Drug discovery intestinal permeability

In conclusion, there are several drawbacks to the use of Caco-2 cells in studies of active drug transport. Despite these drawbacks, we note that a recent comprehensive study comparing various P-glycoprotein drug efflux assays in drug discovery came to the conclusion that the Caco-2 transport assay is the method of choice, since it displays a biased responsiveness towards compounds with low or moderate permeability - in other words, towards compounds whose intestinal permeability is most likely to be significantly affected by drug efflux mechanisms [101]. 

Lead optimization of new chemical entities (NCEs) based on pharmacokinetic behavior plays a major role in modern drug discovery. Despite advancement of drug delivery methods, the oral route remains the most frequent route of administration for approved new drugs. Therefore, during lead optimization it is essential to identify NCEs with sufficient oral absorption predicted using a variety of in vitro and in vivo assays. It is well recognized that in order for a NCE to achieve reasonable oral absorption, it will need to have adequate aqueous solubility, as well as intestinal permeability [1], Recent advancements in chemistry, such as parallel and combinatorial synthesis, have resulted in a multifold increase in the number of compounds that are available for evaluation in new drug discovery. Furthermore, a variety of improved structural chemistry... 

Despite the availability of other cell lines, Caco-2 cells remain the most widely used intestinal cell culture model at present. This model has provided valuable information necessary for lead optimization in the drug discovery process. However, it is important to understand that compounds with high permeability in this model are typically well absorbed, whereas compounds with low solubility and low permeability in this model may not necessarily be poorly absorbed in vivo. Although this type of positive selection limits the usefulness in providing a structure-permeability relationship, the Caco-2 model has the most effect in drug discovery when the screen is implemented early and in conjunction with other types of in vitro and in vivo permeability/absorption screens. 

Caco-2 monolayer, a model for human intestinal permeability, is commonly used in drug discovery to screen discovery compounds.34,35 The method involves measurement of flux of... 

Membrane permeability is one of the most important determinants of pharmacokinetics, not only for oral absorption, but also for renal re-absorption, biliary excretion, skin permeation, distribution to a specific organ and so on. In addition, modification of membrane permeability by formulation is rarely successful. Therefore, membrane permeability should be optimized during the structure optimization process in drug discovery. In this chapter, we give an overview of the physiology and chemistry of the membranes, in vitro permeability models and in silica predictions. This chapter focuses on progress in recent years in intestinal and blood-brain barrier (BBB) membrane permeation. There are a number of useful reviews summarizing earlier work [1-5]. 

Balimane, P.V. and Chong, S. (2005) Cell culture-based models for intestinal permeability A critique. Drug Discovery Today, 10, 335-343. 

The ability of combinatorial chemistry to synthesize large numbers of compounds has stimulated interest in developing in silico methods that can predict bioavailability as part of the drug discovery process. Current computational methods can provide separate estimates of the solubility and intestinal permeability of candidate drug molecules even before they are synthesized (33). However this approach has not yet been perfected/ and the computational requirement of the... 

Three common properties that affect intestinal absorption of drugs after oral administration are solubility, permeability, and p/f. Traditional solubility experiments measure solubility of solids placed into aqueous phases (thermodynamic solubility), but these methods are too slow or they consume too much material for drug discovery. Higher throughput methods must be used. The direct ultraviolet (UV) method [17] adds compound dissolved in dimethyl sulfoxide (DMSO) to an aqueous buffer and measures the UV absorption of the aqueous phase using a 96-well plate reader after equilibration and filtration (kinetic solubility). Lipinski has discussed the pitfalls that inadequate solubility information can have for a drug-discovery organization [18]. 

These automated assays can be used for high-throughput ADME screening in early drug discovery. The double-sink PAMPA permeability assay mimics in vivo conditions by the use of a chemical sink in the acceptor wells and pH gradient in the donor wells. The use of the pION gut-box integrated on the deck has shortened the PAMPA assay incubation time to 30 minutes. The permeability coefficient and rank order correlate well with data obtained using the in vitro Caco-2 assay and in vivo permeability properties measured in rat intestinal perfusions. 

The prediction of the important structural features that affect intestinal permeability is useful information to obtain early in the drug discovery process. The two most common models used to obtain fast, high-throughput measurements are the parallel artificial membrane permeation assay (PAMPA) and the cell line assays that feature cultured human colon adenocarcinoma cells (Caco-2). Each method uses a surrogate model to mimic intestinal absorption followed by LC-MS analysis. 

---