In 2011, approximately 40.3 million people in the United States (roughly one eighth of the population) received an outpatient prescription for the macrolide azithromycin, according to IMS Health. During that year, we at the Food and Drug Administration (FDA) reviewed the labels of azithromycin and other approved macrolide antibacterials in view of cardiovascular risks that had become evident from published studies and reports emerging through postmarketing surveillance. On the basis of its review, the FDA approved revisions to azithromycin product labels regarding risks of QT-interval prolongation and the associated ventricular arrhythmia torsades de pointes. The revised labels advise against using azithromycin in patients with known risk factors such as QT-interval prolongation, hypokalemia, hypomagnesemia, bradycardia, or use of certain antiarrhythmic agents, including class IA (e.g., quinidine and procainamide) and class III (e.g., dofetilide, amiodarone, and sotalol) — drugs that can prolong the QT interval. In March 2013, the FDA announced that azithromycin labels had been further revised to reflect the results of a clinical study showing that azithromycin can prolong the corrected QT interval.
In a 2012 observational study involving Tennessee Medicaid patients, Ray et al.1 quantified the risk of death from cardiovascular causes associated with azithromycin as compared with other antibacterial drugs or nonuse. The study showed that the risks of death, both from any cause and from cardiovascular causes, associated with azithromycin were greater than those associated with amoxicillin. For every 21,000 outpatient prescriptions written for azithromycin, one cardiovascular death occurred in excess of those observed with the same number of amoxicillin prescriptions. The excess risk over amoxicillin varied considerably according to cardiovascular risk factors; the researchers estimated that there was one excess cardiovascular death per 4100 prescriptions among patients at high cardiovascular risk but less than one per 100,000 among patients with lower cardiovascular risk.
The study by Ray et al. has limitations that are intrinsic to observational, nonrandomized clinical studies. In particular, nonrandomized studies cannot exclude the possibility that patients receiving a drug under evaluation differ from control patients in some important but undetected way, causing bias in the results. Such confounding may bias comparisons not only between patients receiving antibacterial drugs and those receiving no antibacterials but also between patients receiving different antibacterials. Although Ray et al. used appropriate analytic methods to address potential confounding, we cannot know for certain whether these methods were fully successful. Replication of the authors’ results, through analysis of a distinct data set, would provide more confidence in the finding of increased cardiovascular mortality among patients receiving azithromycin.
Despite such caveats, the results presented by Ray et al. warrant serious attention. A chief strength of the results is the time-limited pattern of the risk: the azithromycin-associated increase in the rates of death from any cause and from cardiovascular causes spanned days 1 through 5, reflecting the typical 5-day duration of azithromycin administration (e.g., Zithromax Z-Pak). On days 6 through 10, an elevated risk of death from cardiovascular causes was no longer detected. This pattern is consistent with the timing of peak plasma azithromycin concentrations and the concomitant risk of QT-interval prolongation. The elevated risk was statistically significant, regardless of whether azithromycin treatment was compared with amoxicillin or with nonuse of an antibacterial drug. Furthermore, the observed excess mortality was attributable solely to cardiovascular deaths and, in particular, to sudden cardiac death; although sudden cardiac death can result from causes other than arrhythmias, an increase in deaths in this category would be the pattern expected from an arrhythmogenic, QT-interval–prolonging drug. Also, the azithromycin-associated risk was higher among patients with cardiovascular disorders, which is consistent with a drug-related arrhythmia.
A new study by Svanström and colleagues (pages 1704–1712), using Danish national health care data, found no difference between azithromycin and penicillin V in the 5-day risk of cardiovascular death (relative risk, 0.93; 95% confidence interval [CI], 0.56 to 1.55). However, the upper bound of the 95% confidence interval does not exclude an increased risk of as much as 55%. As Svanström et al. point out, the population they studied differed from that studied by Ray et al. with respect to the baseline risk of death and cardiovascular risk factors. Overall, the Danish patients had better cardiovascular health than the Tennessee Medicaid patients. In a subgroup analysis of patients with a history of cardiovascular disease, the risk ratio for azithromycin versus penicillin V was greater than 1, though the difference was not statistically significant (relative risk, 1.35; 95% CI, 0.69 to 2.64). Svanström et al. conclude that their results do not conflict with those of Ray et al. Rather, the effect on cardiovascular mortality may be limited to patients with cardiovascular disease.
One must, of course, weigh any observed drug-associated risk against clinical benefits, so it’s appropriate to consider the possibility that certain offsetting benefits of azithromycin may not have been reflected in the risk data analyzed by Ray et al. For example, other studies have suggested that macrolides have an advantage over other antibacterial agents in terms of overall survival from community-acquired pneumonia. In a recent Canadian observational study, researchers followed 2973 outpatients with community-acquired pneumonia and found significantly lower 30-day mortality among patients receiving macrolides than among those receiving fluoroquinolones (adjusted odds ratio, 0.28; 95% CI, 0.09 to 0.86).2 A recent meta-analysis of observational studies showed a statistically significant 25% difference in mortality among hospitalized patients with community-acquired pneumonia favoring macrolides over nonmacrolide antibacterials.3 Such findings, which must be considered with due regard for the limits of observational studies, do not necessarily contradict the results of Ray et al. Past the 5-day period of risk of azithromycin-associated cardiovascular death, the drug might reduce the longer-term (e.g., more-than-30-day) rate of death due to pneumonia. Pneumonia was an uncommon indication among the Tennessee Medicaid patients treated with azithromycin.
Clinicians must consider the arrhythmogenic potential not only of azithromycin but also of potential alternative antibacterial drugs. An earlier study showed an association between the use of erythromycin and sudden cardiac death, augmented by concomitant use of inhibitors of the cytochrome P-450 3A isozymes that metabolize erythromycin.4 Labels for erythromycin and clarithromycin include warnings regarding QT-interval prolongation and arrhythmias. All labels for fluoroquinolone products similarly have warnings regarding QT-interval prolongation, and grepafloxacin was withdrawn from the market because of that risk. A recent observational study of elderly residents of Quebec, Canada, showed an association between outpatient fluoroquinolone use and serious arrhythmias (as defined by hospital discharge diagnoses of ventricular arrhythmia or sudden or unattended death).5 And although Ray et al. found the risk of cardiovascular death to be greater with azithromycin than with ciprofloxacin, they found the risk with levofloxacin similar to that with azithromycin. The authors interpreted this similarity as evidence that levofloxacin may be proarrhythmic; however, levofloxacin was not implicated as proarrhythmic in the Canadian study.
We investigated the most common ambulatory indications for azithromycin by analyzing data from a survey conducted by Encuity Research of approximately 3200 office-based physicians for the decade from 2002 through 2011. Across all age groups of patients, the two most common indications for azithromycin were chronic sinusitis and bronchitis.
Azithromycin was the leading antibacterial drug for outpatient treatment of bronchitis during this period (even if amoxicillin is combined with amoxicillin–clavulanate). For chronic sinusitis, azithromycin ranked second after amoxicillin. Because the indications are reported by the prescribing physicians, these data don’t allow us to assess the diagnostic certainty regarding the infections being treated.
The risks and benefits of antibacterial therapy should be considered in prescribing decisions. Pharmacologic and epidemiologic data point to lethal arrhythmias as a potential consequence of QT-interval prolongation with use of azithromycin, other macrolides, and fluoroquinolones. This possibility should give clinicians pause when they’re considering prescribing antibacterial drugs, especially for patients with preexisting cardiovascular risk factors or clinical conditions in which antibacterial drug therapy has limited benefits.