Mefloquine in combination with hemin causes severe damage to adult Schistosoma japonicum in vitro

In order to explore the interaction of mefloquine with hemin against adult Schistosoma japonicum in vitro, the 50% and 95% lethal concentration (LC50 and LC95) of mefloquine and hemin against schistosomes, some factors, such as other iron providing agents, iron chelaters, zinc protoporphyrin-IX, and biological relevant reductants, that might impact on antischistosomal activity induced by interaction of mefloquine with hemin, and preliminary analysis of chemical interaction of both compounds were undertaken. The LC50 and LC95 of mefloquine and hemin alone against schistosomes were determined to be 6.5 μg/ml and 7.8 μg/ml as well as 232 μg/ml and 355 μg/ml, respectively. The LC50 and LC95 of mefloquine in the presence of hemin 100 μg/ml was 0.24 μg/ml and 0.59 μg/ml, respectively. On the other hand the LC50 and LC95 of hemin in the presence of mefloquine 1 μg/ml was 23.2 μg/ml and 77.2 μg/ml, respectively. Meanwhile, mefloquine/hemin combinations showed potential synergistic effects against adult S. japonicum, with combination index (CI) values <1. Apart from hemin, zinc protoporphyrin-IX, and other iron providing agents such as ferrous sulfate and ferriammonium sulfate combined with mefloquine exhibited no toxic effect against schistosomes. On the other hand, addition of iron chelators (deferiprone, desferrioxamine mesylate, or 2,2′-bipyridine) to the medium containing mefloquine-hemin resulted in no protective effect on the worms. Furthermore, biological reductants like glutathione, vitamine C or cysteine showed no apparent worm protection effect from toxic mefloquine–hemin even at higher concentrations (242.3–614.6 μg/ml, i.e., 6.4–17.8-fold higher than the concentration of hemin). Chemical interaction of mefloquine with hemin was studied in 40% DMSO–Tris buffer solution. Both UV–Vis spectrum and mass spectrum demonstrated the strong interaction of mefloquine with hemin, which resulted in a reduction of hemin color and emergence of an adduct formed by mefloquine and hemin. The results confirm that mefloquine combined with hemin exhibits potential synergistic effect against adult S. japonicum in vitro.