The emergence of artemisinin partial resistance (ART-R) in Africa threatens to reverse decades of malaria control progress. In addition, widespread resistance to sulfadoxine-pyrimethamine could jeopardise chemoprevention strategies. Gametocytes, the only Plasmodium falciparum life stage capable of infecting mosquitoes, may be present at higher densities and/or exhibit increased infectivity in resistant parasites. Despite this, the development of gametocytocidal drugs receives little attention. Artemisinin-based combination therapies (ACTs) have varying effects on gametocytes and transmission, with primaquine being the only antimalarial currently licensed for Plasmodium falciparum that is highly effective against mature gametocytes. The aim of this thesis is to investigate levels of antimalarial drug resistance, their effect on transmissibility of parasites, and the transmission-blocking activity of common antimalarial regimens with or without a gametocytocidal drug. In this thesis, samples and data from six transmission drug trials conducted between 2013 and 2023 in Ouélessébougou, Mali, were analysed and genotyped. I first establish a detailed characterisation of genome-wide variation and drug resistance profiles in asymptomatic gametocyte carriers from 2019 and 2020 and compare them to publicly available whole-genome data from older Malian (2007-2017) and African-wide P. falciparum isolates. The analysis reveals high multiclonality and low relatedness among 2019-2020 isolates, alongside increased frequencies of molecular markers for lumefantrine and sulfadoxine-pyrimethamine resistance, compared to older Malian isolates. Next, using infected mosquito midguts from the drug trials, as well as participant blood samples upon which the mosquitoes were fed, I investigate the relative transmissibility of different parasite clones and the effect of molecular markers of drug resistance on this transmissibility. The results show that parasite transmission dynamics are highly complex, even after treatment with an ACT, and certain molecular markers of drug resistance appear to confer a transmission advantage or disadvantage. Focusing on blocking transmission with antimalarials, this thesis then presents results from the sixth drug trial conducted at this study site in 2022 testing the effect of the triple artemisinin-based combination therapy artemether-lumefantrine-amodiaquine and the ACT artesunate-amodiaquine, with and without a single low-dose of primaquine, on gametocytes and transmission. We find that artemether-lumefantrine-amodiaquine blocks nearly all mosquito infections within 48 hours, but substantial post-treatment transmission occurs after artesunate-amodiaquine. Adding a single low dose of primaquine is a safe and effective addition to both artemether-lumefantrine-amodiaquine and artesunate-amodiaquine for blocking P. falciparum transmission. Lastly, a pooled analysis from individual patient data from the six trials is presented, to allow for a direct comparison of the transmission-blocking activity of 15 antimalarial regimens, including different ACTs and non-ACTs, alone or in combination with a single low-dose gametocytocide. We show marked differences in the anti-gametocyte and anti-transmission effects between ACTs, with artemetherlumefantrine being superior in blocking transmission. Moreover, the findings validate the rapid effects of a single low-dose primaquine in clearing gametocytes when used in combination with any ACT. The results from all chapters are discussed in the context of the drug resistance threat in Africa, highlighting the urgency to delay its onward transmission.
