Comparative effects of racemic verapamil vs R-verapamil on normal and leukemic progenitors

Pharmacokinetics of fexofenadine enantiomers in healthy subjects

Fexofenadine, a substrate of P-glycoprotein and an organic anion transporter polypeptide, is commonly used to assess P-glycoprotein activity in vivo. The purpose of this study was to elucidate the pharmacokinetics of each fexofenadine enantiomer. After a single oral dose of racemic fexofenadine (60 mg), the plasma and urine concentrations of fexofenadine enantiomers were measured over the course of 24 h in six healthy subjects. The mean plasma concentration of R(+)-fexofenadine was higher than that of S(-)-fexofenadine. The area under the plasma concentration-time curve (AUC(0-infinity)) and the maximum plasma concentration (C(max)) of R(+)-fexofenadine were significantly greater than those of the S(-)-enantiomer (P = 0.0018 and 0.0028, respectively). The R/S ratios of AUC and C(max) of fexofenadine were 1.75 and 1.63, respectively. The oral clearance and renal clearance of S(-)-fexofenadine were significantly greater than that of R(+)-fexofenadine (P = 0.0074 and 0.0036). On the other hand, the stereoselective metabolism of fexofenadine using recombinant CYP3A4 was investigated; however, fexofenadine enantiomers were not metabolized by CYP3A4. Fexofenadine is transported by both P-glycoprotein and OATP and is not metabolized by intestinal CYP3A. Our findings suggest that the affinity of P-glycoprotein for S(-)-fexofenadine is greater than its affinity for the R(+)-enantiomer. Thus, P-glycoprotein is likely to have chiral discriminatory abilities.

The absence of stereoselective P-glycoprotein- and multidrug resistance-associated protein-mediated transport of daunorubicin

Multidrug resistance phenotype in mammalian cells is often correlated with overexpression of P-glycoprotein (P-gp) or multidrug resistance-associated protein (MRP1). Both proteins are energy-dependent drug efflux pumps that efficiently reduce the intracellular accumulation and hence the cytotoxicity of many natural cytotoxins. Thus, both P-gp and MRP1 proteins are able to transport anthracycline but the role of chirality has not, up to now, been addressed. In this study, we compared the P-gp- and MRP1-mediated efflux of daunorubicin and its enantiomer WP900 in multidrug-resistant cells overexpressing either P-gp (K562/ADR cells) or MRP1 (GLC4/ADR cells). Using fluorescence techniques, we showed that in both cell lines the presence of the pump yielded a gradient of drug concentration: the intracellular free drug concentration in the cytosol was lower than the extracellular free drug concentration. Our data showed that the gradient of concentration generated by the pump was the same whether DNR or WP900 was used. This means that P-gp on the one hand and MRP1 on the other recognise WP900 as well as DNR and that the chirality of the molecule plays no role.

Chemotherapy resistance in acute myeloid leukaemia

The development of refractory disease in acute myeloid leukaemia (AML) is frequently associated with the expression of one or several multidrug resistance (MDR) genes. MDR1, MRP1 and LRP have been identified as important adverse prognostic factors in AML. Recently it has become possible to reverse clinical multidrug resistance by blocking P-glycoprotein-mediated drug efflux. The potential relevance of MDR and new approaches to treat refractory disease, are discussed.

Multidrug resistance in haematological malignancies

The development of refractory disease in acute myeloid or lymphoblastic leukaemias (AML, ALL) and multiple myeloma (MM) is frequently associated with the expression of one or several multidrug resistance (MDR) genes. MDR1, MRP1 and LRP have been identified as important adverse prognostic factors in AML, T-ALL and MM. Recently, it has become possible to reverse clinical multidrug resistance by blocking P-glycoprotein-mediated drug efflux. The potential relevance of these reversal agents of MDR and potential new approaches to treat refractory disease are discussed.

Multidrug resistance in acute myeloid leukaemia

Multidrug resistance represents a form of pleiotropic drug resistance that has a prognostic value in untreated AML. It may affect the outcome of current chemotherapy protocols. Therefore, MDR1 expression should be systematically investigated in prospective studies. In addition, restoration of drug sensitivity may be attempted by adding drug resistance modifying agents such as cyclosporins to standard chemotherapy. The clinical value of such an approach has to be established in currently ongoing phase III studies.