P-glycoprotein in helminths: function and perspectives for anthelmintic treatment and reversal of resistance.

Infestation with parasitic helminths is a common problem in human populations of third world countries and is ubiquitous in livestock and other domestic animals. The cell-membrane efflux pump, P-glycoprotein (Pgp), appears to contribute to anthelmintic resistance. Pgp have been identified from both phyla of parasitic helminths, Platyhelmintha and Nematoda, and alterations in expression levels and allele frequencies of Pgp in anthelmintic-resistant populations have been observed in nematodes. Localisation of Pgp has been studied in the free-living nematode Caenorhabditis elegans and in the sheep parasite Haemonchus contortus using specific monoclonal antibodies or lectins. Reversing agents used in human studies, such as the calcium-channel blocker verapamil (VPL), appear to have similar effects in helminths as they do in human cancer cells: the efficacy of drug treatment is increased in drug-resistant parasites when reversing agents are co-administered with the anthelmintic. The functional role of the Pgp glycosylation was also studied using a lectin specific for the alpha-mannosyl residues and showed that resistance can be associated with a decreased affinity of the lectin for Pgp sites and that up to 50% reversion in the resistance to benzimidazoles (BZ) can be obtained using this lectin. Furthermore, the current knowledge on the role of Pgp in molecular mechanisms of drug resistance in the parasitic protozoan genus Trypanosoma is discussed. In some Trypanosoma species it was shown that drug resistance was associated with reduced uptake and in other ones with increased efflux. Several trypanosome Pgp-coding sequences have been described. In contrast to earlier data, most recent observations, based on experimentally overexpressed Pgp in Trypanosoma brucei, indicate a possible involvement in the mechanism of drug resistance in this parasite.