Structure-activity relationship studies of antiplasmodial cyclometallated ruthenium(II), rhodium(III) and iridium(III) complexes of 2-phenylbenzimidazoles

Benzimidazoles, such as albendazole, thiabendazole and omeprazole have antiplasmodial activity against Plasmodium falciparum and are widely used as scaffolds for metal-based drug research. Incorporating substituents with various lipophilic and electronic properties can influence trans-membrane interactions and concomitantly improve the biological activity. To study structure-activity relationships, a series of 2-phenylbenzimidazoles and their corresponding Ru(II), Ir(III) and Rh(III) cyclometallated complexes were synthesised and evaluated for antiplasmodial activity against the chloroquine-sensitive (NF54) strain of the human malaria parasite Plasmodium falciparum. Selected metal complexes were further screened against the multidrug-resistant (K1) strain. In general, the 2-phenylbenzimidazole ligands showed weak antiplasmodial activities (IC50 ∼ 17.66–22.32 μM) while an enhancement of antiplasmodial activity was observed upon coordination of the ligands with either ruthenium, iridium or rhodium. The new cyclometallated complexes were found to be active against both parasite strains, with IC50 values in the low to submicromolar range (0.12–5.17 μM). In addition, the metal complexes have relatively low cytotoxicity against mammalian Chinese Hamster Ovarian (CHO) cells.