Author(s): Shweta Sinha1 , B. D. Radotra2 , Bikash Medhi3 , Daniela I. Batovska 4 , Nadezhda Markova4 and Rakesh Sehgal1
Malaria caused by Plasmodium falciparum species is the most noxious as they can infect all RBCs irrespective of their ages. The species is also most prevalent in the WHO African Region, leading to almost 99.7% malaria cases in 2018 . Although there is a significant reduction in the number of malaria-infected cases and deaths over years of successful efforts through the malaria elimination programme, at the same time with the persistence evidence of growing P. falciparum resistance to artemisinin has led to a global threat [2-5]. Therefore, it is crucial to discover some fresh antimalarial drug entities which have the quality of effectiveness as well as efficiency towards malaria treatment  and can counter the rapidity of the drug resistance phenomenon of the parasite.
Chalcones (1,3-diaryl-2-propen-1-ones), a plant secondary metabolites is well known for its diverse pharmacological activity [7-9], including antimalarial activity . It can also offer a huge repository of bioactive compounds with enormous molecular targets . Earlier our group has reported three potent chalcone derivatives 1, 2 and 3 with antimalarial activity, screened from a series of recently synthesized chalcone derivatives under in vitro conditions on both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium . The study revealed even minor structural changes can increase the activity of a particular pharmacophore. Additionally, these derivatives act on the haeme degradation pathway of the malaria parasite, i.e., in a similar way as chloroquine does . However, to better understand the effect and the mechanism by which these derivatives act on P. falciparum , an ultrastructure study was performed using the in vitro culture system.
Materials and methods
The CQ-sensitive (MRC-2) and CQ-resistant (RKL-9) P. falciparum strains were maintained aseptically in continuous culture as mentioned earlier . The P. falciparum culture was nurtured in A+ erythrocytes at 5% haematocrit. The complete culture medium consisted of RPMI 1640 (with glutamine, but without NaHCO 3 ), constituted of 5.94 g/L of HEPES buffer, 1.00 g/L dextrose, 40.00 mg/L of gentamycin. In addition, supplemented with 5% NaHCO 3 and 10% (v/v) inactivated human AB+ serum. Parasite cultures were kept at 37 [degrees]C with 90% N2 , 5% O 2 and 5% CO2 , and the culture medium was changed at the interval of 22-24 h. Parasites were synchronized to the ring stage using 5% d-sorbitol as described previously . Parasite growth and multiplication were checked by thin smear Giemsa stained slides using a light microscope.
Drugs and drug exposure
The three chalcone derivatives 1, 2 and 3 were synthesized as described previously by our group . Chloroquine phosphate was obtained from Sigma Aldrich and artemisinin from IPCA. Stock solutions of three chalcone derivatives, chloroquine phosphate and artemisinin were separately prepared by dissolving each one of them in the diluted concentration of DMSO (1%) to attain a concentration of 1.00 mg/mL.
Before each experiment, cultures were expanded in sterile cell culture plates, maintaining less than 5%...