PfATP4 and Series 4

The Plasmodium falciparum P-type Na⁺-ATPase (PfATP4) transporter is an essential part of the malaria parasites membrane that allows for the regulation of intracellular ions when inside a red blood cell. A number of publications have indicated that PfATP4 is an important new target for malaria medicines. It appears that PfATP4 active compounds act by disrupting the ability of the ion transporter to export Na⁺, leading to a rapid influx of Na⁺ into the parasite which causes parasite death.

In an assay developed by the Kirk lab at the Australian National University, the 400 compounds contained within the MMV Malaria Box were screened for PfATP4 activity. It was found that 28 compounds exhibited disruption of the parasites intracellular Na⁺ concentration, consistent with PfATP4 inhibition. The number of different chemotypes (including spiroindolones, pyrazoleamides, dihydroisoquinolones and aminopyrazoles) that appear to target PfATP4 is striking (Figure 1), however, as the structure of the protein has yet to be generated pure or crystallised, it is unknown as to how these diverse compounds have the same molecular target.

Figure 1: Diverse chemotypes that have been implicated to target PfATP4.

Among the documents inherited at the beginning of Series 4 in 2013 was data on five compounds which had been evaluated in the Kirk ion regulation assay. The results showed good correlation between in vitro parasite-killing activity and disturbance of ion regulation, suggesting that PfATP4 could be the molecular target for Series 4 (Figure 2).

Figure 2: Set of five inherited compounds evaluated in the PfATP4 assay.

To probe this initial observation, a further seven compounds were evaluated in 2014, with the results showing excellent correlation between in vitro potency and ion regulation activity (Figure 3).

Figure 3: Further set of seven inherited compounds evaluated in the PfATP4 assay.