Approach to nigericin derivatives and their therapeutic potential

Abstract

A new nigericin analogue that has been chemically modified was synthesized through a fluorination process from the parent nigericin, produced from a novel Streptomyces strain DASNCL-29. Fermentation strategies were designed for the optimised production of nigericin molecule and subjected for purification and structural analysis. The fermentation process resulted in the highest yield of nigericin (33% (w/w)). Initially, nigericin produced from the strain DASNCL-29 demonstrated polymorphism in its crystal structure, i.e., monoclinic and orthorhombic crystal lattices when crystallised with methanol and hexane, respectively. Furthermore, nigericin produced has been subjected to chemical modification by fluorination to enhance its efficacy. Two fluorinated analogues revealed that they possess a very potent antibacterial activity against Gram positive and Gram negative bacteria. To date, the nigericin molecule has not been reported for any reaction against Gram-negative bacteria, which are increasingly becoming resistant to antibiotics. For the first time, fluorinated analogues of nigericin have shown promising activity. In vitro cytotoxicity analysis of fluorinated analogues demonstrated tenfold lesser toxicity than the parent nigericin. This is the first type of study where the fluorinated analogues of nigericin showed very encouraging activity against Gram-negative organisms; moreover, they can be used as a candidate for treating many serious infections.

Scheme 1. Reaction scheme for synthesis of nigericin derivatives.

Fig. 1. Antibacterial activity (disc diffusion method). (A–F) Antibacterial against Gram-positive test strain (S. aureus ATCC 9144), (G–L) antibacterial against Gram-negative test strain (E. coli ATCC 8739). (M) Minimum inhibitory concentration of nigericin and its fluorinated analogs. Nigericin is represented as Nig-C, fluorinated analog 3 is represented as Nig-3, and fluorinated analog 5 is represented as Nig-5. (N) Determining the EC₅₀ values for inhibition of blood-stage malaria parasite growth. The number of replicates n = 3 for all samples. Standard nigericin was procured from Sigma Aldrich.

Fig. 2. Cytotoxicity of nigericin and its fluorinated analogues on wild-type mouse embryonic fibroblast cell line (WTMEFs). (A) Effect of molecules upon 2 hour treatment and (F) effect of molecules upon 24 hour treatment. (B) and (G) are WTMEFs untreated cells and (C–E) and (H–J) cell morphology, upon 2 h and 24 h treatment with 10 μM nigericin (DAS29), fluorinated analogue 3 (NIG-3) and fluorinated analogue 5 (NIG-5) respectively.