Identifying a next-generation antimalarial trioxolane in a landscape of artemisinin partial resistance

For over two decades, artemisinin-based combination therapy (ACT) has been the standard of care for the treatment of uncomplicated falciparum malaria. However, artemisinin partial resistance (ART-R) is now prevalent in Southeast Asia and has emerged in eastern Africa, threatening ACT efficacy. Mechanistically, ART-R results from an endocytosis defect that limits concentrations of host-derived free heme in the parasite digestive vacuole, allowing early ring-stage parasites to survive exposure to the artemisinin component of ACT. The artemisinin-inspired 1,2,4-trioxolane artefenomel exhibits an extended pharmacokinetic exposure profile that predicts efficacy against ART-R parasites. Unfortunately, the development of artefenomel was halted recently after almost a decade of clinical trials. Herein, we describe the discovery of RLA-4735 and its single-enantiomer form RLA-5764, next-generation antimalarial trioxolanes that exhibit excellent in vitro potency against Plasmodium falciparum and single-exposure efficacy in a murine P. berghei model, thus retaining many of the favorable pharmacokinetic and pharmacodynamic properties of artefenomel while markedly improving solubility and development potential. In P. falciparum samples collected from patients in Uganda in 2019 and 2023, ex vivo ring-stage survival assays revealed the emergence of the ART-R phenotype over this timeframe, and furthermore demonstrated markedly superior activity of artefenomel and RLA-4735 as compared to dihydroartemisinin (the active metabolite of artemisinin components of ACTs) against ART-R parasites. Overall, our findings suggest a role for next-generation trioxolanes in addressing ART-R, and present a potent new, artefenomel-adjacent chemotype with good potential to deliver new development candidates.