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.