Extending Thioflavin T Fluorescence Probe to 2-Ethenyl-benzothiazole Derivatives: Drug-like Quadruplex Ligands with Potent Antitrypanosomatid Activity

Thioflavin T (ThT) is a well-established fluorescence probe with selectivity for G-quadruplex (G4) structures. Over the past few years, G4 ligands have emerged as promising candidates for the development of antiparasitic agents. Building on this concept, we explored extending ThT's benzothiazole scaffold by introducing various 2-ethenyl aromatic and heteroaromatic moieties, aiming to enhance G4 binding affinity and potential therapeutic effect. A series of benzothiazolium derivatives were synthesized and evaluated for their antiproliferative and antiparasitic activity. Several 2-ethenyl benzothiazole derivatives showed submicromolar activity against <i>Leishmania</i> spp. and <i>Trypanosoma brucei</i> parasites, with up to 200-fold selectivity over MRC-5 human lung fibroblasts. Notably, compound <b>2b</b> demonstrated remarkable potency, with an IC₅₀ of 0.48 nM and a selectivity index of 46,151 against <i>Leishmania major</i> amastigotes, and an IC₅₀ of 0.019 nM and a selectivity index of 79,206 against <i>T. brucei</i>. In fact, compound <b>2b</b> demonstrated superior efficacy and selectivity in comparison to the clinically used drugs suramin, fexinidazole, miltefosine, and amphotericin B. Biophysical studies revealed that all tested derivatives exhibited significant G4 stabilization, surpassing ThT. Location of compound <b>2b</b> inside the nucleus and the kinetoplast, as well as partially in the mitochondria, opens up the possibility of <b>2b</b> acting against the parasite through binding to G4.