Daptomycin Is Highly Effective for Methicillin-resistant Staphylococcus Aureus (MRSA) Infections That Have a High Vancomycin MIC

A recent multicenter retrospective study showed that the antibiotic daptomycin cleared methicillin-resistant Staphylococcus aureus (MRSA) infections that are at the upper range of vancomycin susceptibility to within four days in 80% of patients.

Pamela A. Moise, PharmD, from Cubist Pharmaceuticals, which is headquartered in Lexington, MA, and several US investigators presented this study on October 5, 2013, at IDWeek 2013, a joint meeting of the Infectious Diseases Society of America (IDSA), the Society for Healthcare Epidemiology of America (SHEA), the HIV Medicine Association (HIVMA), and the Pediatric Infectious Diseases Society (PIDS), in San Francisco, CA.

MRSA is a frequent cause of serious infections in hospitalized patients. Vancomycin has been the most common antibiotic used to treat MRSA infections in the hospital, but recent research suggests that vancomycin may be less effective in serious MRSA infections with a moderate to high vancomycin minimum inhibitory concentration (MIC; an indicator of how effective the antibiotic is in inhibiting growth of a bacterial strain in the lab). Few clinical studies have investigated optimal alternative antimicrobial therapies, such as daptomycin, for these patients. The goal of the current study was to investigate the clinical outcomes of MRSA patients with a moderate vancomycin MIC (1.5 or 2.0 µg/mL) who took daptomycin for at least three days.

The investigators studied 93 patients treated for MRSA infections with daptomycin at 11 institutions. All of the MRSA infections studied were in the bloodstream, and many also involved bones or joints, were within the heart, or were on the skin. Fifty-four percent had received two or fewer days of vancomycin therapy before starting daptomycin treatment, and 17% had not received any vancomycin prior to daptomycin treatment.

A key finding, according to the researchers, was the relatively quick clearance of the MRSA bacteria with the daptomycin treatment. Seven days is the time frame commonly used to assess whether the treatment was successful, but daptomycin cleared the bacteria in four days for 80% of patients. The antibiotic was particularly successful in treating extra-vascular infections, such as within the bone or joint, or on the skin or skin structure. Ninety-one percent of these infections were cleared by the fourth day of treatment, and 98% were treated by day seven. The endo-vascular infections, such as those infecting the heart, which are often more difficult to treat, had 63% clearance by day four of treatment and 81% clearance by day seven. Eight patients still had bacteremia present seven days after the therapy.

Treatment failures, defined as death by any cause, failure to clear the bacteria after seven days of treatment, and/or discontinuation of the drug occurred in 28 patients. Although 17 patients died during the 60-day period following the start of daptomycin treatment, only three of these deaths were attributable to MRSA. Only four of the patients switched to an alternative treatment. No patients experienced an adverse event that forced them to discontinue treatment or had an infection relapse within 30 days of the end of therapy.

Moise and colleagues suggest that daptomycin treatment is an important medication for treatment of MRSA infections at the upper limit of vancomycin susceptible range, particularly in those with more severe and complicated infections. This has now been shown in a geographically diverse population from many hospitals throughout the country. According to the researchers, comparing the efficacy of daptomycin with vancomycin will be important for optimizing MRSA treatment for individual patients.

All authors listed financial disclosures.