open or be occluded. Nonoccluded conditions permit an exchange with the environment, such as evaporation of volatile substances and drying of the skin surface. In contrast, a tight occlusion of the skin surface may lead to excessive 

Temperature may be controlled by using a water jacket around each permeation cell, an external water bath, or warm air in a drying oven. Usually, experiments are carried out at 32°C, that is, the temperature of the skin surface, or else a temperature gradient may be applied of 32°C at the skin surface to 37°C in the acceptor compartment, mimicking body temperature. Constant stirring of the acceptor phase ensures that diffusion is unhampered by the buildup of high local concentrations and provides sink conditions throughout the duration of the experiment. 

Various membrane types suitable for permeation experiments are listed below. More details are available in literature [66], 

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In vitro skin permeation studies in a hydrodynamically well-calibrated skin permeation cell (Figure 4) demonstrated that simple pharmaceutical excipients, like capric acid (a saturated straight-chain fatty acid), can substantially enhance the trandermal permeation rate of progesterone. The time lag is significantly re-... 

Peptides penetrating through skin intercellular lipids have been studied using solid-state MAS NMR. In vitro skin permeation studies were performed on rat skin, and sections were collected and analyzed for P NMR signal. The results of the solid-state NMR study were in agreement with confocal microscopy studies. ... 

G. Lee and P. Parlicharla. An examination of excised skin tissues used for in vitro membrane permeation studies. Pharm. Res. 3 356-359 (1986). 

Songkro S, Rades T, Becket G. The effects of p-menthane monoterpenes and related compounds on the percutaneous absorption of propranolol hydrochloride across newborn pig skin I. In vitro skin permeation and retention studies. STP Pharma Set 2003 13(5) 349-357. 

In many respects, risk assessment for components of cosmetic formulations is a more straightforward task than that for environmmtal contaminants (Yourick and Bronaugh, 1999). Exposure periods and amounts appfied to the skin can be readily defined, and the concentration of the substance in question in the product is usually known. There is also a reasonably accurate measure of the frequency of appheation, which can be established by demographic studies. The studies described below illustrate the use of experimentally derived in vitro skin permeation data in generating margins of safety for two commonly used cosmetic types, sunscreen agents and hair dyes. 

It is well known that the actives used in hair dye formulations can penetrate into and permeate across the skin (Dressier, 1998 Yourick and Bronaugh, 2000). The rate and total cumulative amount of dye that has been shown to be absorbed, however, is variable and also dependent on the study protocol. When similar protocols have been used, in inter- and intralaboratory studies variability is considerably reduced (Beck et al., 2000). Calculation of safety margins for hair dyes using in vitro skin permeation data is somewhat more complex than that illustrated above for a sunscreen agent. Whereas a typical sunscreen agent is applied as a leave-on product over a large area of the body, hair dye products are applied to the hair over a relatively small (but certainly an area rich in hair follicles) area of skin and remain in place for a short period of time. Thus, to determine the potential systemic load using in vitro skin permeation measurements, the experiments should follow as closely as possible the in-use scenario. 

The tape-stripping technique has been applied in vivo, as well as in vitro. The used in vitro incubation devices are the same as described for skin permeation studies. A specialized incubation device developed by Loth and coworkers, the Saarbriicken penetration model, allows investigation of skin penetration bypassing the normally occurring nonphysiological hydration of the dermis [64],... 

Shaker, D.S. Ghanem, A.-H. Li, S.K. Warner, K.S. Hashem, F.M. Higuchi, W.I. Mechanistic studies of the effect of hydroxypropyl-P-cyclodextrin on in vitro transdermal permeation of corticosterone through hairless mouse skin. Int. J. Pharm. 2003, 253, 1-11. 

Bronaugh, R. F., and R. F. Stewart. 1985b. Methods for in vitro percutaneous absorption studies. V Permeation through damaged skin./. Pharm. Sci. 74 1062-1066. 

Protocols and Guidelines for In Vitro Skin Penetration and Permeation Studies... 

In vitro human skin permeation studies revealed that microemulsion improved iodide ion diffusion significantly. 

EstracanhoUi EA, Pra a ESG, Cintra AB, Pierre MBR, Lara MG. Liquid crystalline systems for transdermal delivery of Celecoxib In vitro drug release and skin permeation studies. AAPS PharmSciTech. 2014 1—8. 

In the light of these observations and the newer trends in product formulation, it was decided to study the in vitro release and permeation of propranolol hydrochloride from various hydrophilic polymeric matrices using the cellulose membrane and the hairless mouse skin as the diffusion barriers and to evaluate the effects of some of the additive ingredients known to enhance drug release from dermatological bases. 

Bond, J.R., and B.W. Barry. 1986. Limitations of hairless mouse skin as a model for in vitro permeation studies through human skin Hydration damage. J Invest Dermatol 90 486. 

H. Tanojo, J. A. Bouwstra, H. E. Junginger, and H. E. Bodde, In vitro human skin barrier modulation by fatty acids skin permeation and thermal analysis studies, Pharm Res. 14 42-49 (1997). 

Transdermal therapeutic systems (TTS) of nitroglycerin (1), isosorbide dinitrate (2), scopolamine (3) or clonidine (4) have been throughly investigated in vitro and in vivo. However, there have been few studies of skin permeation of ionizable water soluble drugs. 

Marongiu, B., Piras, A. and Porcedda, S. (2004) Cyclic monoterpene extract from cardamom oil as a skin permeation enhancer for indomethacin in vitro and in vivo studies. 1. journal of Agriculture and Food Chemistry 52(20), 6278-6282. 

The ability to test the transdermal penetration of compounds in vitro has been investigated extensively over the last decade. Hundreds of studies have shown that the permeation rate of compounds can be accurately measured using several available in vitro diffusion cells and various types of skin models [17]. 

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