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:
| MIC (µg/mL) |
Interpretation* |
| ≤ 8 |
Susceptible (S) |
| 16 |
Intermediate (I) |
| ≥ 32 |
Resistant (R) |
| *When testing H. influenzae spp. these interpretive
standards are applicable only to broth microdilution method using Haemophilus
Test Medium (HTM)1 |
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:
| Microorganism |
|
MIC (µg/mL) |
| E. coli |
ATCC 25922 |
1-4 |
| E. faecalis |
ATCC 29212 |
> 32 |
| S. aureus |
ATCC 29213 |
1-4 |
| When testing H. influenzae* |
| Microorganism |
|
MIC (µg/mL) |
| H. influenzae |
ATCC 49766 |
1-4 |
| *Broth microdilution test performed using Haemophilus Test
Medium (HTM)1 |
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
| Zone Diameter (mm) |
Interpretation |
| ≥ 18 |
Susceptible (S) |
| 15-17 |
Intermediate (I) |
| ≤ 14 |
Resistant (R) |
When testing H. influenzae*
| Zone Diameter (mm) |
Interpretation |
| ≥ 20 |
Susceptible (S) |
| 17-19 |
Intermediate (I) |
| ≤ 16 |
Resistant (R) |
| *Disk susceptibility test performed using Haemophilus Test
Medium (HTM)2 |
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:
| Microorganism |
Zone Diameter (mm) |
| E. coli ATCC 25922 |
23-27 |
| S. aureus ATCC 25923 |
27-31 |
When testing H. influenzae*
| Microorganism |
|
Zone Diameter (mm) |
| H. influenzae |
ATCC 49766 |
25-31 |
| *Disk susceptibility test performed using Haemophilus Test
Medium (HTM)2 |
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 updated on RxList: 12/12/2007