Clinical Pharmacology for Bactocill
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 mcg/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 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 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 and, consequently, increases the serum concentration.
Microbiology
Mechanism Of Action
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.
Resistance
Resistance to penicillins may be mediated by destruction of the beta-lactam ring by a beta-lactamase, altered affinity of penicillin for target, or decreased penetration of the antibiotic to reach the target site.
Resistance to oxacillin (or cefoxitin) implies resistance to all other beta-lactam agents, except newer agents with activity against methicillin-resistant Staphylococcus aureus.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide cumulative reports of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in the selection of the most appropriate antibacterial drug for treatment.
Dilution Techniques
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method1,2 (broth and/or agar). The MIC values should be interpreted according to the criteria in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized method.2,3 It has been determined that the most accurate method to test the susceptibility of microorganisms to penicillinase-resistant penicillins, including oxacillin, by disk diffusion is achieved using disks impregnated with 30 mcg cefoxitin. Interpretation involves correlation of the diameter obtained in the cefoxitin disk test with the MIC for oxacillin. 2 Results of the standard single-disk susceptibility test with a 30 microgram cefoxitin disk should be interpreted according to the following criteria in Table 1.
Table 1. Susceptibility Test Interpretive Criteria for Oxacillin
| Pathogen |
Antimicrobial Agent |
Disk Content |
Disk Diffusion Zone Diameter (mm) a |
Minimum Inhibitory Concentrations (mcg/mL) |
| S |
I |
R |
S |
I |
R |
| Staphylococcus aureus and S. lugdenensis c |
Oxacillin |
- |
- |
- |
- |
≤ 2 (oxacillin) |
- |
≥ 4 (oxacillin) |
|
|
30 mcg cefoxitin b |
≥ 22 |
- |
≤ 21 |
≤ 4 (cefoxitin) |
- |
≥ 8 (cefoxitin) |
| Coagulase-negative staphylococci except S. lugdenensis |
Oxacillin d |
- |
- |
- |
- |
≤0.25 |
- |
≥ 0.5 |
|
|
30 mcg cefoxitin b |
≥25 |
- |
≤ 24 |
- |
- |
- |
S=susceptible, I=intermediate, R-resistant
a Oxacillin disk testing is not reliable. For disk testing see cefoxitin.2
b Cefoxitin is used as a surrogate for oxacillin; report oxacillin susceptible or resistant based on the cefoxitin result.2
c If both cefoxitin and oxacillin are tested against S. aureus or S. lugdenensis, and either result is resistant, the organism should be reported as oxacillin resistant.2
d Oxacillin MIC interpretive criteria may overcall resistance for some coagulase-negative staphylococci (CoNS), because some non-S. epidermidis strains for which the oxacillin MICs are 0.5 to 2 mcg/ml lack mecA. For serious infections with CoNS other than S. epidermidis, testing for mecA or for PBP 2a or with cefoxitin disk diffusion may be appropriate for strains for which the oxacillin MICs are 0.5 to 2 mcg/ml.2 |
A report of “Susceptible” indicates that the antimicrobial drug is likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the site of infection. A report of “Resistant” indicates that the antimicrobial drug is not likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the infection site; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test.1,2,3 Standard oxacillin powder should provide the following range of MIC values noted in Table 2. For the diffusion technique using the 30 mcg cefoxitin disk, the criteria in Table 2 should be achieved.
Table 2. Acceptable Quality Control Ranges for Oxacillin*
| Quality Control Organism |
Minimum Inhibitory Concentration (mcg/mL) |
Disk Diffusion Zone Diameters (mm) |
Enterococcus faecalis
ATCC® 29212 |
8-32 |
- |
Staphylococcus aureus
ATCC® 25923 |
- |
18-24 |
Staphylococcus aureus
ATCC® 29213 |
0.12 - 0.5 |
- |
Streptococcus pneumoniae
ATCC® 49619a |
- |
≤12b |
*For cefoxitin QC see CLSI document M100-S27. 2
ATCC = American Type Culture Collection
a Despite the lack of reliable disk diffusion interpretive criteria for S. pneumoniae with certain beta-lactams, Streptococcus pneumoniae ATCC® 49619 is the strain designated for QC of all disk diffusion tests with Streptococcus spp.
b Deterioration of oxacillin disk content is best assessed with QC organism S. aureus ATCC® 25923, with an acceptable zone diameter for 18-24. |
REFERENCES
1. Clinical and Laboratory Standards Institute (CLSI). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard- Tenth Edition. CLSI Document M07-A10. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087 USA, 2015.
2. Clinical and Laboratory Standards Institute (CLSI). Performance Standards for Antimicrobial Susceptibility Testing; Twenty-seventh Informational Supplement. CLSI Document M100-S27. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087 USA, 2017.
3. Clinical and Laboratory Standards Institute (CLSI). Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard-Twelfth Edition. CLSI Document M02-A12. Clinical and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087 USA, 2015.