An iterative process produces oxamniquine derivatives that kill the major species of schistosomes infecting humans

Currently there is only one method of treatment for human schistosomiasis, the drug praziquantel. Strong selective pressure has caused a serious concern for a rise in resistance to praziquantel leading to the necessity for additional pharmaceuticals, with a distinctly different mechanism of action, to be used in combination therapy with praziquantel. Previous treatment of Schistosoma mansoni included the use of oxamniquine (OXA), a prodrug that is enzymatically activated in S. mansoni but is ineffective against S. haematobium and S. japonicum. The oxamniquine activating enzyme was identified as a S. mansoni sulfotransferase (SmSULT-OR). Structural data have allowed for directed drug development in reengineering oxamniquine to be effective against S. haematobium and S. japonicum. Guided by data from X-ray crystallographic studies and Schistosoma worm killing assays on oxamniquine, our structure-based drug design approach produced a robust SAR program that tested over 300 derivatives and identified several new lead compounds with effective worm killing in vitro. Previous studies resulted in the discovery of compound CIDD-0066790, which demonstrated broad-species activity in killing of schistosome species. As these compounds are racemic mixtures, we tested and demonstrate that the R enantiomer CIDD-007229 kills S. mansoni, S. haematobium and S. japonicum better than the parent drug (CIDD-0066790). The search for derivatives that kill better than CIDD-0066790 has resulted in a derivative (CIDD- 149830) that kills 100% of S. mansoni, S. haematobium and S. japonicum adult worms within 7 days. We hypothesize that the difference in activation and thus killing by the derivatives is due to the ability of the derivative to fit in the binding pocket of each sulfotransferase (SmSULT-OR, ShSULT-OR, SjSULT-OR) and to be efficiently sulfated. The purpose of this research is to develop a second drug to be used in conjunction with praziquantel to treat the major human species of Schistosoma. Collectively, our findings show that CIDD-00149830 and CIDD-0072229 are promising novel drugs for the treatment of human schistosomiasis and strongly support further development and in vivo testing.
Author summary Schistosomiasis affects more than 229 million people in 78 countries of the world. The main treatment is Mass Drug Administration with praziquantel. With donations to the World Health Organization, approximately 250 million tablets of praziquantel are being administered in sub-Saharan Africa where about 90% of the cases of schistosomiasis occur. The concern with a monotherapy is the development of drug resistance. The need for new drugs with a different mode of action to be used in combination with praziquantel is great. In this regard, we have taken oxamniquine, a drug that was previously used to treat Schistosoma mansoni but was ineffective against S. haematobium and S. japonicum, and determined the enzyme responsible for activation of oxamniquine as a sulfotransferase. With this knowledge and the sulfotransferase crystal structure, we were able to determine the mode of action of the drug and develop an iterative approach of soaking oxamniquine derivatives into sulfotransferase crystals, determining structure function relationships, synthesizing new derivatives and testing them in an in vitro killing assay. The most effective derivatives are soaked into new crystals and the process repeated. We have identified two derivatives, CIDD-0072229 and CIDD-149830 that will kill S. haematobium and S. japonicum in addition to S. mansoni. CIDD-149830 will kill 100% of the worms of all three species within 7 days.