Ofloxacin (Page 2 of 10)

MICROBIOLOGY

Ofloxacin is a quinolone antimicrobial agent. The mechanism of action of ofloxacin and other fluoroquinolone antimicrobials involves inhibition of bacterial topoisomerase IV and DNA gyrase (both of which are type II topoisomerases), enzymes required for DNA replication, transcription, repair and recombination.

Ofloxacin has in vitro activity against a wide range of gram-negative and gram-positive microorganisms. Ofloxacin is often bactericidal at concentrations equal to or slightly greater than inhibitory concentrations.

Fluoroquinolones, including ofloxacin, differ in chemical structure and mode of action from aminoglycosides, macrolides and β-lactam antibiotics, including penicillins. Fluoroquinolones may, therefore, be active against bacteria resistant to these antimicrobials.

Resistance to ofloxacin due to spontaneous mutation in vitro is a rare occurrence (range: 10-9 to 10-11). Although cross-resistance has been observed between ofloxacin and some other fluoroquinolones, some microorganisms resistant to other fluoroquinolones may be susceptible to ofloxacin.

Ofloxacin has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section:

Aerobic Gram-Positive Microorganisms

Staphylococcus aureus (methicillin-susceptible strains)

Streptococcus pneumoniae (penicillin-susceptible strains)

Streptococcus pyogenes

Aerobic Gram-Negative Microorganisms

Citrobacter (diversus) koseri

Enterobacter aerogenes

Escherichia coli

Haemophilus influenzae

Klebsiella pneumoniae

Neisseria gonorrhoeae

Proteus mirabilis

Pseudomonas aeruginosa

As with other drugs in this class, some strains of Pseudomonas aeruginosa may develop resistance fairly rapidly during treatment with ofloxacin.

Other Microorganisms

Chlamydia trachomatis

The following in vitro data are available, but their clinical significance is unknown.

Ofloxacin exhibits in vitro minimum inhibitory concentrations (MIC values) of 2 mcg/mL or less against most (≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of ofloxacin in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled trials.

Aerobic Gram-Positive Microorganisms

Staphylococcus epidermidis (methicillin-susceptible strains)

Staphylococcus saprophyticus

Streptococcus pneumoniae (penicillin-resistant strains)

Aerobic Gram-Negative Microorganisms

Acinetobacter calcoaceticus

Bordetella pertussis

Citrobacter freundii

Enterobacter cloacae

Haemophilus ducreyi

Klebsiella oxytoca

Moraxella catarrhalis

Morganella morganii

Proteus vulgaris

Providencia rettgeri

Providencia stuartii

Serratia marcescens

Anaerobic Microorganisms

Clostridium perfringes

Other Microorganisms

Chlamydia pneumoniae

Gardnerella vaginalis

Legionella pneumophila

Mycoplasma hominis

Mycoplasma pneumoniae

Ureaplasma urealyticum

Ofloxacin is not active against Treponema pallidum (see WARNINGS).

Many strains of other streptococcal species, Enterococcus species, and anaerobes are resistant to ofloxacin.

Susceptibility Tests

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MIC values). These MIC values provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MIC values should be determined using a standardized procedure. Standardized procedures are based on a dilution method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of ofloxacin powder. The MIC values should be interpreted according to the following criteria:

For testing Enterobacteriaceae , methicillin-susceptible Staphylococcus aureus , and Pseudomonas aeruginosa:

MIC (mcg/mL)

Interpretation

≤ 2

Susceptible (S)

4

Intermediate (I)

≥ 8

Resistant (R)

For testing Haemophilus influenzae:a

MIC (mcg/mL)

Interpretation

≤ 2

Susceptible (S)

a This interpretive standard is applicable only to broth microdilution susceptibility tests with Haemophilus influenzae using Haemophilus Test Medium.1

The current absence of data on resistant strains precludes defining any results other than “Susceptible.” Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.

For testing Neisseria gonorrhoeae:b

MIC (mcg/mL)

Interpretation

0.25

Susceptible (S)

0.5 to 1

Intermediate (I)

2

Resistant (R)

b These interpretive standards are applicable only to agar dilution tests using GC agar base and 1% defined growth supplement incubated in 5% CO2 .

For testing Streptococcus pneumoniae and Streptococcus pyogenes:c

MIC (mcg/mL)

Interpretation

≤ 2

Susceptible (S)

4

Intermediate (I)

≥ 8

Resistant (R)

c These interpretive standards are applicable only to broth microdilution susceptibility tests using cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood.

A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentration usually achievable. A report of “Intermediate” indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where a high dosage of drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentration usually achievable; other therapy should be selected.

Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard ofloxacin powder should provide the following MIC values:

Microorganism

MIC Range (mcg/mL)

Escherichia coli

ATCC 25922

0.015 to 0.12

Haemophilus influenzae

ATCC 49247d

0.016 to 0.06

Neisseria gonorrhoeae

ATCC 49226e

0.004 to 0.016

Pseudomonas aeruginosa

ATCC 27853

1 to 8

Staphylococcus aureus

ATCC 29213

0.12 to 1

Streptococcus pneumoniae

ATCC 49619f

1 to 4

d This quality control range is applicable only to H. influenzae ATCC 49247 tested by a microdilution procedure using Haemophilus Test Medium (HTM).1

e This quality control range is applicable only to N. gonorrhoeae ATCC 49226 tested by an agar dilution procedure using GC agar base with 1% defined growth supplement incubated in 5% CO2 .

f This quality control range is applicable only to S. pneumoniae ATCC 49619 tested by a microdilution procedure using cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood.

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