In vivo antimalarial activities of mono- and bis quaternary ammonium salts interfering with Plasmodium phospholipid metabolism

Abstract

We previously showed that quaternary ammonium salts have potent antimalarial activities against the blood stage of drug-resistant Plasmodium falciparum. In the present study, 13 compounds of this series were comparatively assessed in murine in vivo malarial models. Mice infected with Plasmodium berghei were successfully treated with 11 quaternary ammonium salts in a 4-day suppressive test with a once-daily intraperitoneal administration. The dose required to decrease parasitemia by 50% (ED(50)) ranged from 0.04 to 4.5 mg/kg of body weight. For six mono- and three bis-quaternary ammonium salts, the therapeutic indices (i.e., 50% lethal dose and ED(50)) were higher than 5, and at best, around 20 to 30 for five of them (E6, E8, F4, G5, and G25), which is comparable to that of chloroquine under the same conditions. Plasmodium chabaudi was significantly more susceptible to G5, G15, and G25 compounds than P. berghei. Similar therapeutic indices were obtained, regardless of the administration mode or initial parasitemia (up to 11.2%). Parasitemia clearance was complete without recrudescence. Subcutaneously administered radioactive compounds had a short elimination half-life in mice (3.5 h) with low bioavailability (17.3%), which was likely due to the permanent cationic charge of the molecule. The high in vivo therapeutic index in the P. chabaudi-infected mouse model and the absence of recrudescence highlight the enormous potential of these quaternary ammonium salts for clinical malarial treatment.

FIG. 1.

In vivo G25 antimalarial activity against three different rodent strains. Mice were i.p. inoculated with P. chabaudi (▪) or P. berghei strain N (•) or NS (○), as described in Materials and Methods. Animals were treated twice daily for 4 consecutive days; the first drug injection started 1 h after parasite inoculation. G25 was administered i.p. Parasitemia was determined on the day following the last treatment, and the ED₅₀ was estimated from a plot of log dose against activity (expressed as a percentage of the control). The results are expressed as means ± standard errors of the means (n ≥ 3).

FIG. 2.

In vivo G25 antimalarial activity against P. berghei as a function of the route of drug administration to mice. Mice were infected i.v. with 10⁷ P. berghei (strain NS)-infected erythrocytes, and G25 was administered twice daily for 4 days either i.p. (•), s.c. (▪), or orally (▴). The first drug administration began 24 h after parasite inoculation.

FIG. 3.

Curative G25 (0.9 mg/kg) antimalarial activity against P. chabaudi-infected mice as a function of parasitemia and the treatment window. Mice were inoculated i.v. with 10⁷ P. chabaudi-infected erythrocytes at time zero. Animals were then s.c. treated with G25 either for 8 (filled symbols) or 4 (open symbols) consecutive days in comparison with untreated controls (x). The treatment began either on the day of infection (parasitemia = 0.05%) (A), on day 2 (parasitemia = 3.5%) (B), or on day 4 (parasitemia = 11.2%) (C). The parasitemia level was determined daily between time zero and time zero plus 11 days. The points at 100% parasitemia corresponded to the death of control mice. Black arrows correspond to the beginning (upward direction) and end (downward direction) of the 8-day treatment period; white arrows correspond to the end of the 4-day treatment period.

FIG. 4.

Plasma concentration-time profile of tritiated F4 following i.v. (⧫) and s.c. (◊) administration to uninfected mice. Uninfected Swiss mice (male, 34 g) were injected either i.v. (⧫) with 100 μl of [³H]F4 (0.26 μCi/nmol) dissolved in saline or s.c. (◊) (27 μCi/mmol). The single injection contained the radioactive compound at a dose of 0.75 mg/kg (i.v.) or 60 mg/kg (s.c.). The figure shows the results for one typical experiment.