5-(2-Pyrimidinyl)-imidazo[1,2-a]pyridines are antibacterial agents targeting the ATPase domains of DNA gyrase and topoisomerase IV

Leveraging structural information for the discovery of new drugs: computational methods

Escalating problems with drug resistance continue to compromise the effectiveness of commercial antibiotics, necessitating the search for novel classes of antimicrobial agents. To circumvent problems with resistance, a multitarget single-pharmacophore approach has been employed to discover inhibitors that possess balanced activity against multiple target enzymes. In this chapter, we examine the application of computational techniques, in particular, structure-based drug design approaches, to design new dual-targeting antibacterial agents against bacterial topoisomerases.

Alkaloids from the Chinese vine Gnetum montanum

During a high-throughput screening campaign of a prefractionated natural product library, fractions from the Chinese vine Gnetum montanum showed in vitro activity against Pseudomonas aeruginosa wild-type strain, PAO1. UV-directed isolation of the organic extract from the vine leaves resulted in the purification of the new natural products N-methyllaudanosolinium trifluoroacetate (1), 3'-hydroxy-N,N-dimethylcoclaurinium trifluoroacetate (2), 1,9,10-trihydroxy-2-methoxy-6-methylaporphinium trifluoroacetate (3), and 6a,7-didehydro-1,9,10-trihydroxy-2-methoxy-6-methylaporphinium trifluoroacetate (4). Compound 4 is described here for the first time, and this is the first report of compounds 1-3 as natural products. Compounds 1-3 were found to racemize over time. Starting from commercially available (+)-boldine, through a series of semisynthetic reactions, a mechanism for the racemization of the isolated compounds is proposed. The known natural products (-)-latifolian A (5) and magnocurarine (6) were also isolated during these studies. The antibacterial activity was explained by the presence of 5, which displayed an IC50 value of 9.8 μM (MIC = 35 μM).

The challenge of developing robust drugs to overcome resistance

Drug resistance is problematic in microbial disease, viral disease and cancer. Understanding at the outset that resistance will impact the effectiveness of any new drug that is developed for these disease categories is imperative. In this Feature, we detail approaches that have been taken with selected drug targets to reduce the susceptibility of new drugs to resistance mechanisms. We will also define the concepts of robust drugs and resilient targets, and discuss how the design of robust drugs and the selection of resilient targets can lead to successful strategies for combating resistance.

Design, synthesis and in vitro antimicrobial evaluation of novel Imidazo[1,2-a]pyridine and imidazo[2,1-b][1,3]benzothiazole motifs

New antimicrobial agents, imidazo[1,2-a]pyridine and imidazo[2,1-b][1,3]benzothiazole, have been synthesized. Their antimicrobial activities were conducted against various Gram-positive, Gram-negative bacteria and fungi. Compounds 6c, 7a, 10b, 11a, 12b, 14a, 14b, 15a and 15b, exerted strong inhibition of the investigated bacterial and fungal strains compared to control antibiotics amoxicillin and cefixime and the antifungal agent fluconazole. Results from this study showed that the nature of the substituents on the armed aryl groups determines the extent of the activity of the fused imidazopyridine and/or imidazobenzothiazole derivatives. Preliminary structure-activity observations revealed that bromo-fluoro substituents enhanced the antimicrobial activity significantly compared to other substituents.

Challenges of antibacterial discovery

The discovery of novel small-molecule antibacterial drugs has been stalled for many years. The purpose of this review is to underscore and illustrate those scientific problems unique to the discovery and optimization of novel antibacterial agents that have adversely affected the output of the effort. The major challenges fall into two areas: (i) proper target selection, particularly the necessity of pursuing molecular targets that are not prone to rapid resistance development, and (ii) improvement of chemical libraries to overcome limitations of diversity, especially that which is necessary to overcome barriers to bacterial entry and proclivity to be effluxed, especially in Gram-negative organisms. Failure to address these problems has led to a great deal of misdirected effort.

Fully automated continuous flow synthesis of highly functionalized imidazo[1,2-a] heterocycles

The first continuous flow synthesis of imidazo[1,2-a]pyridine-2-carboxylic acids directly from 2-aminopyridines and bromopyruvic acid has been developed, representing a significant advance over the corresponding in-flask method. The process was applied to the multistep synthesis of imidazo[1,2-a]pyridine-2-carboxamides, including a Mur ligase inhibitor, using a two microreactor, multistep continuous flow process without isolation of intermediates.