The overexpression of genes of thiol metabolism contribute to drug resistance in clinical isolates of visceral leishmaniasis (kala azar) in India

Abstract

Background: Visceral leishmaniasis (VL), also called Kala Azar (KA) or black fever in India, claims around 20,000 live every year. Chemotherapy remains one of the most important tools in the control of VL. Current chemotherapy fo Kala Azar in India relies on a rather limited arsenal of drugs including sodium antimony gluconate and amphoterici B in addition to the very expensive drug miltefosine. Pentavalent antimonials have been used for more than half century in the therapy of leishmaniasis as it is relatively safe and inexpensive, however, the spread of resistance to thi drug is forcing clinicians in India to abandon this treatment. Consequently, improvement of antimonial chemotherap has become a major challenging area of study by leishmaniacs worldwide. The alarming emergence of resistance t the commonly used antleishmanial drug, sodium antimony gluconate, in India, has led us to elucidate the resistanc mechanism(s) in clinical isolates. Studies on laboratory mutants have shown that resistance to antimonials is highl dependent on thiol levels. The parasite evades cytotoxic effects of antimonial therapy by enhanced efflux of drug upo conjugation with thiols, through overexpressed membrane proteins belonging to the superfamily of ABC transporters Methods: We have carried out functional studies to determine the activity of the efflux pumps in antimonial resistan clinical isolates collected from disease endemic areas in India and also carried out molecular characterization of thio levels in these parasites Results: Overexpression of the gene coding for ? glutamylcysteine synthetase was observed in these resistant clinica isolates thereby establishing that thiols represent the key determinants of antimonial resistance. The SbIII/thio conjugates can be sequestered by ABC transporter multidrug resistance protein A (MRPA) into intracellular organelle or can be directly pumped out by an uncharacterized transporter Conclusions: Our studies investigating antimonial resistance in different L. donovani clinical isolates suggest that ove functioning of MRP plays a role in generation of antimony resistance phenotype in some L. donovani clinical isolates.