Leishmania and other trypanosomatid protozoa require
reduced pteridines (pterins and folates) for growth, suggesting that
inhibition of these pathways could be targeted for effective chemotherapy.
This goal has not yet been realized, indicating
that pteridine metabolism may be unusual in this lower eukaryote. We have
investigated this possibility using both wild
type and laboratory-selected antifolate-resistant strains, and with
defined genetic knockouts of several pteridine metabolic
genes. In Leishmania, resistance to the antifolate methotrexate
is
mediated through several mechanisms singly or in
combination, including alterations 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 pteridine
reductase (PTR1, encoded by the H-region). All of the proteins
involved are potential targets for antifolate chemotherapy.
Notably, parasites in which the gene encoding dihydrofolate reductase (DHFR)
has
been deleted (dhfr-ts− knockouts) do
not survive in animal models, validating this enzyme as a target for
effective chemotherapy. However, the properties of
pteridine reductase 1 (PTR1) suggest a reason why antifolate chemotherapy
has so far not been successful in trypanosomatids. PTR1, by its ability
to
provide reduced pterins and folates, has the potential to act as a by-pass
and/or modulator
of DHFR inhibition under physiological conditions. Moreover, PTR1 is
less sensitive to many antifolates targeted
primarily against DHFR. These findings suggest that successful antifolate
chemotherapy in Leishmania will have to target
simultaneously both DHFR and PTR1.