One of the most important challenges facing the pharmaceutical industry at present is to develop high-throughput, cost effective and highly predictive screening models of drug absorption that can be used during the decision making process early in drug discovery. Although physicochemical parameters such as lipophilicity, charge, molecular weight, etc. are often used as initial indicators of absorption, they are often not reliable because of their inability to incorporate the physiologic conditions. On the other hand, the conventional experimental methods for assessing the absorption characteristics are not streamlined to keep pace with the large number of compounds synthesized by combinatorial chemistry. The available experimental models have various pros and cons and their judicious use can increase the likelihood of progressing drugs with favorable oral absorption characteristics. Physicochemical and/or in silico methods that are rapid and require no usage of animal tissues should be used as the primary screening models in early drug discovery stage. Subsequently, automated high-throughput in vitro systems (cell culture-based or animal tissue-based) can be used in selecting and optimizing the chemical leads to identify potential drug candidates. Multiple complementary screening models involving in situ and in vivo methods should also be utilized simultaneously at this stage to minimize "false-positives" and "false-negatives" from getting into the development process.
Caco-2 cells and PAMPA are two valuable research tools that are currently the method-of-choice for screening compounds for absorption and P-gp interaction potential. Despite the popularity and acceptability of PAMPA and Caco-2 cell models, it is important to recognize the caveats associated with these models to fully realize their potential. Standardization of experimental variables to develop a "uniform" methodology is an important first step prior to implementation of these models in high-throughput mode. Calibration of the models with appropriate reference probes (known marketed drugs as well as internal research compounds) is essential to maintain the validity of the model. These reference compounds should cover a broad structural and physicochemical space and should cover a range of solubility/permeability. Other probes to represent paracellular and efflux (P-gp) transport are also recommended. These reference compounds should be included as quality control set in every test run performed with unknown compounds. Like all other in vitro models, both PAMPA and Caco-2 cells have strengths and weaknesses. A thorough understanding of the rationale underlying the caveats (such as low recovery, solubility, etc.) associated with these models could certainly help putting the results in the proper perspectives. The results obtained from these assays should not be interpreted in isolation but should be assessed in conjunction with solubility and stability characteristics of the compounds. Finally, it is important to realize that simplistic in vitro models like PAMPA and Caco-2 cells are inadequate to represent the complicated absorption machinery in the human intestinal tract. ultimately, it is the judicious use of these models in combination with in vivo studies that would enhance our ability to predict the drug disposition in humans and ensure that only drug candidates with a high-developability potential are moved forward into the development pipeline.
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