Solithromycin Can Specifically Induce Macrolide–Lincosamide–Streptogramin B Resistance

Introduction

Ketolide antibiotics are derivatives of 14-membered-ring macrolides and are characterized by the substitution of a ketone group for the cladinose moiety. In contrast to macrolides, ketolides are thought to be incapable of inducing macrolide–lincosamide–streptogramin B (MLS_B) resistance mediated by erm genes.^1,2 However, the marketed ketolides telithromycin and cethromycin were shown to induce expression of the erm(B) gene, ³ the predominant MLS_B resistance gene in streptococci. Moreover, expression of erm(C), a common MLS_B resistance gene in staphylococci, was reported to be induced by telithromycin, cethromycin, and some experimental fluoroketolides in Escherichia coli.^4,5

Solithromycin was the first fluoroketolide to enter clinical development, and has a fluorine at the C2 position of the 14-membered macrolactone ring that increases binding to 23S rRNA.^6,7 Consequently, solithromycin has enhanced activity against pneumonia-causing bacteria, including penicillin-, macrolide-, and even telithromycin-resistant strains.^6,7 Solithromycin is also thought not to trigger inducible MLS_B resistance due to its lack of cladinose, like other ketolides. ⁶ Despite the importance of solithromycin for development of new antibiotics that do not induce MLS_B resistance, few studies have evaluated the potential of solithromycin to induce expression of genes related to MLS_B resistance. In this study, a lacZ reporter assay, double-disk test, and determination of the minimal inhibitory concentration (MIC) after incubation in the presence of a subinhibitory concentration of various antibiotics were used to examine the ability of solithromycin to induce expression of erm(A), erm(C), and erm(B), the three major erm genes involved in MLS_B resistance.

Materials and Methods

Results

Expression of erm(A) or erm(C) was not induced by solithromycin or by the 16-membered-ring macrolide rokitamycin, the lincosamide clindamycin, or the ketolides telithromycin and cethromycin (Fig. 1). Only erythromycin could induce erm(A) or erm(C) expression (Fig. 1). In contrast, erm(B) gene expression was induced by all of the MLS_B and ketolide antibiotics tested, including solithromycin, which exhibited significantly higher inducing activity than cethromycin and clindamycin, but less than that for erythromycin, rokitamycin, or telithromycin. This result is consistent with an earlier study showing that erm(B) expression is induced by all MLS_B antibiotics and the ketolides telithromycin and cethromycin. ³ In the double-disk test, no induction potential for solithromycin was seen for any of the S. aureus isolates with either erm(A) or erm(C), but induction was seen for all S. pneumoniae isolates with erm(B) (Table 1). In accordance with the lacZ reporter assay and double-disk test results, none of the ketolide antibiotics tested, including solithromycin, altered the susceptibility of the S. aureus isolates with either erm(A) or erm(C) after incubation in the presence of a subinhibitory concentration (0.01 μg/mL) of ketolides (Table 2). In contrast, for the erm(B)-harboring S. pneumoniae isolate, pregrowth in a subinhibitory concentration (0.01 μg/mL) of ketolides did increase the MICs of MLS_B and the ketolide antibiotics tested. In particular, solithromycin induced a two- to eightfold increase in the MICs of the MLS_B and ketolide antibiotics.

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FIG. 1.

Expression of erm(A)-lacZ, erm(C)-lacZ, and erm(B)-lacZ fusions after incubation with various concentrations of erythromycin (●), rokitamycin (○), clindamycin (■), telithromycin (□), cethromycin (♦), and solithromycin (◊) for 120 minutes. The experiment was performed in triplicate and mean values are plotted.

Discussion

In a previous study, the ketolide antibiotic telithromycin induced no erm(A) or erm(C) expression in S. aureus or S. pyogenes strains, but did induce erm(B) in S. pyogenes by disk diffusion. ¹⁴ Furthermore, this study found that upon pregrowth in the presence of subinhibitory concentrations of telithromycin, the MICs of MLS_B against the S. aureus strain harboring inducible erm(C) were unchanged, whereas MICs against the S. pyogenes strain harboring inducible erm(B) increased. ¹⁴ This study showed that the ketolide antibiotics solithromycin and cethromycin both exhibited the same induction specificity as telithromycin.

Meanwhile, other earlier studies reported that ketolides induced expression of erm(C) in E. coli,^4,5 which is in contrast to results of this study. This discrepancy could be due to the difference between ribosomes of Gram-positive bacteria and Gram-negative E. coli. For example, the uridine at position 746 in domain II (hairpin 35) of 23S rRNA, a telithromycin binding site, is modified to pseudouridine in E. coli.^16,17 The potential role of this modification in erm induction requires further investigation to determine whether it is in fact involved in the action of ketolide antibiotics.

Taken together, the data for this study demonstrate that solithromycin, like telithromycin and cethromycin, does not induce expression of erm(A) and erm(C), but does induce erm(B) expression. These findings suggest that the substitution of a ketone for cladinose, a key feature of ketolides, is sufficient to abolish erm(A) and erm(C) induction. Ketolides could not completely suppress induction of erm(B) expression, but induced lower expression levels than those seen for macrolides. Additional modifications to the ketolide structure will be needed to minimize induction of erm(B) expression.