Differential ex vivo susceptibility of Plasmodium malariae and Plasmodium falciparum clinical isolates from Ghana and Mali to current and lead discovery candidate antimalarial drugs

Abstract

Non-falciparum species causing malaria in humans are considered neglected in the fight toward malaria elimination. Recent data highlight the increasing contribution of Plasmodium malariae to malaria morbidity and mortality. In this study, the susceptibility of P. malariae and Plasmodium falciparum to current antimalarial drugs was compared to advanced lead candidate drugs using field isolates. The blood samples were collected from the Central region of Ghana and Faladje and Kati in Mali. Following this, an ex vivo drug efficacy assay was conducted by screening mono-infected isolates against a panel of antimalarials. In Ghana, the susceptibility of the two species to most of the current antimalarial drugs was comparable, except for artemether, sulfadoxine, and atovaquone, for which the drugs were less potent against P. malariae than P. falciparum (7.12 vs 2.15 nM, 25.72 vs 7.86 nM, and 10.38 vs 2.51 nM, respectively). In Mali, quinine was significantly more potent against P. malariae than P. falciparum (18.35 and 26.84 nM), and tafenoquine was less potent against P. malariae than P. falciparum (5.50 and 2.85 nM). Among the candidate drugs, except INE963, whose inhibitory potency was comparable between both species, the other compounds significantly inhibited P. malariae more than P. falciparum. The data showed that current drugs investigated against the isolates from Ghana may be suitable for curing P. malariae infections. However, in Mali, chloroquine resistance appeared to have affected the suitability of quinine-based compounds for non-falciparum malaria treatment. Therefore, additional studies are required to establish the efficacy of artemether-lumefantrine for the treatment of P. malariae infections.

Importance: One major hurdle to research in the community is our inability to have continuous culture for parasites such as Plasmodium malariae and Plasmodium ovale. These two are common in the West African region and co-occur with Plasmodium falciparum in driving both clinical or asymptomatic infections as either mono-infections or mixed infections. This manuscript is a buildup of our efforts at using ex vivo methods to study the susceptibility of P. malariae and P. falciparum to conventional and lead compounds, comparing the isolates from Ghana and Mali. This is necessary to facilitate drug discovery efforts in combating malaria holistically. The community will greatly see this work as a step in the right direction, stimulating more research into these other parasites causing malaria.

Keywords: Plasmodium; antimalarial agents; malaria; non-falciparum.

Fig 1

Scheme of the study. The figure shows the processes that the samples are taken through from their collection to conducting the drug assay and representation of the data.

Fig 2

P. malariae and P. falciparum comparative susceptibility to current antimalarials from isolates in Ghana. The comparative susceptibility (50% inhibitory concentration [IC₅₀, nM]) of the P. malariae and P. falciparum Ghanaian clinical isolates to conventional antimalarial drugs (A) chloroquine, (B) quinine, (C) artemether, (D) lumefantrine, (E) sulfadoxine, (F) pyrimethamine, (G) atovaquone, and (H) tafenoquine. The number of samples n = 11 for P. malariae and n = 10 for P. falciparum. P values have been indicated between the two parasite species (red line = significant difference, violet = not significant difference).

Fig 3

P. malariae and P. falciparum comparative susceptibility to current antimalarials from isolates in Mali. The drug compounds activity (IC50 (nM)) to the conventional antimalarial drugs (A) chloroquine, (B) quinine, (C) artemether, (D) lumefantrine, (E) sulfadoxine, (F) pyrimethamine, (G) atovaquone, (H) tafenoquine, and (I) piperaquine against P. malariae and P. falciparum clinical isolates from Mali. The number of samples n=11 for P. malariae and 10 for P. falciparum. P-values have been indicated between the two parasite species (red line= significant difference, violet= not significant difference).

Fig 4

Susceptibility P. malariae and P. falciparum to candidate antimalarials drug from isolates in Mali. The different lead compounds were tested for their activity against P. malariae (n=12) and P. falciparum (n=11) with the IC50 values (nM) plotted against the parasite species for (A) INE963, (B) MMV1579167, (C) MMV1581373, and (D) MMV1793609. The p-values between P. malariae and P. falciparum isolates have been indicated above the line (red line= significant difference, blue= not significant difference).