Reversal of Chloroquine Resistance in Plasmodium falciparum in Gabon: A Phenotype-Genotype Relationship over the Last 20 Years.

The emergence of chloroquine-resistant Plasmodium falciparum in the 1950s posed a global threat to malaria control. Ceasing chloroquine use restored chloroquine-sensitive strains in many African countries. To assess whether chloroquine sensitivity re-emerged in Lambaréné, Gabon, we compiled published and new data on P. falciparum ex vivo chloroquine susceptibility, quantified as half-maximal inhibitory concentration (IC50), from four time points between 2004 and 2024. We then assessed the prevalence of pfcrt and pfmdr1 polymorphisms associated with chloroquine resistance via real-time PCR and Sanger sequencing, respectively, at six different time points between 2009 and 2024. Ex vivo chloroquine susceptibility data revealed a stepwise decrease in the median chloroquine IC50 from 2004 (113.8 nM; IQR: 79.6-163.5 nM, n = 42), 2009 (46.7 nM; IQR: 27.4-76.9 nM, n = 26), and 2017-2018 (15.6 nM; IQR: 6.1-40.9 nM, n = 46) to 2024 (2.1 nM; IQR: 0.8-6.1 nM, n = 39). The chloroquine-sensitive pfcrt haplotype CVMNK increased from 3.6% (1/28) in 2009 to 98.2% (56/57) in 2024, as well as the wild-type pfmdr1 N86 (23.1% (6/16), 2009; 100% (19/19), 2024). Paired molecular and ex vivo analyses revealed an association between IC50 values and CVMNK pfcrt and pfmdr1 N86. Our data provide evidence for the reestablishment of chloroquine-sensitive P. falciparum in Lambaréné, Gabon.