CLINICAL PHARMACOLOGY
Intravenous administration provides peak serum levels approximately 5 minutes
after the injection is completed. Slow I.V. administration of 500 mg gives a
peak serum level of 43 µg/mL after 5 minutes with a half-life of 20-30
minutes.
The penicillinase-resistant penicillins bind to serum protein, mainly albumin.
The degree of protein binding reported for oxacillin (oxacillin (oxacillin for injection) for injection) is 94.2% ± 2.1%.
Reported values vary with the method of study and the investigator.
The penicillinase-resistant penicillins vary in the extent to which they are
distributed in the body fluids. With normal doses, insignificant concentrations
are found in the cerebrospinal fluid and aqueous humor. All the drugs in this
class are found in therapeutic concentrations in the pleural, bile, and amniotic
fluids.
The penicillinase-resistant penicillins are rapidly excreted primarily as unchanged
drug in the urine by glomerular filtration and active tubular secretion. The
elimination half-life for oxacillin (oxacillin (oxacillin for injection) for injection) is about 0.5 hours. Nonrenal elimination
includes hepatic inactivation and excretion in bile.
Probenecid blocks the renal tubular secretion of penicillins. Therefore, the
concurrent administration of probenecid prolongs the elimination of oxacillin (oxacillin (oxacillin for injection) for injection)
and, consequently, increases the serum concentration.
Microbiology
Penicillinase-resistant penicillins exert a bactericidal action against penicillin
susceptible microorganisms during the state of active multiplication. All penicillins
inhibit the biosynthesis of the bacterial cell wall.
The drugs in this class are highly resistant to inactivation by staphylococcal
penicillinase and are active against penicillinase producing strains of Staphylococcus
aureus. The penicillinase-resistant penicillins are active in vitro
against a variety of other bacteria.
Susceptibility tests
Diffusion techniques
Quantitative methods that require measurement of zone diameters provide reproducible
estimates of the susceptibility of bacteria to antimicrobial compounds. One
such standardized procedure1 that has been recommended for use with
disks to test susceptibility of microorganisms to oxacillin uses the 1- µg
oxacillin (oxacillin (oxacillin for injection) for injection) disk. Interpretation involves the correlation of the diameter obtained
in the disk test with the minimum inhibitory concentration (MIC) for oxacillin (oxacillin (oxacillin for injection) for injection) .
Reports from the laboratory providing results of the standard single-disk susceptibility
test for S. aureus with a 1-µg oxacillin (oxacillin (oxacillin for injection) for injection) disk should be interpreted
according to the following criteria:
Zone Diameter (mm) |
Interpretation |
≥ 13 mm |
Susceptible (S) |
11-12 mm |
Intermediate (I) |
≤ 10 mm |
Resistant (R) |
Reports from the laboratory providing results of the standard single-disk susceptibility
test for coagulase-negative staphylococcus with a 1-µg oxacillin (oxacillin (oxacillin for injection) for injection)
disk should be interpreted according to the following criteria:
Zone Diameter (mm) |
Interpretation |
≥ 18 mm |
Susceptible (S) |
≤ 17 mm |
Resistant (R) |
A report of "Susceptible" indicates that the pathogen is likely to be inhibited
by usually achievable concentrations of the antimicrobial compound in blood.
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 high dosage of drug can be used. This category also provides
a buffer zone that prevents small uncontrolled technical factors from causing
major discrepancies in interpretation. A report of "Resistant" indicates that
usually achievable concentrations of the antimicrobial compound in the blood
are unlikely to be inhibitory and that other therapy should be selected.
Measurement of MIC or MBC and achieved antimicrobial compound concentrations
may be appropriate to guide therapy in some infections. (See CLINICAL PHARMACOLOGY
section for further information on drug concentrations achieved in infected
body sites and other pharmacokinetic properties of this antimicrobial drug product.)
Standardized susceptibility test procedures require the use of laboratory control
microorganisms. The 1-µg oxacillin (oxacillin (oxacillin for injection) for injection) disk should provide the following zone
diameters in these laboratory test quality control strains:
Microorganism |
Zone Diameter (mm) |
S. aureus ATCC 25923 |
18-24 mm |
Dilution techniques
Quantitative methods that are used to determine minimum inhibitory concentrations
provide reproducible estimates of the susceptibility of bacteria to antimicrobial
compounds. One such standardized procedure uses a standardized dilution method2
(broth, agar, or microdilution) or equivalent with oxacillin (oxacillin (oxacillin for injection) for injection) powder. The MIC
values obtained for S. aureus should be interpreted according to the following
criteria:
MIC (µg/mL) |
Interpretation |
≥ 2 |
Susceptible |
≤ 4 |
Resistant |
The MIC values obtained for coagulase-negative staphylococcus should be interpreted
according to the following criteria:
MIC (µg/mL) |
Interpretation |
≤ 0.25 |
Susceptible |
≥ 0.5 |
Resistant |
Interpretation should be as stated above for results using diffusion techniques.
As with standard diffusion techniques, dilution methods require the use of
laboratory control microorganisms. - Standard oxacillin (oxacillin (oxacillin for injection) for injection) powder should provide
the following MIC values:
Microorganism |
MIC (µg/mL) |
S. aureus ATCC 29213 |
0.12-0.5 |
REFERENCES
1. National Committee for Clinical Laboratory Standards, Performance Standards
for Antimicrobial Disk Susceptibility Tests, Eighth Edition. Approved Standard
NCCLS Document M2-A8, Vol. 23, No. 1 NCCLS, Wayne, PA, January, 2003.
2. National Committee for Clinical Laboratory Standards, Methods for Dilution
Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically, Sixth
Edition. Approved Standard NCCLS Document M7-A6, Vol. 23, No. 2 NCCLS, Wayne,
PA, January, 2003.