Design, Synthesis And Antimalarial Activity Of Dihydroartemisinin Derivatives

Malaria is one of the most prevalent parasitic diseases and especially Plasmodium falciparum malaria remains a threat to human health. To against the growing resistance to antimalarial agents, the recommended combination therapies with rational use of existing drugs are effective and the development of antimalarial drugs against new targets is an urgent priority.Among the promising new targets, P. falciparum cysteine protease, falcipain-2, which plays a pivotal role for the parasite survival as a major peptide hydrolase within the hemoglobin degradation pathway, is one of the most attractive targets for antimalarial drug design. Falcipain-2 inhibitors can block parasite protein biosynthesis by preventing host hemoglobin hydrolysis. Therefore, it is significant to discover new falcipain-2 inhibitors with acceptable properties.The new endoperoxide sesquiterpene lactone artemisinin isolated from Artemisia annua L. and its derivatives as a new generation antimalarial drugs have been used successfully in the treatment of multidrug-resistant P. falciparum strains. However, since artemisinin derivatives are being widely used, to lower the risk of drug resistance, development of antimalarial drugs with new mechanism is important.Based on three-dimensional computer model of P. falciparum cysteine protease, the (thio)semicarbazone moiety has shown potential activity against falcipain-2. According to the SAR of artemisinin and its derivatives, endoperoxide is critical for the activity and the C-10 carbonyl group is not the key structure for the activity. In this work, a total of 89 novel dihydroartemisinin derivatives (A, B, C and D series) were designed and synthesized by incorporating above two biologically active scaffolds, dihydroartemisinin and (thio)semicarbazone. A, C are thiosemicarbazones and B, D are semicarboznes; A, B are ether linker in compounds and C, D are ester linker in compounds. The target compounds were synthesized from dihydroartemisinin and different (thio)semicarbazides and their structures were confirmed by MS, IR,1H-NMR and 13C-NMR. All the target compounds were identified as single isomers conformed by 1H-NMR. The configuration at C-10 was assigned based on the coupling constant between H-9 and H-10 that indicated A, B wereβ-isomers and the C, D were a-isomers.The in vitro inhibition assay against P. falciparum falcipain-2 was carried out. Activity of the 53 (A, B and D series) tested compounds show that all the tested compounds were found to be inhibitors of falcipain-2 with inhibition rates greater than 20% at 10μM concentration level and IC50 values ranging from 0.22 to 10.63μM. According to the result of in vitro inhibition assay, the 12 selected compounds were screened antimalarial activity in animal model. The result showed that when the single daily dose in mice was 50 mg/kg and 75 mg/kg respectively, only LY-A03 and LY-A05 exhibited the inhibition of parasite in mice with the reduction rates greater than 90%; while the single daily dose in mice was reduced to 3 mg/kg,9 mg/kg,15 mg/kg and 27 mg/kg respectively, the in vivo inhibition activity was not obvious with reduction rates less than 90%.The preliminary SARs showed that the ester linker in compounds (D series) exhibited more effective than ether linker in compounds (A and B series) with the same substitution on aromatic ring and the latter displayed equivalent efficacy between semicarbazones and thiosemicarbazones. Furthermore, in the same kind of scaffolds, the introduction of fluoro, chloro, nitro, methoxyl or ethoxyl group at C-4 position as mono substitution on aromatic ring could improve the activity and among those disubstituted compounds, the ones with methyl group disubstitution at C-2 and C-4, or C-3 and C-5 positions on the aromatic ring presented better activities. These implications would benefit a drug design in future research.Molecular docking studies were performed to investigate the binding affinities and interaction modes for the inhibitors using AutoDock 3.05. As illustrated in model, several residues of falcipain-2 could form a pocket for ligand binding and the most amino group on the side chain of the target compound could form hydrogen bonds with residues as donators.