Mutants of Toxoplasma gondii Resistant to Atovaquone (566C80) or Decoquinate

Abstract

Mutants of Toxoplasma gondii resistant to drugs that appear to affect the mitochondrial bc1 complex were isolated with the aid of mutagenesis with ethylnitrosourea. Mutant DeqR-1 was > 1,000-fold more resistant to decoquinate than was the wild type but more sensitive to atovaquone (formerly called 566C80). Mutant AtoR-1 was 20-fold more resistant to atovaquone than was the wild type and was also partially cross-resistant to decoquinate. Both drugs rapidly inhibited oxygen uptake by freshly prepared extracellular parasites, and the mutants were resistant to this inhibition. Neither the addition of uracil to the medium nor the use of a mutant of T. gondii with a defect in pyrimidine salvage had a substantial effect on the in vitro anti-parasitic activity of these drugs, suggesting that de novo pyrimidine synthesis was not the major biochemical target of either drug.

In Vitro Susceptibility of Various Genotypic Strains of Toxoplasma gondii to Pyrimethamine, Sulfadiazine, and Atovaquone

“…Other authors arbitrarily used an eightfold increase in the inhibitory concentration compared to that of sensitive strains (15). For SDZ and atovaquone, no criterion has been defined, but resistant mutants obtained by mutagenesis have IC 50 s that are 20-to 300-fold higher than that of the wild type (6,38,39).…”

“…Up to now, very few data document the two latter hypotheses. Drug-resistant mutants have been obtained in vitro by mutagenesis under drug pressure for sulfonamides (38), pyrimethamine (41,42), and atovaquone (32,39). However, the demonstration of a mutation responsible for substantial resistance to sulfonamides was demonstrated for only one clinical isolate (6).…”

Atovaquone Nanosuspensions Show Excellent Therapeutic Effect in a New Murine Model of Reactivated Toxoplasmosis

“…However, to date, insufficient passage through the blood-brain barrier (BBB) and/or insufficient bioavailability of these drugs has limited their in vivo use. The hydroxynaphthoquinone atovaquone is a potent inhibitor of the respiratory chain of parasites (17,38) and is used for patients with Pneumocystis carinii pneumonia (46). It has potent in vitro activity against both the tachyzoite and cyst forms of T. gondii (2,24).…”

Respiration and Oxidative Phosphorylation in the Apicomplexan Parasite Toxoplasma gondii

“…It has also been suggested that the plastid-like organelles that occur in apicomplexan parasites (13)(14)(15), which have received considerable attention recently (16,17), may contain components of a respiratory chain (18), which could contribute to the respiration of the cell (12). Although the importance of the respiratory chain in T. gondii is unknown, it is thought that it is the target of the anti-Toxoplasma activity of the naphthoquinone atovaquone (19,20), which is currently used against toxoplasmosis in AIDS patients (19).…”

“…Given the lack of information on mitochondrial physiology in T. gondii and the apparent relevance of mitochondrial activity for its differentiation (7,8) and for the chemotherapy of toxoplasmosis (19,20), it is extremely important to develop strategies to study the mitochondrial bioenergetics of these cells. In this work we report that the use of tachyzoites permeabilized with digitonin, a procedure previously established to investigate in situ mitochondrial bioenergetics in trypanosomatids (25)(26)(27)(28), allowed for the first time the functional characterization of the respiratory chain of an apicomplexan parasite.…”