ArticlesEffect of azithromycin and clarithromycin therapy on pharyngeal carriage of macrolide-resistant streptococci in healthy volunteers: a randomised, double-blind, placebo-controlled study
Introduction
Resistance to antibiotics is a major public-health problem.1 Many ecological studies have shown a clear relation between antimicrobial use and resistance.2 However, these studies are commonly confounded by a number of variables, and they show, at best, an association, not a causal effect. Moreover, they do not link antibiotic exposure in an individual to the outcome for that individual, which creates the so-called ecological fallacy.3 Randomised clinical trials and, to some extent, observational studies that examine antibiotic-exposed versus non-exposed individuals are crucial to study definitively the link between antibiotic use and resistance as well as to provide in-vivo biological samples to study the molecular basis of resistance.4, 5, 6 Previously, such evidence was provided solely by animal experimentation.7, 8
Two macrolides—clarithromycin and azithromycin—are among the drugs of choice for the treatment of respiratory tract infections. Study of the link between antibiotic use and resistance—as well as the molecular mechanisms of resistance—is especially important because resistance to macrolides in common respiratory pathogens (eg, Streptococcus pneumoniae and Streptococcus pyogenes) is increasing,9, 10 and is most likely due to their inappropriate use.2
There is much debate as to which of these macrolides has greater potential for selecting resistant organisms,11 one of the decisive factors for eventual preference for clinical use. Azithromycin has a long half-life,12 and therefore a convenient dose regimen (once daily for 3 days, compared with twice daily for 7 days for clarithromycin). Theoretically, however, shorter drug exposure decreases the chance of the development of resistance, whereas higher tissue persistence and slowly receding azithromycin concentrations increase the chance of development of drug-resistant organisms. Some ecological studies have identified a strong relation between azithromycin use and macrolide resistance,13 whereas others did not find a correlation.14
The few studies that have compared the effect of azithromycin and clarithromycin on the selection of resistance have also shown conflicting results.15, 16, 17 Kastner and Guggenbichler16 showed that a significantly higher proportion of paediatric patients carried resistant organisms for 6 weeks after azithromycin treatment than did those treated with clarithromycin. King and co-workers17 were unable to show any difference between the two macrolides because of a lack of power, whereas Matute and colleagues,15 in a randomised double-blind study on 18 individuals, showed that neither clarithromycin nor azithromycin use selected for any resistant organisms in either the faecal or the oropharyngeal flora.
Resistance to macrolides in streptococci occurs via two main mechanisms. The first is active drug efflux mediated by a pump encoded by the mef (macrolide efflux) gene that confers low to moderate resistance against macrolides, with minimum inhibitory concentrations (MIC) to erythromycin (a prototype macrolide) ranging from 0·5 μg/mL to 32 μg/mL. In the second mechanism, a methylase encoded by the erm(B) gene modifies the macrolide binding site on the bacterial ribosome, generally conferring a high degree of resistance, with erythromycin MIC typically ranging from 32 μg/mL to more than 512 μg/mL.18
Various studies have shown that the oral commensal streptococcal flora endemically harbours the same macrolide resistance genes seen in the genetically related pathogenic streptococci.19, 20 Thus, we used the oral commensal streptococcal flora as model organisms to study the effect of different macrolides in selecting macrolide resistance in a healthy population. We did a randomised double-blind placebo-controlled trial with azithromycin and clarithromycin to investigate the direct effect of antibiotic exposure on resistance in the oral streptococcal flora of healthy volunteers. We also aimed to investigate the molecular basis for any differences in selection for resistance.
Section snippets
Participants
We did a randomised, double-blind, placebo-controlled trial between October, 2002, and May, 2003 (ie, during winter) and between March, 2003, and October, 2003 (ie, during summer) at the University of Antwerp, Belgium. Volunteers were selected on the basis of Belgian identity cards (to exclude those younger than 18 years) and a written questionnaire that included information on their sex, age, smoking status, previous antibiotic use, and employment in hospital with contact with patients. Of the
Results
Figure 1 shows the trial profile. 224 volunteers were randomly assigned a course of clarithromycin, azithromycin, or placebo. Age, sex, mean proportion of macrolide-resistant streptococci, and other characteristics of the volunteers were much the same in all three groups at baseline (table 1).
20 volunteers did not continue with the study; of these individuals, reasons for 15 dropouts were not known, two experienced side-effects due to the antibiotics being administered, one was diagnosed with
Discussion
By use of oral streptococci as model organisms, we have shown that macrolide use is the single most important driver of the emergence of macrolide resistance in human beings. Our results also show important differences in the outcome of selection of resistance by two antibiotics in the same class: azithromycin selected quantitatively more resistant organisms in the early post-therapy phases, whereas clarithromycin qualitatively selected for the higher resistance-conferring erm(B) gene. Finally,
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