Endochin-like quinolones are highly efficacious against acute and latent experimental toxoplasmosis

Abstract

Toxoplasma gondii is a widely distributed protozoan pathogen that causes devastating ocular and central nervous system disease. We show that the endochin-like quinolone (ELQ) class of compounds contains extremely potent inhibitors of T. gondii growth in vitro and is effective against acute and latent toxoplasmosis in mice. We screened 50 ELQs against T. gondii and selected two lead compounds, ELQ-271 and ELQ-316, for evaluation. ELQ-271 and ELQ-316, have in vitro IC(50) values of 0.1 nM and 0.007 nM, respectively. ELQ-271 and ELQ-316 have ED(50) values of 0.14 mg/kg and 0.08 mg/kg when administered orally to mice with acute toxoplasmosis. Moreover, ELQ-271 and ELQ-316 are highly active against the cyst form of T. gondii in mice at low doses, reducing cyst burden by 76-88% after 16 d of treatment. To investigate the ELQ mechanism of action against T. gondii, we demonstrate that endochin and ELQ-271 inhibit cytochrome c reduction by the T. gondii cytochrome bc(1) complex at 8 nM and 31 nM, respectively. We also show that ELQ-271 inhibits the Saccharomyces cerevisiae cytochrome bc(1) complex, and an M221Q amino acid substitution in the Q(i) site of the protein leads to >100-fold resistance. We conclude that ELQ-271 and ELQ-316 are orally bioavailable drugs that are effective against acute and latent toxoplasmosis, likely acting as inhibitors of the Q(i) site of the T. gondii cytochrome bc(1) complex.

Fig. 1.

Chemical structures of endochin, ELQ-271, and ELQ-316.

Fig. 2.

The efficacy of ELQ-271, ELQ-316, and atovaquone against acute T. gondii infection in mice. The T. gondii burden reflects the percentage of T. gondii infection. Treatment groups consisted of the control (n = 7), 0.04 mg/kg (n = 4), 0.2 mg/kg (n = 4; one value from the atovaquone 0.2 mg/kg group was excluded from analysis as an outlier because it was excessively high), 1 mg/kg (n = 8), 5 mg/kg (n = 8), 20 mg/kg (n = 4), and 50 mg/kg (n = 4) of each drug. ED₅₀ and ED₉₀ values for each compound are listed in Table 1. Error bars represent SEM. ATQ, atovaquone.

Fig. 3.

The efficacy of ELQ-271, ELQ-316, and atovaquone against latent T. gondii infection. The number of T. gondii cysts per brain was reduced by atovaquone, ELQ-271, and ELQ-316 compared with the control. Five weeks after inoculation with the T. gondii ME49 strain, mice were treated with each drug for 16 d. Brain cysts were counted 2 wk after the final day of drug administration. The number of cysts per brain was significantly lower for each ELQ group than for the atovaquone group (P < 0.0001). Error bars represent SEM. ATQ, atovaquone.

Fig. 4.

Comparison of growth inhibition by disk diffusion of ELQ-271 against S. cerevisiae strain AD 1-9 with and without the M221Q point mutation. ELQ-271 concentrations are listed below the disks.