Abstract
Background: Currently, only two drugs are recommended for treatment of
infection with Trypanosoma cruzi, the etiologic agent of Chagas’
disease. These compounds kill the
trypomastigote forms of the parasite circulating in the bloodstream, but are
relatively ineffective against the intracellular stage of the parasite life
cycle. Neither drug is approved by
the FDA for use in the U.S.. The
hypoxanthine phosphoribosyltransferase from T. cruzi is a possible new target for antiparasite
chemotherapy. The crystal
structure of the HPRT in a conformation approximating the transition state
reveals a closed active site that provides a well defined target for
computational structure-based drug discovery.
Results: A
flexible ligand docking program incorporating a desolvation correction was used
to screen the Available Chemicals Directory for inhibitors targeted to the
closed conformation of the trypanosomal HPRT. Of 22 potential inhibitors identified, acquired and tested,
16 yielded Ki’s between 0.5 and 17 µM versus the
substrate phosphoribosyl pyrophosphate (PRPP). Surprisingly, three of nine compounds tested were effective
in inhibiting the growth of parasites in infected mammalian cells.
Conclusions: This
structure-based docking method provided a remarkably efficient path for the
identification of inhibitors targeting the closed conformation of the
trypanosomal HPRT. The inhibition
constants of the lead inhibitors identified are unusually favorable, and the
trypanostatic activity of three of the compounds in cell culture suggests that
they may provide useful starting points for drug design for the treatment of
Chagas’ disease.
Comment
We carried out computational studies using DOCK with the
advice of members of Brian Shoichet's lab (now at UCSF) to see if the
'closed' active site of the trypanosomal enzyme hypoxanthinephosphoribosyl
transferase (previously determined by Focia et al (1998) 'Approaching the transition state in the crystal
structure of a phosphoribosyltransferase'. Biochemistry 37, p.17120) would
present a good target for structure based drug discovery. Indeed it appeared
to; we identified several candidate inhibitors which proved to inhibit growth
of trypansosomes in culture.