Schistosomiasis is one of the most widely spread parasitic diseases in the world, as well as is one of the main tropical endemic disease in Asia, Africa and South America . It is estimated that there are approximately 779 million individuals around the world bearing the risk of schistosomiasis infection . Schistosomiasis is caused by five species of the trematode flatworms, Schistosoma japonicum (S. japonicum), Schistosoma mansoni (S. mansoni), Schistosoma haematobium, Schistosoma mekongi and Schistosoma intercalatum. When human body is infected, the worms will persist in the liver and hepatic portal system or the urinary tract system. Mature period schistosomiasis lay eggs inside their host body, and the eggs are often trapped in the host’s tissues, leading to inflammation and obstructive disease in the infected organs . In China, schistosomiasis is mainly caused by Schistosoma japonicum and more than 77 thousand people are infected .
The most effective drug for treatment of schistosomiasis is PZQ (1), which has been extensively used since the 1980s  in clinic without any back-up drugs. The overuse of praziquantel has caused and accelerated the emergence of drug resistance. Furthermore, PZQ is very effective against the adult forms of all schistosome species, but it is weak for juvenile forms of all schistosome, which might be the reason that PZQ can not cure schistosomasis completely. Although R-PZQ was approved by the FDA as an orphan drug in 2018, it is also at risk of resistance and there still no relative drug against migratory juvenile and sub-adult worms . Therefore, it is very important and urgent to look for potential reagent in order to discover the novel alternatives to PZQ for treatment of schistosomasis.
It has been proved that instead of separate thioredoxin reductase (TrxR) and glutathionereductase (GR) enzymes of mammalian hosts, a single multifunctional selenocysteine-containing flavoenzyme designated TGR in both S. mansoni TGR (SmTGR) and S. japonicum TGR (SjTGR) play critical role in maintaining proper redox balance for parasite survival [1, 7] (Fig. 1). Analysis showed that schistosomiasis TGR is similar to mammalian TrxR and GR with amino acid sequence and domain structure distribution. The protein sequence contains the binding domain of nicotinamide adenine dinucleotide phosphate (NADPH) and flavin adenin dinucleotide (FAD), the transformation center between –SH and disulfide and the dimer interface that in contact with pyridine nucleotide disulfide oxidoreductase. A carboxyl-terminal GCUG active site motif exists in the schistosomiasis TGR that has both TR and GR functions. Moreover, schistosomiasis TGR also has an additional extension of glutaredoxin (Grx) domain, it’s made up of approximately 110 amino acids, with typical CPYC activity [7d, 8].
The study of the anti-oxidative protection mechanism of S. mansoni indicated that schistosomiasis and mammalian host have obvious biochemical differences in the characteristics of reactive oxygen species (ROS) defense. The specificity of schistosomiasis in the ROS defense causes the biochemical differences of REDOX pathway between schistosomiasis and host (mammalian). Schistosomiasis TGR is in critical position in the REDOX pathway  and is inferred to be potential target for new drug design.
In the previous study [1–2, 7] D. L. William and co-workers have proved that some oxadiazole-2-oxide analogues were effective on inhibiting SmTGR and SjTGR, even had good antischistosomal activity in vive. Modifications around oxadiazole-2-oxide skeleton have demonstrated to be the valuable strategy for discovering potential antischistosomal agents. The application of bioisosteres is an effective strategy in drug design. The previous researchers mainly substituted on the benzene ring and changed in the CN moiety when designing the compound [1–2, 7], but did not pay attention to modification of benzene ring and other moieties in the furoxan structure with corresponding bioisosteres. Therefore, in this work, the strategy of application of bioisosteres was exploited to design novel oxadiazole-2-oxide derivatives.
On the other hand, prokaryotic expression system has been used extensively for protein expression due to its rapid growth rate, capacity for continuous fermentation, and relatively low cost. Due to lack of the post-translational modifications of this expression system, the bioactivity, function, structure, solubility and so on will be affected to the expressed functional products. Although eukaryotice expression systems could resolve this problem, just like a two-edged sword, some disadvantages such as low yield, high demanding culture conditions and higher cost cannot be avoided totally . Thus, the special SWAP containing wtSjTGR extracted from the host cells might be a better alternative target for studying the bioactivity. It would reflect the actual situation in advance when Schistosoma japonicum is treated with chemical compounds. Therefore, the target compounds herein was also applied to test the inhibition activity on SWAP containing wtSjTGR besides rSjTGR-Sec.
In this work, two series of novel furoxan derivatives (7 and 9) were obtained using two ways, one by replacing pheny moiety of furoxan with its bioisosteres pyridine or substituted pyridine, and the other by modifying the cyano moiety into an ester or amine group. In addition, fluorine atom prosses strong electron-withdrawing effect and high lipophilicity, appropriate introduction of fluorine atom into molecules can improve their biofunctions . Therefore, several fluorine-containing derivatives were also intentionally synthesized. The target compounds (7 and 9) and their corresponding intermediates (6) were tested the inhibition activity on TrxR from rSjTGR-Sec and SWAP containing wtSjTGR respectively. Based on the result obtained, the SAR of these novel furoxan derivatives was preliminarily analyzed.