Cefaclor is well absorbed after oral administration to fasting subjects. Total absorption is the same whether the drug is given with or without food; however, when it is taken with food, the peak concentration achieved is 50% to 75% of that observed when the drug is administered to fasting subjects and generally appears from three fourths to 1 hour later. Following administration of 250-mg, 500-mg, and 1-g doses to fasting subjects, average peak serum levels of approximately 7, 13, and 23 µg/mL respectively were obtained within 30 to 60 minutes. Approximately 60% to 85% of the drug is excreted unchanged in the urine within 8 hours, the greater portion being excreted within the first 2 hours. During this 8-hour period, peak urine concentrations following the 250-mg, 500-mg, and 1-g doses were approximately 600, 900, and 1,900 µ/mL, respectively. The serum half-life in normal subjects is 0.6 to 0.9 hour. In patients with reduced renal function, the serum half-life of cefaclor is slightly prolonged. In those with complete absence of renal function, the plasma half-life of the intact molecule is 2.3 to 2.8 hours. Excretion pathways in patients with markedly impaired renal function have not been determined. Hemodialysis shortens the half-life by 25% to 30%.

Microbiology

In vitro tests demonstrate that the bactericidal action of the cephalosporins results from inhibition of cell-wall synthesis. Cefaclor has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS section.

Aerobes, Gram-positive

Staphylococci, including coagulase-positive, coagulase-negative, and penicillinase-producing strains
Streptococcus pneumoniae
Streptococcus pyogenes (group A (β-hemolytic streptococci)

Aerobes, Gram-negative

Escherichia coli
Haemophilus influenzae, excluding {β-lactamase-negative ampicillin-resistant strains
Klebsiella spp.
Proteus mirabilis

The following in vitro data are available, but their clinical significance is unknown.

Cefaclor exhibits in vitro minimal inhibitory concentrations (MICs) of ≤ 8 µglmL against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of cefaclor in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.

Aerobes, Gram-negative

Citrobacter diversus
Moraxella (Branhamella)
catarrhalis Neisseria gonorrhoeae

Anaerobes, Gram-positive

Bacteroides spp. (excluding Bacteroides fragilis)
Peptococcus
Peptostreptococcus
Propionibacterium acnes

Note: Pseudomonas spp., Acinetobacter calcoaceticus and most strains of enterococci (Enterococcus faecalis, group D streptococci), Enterobacter spp., indole-positive Proteus, Morganella morganii (formerly Proteus morganii), Providencia rettgeri (formerly Proteus rettgeri), and Serratia spp. are resistant to cefaclor. When tested by in vitro methods, staphylococci exhibit cross-resistance between cefaclor and methicillin-type antibiotics.

Susceptibility Testing

Dilution Techniques - Quantitative methods that are used to determine minimum inhibitory concentrations (MIC) provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure that has been recommended for use with cefaclor powder uses a standardized dilution method¹ (broth, agar, or microdilution). The MIC values obtained should be interpreted according to the following criteria:

Note:β-lactamase-negative, ampicillin-resistant strains of H. influenzae should be considered resistant to cefaclor despite apparent in vitro susceptibility to this agent.

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.

Standardized susceptibility test procedures require the use of laboratory control microorganisms. Standard cefaclor powder should provide the following MIC values:

Diffusion Techniques - Quantitative methods that require measurement of zone diameters provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure² that has been recommended for use with disks to test the susceptibility of microorganisms to cefaclor uses the 30-µg cefaclor disk. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for cefaclor. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30-µg cefaclor disk should be interpreted according to the following criteria:

When Testing Organisms Other Than Haemophilus spp. and Streptococci

When testing H. influenzae*

Note: β-lactamase-negative, ampicillin-resistant strains of H. influenzae should be considered resistant to cefaclor despite apparent in vitro susceptibility to this agent.

Interpretation should be as stated above for results using dilution techniques.

As with standard dilution techniques, diffusion methods require the use of laboratory control microorganisms. The 30-µg cefaclor disk should provide the following zone diameters in these laboratory test quality control strains:

When testing H. influenzae*

REFERENCES

1. National Committee for Clinical Laboratory Standards. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically - Fourth Edition. Approved Standard NCCLS Document M7-A4, Vol. 17, No. 2, NCCLS, Wayne, PA, January, 1997.

2. National Committee for Clinical Laboratory Standards. Performance Standards for Antimicrobial Disk Susceptibility Tests - Sixth Edition. Approved Standard NCCLS Document M2-A6, Vol. 17, No. 1, NCCLS, Wayne, PA, January, 1997.

Last reviewed on RxList: 12/12/2007
This monograph has been modified to include the generic and brand name in many instances.