Human Pharmacology
Keflex is acid stable and may be given without regard to meals. It is rapidly
absorbed after oral administration. Following doses of 250 mg, 500 mg, and 1
g, average peak serum levels of approximately 9, 18, and 32 μg/mL respectively
were obtained at 1 hour. Measurable levels were present 6 hours after administration.
Cephalexin is excreted in the urine by glomerular filtration and tubular secretion.
Studies showed that over 90% of the drug was excreted unchanged in the urine
within 8 hours. During this period, peak urine concentrations following the
250-mg, 500-mg, and 1-g doses were approximately 1000, 2200, and 5000 μg/mL
respectively.
Microbiology
In vitro tests demonstrate that the cephalosporins are bactericidal
because of their inhibition of cell-wall synthesis. Cephalexin 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
AND USAGE section.
Aerobes, Gram-positive:
Staphylococcus aureus (including penicillinase-producing strains)
Streptococcus pneumoniae (penicillin-susceptible strains)
Streptococcus pyogenes
Aerobes, Gram-negative:
Escherichia coli
Haemophilus influenzae
Klebsiella pneumoniae
Moraxella (Branhamella) catarrhalis
Proteus mirabilis
Note — Methicillin-resistant staphylococci and most strains of enterococci
(Enterococcus faecalis [formerly Streptococcus faecalis]) are
resistant to cephalosporins, including cephalexin. It is not active against
most strains of Enterobacter spp., Morganella morganii, and Proteus
vulgaris. It has no activity against Pseudomonas spp. or Acinetobacter
calcoaceticus. Penicillin-resistant Streptococcus pneumoniae is usually
cross-resistant to beta-lactam antibiotics.
Susceptibility Tests
Dilution techniques — Quantitative methods are used to determine
antimicrobial minimal inhibitory concentrations (MIC's). These MIC's provide
estimates of the susceptibility of bacteria to antimicrobial compounds. The
MIC's should be determined using a standardized procedure. Standardized procedures
are based on a dilution method1-3 (broth or agar) or equivalent with standardized
inoculum concentrations and standardized concentrations of cephalothin powder.
The MIC values should be interpreted according to the following criteria:
| MIC (μg/mL) |
Interpretation |
| ≤ 8 |
Susceptible (S) |
| 16 |
Intermediate (I) |
| ≥ 32 |
Resistant (R) |
A report of “Susceptible” indicates that the pathogen is likely to
be inhibited if the antimicrobial compound in the blood reaches the concentrations
usually achievable. 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 which prevents small
uncontrolled technical factors from causing major discrepancies in interpretation.
A report of “Resistant” indicates that the pathogen is not likely
to be inhibited if the antimicrobial compound in the blood reaches the concentrations
usually achievable; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory control
microorganisms to control the technical aspects of the laboratory procedures.
Standard cephalothin powder should provide the following MIC values:
| Microorganism |
MIC (μg/mL) |
| E. coliATCC 25922 |
4-16 |
| S. aureus ATCC 29213 |
0.12-0.5 |
Diffusion techniques — Quantitative methods that require measurement
of zone diameters also provide reproducible estimates of the susceptibility
of bacteria to antimicrobial compounds. One such standardized procedure2,3
requires the use of standardized inoculum concentrations. This procedure uses
paper disks impregnated with 30-μg cephalothin to test the susceptibility
of microorganisms to cephalexin.
Reports from the laboratory providing results of the standard single-disk susceptibility
test with a 30-μg cephalothin disk should be interpreted according to the
following criteria:
| Zone Diameter (mm) |
Interpretation |
| ≥ 18 |
Susceptible (S) |
| 15-17 |
Intermediate (I) |
| ≤ 14 |
Resistant (R) |
Interpretation should be as stated above for results using dilution techniques.
Interpretation involves correlation of the diameter obtained in the disk test
with the MIC for cephalexin.
As with standard dilution techniques, diffusion methods require the use of
laboratory control microorganisms that are used to control the technical aspects
of the laboratory procedures. For the diffusion technique, the 30-μg cephalothin
disk should provide the following zone diameters in these laboratory test quality
control strains:
| Microorganism |
Zone Diameter (mm) |
| E. coli ATCC 25922 |
15-21 |
| S. aureus ATCC 25923 |
29-37 |
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.
3. National Committee for Clinical Laboratory Standards. Performance
Standards for Antimicrobial Susceptibility Testing — Eighth Informational Supplement.
Approved Standard NCCLS Document M100-S8, Vol. 18, No. 1, NCCLS, Wayne, PA,
January, 1998.
Last updated on RxList: 12/19/2008