CLINICAL PHARMACOLOGY
Chloramphenicol administered orally is absorbed rapidly from the intestinal
tract. In controlled studies in adult volunteers using the recommended dosage
of 50 mg/kg/day, a dosage of 1 g every 6 hours for 8 doses was given. Using
the microbiological assay method, the average peak serum level was 11.2 mcg/mL
one hour after the first dose. A cumulative effect gave a peak rise to 18.4
mcg/mL after the fifth dose of 1 g. Mean serum levels ranged from 8 to 14 mcg/mL
over the 48-hour period. Total urinary excretion of chloramphenicol in these
studies ranged from a low of 68% to a high of 99% over a three-day period. From
8% to 12% of the antibiotic excreted is in the form of free chloramphenicol;
the remainder consists of microbiologically inactive metabolites, principally
the conjugate with glucuronic acid. Since the glucuronide is excreted rapidly,
most chloramphenicol detected in the blood is in the microbiologically active
free form. Despite the small proportion of unchanged drug excreted in the urine,
the concentration of free chloramphenicol is relatively high, amounting to several
hundred mcg/mL in patients receiving divided doses of 50 mg/kg/day. Small amounts
of active drug are found in bile and feces. Chloramphenicol diffuses rapidly,
but its distribution is not uniform. Highest concentrations are found in liver
and kidney, and lowest concentrations are found in brain and cerebrospinal fluid.
Chloramphenicol enters cerebrospinal fluid even in the absence of meningeal
inflammation, appearing in concentrations about half of those found in the blood.
Measurable levels are also detected in pleural and in ascitic fluids, saliva,
milk, and in the aqueous and vitreous humors. Transport across the placental
barrier occurs with somewhat lower concentration in cord blood of neonates than
in maternal blood.
Microbiology
Chloramphenicol is a broad-spectrum antibiotic originally isolated from Streptomyces
venezuelae. It inhibits bacterial protein synthesis by interfering with
the transfer of activated amino acids from soluble RNA to ribsomes. In vitro,
chloramphenicol exerts mainly a bacteriostatic effect on a wide range of gram-negative
and gram-positive bacteria. Bacteriological studies should be performed to determine
the causative organisms and their susceptibilities to chloramphenicol.
Chloramphenicol 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.
Aerobic gram-negative microorganisms
Haemophilus influenzae
Salmonella species, including Salmonella typhi
Other microorganisms
Lymphogranuloma-psittacosis group
Rickettsia
Susceptibility Testing Methods
Dilution Techniques: Quantitative methods are used to determine antimicrobial
minimum inhibitory concentrations (MICs). These MICs provide estimates of the
susceptibility of bacteria to antimicrobial compounds. The MICs 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 chloramphenicol powder. The
MIC values should be interpreted according to the following criteria:
For testing extraintestinal isolates of Salmonella species
MIC (μg/mL) |
Interpretation |
< 8 |
Susceptible (S) |
16 |
Intermediate (I) |
> 32 |
Resistant (R) |
For testing Haemophilus influenzaea
MIC (μg/mL) |
Interpretation |
< 2 |
Susceptible (S) |
4 |
Intermediate (I) |
> 8 |
Resistant (R) |
aThese MIC interpretative standards
are applicable only to broth microdilution susceptibility tests with Haemophilus
influenzae using Haemophilus Test Medium (HTM)1. |
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 chloramphenicol powder should provide the following MIC values:
Microorganism |
MIC (μg/mL) |
Escherichia coli ATCC 25922 |
2-8 |
Haemophilus influenzaeb ATCC 49247 |
0.25-1 |
bThis MIC quality control range
is applicable to only Haemophilus influenzae ATCC 49247 tested
by broth microdilution procedure using Haemophilus Test Medium
(HTM)1. |
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 30μg chloramphenicol to test the susceptibilty
of microorgansims to chloramphenicol. Reports from the laboratory providing
results of the standard single-disk susceptibility test with a 30-μg chloramphenicol
disk should be interpreted according to the following criteria:
For testing extraintestinal isolates of Salmonella species
Zone Diameter (mm) |
Interpretation |
> 18 |
Susceptible (S) |
13-17 |
Intermediate (I) |
< 12 |
Resistant (R) |
For testing Haemophilus influenzaec
Zone Diameter |
Interpretation |
≥ 29 |
Susceptible (S) |
26-28 |
Intermediate (I) |
< 25 |
Resistant (R) |
cThese zone diameter interpretative
standards are applicable only to susceptibility tests performed by disk
diffusion with Haemophilus influenzae using Haemophilus Test
Medium (HTM)2. |
As with standardized dilution techniques, diffusion methods require the use
of laboratory control microorganisms to control the technical aspects of the
laboratory procedures. For the diffusion technique, the 30μg chloramphenicol
disk should provide the following zone diameters in these laboratory test quality
control strains:
Microorgansim |
Zone diameter (mm) |
Escherichia coli ATCC 25922 |
21-27 |
Haemophilus influenzaed ATCC 49247 |
31-40 |
dThese zone diameter quality control
limits are applicable to only Haemophilus influenzae ATCC 49247
tested by a disk diffusion method 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.
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.