Inhibition of Plasmodium falciparum pH regulation by small molecule indole derivatives results in rapid parasite death

The V-type H+-ATPase is critical during the intraerythrocytic stage of the human malaria parasite Plasmodium falciparum. It is responsible for maintaining a near-neutral cytosolic pH (pH 7.3), an acidic digestive vacuole (pH 4.5–5.5) and the generation of an inside-negative plasma membrane potential (∼−95 mV). Inhibition of this pump is therefore likely to result in profound physiological disturbances within the parasite and parasite death, as illustrated previously by the antiplasmodial activity of the potent and specific inhibitors of the V-type H+-ATPase, bafilomycin A1 and concanamycin A. In this study we examined the antiplasmodial activity of a series of compounds previously designed, on the basis of the active structural constituents of bafilomycin A1, to inhibit the osteoclast V-type H+-ATPase. The compounds were tested against up to 4 strains of P. falciparum with varying chloroquine sensitivities. Of the 30 novel compounds tested, 9 had sub-micromolar antiplasmodial IC50 values, with the most active compound having an IC50 of 160 ± 20 nM. The activity of a number of these compounds was investigated in more detail. We show that these inhibitors acidify the parasite cytosol within seconds and that some inhibitors irreversibly kill the parasite within 0.5–4 h. The antiplasmodial activity of the V-type H+-ATPase inhibitors was strongly correlated with their ability to acidify the parasite cytosol (correlation coefficient 0.98). In combination studies, we show that the inhibitors act indifferently when combined with current antimalarials. Our data support the disruption of parasite pH regulation through inhibition of its V-type H+-ATPase as an antimalarial approach.

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