Design of 2-aminobenzothiazole derivatives targeting trypanosomatid PTR1 by a multidisciplinary fragment hybridization 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 leishmaniasis. 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 Trypanosoma 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 Leishmania major PTR1 (LmPTR1). Five selected compounds were synthesized, two of which, 1a and 1b, emerged as promising inhibitors of TbPTR1, with 1b also showing a micromolar inhibition level against LmPTR1 and moderate activity against Leishmania infantum (close to 50% at 10 µM), demonstrating its potential as a pan-parasite PTR1 inhibitor, which is rather unusual for 2-aminobenzothiazole derivatives. 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 hybridization 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.