Enhanced Killing of Methicillin-Resistant Staphylococcus aureus (MRSA) Strains With Gatifloxacin and Benzalkonium Chloride

1
Enhanced Killing of Methicillin-Resistant
Staphylococcus aureus (MRSA) Strains With
Gatifloxacin and Benzalkonium Chloride

• Joseph M. Blondeau, PhD Christine Hesje, BSc
• Royal University Hospital and the University of
  Saskatchewan, Saskatoon, Saskatchewan, Canada

Financial Disclosures This study was supported
by an unrestricted educational grant from
Allergan, Inc., Irvine, CA. Authors have no
financial interest.
2
INTRODUCTION

• Gram-positive staphylococcal species are common
  bacteria cultured from normal ocular flora and
  patients with endophthalmitis, keratitis, and
  conjunctivitis.1
• Antibacterial resistance among staphylococci such
  as Staphylococcus aureus is on the rise.
• Among ocular isolates of S aureus, resistance to
  methicillin has significantly increased over the
  last decade, from less than 5.0 to over
  15.0.2,3
• Methicillin-resistant S aureus (MRSA) strains are
  now prevalent in both the hospital and community
  settings.4
• Effective therapeutic strategies are, therefore,
  necessary to prevent and treat ocular infections
  that result from resistant strains of bacteria.

3
INTRODUCTION

• Gatifloxacin ophthalmic solution 0.3 (Zymar
  Allergan, Inc. Irvine, CA) is a
  fourth-generation fluoroquinolone that is
  frequently used against ocular infections.5
• Conventionally, the efficacy of ophthalmic
  antibiotic preparations against ocular pathogens
  is evaluated by considering only the active
  antibacterial ingredient.6,7
• Zymar, however, contains 0.005 benzalkonium
  chloride (BAK) (50 µg/mL) as a preservative
  agent.5
• Preservatives exert antibacterial activity
  against Gram-positive bacteria and thus may
  augment the potency of the active antibacterial
  ingredient.8
• This study was designed to determine if killing
  of MRSA strains by gatifloxacin is enhanced by
  BAK.

4
METHODS

• Two MRSA isolates collected through the clinical
  microbiology laboratory at Royal University
  Hospital in Saskatoon, Saskatchewan, Canada, were
  used in this study.
• Bacteria were identified by using the reference
  procedures as indicated in the Manual of Clinical
  Microbiology.9
• MRSA isolates (105 colony-forming units) were
  incubated with various concentrations of BAK,
  gatifloxacin, gatifloxacin plus BAK, or Zymar.
• The percentage of killed bacteria was determined
  after 20 or 30 minutes incubation at 35oC to 37oC
  in ambient air.
• Each timepoint was tested in triplicate and the
  values averaged, as were the values for both
  strains.

5
MRSA Killing by BAK, Gatifloxacin, and
GatifloxacinBAK combination
Gatifloxacin (0.063-4 µg/mL) plus BAK (10
µg/mL)
50
Gatifloxacin (0.063-4 µg/mL)
29
22
15
BAK (µg/mL)
10
13
Percentage of Bacteria Killeda
aAfter 30 minutes incubation.

• Gatifloxacin at concentrations from 0.063 µg/mL
  to 4 µg/mL killed MRSA isolates more effectively
  in the presence of than in the absence of 10
  µg/mL BAK.

6
MRSA Killing by Gatifloxacin, Gatifloxacin Plus
BAK, and Zymar
Gatifloxacin Dose (µg/mL)
Zymar (gatifloxacin equivalent containing 15-20
of µg/mL BAK)
600
95
300
90
600
95
Gatifloxacin plus 15-20 µg/mL of BAK
300
90
600
85
Gatifloxacin alone
300
51
Percentage of Bacteria Killeda
aAfter 20 minutes incubation.

• Gatifloxacin plus BAK and Zymar were more
  effective than gatifloxacin alone against MRSA
  isolates.

7
DISCUSSION

• Gatifloxacin plus BAK and Zymar were more
  effective than either gatifloxacin or BAK alone
  against MRSA.
• Consistent with these findings, we recently
  demonstrated that the addition of BAK
  dramatically lowered the minimum inhibitory
  concentration of gatifloxacin against MRSA.10
• These findings suggest that the presence of BAK
  in Zymar may serve to enhance antibacterial
  efficacy of gatifloxacin.
• A recent study demonstrated that gatifloxacin
  plus BAK was significantly more effective than
  either gatifloxacin or BAK alone in killing
  gatifloxacinresistant MRSA in vivo.11
• The contribution of BAK may also explain the in
  vivo efficacy of Zymar against S aureus that were
  resistant to the gatifloxacin molecule in
  vitro.12,13
• These findings point out the importance of
  evaluating the efficacy of ocular antibiotics on
  the basis of their ophthalmic formulations rather
  than simply the active ingredients.

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CONCLUSIONS

• Gatifloxacin plus BAK killed more bacteria than
  gatifloxacin alone, indicating that BAK enhanced
  bactericidal activity of gatifloxacin.
• These findings suggest that Zymar, which contains
  BAK, is effective against resistant pathogens.

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