Design of 2-aminobenzothiazole derivatives targeting trypanosomatid PTR1 by a multidisciplinary fragment-based approach

Pteridine reductase 1 (PTR1) is a key folate pathway enzyme of pathogenic trypanosomatids that reduces biopterin to dihydro- and tetrahydrobiopterin. It is a promising target for drug design against diseases such as sleeping sickness or leishmaniases. Amongst known PTR1 inhibitors, 2-aminobenzothiazole derivatives that target the PTR1 biopterin pocket were previously found to show good overall toxicity profiles and some of them display promising anti-parasite activity. On the other hand, compounds containing a 3,4-dichlorophenyl moiety, such as 1-(3,4-dichlorobenzyl)-1H-benzimidazol-2-amine (I), interact in a different region of the T. brucei PTR1 (TbPTR1) binding pocket. In this work, we develop two compound series in which a 2-aminobenzothiazole moiety is connected to a 3,4-dichlorophenyl moiety via different linkers. A bespoke in-house compound library was virtually screened by computational docking against TbPTR1 and L. major PTR1 (LmPTR1). Five selected compounds were synthesized, three of which display low-micromolar activity against TbPTR1, and two of these were also found to be low-micromolar inhibitors of T. brucei. In addition, one of the latter compounds shows inhibitory activity against LmPTR1 and L. infantum, demonstrating dual-inhibition potential. Quantum chemical calculations and crystallography guided the design of further compounds whose structure-activity relationship (SAR) showed that the halogen at the meta position of the phenyl ring is energetically more favorable for anti-PTR1 activity than at the para position, but that single halogen substitutions resulted in lower anti-parasite activities. Overall, our multidisciplinary fragment-based approach resulted in compounds with experimentally validated designed binding modes, that were less toxic and more active against the T. brucei parasite than the parent compound I.