Vancomycin: A 50-something-year-old antibiotic we still don’t understand

‘MIC creep’: Is it real?

Also worrisome, the average vancomycin MIC for S aureus has been shifting upward, based on reports from several institutions, although it is still within the susceptible range.^19,20 However, this “MIC creep” likely reflects, at least in part, differences in MIC testing and varying methods used to analyze the data.^19,20

Holmes and Jorgensen,²¹ in a single-institution study of MRSA isolates recovered from bacteremic patients from 1999 to 2006, determined that no MIC creep existed when they tested vancomycin MICs using the broth microdilution method. The authors found the MIC₉₀ (ie, the MIC in at least 90% of the isolates) remained less than 1 mg/L during each year of the study.

Sader et al,²² in a multicenter study, evaluated 1,800 MRSA bloodstream isolates from nine hospitals across the United States from 2002 to 2006. Vancomycin MICs were again measured by broth microdilution methods. The mode MIC remained stable at 0.625 mg/L during the study period, and the authors did not detect a trend of rising MICs.

The inconsistency between reports of MIC creep at single institutions and the absence of this phenomenon in large, multicenter studies seems to imply that vancomycin MIC creep is not occurring on a grand scale.

Vancomycin tolerance

Another troubling matter with S aureus and vancomycin is the issue of tolerance. Vancomycin tolerance, defined in terms of increased minimum bactericidal concentration, represents a loss of bactericidal activity. Tolerance to vancomycin can occur even if the MIC remains in the susceptible range.²³

Safdar and Rolston,²⁴ in an observational study from a cancer center, reported that of eight cases of bacteremia that was resistant to vancomycin therapy, three were caused by S aureus.

Sakoulas et al²⁵ found that higher levels of vancomycin bactericidal activity were associated with higher rates of clinical success; however, they found no effect on the mortality rate.

The issue of vancomycin tolerance remains controversial, and because testing for it is impractical in clinical microbiology laboratories, its implications outside the research arena are difficult to ascertain at present.

IS VANCOMYCIN STILL THE BEST DRUG FOR S AUREUS?

MIC break points have been lowered

In 2006, the Clinical Laboratories and Standards Institute lowered its break points for vancomycin MIC categories for S aureus:

• Susceptible: ≤ 2 mg/L (formerly ≤ 4 mg/L)
• Intermediate: 4–8 mg/L (formerly 8–16 mg/L)
• Resistant: ≥ 16 mg/L (formerly ≥ 32 mg/L).

The rationales for these changes were that the lower break points would better detect hVISA, and that cases have been reported of clinical treatment failure of S aureus infections in which the MICs for vancomycin were 4 mg/L.²⁶

Since 2006, the question has been raised whether to lower the break points even further. A reason for this proposal comes from an enhanced understanding of the pharmacokinetics and pharmacodynamics of vancomycin.

The variable most closely associated with clinical response to vancomycin is the AUC-MIC ratio. An AUC-MIC ratio of 400 or higher may be associated with better outcomes in patients with serious S aureus infection. A study of 108 patients with S aureus infection of the lower respiratory tract indicated that organism eradication was more likely if the AUC-MIC ratio was 400 or greater compared with values less than 400, and this was statistically significant.²⁷ However, in cases of S aureus infection with a vancomycin MIC of 2 mg/L or higher, this ratio may not be achievable.

A prospective study of 414 MRSA bacteremia episodes found a vancomycin MIC of 2 mg/L to be a predictor of death.²⁸ The authors concluded that vancomycin may not be the optimal treatment for MRSA with a vancomycin MIC of 2 mg/L.²⁸ Additional studies have also suggested a possible decrease in response to vancomycin in MRSA isolates with elevated MICs within the susceptible range.^25,29

Recent guidelines from the IDSA recommend using the clinical response, regardless of the MIC, to guide antimicrobial selection for isolates with MICs in the susceptible range.²

Combination therapy with vancomycin

As vancomycin use has increased, therapeutic failures with vancomycin have become apparent. Combination therapy has been suggested as an option to increase the efficacy of vancomycin when treating complicated infections.

Rifampin plus vancomycin is controversial.³⁰ The combination is theoretically beneficial, especially in infections associated with prosthetic devices. However, clinical studies have failed to convincingly support its use, and some have suggested that it might prolong bacteremia. In addition, it has numerous drug interactions to consider and adverse effects.³¹

Gentamicin plus vancomycin. The evidence supporting the use of this combination is weak at best. It appears that clinicians may have extrapolated from the success reported by Korzeniowski and Sande,³² who found that methicillin-susceptible S aureus bacteremia was cleared faster if gentamicin was added to nafcillin. A more recent study³³ that compared daptomycin (Cubicin) monotherapy with combined vancomycin and gentamicin to treat MRSA bacteremia and endocarditis showed a better overall success rate with daptomycin (44% vs 32.6%), but the difference was not statistically significant.

Gentamicin has some toxicity. Even short-term use (for the first 4 days of therapy) at low doses for bacteremia and endocarditis due to staphylococci has been associated with a higher rate of renal adverse events, including a significant decrease in creatinine clearance.³⁴

Clindamycin or linezolid plus vancomycin is used to decrease toxin production by S aureus.³⁰

While combination therapy with vancomycin is recommended in specific clinical situations, and the combinations are synergistic in vitro, information is lacking about clinical outcomes to support their use.