Tolerance of Plasmodium falciparum mefloquine-resistant clinical isolates to mefloquine-piperaquine with implications for triple artemisinin-based combination therapies

Triple artemisinin-based combination therapies (TACTs) have been proposed to delay the emergence of multidrug-resistant Plasmodium falciparum by combining two partner drugs with an artemisinin derivative. Among these, mefloquine–piperaquine (MQ–PPQ) is a leading candidate, based on the assumption that simultaneous resistance to both partner drugs would be difficult to develop. 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, whereas PPQ-resistant and -sensitive strains remain susceptible. Under repeated MQ–PPQ pressure for four months, 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, calling into question the strategic rationale of this combination and underscoring the need for alternative regimens with a lower risk of resistance selection.