Insights into the Mode of Action of Novel Fluoroketolides, Potent Inhibitors of Bacterial Protein Synthesis

Abstract

Ketolides, the third generation of expanded-spectrum macrolides, have in the last years become a successful weapon in the endless war against macrolide-resistant pathogens. Ketolides are semisynthetic derivatives of the naturally produced macrolide erythromycin, displaying not only improved activity against some erythromycin-resistant strains but also increased bactericidal activity as well as inhibitory effects at lower drug concentrations. In this study, we present a series of novel ketolides carrying alkyl-aryl side chains at the C-6 position of the lactone ring and, additionally, one or two fluorine atoms attached either directly to the lactone ring at the C-2 position or indirectly via the C-13 position. According to our genetic and biochemical studies, these novel ketolides occupy the known macrolide binding site at the entrance of the ribosomal tunnel and exhibit lower MIC values against wild-type or mutant strains than erythromycin. In most cases, the ketolides display activities comparable to or better than the clinically used ketolide telithromycin. Chemical protection experiments using Escherichia coli ribosomes bearing U2609C or U754A mutations in 23S rRNA suggest that the alkylaryl side chain establishes an interaction with the U2609-A752 base pair, analogous to that observed with telithromycin but unlike the interactions formed by cethromycin. These findings reemphasize the versatility of the alkyl-aryl side chains with respect to species specificity, which will be important for future design of improved antimicrobial agents.

Targeting Antibiotic Resistance

“…The aniline moiety, which replaces the detrimental pyridine is shown to form additional hydrogen bonds, notably to A752 and G748, and results in tighter binding. Finally the fluorine atom at the C2 position was shown to contribute to drug binding as well as chemical properties such as solubility and cellular uptake 73. Indeed, in comparison to non‐fluorinated analogues, the fluorinated versions showed stronger inhibition of the growth of streptococci carrying the erm gene.…”

Macrolide resistance mechanisms inEnterobacteriaceae: Focus on azithromycin

“…Additionally, other alterations have been described in Enterobacteriaceae, such as those affecting positions A745, A752, U754, G2057, A2032, A2062, A2503, U2609, C2610 or C2611, which are able to affect the activity of macrolides either by increasing the levels of resistance, or resulting in enhanced activity. Moreover, an association has been described between a specific 23S rRNA deletion (D1219-1230) and the development of erythromycin resistance (Douthwaite et al, 1985;Krokidis et al, 2014;Novotny et al, 2004;V azquez-Laslop et al, 2008V azquez-Laslop et al, , 2010V azquez-Laslop et al, , 2011Vannuffel et al, 1992, Weisblum, 1995 (Table 3). Alterations present in domain II of the 23S rRNA affect specific macrolides like tylosin and ketolide antibiotics but not erythromycin or azithromycin.…”

“…Curiously, the concomitant presence of any A752 alteration in combination with the presence of A2058G results in the loss of telithromycin resistance but with an increase in tylosin resistance (Novotny et al, 2004). Meanwhile, the U754A alteration has a slight affect on the MIC of some ketolides, such as telithromycin, but not on that of others as in the case of the new ketolide K1835 (Krokidis et al, 2014). Regarding this specific substitution, its effect on the MIC of erythromycin remains controversial, with some authors describing a slight effect on its MIC levels (Xiong et al, 1999), while others do not observe any effect (Krokidis et al, 2014).…”

The slow dissociation rate of K-1602 contributes to the enhanced inhibitory activity of this novel alkyl–aryl-bearing fluoroketolide

“…Recently, we published data concerning new ketolides carrying an alkyl-aryl group via an ÀOÀ bridge at the C6 position of the lactone ring and one or two fluoride atoms at the C3 or C13 position of the lactone ring 13 . Among these compounds, which are all strong inhibitors of protein synthesis in vivo and in vitro, K-1602 ( Figure 1) was shown to be the only compound that did not induce expression of the macrolide resistance gene erm(C) and is, therefore, effective against bacteria with macrolide-inducible resistance 13 .…”

“…Among these compounds, which are all strong inhibitors of protein synthesis in vivo and in vitro, K-1602 ( Figure 1) was shown to be the only compound that did not induce expression of the macrolide resistance gene erm(C) and is, therefore, effective against bacteria with macrolide-inducible resistance 13 . It is well known that a small number of specific short nascent peptides, such as those encoded in the regulatory cistrons of macrolide resistance genes, can induce ribosome stalling, which retains the peptidyl-tRNA, but prevents peptide bond formation with the incoming aminoacyl-tRNA 14,15 .…”