In Vitro Antimalarial Activity of Inhibitors of the Human GTPase Rac1

Abstract

Malaria accounts for millions of cases and thousands of deaths every year. In the absence of an effective vaccine, drugs are still the most important tool in the fight against the disease. Plasmodium parasites developed resistance to all classes of known antimalarial drugs. Thus, the search for antimalarial drugs with novel mechanisms of action is compelling. The human GTPase Rac1 plays a role in parasite invasion of the host cell in many intracellular pathogens. Also, in Plasmodium falciparum, the involvement of Rac1 during both the invasion process and parasite intracellular development was suggested. The aim of this work is to test a panel of Rac1 inhibitors as potential antimalarial drugs. Fourteen commercially available or newly synthesized inhibitors of Rac1 were tested for antimalarial activity. Among these, EHop-016 was the most effective against P. falciparum in vitro, with nanomolar 50% inhibitory concentrations (IC₅₀s) (138.8 ± 16.0 nM on the chloroquine-sensitive D10 strain and 321.5 ± 28.5 nM on the chloroquine-resistant W2 strain) and a selectivity index of 37.8. EHop-016 did not inhibit parasite invasion of red blood cells but affected parasite growth inside them. Among the tested Rac1 inhibitors, EHop-016 showed promising activity that raises attention to this class of molecules as potential antimalarials and deserves further investigation.

Keywords: Plasmodium; Rac1; antimalarial drug.

FIG 1

The most effective Rac1 inhibitors (EHop-016, Aza-1, and compound 3) on P. falciparum and comparison between the structures of compound 3 and compound 3′.

FIG 2

Effect of EHop-016 on P. falciparum invasion and intraerythrocytic growth. (A) P. falciparum invasion rates in the presence of 2.5 μM EHop-016 compared to the untreated control. (B) Levels of active Rac1 in uninfected erythrocytes treated with 2.5 μM EHop-016 compared to the untreated control. (C) Parasitemia of infected erythrocytes treated with 1.4 μM EHop-016 expressed as a percentage of the untreated control, at 20, 27, and 39 hpi. *, P < 0.02; **, P < 0.001; ***, P < 0.01 (by Student's t test). (D) Levels of active Rac1 in 1.4 μM EHop-016-treated parasites at 20 hpi, expressed as a percentage of the untreated control. *, P < 0.001 (by Student's t test).

FIG 3

Isobologram analysis of the antimalarial activity of EHop-016 in combination with 1A116. The two compounds showed additive activity. Results are the means from three independent experiments.