4(1H)-Quinolones as new antimycobacterial drug leads

This proposal outlines a research project whose purpose is to synthesise a new antibacterial subclass of quinolone alkaloids possessing diverse sets of alkyl, alkenyl, alkynoic acid and alkanolyl substituents at C-2 position of the N-alkyl-4(1H)-quinolone and investigate their antimycobacterial and cytotoxic activities in vitro. Accumulated experience has revealed that natural products are rich sources for lead structures in drug discovery and development. In our previous investigations N-methly-2-alkyl-4(1H)quinolones, isolated from the fruit of the Chinese medicinal plant Euodia rutaecarpa (Juss.) Benth. (Rutaceae) (syn. = Evodia rutaecarpa, Tetradium ruticarpum), have been shown to possess highly interesting antimycobacterial and anti-inflammatory activities in vitro. Our in vitro antimycobacterial assay enabled us to select evocarpine as a hit compound which is now the base for further investigations. Tested against rapidly growing strains of mycobacteria in vitro including Mycobacterium fortuitum, which has been used as a testing model for M. tuberculosis, evocarpine displayed superior activity compared to the standard antimycobacterial drugs ethambutol and isoniazid. Our studies have shown that the alkenyl side chain at the C-2 position plays a crucial role in the antimycobacterial properties of N- methyl-4(1H)-quinolone alkaloids, which pressed us to synthesize a series of evocarpine analogues with special emphasize on the lipophilic side chain at C-2 so as to further optimize the antimycobacterial activity by structural variation and subsequent testing. Several methods for the synthesis of N-alkyl-2-alkyl-4(1H)-quinolone derivatives are outlined in this proposal. Although a series of fluoroquinolones derivatives containing a carboxyl group at C-3 have been synthesized and commercially available for the treatment of many types of infectious diseases, including tuberculosis, no attempt has been made to assess the potential of C-2 substituted N-alkyl-4(1H)-quinolones as antimycobacterial agents. Therefore, N-alkyl-4(1H)-quinolones having an aliphatic side chain at position 2 constitute a new subclass of compounds, which hold promise as antimycobacterial agents. The discovery of a potential new class of antituberculosis drug is exciting because no new drugs have been put into first-line usage against TB since rifampin in the 1960s. Since some species of the genus Euodia and genera of the family Rutaceae are known to contain N-methyl-4(1H)- quinolones, this project will also focus on the search for antimycobacterial quinolone alkaloids in taxonomically related species of the family Rutaceae. Selected plant species will be evaluated for their antimycobacterial constituents using a bioassay-guided isolation strategy and a microdilution assay with fast-growing strains of mycobacteria to identify further hit compounds. The project will be carried out by a consortium of phytochemists, pharmaceutical chemists and microbiologists.

This project achieved the characterization of a new subclass of antimycobacterial compounds starting from plant natural products. In the search for new antibacterials plants offer a largely unexplored source of new chemical entities. Accumulated experience has revealed that natural products are rich sources for lead structures in drug discovery and development. In our previous investigations certain plant compounds, quinolone alkaloids like evocarpine, isolated from the fruits of the Chinese medicinal plant Euodia rutaecarpa (synonym = Evodia rutaecarpa) have been shown to possess highly interesting antimycobacterial and anti-inflammatory activities. In course of this research project synthetic analogues of this new antibacterial subclass of quinolone alkaloids were prepared and investigated for their antimycobacterial and cytotoxic activities in vitro. Mycobacteria such as Mycobacterium tuberculosis, the causing agent of tuberculosis, are a major health threat and show increasing resistance to antibiotics. This urges the search for new chemical tools to combat mycobacterial diseases. The project was carried out by a consortium of phytochemists, medicinal chemists and microbiologists from the University of Graz, Austria, and the University of London, UK. We were able to synthesize a high number of variants of the hit compound evocarpine with special emphasize on the lipophilic side chain at C-2 so as to further optimize the antimycobacterial activity by structural variation and subsequent testing. As a result, chemical structures with optimised chain length and up to four-fold improved antibacterial activity could be found. The most important achievement during this project was the discovery of a possible mechanism of action of this class of compounds. Selected quinolone alkaloids which could inhibit the growth of different strains of mycobacteria in vitro, including M. tuberculosis, proved to be inhibitors of an enzyme which is essential in the cell wall biosynthesis, i.e. the ATP dependent MurE ligase. This enzyme was just recently characterized for M. tuberculosis and is regarded as a new target for antitubercular drugs. Therefore, N-alkyl-4(1H)-quinolones having an aliphatic side chain at position 2 constituted a new subclass of compounds, which hold promise as antimycobacterial agents. In addition, these compounds showed significant growth inhibition of methicillin resistant strains of Staphylococcus aureus (MRSA), this gives way to explore the potential of the quinolone alkaloids for treatment of nosocomial infections.