Mechanism of Action:
Cefpodoxime is a bactericidal agent that acts by inhibition of bacterial cell wall synthesis. Cefpodoxime has activity in the presence of some beta-lactamases, both penicillinases and cephalosporinases, of Gram-negative and Gram-positive bacteria.
Mechanism of Resistance:
Resistance to Cefpodoxime is primarily through hydrolysis by beta-lactamase, alteration of penicillin-binding proteins (PBPs), and decreased permeability.
Cefpodoxime has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections as described in the Indications and Usage (1) section:
Gram-positive bacteria:
Staphylococcus aureus (methicillin-susceptible strains, including those producing penicillinases)
Staphylococcus saprophyticus
Streptococcus pneumoniae (excluding penicillin-resistant isolates)
Streptococcus pyogenes
Gram-negative bacteria:
Escherichia coli
Klebsiella pneumoniae
Proteus mirabilis
Haemophilus influenzae (including beta-lactamase producing isolates)
Moraxella catarrhalis
Neisseria gonorrhoeae (including penicillinase-producing isolates)
The following in vitro data are available, but their clinical significance is unknown. At least 90 percent of the following microorganisms exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for Cefpodoxime. However, the efficacy of Cefpodoxime in treating clinical infections due to these microorganisms has not been established in adequate and well-controlled clinical trials.
Gram-positive bacteria:
Streptococcus agalactiae
Streptococcus spp. (Groups C, F, G)
Gram-negative bacteria:
Citrobacter diversus
Klebsiella oxytoca
Proteus vulgaris
Providencia rettgeri
Haemophilus parainfluenzae
Anaerobic Gram-positive bacteria:
Peptostreptococcus magnus
Susceptibility Test Methods:
When available, the clinical microbiology laboratory should provide the results 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 antimicrobial minimal inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method 1,3. The MIC values should be interpreted according to criteria provided in Table 1.
Diffusion techniques:
Quantitative methods that require measurement of zone diameters 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 test method 2,3. This procedure uses paper disks impregnated with 10 mcg Cefpodoxime to test the susceptibility of microorganisms to Cefpodoxime. The disk diffusion interpretive criteria are provided in Table 1.
Table 1. Susceptibility Test Interpretive Criteria for Cefpodoxime 2.
Pathogen
| Minimum Inhibitory Concentrations (mcg/mL)
| Disk Diffusion Diameters (mm)
|
S
| I
| R
| S
| I
| R
|
Enterobacteriaceae
| ≤ 2
| 4
| ≥ 8
| ≥21
| 18 to 20
| ≤ 17
|
Haemophilus influenzaea
| ≤ 2
| -
| -
| ≥21
| -
| -
|
Streptococcus pneumoniae
| ≤0.5
| 1
| ≥ 2
| -
| -
| -
|
Neisseria gonorrhoeaea
| ≤0.5
| -
| -
| ≥29
| -
| -
|
Susceptibility of staphylococci to Cefpodoxime may be deduced from testing only penicillin and either cefoxitin or oxacillin.
a = The current absence of resistant isolates precludes defining any results other than “Susceptible.” Isolates yielding MIC results other than “Susceptible” should be submitted to a reference laboratory for further testing.
A report of Susceptible indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration 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 is physiologically concentrated or in situations where a 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 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 Cefpodoxime powder should provide the following range of MIC values noted in Table 2. For the diffusion technique using the 10 mcg disk, the criteria in Table 2 should be achieved.
Table 2. Acceptable Quality Control Ranges for Cefpodoxime
QC Strains
| Minimum Inhibitory Concentrations (mcg/mL)
| Disk Diffusion Zone diameters (mm)
|
Escherichia coli ATCC 25922
| 0.25 to 1
| 23 to 28
|
Haemophilus influenzae ATCC 49247
| 0.25 to 1
| 25 to 31
|
Streptococcus pneumoniae ATCC 49619
| 0.03 to 0.12
| 28 to 34
|
Neisseria gonorrhoeae ATCC 49226
| 0.03 to 0.12
| 35 to 43
|
Staphylococcus aureus ATCC 25923
| ---------
| 19 to 25
|
Staphylococcus aureus ATCC 29213
| 1 to 8
| --------
|
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