12.2 Pharmacodynamics
The pharmacokinetic/pharmacodynamic relationship for cefazolin has not been evaluated in patients.
12.3 Pharmacokinetics
After intramuscular administration of Cefazolin for Injection to normal volunteers, the mean serum concentrations were 37 mcg/mL at 1 hour and 3 mcg/mL at 8 hours following a 500 mg dose, and 64 mcg/mL at 1 hour and 7 mcg/mL at 8 hours following a 1-gram dose.
Studies have shown that following intravenous administration of Cefazolin for Injection to normal volunteers, mean serum concentrations peaked at approximately 185 mcg/mL and were approximately 4 mcg/mL at 8 hours for a 1 gram dose.
The serum half-life for Cefazolin for Injection is approximately 1.8 hours following IV administration and approximately 2 hours following IM administration.
In a study (using normal volunteers) of constant intravenous infusion with dosages of 3.5 mg/kg for 1 hour (approximately 250 mg) and 1.5 mg/kg the next 2 hours (approximately 100 mg), Cefazolin for Injection produced a steady serum level at the third hour of approximately 28 mcg/mL.
Studies in patients hospitalized with infections indicate that Cefazolin for Injection produces mean peak serum levels approximately equivalent to those seen in normal volunteers.
Bile levels in patients without obstructive biliary disease can reach or exceed serum levels up to 5 times; however, in patients with obstructive biliary disease, bile levels of Cefazolin for Injection are considerably lower than serum levels (< 1 mcg/mL).
In synovial fluid, the level of Cefazolin for Injection becomes comparable to that reached in serum at about 4 hours after drug administration.
Studies of cord blood show prompt transfer of Cefazolin for Injection across the placenta. Cefazolin for Injection is present in very low concentrations in the milk of nursing mothers.
Cefazolin for Injection is excreted unchanged in the urine. In the first 6 hours approximately 60% of the drug is excreted in the urine and this increases to 70% to 80% within 24 hours. Cefazolin for Injection achieves peak urine concentrations of approximately 2,400 mcg/mL and 4,000 mcg/mL respectively following 500 mg and 1 gram intramuscular doses.
In patients undergoing peritoneal dialysis (2 L/hr.), Cefazolin for Injection produced mean serum levels of approximately 10 and 30 mcg/mL after 24 hours' instillation of a dialyzing solution containing 50 mg/L and 150 mg/L, respectively. Mean peak levels were 29 mcg/mL (range 13 to 44 mcg/mL) with 50 mg/L (3 patients), and 72 mcg/mL (range 26 to 142 mcg/mL) with 150 mg/L (6 patients). Intraperitoneal administration of Cefazolin for Injection is usually well tolerated.
Controlled studies on adult normal volunteers, receiving 1 gram 4 times a day for 10 days, monitoring CBC, SGOT, SGPT, bilirubin, alkaline phosphatase, BUN, creatinine, and urinalysis, indicated no clinically significant changes attributed to Cefazolin for Injection.
12.4 Microbiology
Mechanism of Action
Cefazolin is a bactericidal agent that acts by inhibition of bacterial cell wall synthesis.
Mechanism of Resistance
Predominant mechanisms of bacterial resistance to cephalosporins include the presence of extended-spectrum beta-lactamases and enzymatic hydrolysis.
Lists of Microorganisms
Cefazolin has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections as described in the Indications and Usage (1) section.
- Gram-Positive Bacteria
Staphylococcus aureus
Staphylococcus epidermidis
Streptococcus pyogenes
Streptococcus agalactiae
Streptococcus pneumoniae
Methicillin-resistant staphylococci are uniformly resistant to cefazolin.
- Gram-Negative Bacteria
Escherichia coli
Proteus mirabilis
Most isolates of indole positive Proteus (Proteus vulgaris), Enterobacter spp., Morganella morganii, Providenciarettgeri, Serratia spp., and Pseudomonas spp. are resistant to cefazolin.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide cumulative reports of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.
Dilution Techniques
Quantitative methods are used to determine minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standard test1, 2 (broth and/or agar). The MIC values obtained should be interpreted according to criteria as provided in Table 4.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters 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 interpreted using a standard test method2, 3. This procedure uses paper disks impregnated with 30 mcg cefazolin to test the susceptibility of microorganisms to cefazolin. The disk diffusion interpretive criteria are provided in Table 4.
NOTE:
S. pyogenes and S. agalactiae that have a penicillin MIC of ≤0.12 mcg/mL, or disk diffusion zone diameters of ≥24 mm with a 10 mcg penicillin disk, may be interpreted as susceptible to cefazolin. NOTE: Susceptibility of staphylococci to cefazolin may be deduced from testing either cefoxitin or oxacillin.
A report of Susceptible indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the infection site necessary to inhibit growth of the pathogen. 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 product is physiologically concentrated or in situations where a high dosage of the drug product 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 the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations 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 supplies and reagents used in the assay, and the techniques of the individual performing the test1, 2, 3. Standard cefazolin powder should provide the following MIC values noted in Table 5. For the diffusion technique using the 30 mcg disk, the criteria in Table 5 should be achieved.
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