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Low Propensity of Gatifloxacin-BAK Low Propensity of Gatifloxacin-BAK Combination to Select for Combination to Select for

Fluoroquinolone Resistance Among Fluoroquinolone Resistance Among Methicillin-Resistant Methicillin-Resistant Staphylococcus Staphylococcus

aureusaureus

Christine Hesje, BSc; Joseph M. Blondeau, PhD

Department of Clinical Microbiology, Royal University Hospital and the Departments of Microbiology and Immunology and Pathology, University of

Saskatchewan, Saskatoon, Saskatchewan, Canada

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Financial DisclosuresFinancial Disclosures

• Study supported by an unrestricted Study supported by an unrestricted educational grant from Allergan, Inc.educational grant from Allergan, Inc.

• Christine Hesje and Joseph Blondeau Christine Hesje and Joseph Blondeau have no financial interests in any product have no financial interests in any product mentioned in this studymentioned in this study

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INTRODUCTIONINTRODUCTION• Staphylococcus aureusStaphylococcus aureus is a common is a common

cause of ocular infections cause of ocular infections

– Methicillin-resistant Methicillin-resistant S aureusS aureus (MRSA) strains (MRSA) strains are now prevalent in both the hospital and are now prevalent in both the hospital and community settingscommunity settings11

– Co-resistance of MRSA to third-generation Co-resistance of MRSA to third-generation fluoroquinolones is well knownfluoroquinolones is well known22

• The fourth-generation fluoroquinolone The fourth-generation fluoroquinolone gatifloxacin has a reduced probability of gatifloxacin has a reduced probability of resistance because 2 mutations are resistance because 2 mutations are necessary for resistance to developnecessary for resistance to develop33

– Older fluoroquinolones develop resistance Older fluoroquinolones develop resistance with only 1 mutation with only 1 mutation

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PurposePurpose• The ability of an antibiotic to overcome antimicrobial The ability of an antibiotic to overcome antimicrobial

resistance can be evaluated using the mutant prevention resistance can be evaluated using the mutant prevention concentration (MPC)concentration (MPC)4,54,5

– MPC is defined as the drug concentration that prevents the growth of MPC is defined as the drug concentration that prevents the growth of the most resistant first-step mutants in a large heterogeneous bacterial the most resistant first-step mutants in a large heterogeneous bacterial populationpopulation

• In vitro potency of the gatifloxacin commercial In vitro potency of the gatifloxacin commercial formulation has been evaluated with the active ingredient formulation has been evaluated with the active ingredient alonealone

– Commercial formulation of gatifloxacin (ZymarCommercial formulation of gatifloxacin (Zymar®®; Allergan Inc.; Irvine, ; Allergan Inc.; Irvine, CA) contains 0.005% benzalkonium chloride (BAK) as a preservativeCA) contains 0.005% benzalkonium chloride (BAK) as a preservative66

– Recent studies from our laboratory demonstrated that the presence of Recent studies from our laboratory demonstrated that the presence of BAK increases antimicrobial activity of gatifloxacinBAK increases antimicrobial activity of gatifloxacin77

• The purpose of this study was to determine the minimal The purpose of this study was to determine the minimal inhibitory concentration (MIC) and MPC values of inhibitory concentration (MIC) and MPC values of gatifloxacin, BAK, and the gatifloxacin-BAK combination gatifloxacin, BAK, and the gatifloxacin-BAK combination against clinical isolates of MRSAagainst clinical isolates of MRSA

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METHODSMETHODS

• Seventeen clinical isolates of MRSA were testedSeventeen clinical isolates of MRSA were tested

• MIC testingMIC testing– Bacteria (10Bacteria (1055 colony-forming units [CFU]/mL) were inoculated in colony-forming units [CFU]/mL) were inoculated in

Mueller-Hinton broth containing 2-fold concentration increments Mueller-Hinton broth containing 2-fold concentration increments of the test agents of the test agents

– The lowest concentration that prevented growth of 90% of The lowest concentration that prevented growth of 90% of bacteria was recorded as the MICbacteria was recorded as the MIC9090

• MPC testingMPC testing– Bacteria (10Bacteria (101010 CFU/mL) were inoculated onto agar plates in the CFU/mL) were inoculated onto agar plates in the

presence of increasing concentrations of the test agents presence of increasing concentrations of the test agents

– The lowest drug concentration preventing bacterial growth was The lowest drug concentration preventing bacterial growth was recorded as the MPC recorded as the MPC

• The range of gatifloxacin concentrations tested was from The range of gatifloxacin concentrations tested was from 8 µg/mL to < 0.004 µg/mL 8 µg/mL to < 0.004 µg/mL

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RESULTSRESULTSThe MIC90 of Gatifloxacin, BAK, and the Gatifloxacin-BAK Combination Against Clinical Isolates of MRSA

3.0

4.0

Gatifloxacin BAK Gatifloxacin-BAKCombination

MIC

90 (

µg/

mL)

µg/

mL)

0.125

3.1

< 0.004

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RESULTSRESULTSThe MPC of Gatifloxacin, BAK, and the Gatifloxacin-BAK Combination Against Clinical Isolates of MRSA

aRanged from 6 to 10 µg/mL.µg/mL.bThe concentration of BAK was 10 µg/mL.µg/mL.

0

1

2

3

4

5

6

≥ 4

6a

< 0.004b

Gatifloxacin BAK Gatifloxacin-BAKCombination

MP

C (

µg/

mL)

µg/

mL)

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CONCLUSIONS

• The combination of gatifloxacin and BAK was highly active against The combination of gatifloxacin and BAK was highly active against MRSA in vitro MRSA in vitro

– The MIC of the gatifloxacin-BAK combination was over 30-fold and 775-The MIC of the gatifloxacin-BAK combination was over 30-fold and 775-fold lower than the MIC of gatifloxacin and BAK alone, respectivelyfold lower than the MIC of gatifloxacin and BAK alone, respectively

– The MPC of the gatifloxacin-BAK combination was at least 1000-fold The MPC of the gatifloxacin-BAK combination was at least 1000-fold and 1500-fold lower than the MPC of gatifloxacin and BAK alone, and 1500-fold lower than the MPC of gatifloxacin and BAK alone, respectivelyrespectively

• These findings suggest that ZymarThese findings suggest that Zymar®® may have low propensity to may have low propensity to select for fluoroquinolone-resistant MRSAselect for fluoroquinolone-resistant MRSA

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REFERENCES1.1. Blomquist PH. Methicillin-resistant Blomquist PH. Methicillin-resistant Staphylococcus aureusStaphylococcus aureus infections of the infections of the

eye and orbit. eye and orbit. Trans Am Ophthalmol Soc.Trans Am Ophthalmol Soc. 2006;104:322-345. 2006;104:322-345.

2.2. Marangon FB, Miller D, Muallem MS, Romano AC, Alfonso EC. Ciprofloxacin Marangon FB, Miller D, Muallem MS, Romano AC, Alfonso EC. Ciprofloxacin and levofloxacin resistance among methicillin-sensitive and levofloxacin resistance among methicillin-sensitive Staphylococcus Staphylococcus aureusaureus isolates from keratitis and conjunctivitis. isolates from keratitis and conjunctivitis. Am J Ophthalmol.Am J Ophthalmol. 2004;137(3):453-458. 2004;137(3):453-458.

3.3. Hooper DC. Mechanisms of action and resistance of older and newer Hooper DC. Mechanisms of action and resistance of older and newer fluoroquinolones. fluoroquinolones. Clin Infect Dis.Clin Infect Dis. 2000;31 (suppl 2):S24-S28. 2000;31 (suppl 2):S24-S28.

4.4. Hansen G, Blondeau JM. Mutant prevention concentrations as a strategy to Hansen G, Blondeau JM. Mutant prevention concentrations as a strategy to minimize antimicrobial resistance: a timely concept but will its acceptance be minimize antimicrobial resistance: a timely concept but will its acceptance be too late? too late? Therapy. Therapy. 2005;2:61-66.2005;2:61-66.

5.5. Blondeau JM, Zhao X, Hansen GT, Drlica K. Mutant prevention Blondeau JM, Zhao X, Hansen GT, Drlica K. Mutant prevention concentrations (MPC) of fluoroquinolones for clinical isolates of concentrations (MPC) of fluoroquinolones for clinical isolates of Streptococcus pneumoniae. Antimicrob Agents Chemother.Streptococcus pneumoniae. Antimicrob Agents Chemother. 2001;45(2):433- 2001;45(2):433-438.438.

6.6. ZYMARZYMAR®® [package insert]. Irvine, CA: Allergan, Inc.; 2004. [package insert]. Irvine, CA: Allergan, Inc.; 2004.

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