In Vitro Activities of Position 2 Substitution-Bearing 6-Nitro- and 6-Amino-Benzothiazoles and Their Corresponding Anthranilic Acid Derivatives against Leishmania infantum and Trichomonas vaginalis

6-Nitro- and 6-amino-benzothiazoles bearing different chains in position 2 and their corresponding anthranilic acid derivatives were investigated for their in vitro antiparasitic properties against parasites of the species Leishmania infantum and Trichomonas vaginalis compared to their toxicity towards human monocytes. Biological investigations established that the antiprotozoal properties depended greatly on the chemical structure of the position 2 substitution-bearing group. Compound C1, 2-[(2-chloro-benzothiazol-6-yl) amino] benzoic acid, demonstrated an interesting antiproliferative activity towards parasites of the species T. vaginalis, while compound C11, 2-({2-[(2-hydroxyethyl) amino]-benzothiazol-6-yl} amino) benzoic acid, exhibited a promising activity against parasites of the species L. infantum in their intracellular amastigote form. Additional experiments established that compound C11, which was poorly toxic against the promastigote and the extracellular amastigote forms of the parasite, could improve host-protective mechanisms against Leishmania by preventing parasite internalization by macrophages and stimulating NO production, by means of a mechanism synergistically enhanced by the presence of gamma interferon.