Cell culture models drug transport studies

The appropriateness of any cell culture model used to study drug delivery processes, whether a primary or a cell line, should be based on certain basic criteria. These include but are not limited to the presence of a restrictive paracellular pathway that allows effective characterization of transcellular permeability, the presence of physiologically realistic cell architecture reflective of the tissue barrier of interest, the expression of functional transporter mechanisms representative of the tissue barrier of interest, and the ease, convenience, and reproducibility of the culture methods.4 However, pharmaceutical scientists must realize that cell culture models in use today are not ideal with respect to these criteria, and there will be advantages and limitations with any choice. 

The enthusiasm for using Caco-2 cells and other epithelial cell cultures in studies of drug transport processes has been explained by the ease with which new information can be derived from these fairly simple in vitro models [7]. For instance, drug transport studies in Caco-2 cells grown on permeable supports are easy to perform under controlled conditions. This makes it possible to extract information about specific transport processes that would be difficult to obtain in more complex models such as those based on whole tissues from experimental animals. Much of our knowledge about active and passive transport mechanisms in epithelia has therefore been obtained from Caco-2 cells and other epithelial cell cultures [10-15]. This has been possible since Caco-2 cells are unusually well differentiated. In many respects they are therefore functionally similar to the human small intestinal enterocyte, despite the fact that they originate from a human colorectal carcinoma [16, 17]. 

Another limitation is that there is no quantitative relationship between active drug transport in the cell culture models and in vivo e.g. [92, 93]. The reason may be that the expression level of the transporter in Caco-2 cells is not comparable to that in vivo or that there is a difference in effective surface area (see Section 4.3.2.2 below). One solution to this problem is to determine the apparent transport constants, Km and Vmax, for each transporter and subsequently, to determine a scaling factor. However, this is not readily done. In addition these studies are further complicated by the lack of specific substrates. For example, there are almost no specific substrates for the drug efflux transporters [18]. Therefore, other epithelial... 

Tavelin, S., J. Taipalensuu, F. Hallbook, K. Vellonen, V. Moore, and P. Artursson. An Improved Cell Culture Model Based on 2/4/A1 Cell Monolayers for Studies of Intestinal Drug Transport. Characterization of Transport Routes., Pharm. Res. 2003, 20, 373-381. 

Tavelin S, Taipalensuu J, Hallbook F, Vellonen KS, Moore V, Artursson P (2003a) An improved cell culture model based on 2/4/A1 cell monolayers for studies of intestinal drug transport Characterization of transport routes. Pharm Res 20 373-381. 

Ethical issues as well as difficulty in obtaining enough human nasal tissue specimens have called for the need to use alternative in vitro and in vivo methods. Various in vivo animal models and in vitro excised tissue models have been described in the literature for nasal drug transport studies. However, due to the difficulty in both controlling the experimental conditions in in vivo animal models and obtaining intact excised tissue samples, in vitro cell culture models are also being actively developed. 

Cultured nasal cells are reliable models for drug transport and metabolism studies, since they are known to express important biological features (e.g. tight junctions, mucin secretion, cilia, and various transporters), resembling those found in vivo systems. Moreover, easy control of experimental conditions as well as separation of the permeation step from the subsequent absorption cascade is also possible. A relatively simple primary culture condition using human nasal epithelial cells for in vitro drug transport studies has been established and applied in transport and metabolism studies of drugs. It is known that the culture condition and/or selection of culture media are critical in the recapitulation of well-differentiation features of in vivo nasal mucosal epithelium [46],... 

U. Werner and T. Kissel. In-vitro cell culture models of the nasal epithelium A comparative histochemical investigation of their suitability for drug transport studies. Pharm Res 13 978-988 (1996). 

For in vitro toxicity studies and assessment of the barrier function, drug transport, cell physiology, and metabolism as well as the development of delivery systems, cell culture models provide powerful systems for scientific research. As the corneal epithelium is the main barrier for ocular penetration, various corneal epithelial cell cultures were established besides the corneal constructs that mimic the whole cornea and serve as reductionist models for the ocular barrier. In general, two types of cell culture models are available primary cell cultures and immortalized, continuous cell lines. 

Reconstituted HCE (order no. RHC/S/5) is available from Skinethic, Nice, France. It consists of transformed human corneal epithelial cells immortalized by Roger Beuerman from the Louisiana State University-Eye Center, New Orleans, USA. This reconstituted human corneal epithelium forms a multilayered cell culture model that does not exhibit tight junctions and is therefore unsuitable for in vitro drug transport studies. Main focus of the model is eye irritation and toxicity testing and it is commonly used in this area with good prediction qualities. Major advantage of this model is the high reproducibility and uniform appearance. The commercial availability provides a ready-to-use model that is easy to handle. 

D.N. Karunaratne, P.S. Silverstein, V. Vasandani, A.M. Young, E. Rytting, B. Yops, and K.L. Audus. Cell Culture Models for Drug Transport Studies. Drug Delivery Principles and Applications (Wang, B, Siahaan, TJ, and Soltero, RA, Eds), John Wiley Sons, Inc, New York 103-125 (2005). 

The primary cell culture model is a more valid model for the study of absorption and transport processes of a drug via the pulmonary route. It provides a tight epithelial barrier with morphological and functional properties resembling those of the in-vivo condition. Primary alveolar epithelial cells from rats [39], rabbits [40] and humans [41] which display morphological and biochemical characteristics similar to the native epithelium have been isolated and can be used for drug transport studies. 

Classic in vitro cell culture models were used earlier to test the permeability of drugs themselves (9). Nowadays, these systems display not only an alternative to animal testing in respect to ethical concerns, but are also applied as advanced models to develop new delivery strategies and to study drug delivery properties of nanoparticulate carriers or chemical delivery enhancers. Thereby, interactions with the biological barrier, specifically binding, uptake, and transport can be evaluated in a well-defined and standardized system. While oral delivery of drugs is still the commonly used and most accepted method of choice due to the... 

Chen W, Tang F, Horie K, Borchardt RT (2002) CACO-2 cell monolayers as a model for studies of drug transport across human intestinal epithelium. In Lehr, KM (ed) Cell Culture models of biological barriers. Taylor and Francis, New York, pp 143-163... 

The advantage of cell culture models is that they are able to measure active transport processes across the cell membranes and not just the interaction of a drug with a lipid bilayer. They can also be used to study passive and active transport routes indeed, much of the knowledge as to the active transport mechanisms in the intestine has been derived from cell culture studies. Despite the predominant route being passive diffusion, the research into transport mechanisms indicates that there are a large number of drugs that are used as substrates for active transporter and efflux systems, and it must therefore be appreciated that multiple transport routes may be involved in the intestinal drug transport. 

CELL CULTURE MODELS FOR DRUG TRANSPORT STUDIES... 

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