P-gp inhibitor verapamil

In contrast to P-gp and the MRP proteins, the breast cancer resistance protein (BCRP) contains six transmembrane domains and only one ATP-binding domain. It was first cloned from the breast cancer cell line MCF-7 selected in doxombicin, in the presence of the P-gp inhibitor verapamil. It is found in many human tissues, such as the placenta, small intestine, colon, and liver [133], It is localized to the apical membrane of epithelial cells of the small intestine and colon and to the bile canalicular membrane in the liver and is involved in reducing intestinal uptake, increasing hepatobiliary excretion, etc., leading to diminished oral bioavailability. cDNA sequences identical to BCRP and named MXR and ABCP, respectively, were independently isolated from human colon carcinoma cells and human placenta [134], BCRP requires... 

Pancreatic cholesterol esterase (3.1.1.3.) aids in transporting cholesterol to the enterocyte. By utilizing a selective and potent cholesterol esterase inhibitor 6-chloro-3-(l-ethyl-2-cyclohexyl)-2-pyrone, the absorption of cholesterol in hamsters could be reduced [71]. Wadkins et al. [72] synthesized novel sulfonamide derivatives, which demonstrated greater than 200-fold selectivity for human intestinal carboxylesterase compared with the human liver carboxylesterase hCEl, and none of them was an inhibitor of human acetylcholinesterase or butyrylcholinester-ase. Maybe these agents can serve as lead compounds for the development of effective, selective carboxylesterase inhibitors for clinical applications. Also the potent P-gp inhibitor verapamil [73] as well as S,S,S-tributylphosphortrithionate (DEF) [74] may exhibit carboxylesterase inhibitory properties. Various other inhibitors of human esterases are listed in Table 5.6. 

An additional example of a bioavailability-predicted absorption plot is shown for a series of calcium antagonists (Fig. 19.8). Again there is considerable scatter in the data, and the four compounds - felodipine, nisoldipine, diltiazem, and verapamil -are predicted to be much better absorbed than was actually observed. Some of these compounds are known to undergo rapid first-pass metabolic clearance, and are also P-gp inhibitors or substrates (diltiazem and felodipine are P-gp substrates nicardipine and nitrendipine are P-gp inhibitors [25] verapamil is a P-gp inhibitor), and this might contribute to the scatter obtained in the graph. 

A more recent example of this technique has been the study on human absorption characteristics of fexofenadine [109], Fexofenadine has been shown to be a substrate for P-gp in the in vitro cell lines its disposition is altered in knockout mice lacking the gene for MDRla, and co-administration of P-gp inhibitors (e.g. ketoconazole and verapamil) was shown to increase the oral bioavailability of fexofenadine [110-113], Hence, it is suggested that the pharmacokinetics of fexofenadine appears to be determined by P-gp activity. In the human model, the intestinal permeability estimated on the basis of disappearance kinetics from the jejunal segment is low, and the fraction absorbed is estimated to be 2% [114], Co-administration of verapamil/ketoconazole did not affect the intestinal permeability estimates however, an increased extent of absorption (determined by de-convolution) was demonstrated. The increased absorption of fexofenadine was not directly related to inhibition of P-gp-mediated efflux at the apical membrane of intestinal cells as intestinal Peff was unchanged. Furthermore, the effect cannot be explained by inhibition of intestinal based metabolism, as fexofenadine is not metabolised to any major extent. It was suggested that this may reflect modulation of efflux transporters in hepatocyte cells, thereby reducing hepatobiliary extraction of fexofenadine. 

The ratio of renal clearance of digoxin to creatinine clearance decreased with the coadministration of clarithromycin (0.64 and 0.73), and was restored (1.30) after administration of clarithromycin had stopped (326). The role of P-gp efflux in this interaction was confirmed using an in vitro kidney epithelial cell line (326). The administration of itraconazole, a P-gp inhibitor, with digoxin resulted in an increased trough concentration and a decrease in the amount of renal clearance, possibly by an inhibition of the renal tubular secretion of digoxin via P-gp (329). The P-gp modulator verapamil has also been shown to decrease the renal clearance of digoxin (330). 

Based on the specificity and affinity, low molecular mass efflux pump inhibitors are classified into three generations (Werle 2008a). First-generation P-gp inhibitors such as verapamil or cyclosporine are compounds that are in clinical use for other indications and exhibit additional inhibitory properties. 

Figure 20.2 Pharmacophore models for P-gp inhibition. A. P-gp inhibition pharmacophore aligned with the potent inhibitor LY335979. B. P-gp substrate pharmacophore aligned with verapamil. C. P-gp inhibition pharmacophore 2 aligned with LY335979. Green indicates H-bond acceptor feature, and cyan indicates hydrophobic feature. See color plate.

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