Drug repositioning offers an effective alternative to de novo drug design to tackle the urgent need for novel antimalarial treatments. The antiamoebic compound emetine dihydrochloride has been identified as a potent in vitro inhibitor of the multidrug-resistant strain K1 of Plasmodium falciparum (50% inhibitory concentration [IC 50 ], 47 nM Ϯ 2.1 nM [mean Ϯ standard deviation]). Dehydroemetine, a synthetic analogue of emetine dihydrochloride, has been reported to have lesscardiotoxic effects than emetine. The structures of two diastereomers of dehydroemetine were modeled on the published emetine binding site on the cryo-electron microscopy (cryo-EM) structure with PDB code 3J7A (P. falciparum 80S ribosome in complex with emetine), and it was found that (Ϫ)-R,S-dehydroemetine mimicked the bound pose of emetine more closely than did (Ϫ)-S,S-dehydroisoemetine. (Ϫ)-R,Sdehydroemetine (IC 50 71.03 Ϯ 6.1 nM) was also found to be highly potent against the multidrug-resistant K1 strain of P. falciparum compared with (Ϫ)-S,S-dehydroisoemetine (IC 50 , 2.07 Ϯ 0.26 M), which loses its potency due to the change of configuration at C-1=. In addition to its effect on the asexual erythrocytic stages of P. falciparum, the compound exhibited gametocidal properties with no cross-resistance against any of the multidrug-resistant strains tested. Drug interaction studies showed (Ϫ)-R,S-dehydroemetine to have synergistic antimalarial activity with atovaquone and proguanil. Emetine dihydrochloride and (Ϫ)-R,S-dehydroemetine failed to show any inhibition of the hERG potassium channel and displayed activity affecting the mitochondrial membrane potential, indicating a possible multimodal mechanism of action.

26Drug repositioning offers an effective alternative to de novo drug design to 27 tackle the urgent need for novel anti-malarial treatments. The anti-amoebic compound, 28 emetine dihydrochloride, has been identified as a potent in-vitro inhibitor of the multi-29 drug resistant strain K1 of Plasmodium falciparum (IC 50 : 47 nM + 2.1 nM). 2,3-30 dehydroemetine, a synthetic analogue of emetine dihydrochloride has been claimed 31 to have less cardiotoxic effects than emetine. The structures of two diastereoisomers 32 of 2,3-dehydroemetine were modelled on the reported emetine binding site on cryo-33 EM structure 3J7A and it was found that (-)-R,S-dehydroemetine mimicked the bound 34 pose of emetine more closely than (-)-S,S-dehydroisoemetine. (-)-R,S- 35 dehydroemetine was also found to be highly potent against the multi-drug resistant K1 36 strain of P. falciparum in comparison with (-)-S,S-dehydroisoemetine, which loses its 37 potency due to the change of configuration at C-1´. In addition to its effect on the 38 asexual erythrocytic stages of P. falciparum, the compounds exhibited gametocidal 39 properties with no cross-resistance against any of the multi-drug resistant strains 40 tested. Drug interaction studies showed (-)-R,S-dehydroemetine to have synergistic 41 antimalarial activity with atovaquone and proguanil. Emetine dihydrochloride, and (-)-42 R,S-dehydroemetine failed to show any inhibition of the hERG potassium channel and 43 displayed atovoquone-like activity on the mitochondrial membrane potential. 44 45 46 47 3 48Author Summary: 49 Malaria is one of the oldest diseases on earth and has taken many lives. With 50 nearly half of the world's population at risk, the spread of drug resistance and 51 insecticide resistance poses a big threat to malaria control measures. With resistance 52 emerging to the artemisinin combination therapy (ACT), the world is in dire need of a 53 new antimalarial. Due to long time-lines for de novo drug discovery, repositioning of 54 drugs provides a viable alternative way. Here, we followed up on the previous 55 repositioning work done in our lab and tested a known anti-amoebic compound for 56 repositioned use in malaria. We found, the synthetic analogue of emetine, 57 dehydroemetine is highly potent against the K1 multi-drug resistant strain and shows 58 gametocidal activity with no cross-resistance. Our study provides a rationale for further 59 optimisation of dehydroemetine for use in complicated cases of malaria. 60 61 62 63 64 65 66 67 68 4 69 Introduction: 70 Malaria presents a huge burden on the economic development of endemic 71 countries (1). In 2016, there were 216 million malaria cases reported globally, with an 72 estimated 445,000 deaths occurring mostly amongst african children (57). The 20 th 73 century witnessed the development of a range of antimalarials including quinine 74 alternatives such as mepacrine, chloroquine and primaquine, antifolates such as 75 sulphadoxine and pyrimethamine, and artemisinin (2) (3) (4) (5) (6) (7). However, the 76 emergence ...