Stephen S. Lane, M.D., adjunct professor of ophthalmology, University of Minnesota, St. Paul.
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Minimum bactericidal concentration (MBC) levels are similar to minimum inhibitory concentrations (MIC) in terms of measurement but vary slightly in what they measure. MIC levels determine how much of a particular antibiotic is necessary to inhibit the growth of specific bacteria; MBC levels determine how much of the antibiotic is necessary to kill the bacteria outright. Typically, the MBC is four to five times that of the MIC (i.e., more drug is necessary to kill than inhibit).
“MBC is a more powerful measurement,” said Stephen S. Lane, M.D., adjunct professor of ophthalmology, University of Minnesota, St. Paul. “These data have important implications with regard to post-op endophthalmitis. As surgeons, we don't want to just prevent bacteria from growing after cataract surgery; we want to kill what is there.”
Clinically, this study shows the two antibiotics tested at a dosing schedule typically seen in a clinical setting, Dr. Lane said. Most patients are dosed three to four times on the day of surgery with an antibiotic before undergoing cataract surgery, he said.
“Both fluoroquinolones have low MBC values, and they are similar with respect to common organisms,” he said. “But the green bar overlay shows that with moxifloxacin (VIGAMOX® solution, Alcon, Fort Worth, Texas) the amount inside the eye is more than enough to kill all the organisms. Gatifloxacin (ZYMAR*) doesn’t cover three of the organisms.”
The results of the study indicate that “if they’re dosed in the same amounts and dosed equally, moxifloxacin penetrates into the aqueous humor better and is therefore more available to kill bacteria than gatifloxacin,” Dr. Lane said.
The study confirms that both fluoroquinolones have good kill rates and remain effective, but “the higher penetration of moxifloxacin leads me to use it preferentially over gatifloxacin because, as this study shows, it gets into the eye better,” Dr. Lane said. “I just want to use an antibiotic that is effective against the most common organisms and gets to the potential site of these organisms in the highest concentration possible. In this way, I feel I provide the best prophylaxis for my patients.”
1. Stroman DW, Cupp G, Dahlin DC, Schlech BA, D'Arienzo PA. Human Ocular Concentrations Following Topical Fluoroquinolone Administration Relative to Susceptibility of Ocular Pathogens. Invest. Ophthalmol. Vis. Sci. 2006;47: E-Abstract 1881
2. Kim DH, Stark WJ, O'Brien TP, Dick JD. Aqueous Penetration and Biological Activity of Moxifloxacin 0.5% Ophthalmic Aolution and Gatifloxacin 0.3% Solution in Cataract Surgery Patients. Ophthalmology, 2005;112:1992-6 Epub 2005, Sep 23