Concentration of 2'C-methyladenosine triphosphate by Leishmania guyanensis enables specific inhibition of Leishmania RNA virus 1 by its RNA polymerase.

Leishmania is a widespread trypanosomatid protozoan parasite causing significant morbidity and mortality in humans. The endobiont dsRNA virus Leishmania RNA virus 1 (LRV1) chronically infects some strains, where it increases parasite numbers and virulence in murine leishmaniasis models, and correlates with increased treatment failure in human disease. Previously, we reported that 2'-C-methyladenosine (2CMA) potently inhibited LRV1 in Leishmania guyanensis (Lgy) and L. braziliensis, leading to viral eradication at concentrations above 10 µM.  Here we probed the cellular mechanisms of 2CMA inhibition, involving metabolism, accumulation and inhibition of the viral RNA dependent RNA polymerase (RDRP) .  Activation to 2CMA triphosphate (2CMA-TP) was required, as 2CMA showed no inhibition of RDRP activity from virions purified on cesium chloride gradients.  In contrast, 2CMA-TP showed IC50s ranging from 150 to 910 µM, depending on the CsCl density of the virion (empty, ssRNA- and dsRNA-containing). Lgy parasites incubated in vitro with 10 µM 2CMA accumulated 2CMA-TP to 410 µM, greater than the most sensitive RDRP IC50 measured.  Quantitative modeling showed good agreement between the degree of LRV1 RDRP inhibition and LRV1 levels.  These results establish that 2CMA activity is due to its conversion to 2CMA-TP, which accumulates to levels that inhibit RDRP and cause LRV1 loss. This attests to the impact of the Leishmania purine uptake and metabolism pathways, which allow even a weak RDRP inhibitor to effectively eradicate LRV1 at micromolar concentrations. Future RDRP inhibitors with increased potency may have potential therapeutic applications for ameliorating the increased Leishmania pathogenicity conferred by LRV1.