Tolerance of Plasmodium falciparum mefloquine-resistant clinical isolates to mefloquine-piperaquine: implications for triple artemisinin-based combination therapy strategies.

Triple artemisinin-based combination therapies (TACTs) have been proposed to delay the emergence of multidrug-resistant Plasmodium falciparum by combining two partner drugs with artemisinin derivatives. Among these, mefloquine–piperaquine (MQ–PPQ) is a leading candidate, based on the assumption that resistance to both partner drugs would be difficult to develop simultaneously. Here, we assess the efficacy and resistance potential of MQ–PPQ using Cambodian clinical isolates with distinct resistance profiles. We find that MQ resistance confers significant cross-tolerance to the MQ–PPQ combination, while PPQ-resistant and sensitive strains remain susceptible. Under repeated MQ–PPQ pressures, parasites rapidly acquire MQ-PPQ tolerance, driven by pfmdr1 amplification. Mechanistic investigations reveal that MQ inhibits PPQ accumulation in a dose-dependent manner, providing a functional explanation for the compromised efficacy of the combination. These findings demonstrate that MQ resistance alone can undermine MQ–PPQ TACT efficacy, which question the strategic foundation of this combination and underscore the need for alternative combinations with lower resistance selection risk.