Genotype Complexity Modulates Antimalarial Susceptibility in vitro

Increased tolerance and resistance to antimalarial drugs is one of the major threats responsible for the stagnation in the fight against malaria. The role of Plasmodium falciparum strain complexity to the overall efficacy of antimalarial drugs in naturally occurring infections is not fully understood.

This study investigated the impact of mixed infections of resistant and susceptible strains to antimalarials.

Fifteen Gambian field isolates and two laboratory strains (Dd2 and 3D7) were used for the complexity assays. IC₅₀ and malaria drug survival assay (mDSA) were used to determine the survival and erythrocyte reinvasion rates of parasites following sub-therapeutic exposure to conventional antimalarials (CQ - 200 nM; PPQ - 200 nM, DHA - 24 nM, and LUM - 200 nM). The complexity of infection (COI) in the field isolates was confirmed by McCOIL tool, and the lab strains co-cultured at different proportions (100:0, 75:25, 50:50, 25:75, and 0:100) were exposed to CQ, DHA, and LUM.

Our findings show that the resistant haplotype in a mixed infection drives the drug response through a significant (p < 0.05) shift in the IC₅₀ of drugs. This shift in IC₅₀ was most distinct for chloroquine using mixed infections of 3D7/Dd2. There was high reinvasion after subtherapeutic exposure to conventional antimalarial drugs in the field isolates. Isolates with resistant haplotypes against chloroquine, even in mixtures with susceptible strains, survived more.

Responses under drug pressure are strongly driven by the resistant haplotypes circulating alongside the wild types of Plasmodium falciparum. There is a need for continuous surveillance.