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Oxacillin

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Oxacillin

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 (g/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.

Last reviewed on RxList: 8/26/2008
This monograph has been modified to include the generic and brand name in many instances.

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