Synthesis and antimalarial evaluation of a quinoline-trioxane hybrid drug
Wamakima, Hannah Nyakio
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The global malaria situation is being exacerbated by emergence of drug resistance to conventional antimalarials, necessitating search for novel drugs. A recent rational approach of antimalarial drug design characterized as „„covalent bitherapy‟‟ involves linking two molecules with individual intrinsic activity into a single agent, thus packaging dual-activity into a single hybrid molecule. This study proposed to synthesize a dual drug based on quinoline and trioxane pharmacophoric scaffold of quinolines and artemisinins, respectively. The synthetic design involved introduction of a linker to 4,7-dichloroquinoline and subsequent coupling with artesunate to form the dual drug. The reactions were monitored by TLC and the compounds purified by recrystallisation and silica gel column. The yield for the dual drug was between trace and 71% depending on the coupling agent and solvent used. A combination of 1Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC - coupling agent), 1-Hydroxy-benzotriazole (HOBt - additive) and dichloromethane (solvent) gave the highest yield of 71%. The chemical structures of the synthesized compounds were confirmed by 13C, and 1H-NMR and GC-MS studies. The dual drug was tested in vitro against chloroquine (CQ)-susceptible (CQS, D6) andresistant (CQR, W2) Plasmodium falciparum strains. The 50% inhibitory concentration (IC50) values revealed that the drug was as active against both strains as artesunate (p>0.05) irrespective of the strain. The dual drug was more potent than CQ against both strains, with IC50 of 6.89 ηg ml-1 for the CQ-sensitive (CQ; 8.02 ηg ml-1) and 3.62 ηg ml-1 for the CQ-resistant (CQ; 65.35 ηg ml-1). The dual drug‟s activity was also superior to that of 1:1 mixture of artesunate and 4,7dichloroquinoline, its precursors. The in vivo antiplasmodial activity of the dual drug was studied against CQ-sensitive P. berghei ANKA-infected mice. The drug produced significant dose-dependent activity against the parasite in the suppressive studies, with an ED50 and ED90 of 5.5 and 13.5 mg kg-1, respectively. The drug was also active against lumefantrine (LU)-sensitive and -resistant parasites with a suppressive effect of 98% (LU; 84%) and 67% (LU; 0%), respectively. The fact that the drug was equally active (p>0.05) against both CQS and CQR strains of P. falciparum, and in vivo against both LUS and LUR strains of P. berghei point to its potential as a suitable replacement of ALU (Coartem®) in case of pervasive resistance against the quinoline partner (LU) in ACT. The Dual drug showed a good safety profile, since at 10 000 ηg ml-1 (the highest concentration used) the drug showed low inhibition of Hep2 cell proliferation (29.3%), and IC50 values >10 000 ηg ml-1. The promising activity of the hybrid drug against drug-resistant malaria parasites as well as its good safety profile validate the concept of „„covalent bitherapy‟‟ as a feasible and novel strategy in antimalarial drug development. The ease of synthesis of the dual drug should also be considered an additional advantage since it would make it affordable and without perturbations of drug supply, if the drug is shown to be efficacious in clinical studies.