Restyling an old scaffold: Ebsulfur analogs with improved activity and selectivity against the infective stage of trypanosomes

Abstract

Several species of trypanosomatids cause fatal and disabling diseases in humans and livestock animals. The current chemotherapy is limited and new drug candidates with improved efficacy and safety are needed. The benziso-thiazolone (e.g. Ebsulfur, EbS) and -selenazolone (e.g. Ebselen, EbSe) have been extensively investigated for their promising action towards transmissible and non-transmissible diseases. Here, we synthetized 23 benzisothiazolones and tested their anti-trypanosomatid activity against the clinically relevant stages of three major trypanosomatid species (Trypanosoma brucei brucei, Trypanosoma cruzi and Leishmania infantum). Several compounds presented nM or low μM activity and, at least a two-digit selectivity against Trypanosoma sp. but most proved inactive towards L. infantum. Structure-activity relationship analysis reveals that the chemotype of the top hits consisted of phenyl and benzyl rings occupying the N² position of the benzisothiazolone scaffold and harboring polar substituents in the para position. Most compounds from these two clusters induced a rapid redox unbalance in the intracellular pool of low molecular weight thiols. None of the hits, but EbSe and EbS, affected Trypanothione synthetase activity (the enzyme producing the major low molecular weight thiol of trypanosomatids). However, at large enzyme:compound ratios, some inhibited irreversibly (and covalently) Trypanothione reductase (the enzyme maintaining trypanothione in a reduced state). Some hits exerted a minor effect on the rate of glucose consumption. Preliminary assessment of therapeutic efficacy in a murine infection model of acute African trypanosomiasis, the top candidate could not reduce parasite burden (monitored by in vivo imaging) but extended animal survival.

Keywords: Ebsulfur; Redox biosensor; Thiol-redox homeostasis; Trypanosomatid parasites; Trypanothione reductase; Trypanothione synthetase.