NEW APPROACHES TO LEISHMANIA CHEMOTHERAPY - PTERIDINE REDUCTASE-1 (PTR1) AS A TARGET AND MODULATOR OF ANTIFOLATE SENSITIVITY

Leishmania and other trypanosomatid protozoa require reduced pteridine s (pterins and folates) for growth, suggesting that inhibition of thes e pathways could be targeted for effective chemotherapy. This goal has not yet been realized, indicating that pteridine metabolism may be un usual in this lower eukaryote. We have investigated this possibility u sing both wild type and laboratory-selected antifolate-resistant strai ns, and with defined genetic knockouts of several pteridine metabolic genes. In Leishmania, resistance to the antifolate methotrexate is med iated through several mechanisms singly or in combination, including a lterations in transport leading to reduced drug influx, overproduction (R-region amplification) or point mutation of dihydrofolate reductase -thymidylate synthase (DHFR-TS), and amplification of a novel pteridin e reductase (PTR1, encoded by the H-region). All of the proteins invol ved are potential targets for antifolate chemotherapy. Notably, parasi tes in which the gene encoding dihydrofolate reductase (DHFR) has been deleted (dhfr-ts(-) knockouts) do not survive in animal models, valid ating this enzyme as a target for effective chemotherapy. However, the properties of pteridine reductase 1 (PTR1) suggest a reason why antif olate chemotherapy has so far not been successful in trypanosomatids. PTR1, by its ability to provide reduced pterins and folates, has the p otential to act as a by-pass and/or modulator of DHFR inhibition under physiological conditions. Moreover, PTR1 is less sensitive to many an tifolates targeted primarily against DHFR. These findings suggest that successful antifolate chemotherapy in Leishmania will have to target simultaneously both DHFR and PTR1.