Stability and toxicity of heteroleptic organometallic Bi(V) complexes towards Leishmania major

Eleven heteroleptic Bi(V) complexes of the form [BiPh3(O2CR)2] have been synthesised and fully characterised. The carboxylate ligands are derived from a series of simple substituted benzoic acids, four of which are common non-steroidal anti-inflammatories (NSAIDs). The solid-state structures of eight of the complexes were determined by single crystal X-ray diffraction, and all were shown to adopt a typical trigonal bipyramidal geometry with chelating carboxylate ligands. Nine of the complexes were assessed for their anti-parasitic activity against Leishmania major promastigotes and their cytotoxicity towards human fibroblasts. The assays indicated that while the complexes showed good anti-leishmanial activity with IC50 values ranging from 0.6 to 2.5 μM they were also non-selectively toxic towards the fibroblasts at similar or slightly higher concentrations. Using (1)H NMR, the stability of one of the complexes, [Bi(C6H5)3(O2CC6H3(m-OH)2)2] was studied in water, DMSO and in the DMEM culture medium. This showed that while the Bi(v) complex was stable in D2O and DMSO, the complex slowly decomposed in the culture medium undergoing reduction to give BiPh3 and the free acid. Since the acids and BiPh3 were not toxic to either the parasites or fibroblasts at the concentrations studied, the implication is that the Bi(v) complexes are stable enough for long enough to have significant in vitro anti-parasitic activity.