Expanding the molecular landscape of fragments binding to trypanothione reductase, a legitimate target for drug design against human African trypanosomiasis

Crystallographic fragment screening is a powerful methodology that enables the identification of low molecular weight ligands and has shown great promises in drug discovery. In this work we report the results of a fragment screening carried out in an effort to further map the cavities of trypanothione reductase from Trypanosoma brucei (TbTR), a critical target for drug design against human African trypanosomiases (HAT), for which efficient and non-toxic trypanocidal drugs are lacking. Moreover, the conservation of trypanothione reductase among trypanosomatids, including Leishmania, could facilitate the design of a wide-spectrum drug against many parasitic diseases. At the XCHEM facility (Diamond Light Sources, United Kingdom) we performed the soaking of TbTR monoclinic crystals with fragments from DSIpoised and EubOPEN DSIp libraries and we identified eight new hits binding to different cavities of TR including the trypanothione and the NADPH binding cavities. These fragments exhibited affinities ranging from submillimolar to millimolar, as determined by surface plasmon resonance (SPR). While the newly identified fragments did not significantly alter TbTR’s enzymatic activity—consistent with the nature of low-affinity ligands—they provide valuable insights into key interactions of fragments with TR and, together with prior fragment screening campaigns, pave the way towards follow-up chemical optimization into lead compounds.