Inhibitors of Leishmania major Inositol Phosphorylceramide Synthase: New Therapies for Leishmaniasis

Abstract

Leishmaniasis is an insect vector-borne Neglected Tropical Disease (NTD) caused by the protozoan Leishmania spp. This disease is endemic in 98 countries, claiming 30,000 lives each year and putting a further 350 million people at risk. With no vaccine currently available, treatment typically requires long, expensive courses of exposure to toxic medicines via parenteral administration. Furthermore, resistance to some antileishmanials is an emerging threat. There is an urgent need for novel treatments that are inexpensive and free of side effects.

Our group has previously identified and validated the Leishmania major inositol phosphorylceramide synthase (LmjIPCS) enzyme as an attractive drug target. This essential membrane-bound enzyme has a differential function to the mammalian orthologue (sphingomyelin synthase), potentiating the development of safe, selective antileishmanials. Using a microtiter plate compatible assay, a set of 1040 pharmacologically active compounds were screened for activity against LmjIPCS and L. major parasites. Clemastine, an over the counter medicine, was found to be effective against L. major promastigotes (ED50 = 0.47 µM) and amastigotes (ED50 = 3.06 µM). With well studied pharmacokinetic and mammalian safety profiles, clemastine was chosen as an ideal candidate for further development.

Initial efforts focussed on the synthesis of a series of chimeric analogues, coupling 2 pyrrolidinyl and 4 chlorobenzhydryl fragments via ether, triazole, amide, ester and olefin linkers. Amides presented the most attractive linker group, proving to be robust and amenable to parallel synthetic efforts. Despite being well tolerated by the LmjIPCS enzyme, the synthesised amides exhibited low in cellulo activity against L. major promastigotes.

Two series of amide linked clemastine analogues were synthesised, exploring substitution of the pyrrolidine and benzhydryl moieties. From a synthesised library of 59 compounds, the indazole N-(1′′-[2′′′-dimethylaminoethyl]indazol-6′′-yl)-2-(4′-chlorophenyl)-2-phenylacetamide exhibited LmjIPCS inhibition (IC50 = 2.41 µM) comparable to the parent molecule, clemastine (IC50 = 2.87 µM). However, this analogue proved to be ineffective against L. major promastigotes. In an attempt to restore antiparasitic action, the amide was replaced by a flexible ether linker, providing 1-(2′-[dimethylamino]ethyl)-6-([methoxy]methyl)indazole. This analogue exhibited inhibition of LmjIPCS activity, with an IC50 of 4.72 µM, but only hindered growth of L. major promastigotes above 5 µM.