Variance in IC50 in in vitro / ex vivo antimalarial drug susceptibility assays in laboratories across Africa

Surveillance of antimalarial drug efficacy is essential for drug policy in the fight against malaria. This includes clinical trials of drug efficacy, molecular marker typing and ex vivo/in vitro drug susceptibility tests such as 50% inhibitory concentration (IC50) assay. The goal of this review was to elaborate on the variance in the IC50 assay across labs in sub-Saharan Africa (sSA) where malaria is endemic. We systematically reviewed 71 articles, published between 2015 and 2025, evaluating IC50 in sSA where only 56 were performed in labs in Western (WA), Central (CA), East (EA) and Southern Africa (SA). The IC50 values of the major antimalarial drugs, including dihydroartemisinin (DHA), lumefantrine (LUM), mefloquine (MFQ), amodiaquine (AMD), chloroquine (CQ), piperaquine (PPQ), artesunate (ATS), artemisinin (ART) and quinine (QN), were reported. An F-test performed on the IC50 values from WA and EA, where ex vivo assays were conducted, revealed a statistically significant variation (P value <0.05) in the IC50 values of some major antimalarials (DHA, CQ, LUM and AMD). The geometric means for DHA, CQ, LUM and AMD in WA versus EA were 2.46 versus 2.21, 83.15 versus 34.34, 10.06 versus 10.47, and 10.63 versus 17.61 nM, respectively. The overall ex vivo IC50 values of CQ were generally below the known resistance thresholds. Differences in drug reconstitution solvents, the range of drug concentrations, period of drug exposure (48 or 72 h), and curve-fitting tools and assay methods were observed. These differences together could account for the variance in IC50 values across sSA. We therefore recommend regional harmonization of the IC50 protocol for antimalarial drug efficacy surveillance and formation of a regional quality assessment network of malaria IC50 labs, as a step towards reliable data sharing and integration into strategic plans for malaria elimination.