Physicochemical PAMPA

One decade has passed since the parallel artificial membrane permeation assay (PAM PA) was first introduced in 1998 [47]. Since then, PAM PA rapidly gained wide popularity in drug discovery [3, 48-51]. Today, PAMPA is the most widely used physicochemical membrane permeation model. The term PAMPA is nowusedas the general name for a plate-based (HTS enabled), biter-supported (filter immobilized) artificial membrane. Typically, phospholipids dissolved in an organic solvent are impregnated into the filter to construct a PAMPA membrane. 

Many types of modeling techniques are available in the discovery phase of drug development, from structure activity relationships (SAR) to physiology based pharmacokinetics (PBPK) and pharmacokinetics-/pharmacodynamics (PK/PD) to help choosing some of the lead compounds. Some tests that are carried out by discovery include techniques related to structure determination, metabolism, and permeability NMR, MS/MS, elemental analysis, PAMPA, CACO-2, and in vitro metabolic stability. Although they are important as a part of physicochemical molecular characterization under the biopharmaceutics umbrella, they will not be discussed here. The reader can find relevant information in numerous monographs [9,10]. 

Similar approaches to enhance solubility have been studied in the PAMPA system. For instance, Kansy et al. (2001) explored the use of glycocholic acid to solubilize compounds. As PAMPA is a completely artificial system, it is expected that, compared to cell-based models, higher concentrations of cosolvents can be used. Sugano and coworkers reported that DMSO, ethanol and PEG 400 could be used up to 30% without causing disruptions of the lipid layer (Sugano et al., 2001). An effect of these cosolvents on the physicochemical properties of the test compounds (e.g. impact on pKa) may however lead to an unpredictable effect on drug permeability (Sugano et al., 2001). Currently, DMSO is commonly used as a cosolvent in the PAMPA system at a concentration of 1 to 2%. 

Hit to lead Physicochemical properties (solubility, Log P) PAMPA Plasma protein binding Efficacy species and human liver microsomal stability, CAR and PXR transactivation assay... 

Table 1 Physicochemical (logD7 4, solubility, PAMPA, and Caco-2), in vitro pharmacokinetic (microsomal stability) and pharmacodynamics (IC50) parameters of FimH antagonists ring A is adjacent to the anomeric center, whereas ring B is in the terminal position.

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