MDR1 P-glycoprotein transports endogenous opioid peptides

MDR1 P-glycoprotein is generally regarded as an efflux pump for amphipathic toxic compounds. The question remains, however, whether certain endogenous compounds are also substrates for this transporter. Certain peptides have been shown to interact with MDR1 Pgp as well and we have therefore investigated whether endogenous bioactive peptides are substrates. We demonstrate here that the synthetic μ-opioid peptide DAMGO is a good substrate for MDR1 Pgp. In view of its low interaction with the membrane it is an attractive ligand for measurement of MDR1 Pgp-mediated transport activity in membrane vesicles. Various linear peptides with amidated C-termini were found to inhibit MDR1 Pgp-mediated DAMGO transport. This group includes endogenous opioid peptides such as adrenorphin and endomorphin 1 and 2, as well as the neurokinin, Substance P. The latter bioactive peptides have a relatively high affinity for the transporter. Transport of endomorphin 1 and 2 could be directly demonstrated by the uptake of the radiolabeled opioid peptides in membrane vesicles from MDR1-transfected cells with a K_m of 15 and 12 μM, respectively. This opens the possibility that MDR1 Pgp is involved in the elimination and/or tissue distribution of these bioactive peptides.     

Molecular cytogenetics of multiple drug resistance

The refractory nature of many human cancers to multi-agent chemotherapy is termed multidrug resistance (MDR). In the past several decades, a major focus of clinical and basic research has been to characterize the genetic and biochemical mechanisms mediating this phenomenon. To provide model systems in which to study mechanisms of multidrug resistance,in vitro studies have established MDR cultured cell lines expressing resistance to a broad spectrum of unrelated drugs. In many of these cell lines, the expression of high levels of multidrug resistance developed in parallel to the appearance of cytogenetically-detectable chromosomal anomalies resulting from gene amplification. This review describes cytogenetic and molecular-based studies that have characterized DNA amplification structures in MDR cell lines and describes the important role gene amplification played in the cloning and characterization of the mammalian multidrug resistance genes (mdr). In addition, this review discusses the genetic selection generally used to establish the MDR cell lines, and how drug selections performed in transformed cell lines generally favor the genetic process of gene amplification, which is still exploited to identify drug resistance genes that may play an important role in clinical MDR.     

Multidrug resistance in parasites: ABC transporters, P-glycoproteins and molecular modelling

Parasitic diseases, caused by protozoa, helminths and arthropods, rank among the most important problems in human and veterinary medicine, and in agriculture, leading to debilitating sicknesses and loss of life. In the absence of vaccines and with the general failure of vector eradication programs, drugs are the main line of defence, but the newest drugs are being tracked by the emergence of resistance in parasites, sharing ominous parallels with multidrug resistance in bacterial pathogens. Any of a number of mechanisms will elicit a drug resistance phenotype in parasites, including: active efflux, reduced uptake, target modification, drug modification, drug sequestration, by-pass shunting, or substrate competition. The role of ABC transporters in parasitic multidrug resistance mechanisms is being subjected to more scrutiny, due in part to the established roles of certain ABC transporters in human diseases, and also to an increasing portfolio of ABC transporters from parasite genome sequencing projects. For example, over 100 ABC transporters have been identified in the Escherichia coli genome, but to date only about 65 in all parasitic genomes. Long established laboratory investigations are now being assisted by molecular biology, bioinformatics, and computational modelling, and it is in these areas that the role of ABC transporters in parasitic multidrug resistance mechanisms may be defined and put in perspective with that of other proteins. We discuss ABC transporters in parasites, and conclude with an example of molecular modelling that identifies a new interaction between the structural domains of a parasite P-glycoprotein.     

Hypothesis: An important role for R-verapamil in pharmacoresistant epilepsy

Drugs that block P-glycoprotein-modulated efflux of antiepileptic drugs in the brain, lead to increase their intracellular concentration. Addition of R-verapamil as one enantiomer of verapamil to antiepileptic drugs may facilitate the penetration of drugs to the brain and prevents their efflux from CNS without serious cardiovascular side effects.