Molecular docking, structure-activity relationship and biological evaluation of the anticancer drug monastrol as a pteridine reductase inhibitor in a clinical isolate of Leishmania donovani.

OBJECTIVES: Using the pteridine reductase (PTR1) enzyme of Leishmania as the target, the objective of our study was to find a drug candidate that can enter the clinical development process after being evaluated for safety and efficacy in animals.

METHODS: Monastrol (R) and (S) enantiomers were docked using the QUANTUM program into the active site of a Leishmania donovani PTR1 (LdPTR1) homology model. A structure-activity relationship based on a homology model of a recombinant enzyme was substantiated by a recombinant enzyme inhibition assay. We adapted an L. donovani (transfected with green fluorescent protein) intramacrophage amastigote screening assay as a cellular model for leishmaniasis. Furthermore, since the clinicopathological features and immunopathological mechanisms of visceral leishmaniasis (VL) in a hamster model are remarkably similar to those of human disease, systemic infection of hamsters with L. donovani was utilized to collect in vivo data for monastrol.

RESULTS: Both monastrol (R) and (S) enantiomers fit well in the ligand-binding pocket of LdPTR1. Monastrol exhibits a K(i) value of 0.428 microM in the recombinant enzyme inhibition assay. We confirm monastrol as a potent inhibitor of PTR1 in Leishmania; it inhibits proliferation of amastigotes with an IC(50) (50% inhibitory concentration) of 10 microM in macrophage cultures infected with an L. donovani clinical isolate, with no host cytotoxicity. We also show that in experimental animals, oral administration of a 5 mg/kg dose of monastrol on two alternate days inhibits 50% of parasite growth, giving therapeutic backing to the use of monastrol as a potent antileishmanial in human VL cases.

CONCLUSIONS: To our knowledge, this is the first report presenting monastrol as a potent oral antileishmanial.