Synthesis and toxicity of monothiooxalamides against human red blood cells, brine shrimp (Artemia salina), and fruit fly (Drosophila melanogaster)

A new family of monothiooxalamides derived from 2-aminobenzimidazole was synthesized, and their structures were confirmed by ¹H and ¹³C one-dimensional and 2D NMR experiments (COSY, HSQC, and HMBC). The antioxidant capacity was evaluated by free radical scavenging assays: 1,1-diphenyl-2-picrylhydrazyl (DPPH•), 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS•⁺), ferric reducing antioxidant power (FRAP), oxygen radical absorbance capacity (ORAC), and the Fe(II) chelating ability. Our work group has previously reported the synthesis and antioxidant activity of monothiooxalamides derived from 2-aminopyridine (<b>I</b>). In this study, the <i>in vitro</i> hemolytic activity of compounds from the 2-aminopyridine (<b>I</b>) and 2-aminobenzimidazole (<b>II</b>) families was evaluated against human red blood cells (RBCs). The concentration at which monothiooxalamides showed no hemolytic activity was chosen to assess their ability to inhibit free radical-induced membrane damage in human RBCs, acute toxicity in brine shrimp, and <i>in vivo</i> toxicity against <i>Drosophila melanogaster</i>. Compounds with morpholine fragments (<b>1g</b>, <b>1h</b>, <b>2g</b>, and <b>2h</b>) showed time- and concentration-dependent protective effects against radical-induced oxidative hemolysis. Moreover, they had the lowest acute toxicity in the brine shrimp lethality assay and a significant increase in chelating activity compared with the other molecules. In particular, monothiooxalamide <b>2g</b> showed lower toxicity and can be considered for further biological screening and application trials.