Multiple synergistic interactions between atovaquone and antifolates against Plasmodium falciparum in vitro: a rational basis for combination therapy.

The use of synergistic drug combinations for the treatment of drug-resistant malaria is a major strategy to slow the selection and spread of Plasmodium falciparum resistant strains. In order to investigate synergistic compounds, with different modes of action, as alternative candidates for combination therapy, we used standard in vitro P. falciparum cultures and an established synergy testing method to define interactions among dapsone (DDS), atovaquone (ATQ), chlorproguanil (CPG) and its triazine metabolite chlorcycloguanil (CCG). Strong synergy was observed in the combinations DDS/CCG and ATQ/CPG. Multiple combination of these drugs, DDS/CCG/CPG/ATQ also exhibited high synergy although not higher than that of either of the two drug combinations separately. The use of this triple combination DDS/CPG/ATQ, even without an increase in synergy over their double combinations, ATQ/CPG and DDS/CCG, would contribute towards slowing the selection pressure since these drugs act against different targets and would delay the selection of parasites resistant to the three drugs, extending the useful therapeutic life of these valuable compounds.