Amoxicillin is stable in the presence of gastric acid and is
rapidly absorbed after oral administration. The effect of food on the absorption
of amoxicillin from amoxicillin tablets and amoxicillin suspension has been
partially investigated. The 400 mg and 875 mg formulations have been studied
only when administered at the start of a light meal. However, food effect
studies have not been performed with the 200 mg and 500 mg formulations.
Amoxicillin diffuses readily into most body tissues and fluids, with the
exception of brain and spinal fluid, except when meninges are inflamed. The
half-life of amoxicillin is 61.3 minutes. Most of the amoxicillin is excreted
unchanged in the urine; its excretion can be delayed by concurrent
administration of probenecid. In blood serum, amoxicillin is approximately 20%
protein-bound.
Orally administered doses of 250 mg and 500 mg amoxicillin capsules result in
average peak blood levels 1 to 2 hours after administration in the range of 3.5
mcg/mL to 5 mcg/mL and 5.5 mcg/mL to 7.5 mcg/mL, respectively.
Mean amoxicillin pharmacokinetic parameters from an open, two-part,
single-dose crossover bioequivalence study in 27 adults comparing 875 mg of
amoxicillin with 875 mg of amoxicillin/ clavulanate potassium showed that the
875 g tablet of amoxicillin produces an AUC0-∞ of 35.4 ±8.1 mcg•hr/mL and a Cmax of
13.8 ±4.1 mcg/mL. Dosing was at the start of a light meal following an overnight
fast.
Orally administered doses of amoxicillin suspension, 125 mg/5 mL and 250 mg/5
mL, result in average peak blood levels 1 to 2 hours after administration in the
range of 1.5 mcg/mL to 3 mcg/mL and 3.5 mcg/mL to 5 mcg/mL, respectively.
Oral administration of single doses of 400 mg amoxicillin chewable tablets
and 400 mg/5 mL suspension to 24 adult volunteers yielded comparable
pharmacokinetic data:
| Dose† | AUC0-∞ (mcg•hr/mL) | Cmax (mcg/mL)‡ |
| Amoxicillin | Amoxicillin (± S.D.) | Amoxicillin (±S.D.) |
| 400 mg (5 mL of suspension) | 17.1 (3.1) | 5.92 (1.62) |
| 400 mg (1 chewable tablet) | 17.9 (2.4) | 5.18 (1.64) |
† Administered at the start of a light meal.
‡ Mean values of 24 normal volunteers. Peak
concentrations occurred approximately 1 hour after the dose.
Detectable serum levels are observed up to 8 hours after an orally
administered dose of amoxicillin. Following a 1 gram dose and utilizing a
special skin window technique to determine levels of the antibiotic, it was
noted that therapeutic levels were found in the interstitial fluid.
Approximately 60% of an orally administered dose of amoxicillin is excreted in
the urine within 6 to 8 hours.
Microbiology: Amoxicillin is similar to ampicillin in
its bactericidal action against susceptible organisms during the stage of active
multiplication. It acts through the inhibition of biosynthesis of cell wall
mucopeptide. Amoxicillin 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-Positive
Microorganisms:
Enterococcus faecalis
Staphylococcus spp.†
(β-lactamase-negative strains only)
Streptococcus pneumoniae
Streptococcus spp. (α- and β-hemolytic strains
only)
† Staphylococci which are susceptible to amoxicillin
but resistant to methicillin/oxacillin should be considered as resistant to
amoxicillin.
Aerobic Gram-Negative
Microorganisms:
Escherichia coli (β-lactamase-negative strains
only)
Haemophilus influenzae (β-lactamase-negative
strains only)
Neisseria gonorrhoeae (β-lactamase-negative
strains only)
Proteus mirabilis (β-lactamase-negative strains
only)
Helicobacter:
Helicobacter pylori
Susceptibility Tests: 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 (broth or agar) or equivalent with standardized
inoculum concentrations and standardized concentrations of ampicillin powder. Ampicillin is sometimes used to predict
susceptibility of S. pneumoniae to amoxicillin;
however, some intermediate strains have been shown to be susceptible to
amoxicillin. Therefore, S. pneumoniae susceptibility
should be tested using amoxicillin powder. The MIC values should be interpreted
according to the following criteria:
For Gram-Positive Aerobes:
Enterococcus
| MIC (mcg/mL) | Interpretation |
| ≤ 8 | Susceptible (S) |
| ≥ 16 | Resistant (R) |
Staphylococcusa
| MIC (mcg/mL) | Interpretation |
| ≤ 0.25 | Susceptible (S) |
| ≥ 0.5 | Resistant (R) |
Streptococcus (except S.
pneumoniae)
| MIC (mcg/mL) | Interpretation |
| ≤ 0.25 | Susceptible (S) |
| 0.5 to 4 | Intermediate (I) |
| ≥ 8 | Resistant (R) |
S. pneumoniaeb (From
non-meningitis source.)
(Amoxicillin powder should be used to determine
susceptibility.)
| MIC (mcg/mL) | Interpretation |
| ≤ 2 | Susceptible (S) |
| 4 | Intermediate (I) |
| ≥ 8 | Resistant (R) |
NOTE: These interpretive criteria are based on the
recommended doses for respiratory tract infections.
For Gram-Negative Aerobes:
Enterobacteriaceae
| MIC (mcg/mL) | Interpretation |
| ≤ 8 | Susceptible (S) |
| 16 | Intermediate (I) |
| ≥ 32 | Resistant (R) |
H. influenzaec
| MIC (mcg/mL) | Interpretation |
| ≤ 1 | Susceptible (S) |
| 2 | Intermediate (I) |
| ≥ 4 | Resistant (R) |
a Staphylococci which are susceptible to amoxicillin
but resistant to methicillin/oxacillin should be considered as resistant to
amoxicillin.
b These interpretive standards are applicable only to
broth microdilution susceptibility tests using cation-adjusted Mueller-Hinton
broth with 2-5% lysed horse blood.
c These interpretive standards are applicable only to
broth microdilution test with H. 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 ampicillin powder should provide
the following MIC values:
| Microorganism | MIC
Range (mcg/mL) |
| E. coli | ATCC 25922 | 2 to 8 |
| E. faecalis | ATCC 29212 | 0.5 to 2 |
| H. influenzae | ATCC 49247d | 2 to 8 |
| S. aureus | ATCC 29213 | 0.25 to 1 |
Using amoxicillin to determine susceptibility
| Microorganism | MIC
Range (mcg/mL) |
| S. pneumoniae | ATCC 49619e | 0.03 to 0.12 |
d This quality control range is applicable to only
H. influenzae ATCC 49247 tested by a broth
microdilution procedure using HTM.1
e This quality control range is applicable to only
S. pneumoniae ATCC 49619 tested by the broth
microdilution procedure using cation-adjusted Mueller-Hinton broth with 2 to 5%
lysed horse blood.
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 requires the use of standardized inoculum concentrations.
This procedure uses paper disks impregnated with 10 mcg ampicillin to test the
susceptibility of microorganisms, except S.
pneumoniae, to amoxicillin. Interpretation involves correlation of the
diameter obtained in the disk test with the MIC for ampicillin.
Reports from the laboratory providing results of the standard single-disk
susceptibility test with a 10 mcg ampicillin disk should be interpreted
according to the following criteria:
For Gram-Positive Aerobes:
Enterococcus
| Zone Diameter
(mm) | Interpretation |
| ≥ 17 | Susceptible (S) |
| ≤ 16 | Resistant (R) |
Staphylococcusf
| Zone Diameter
(mm) | Interpretation |
| ≥ 29 | Susceptible (S) |
| ≤ 28 | Resistant (R) |
β-hemolytic streptococci
| Zone Diameter
(mm) | Interpretation |
| ≥ 26 | Susceptible (S) |
| 19 to 25 | Intermediate (I) |
| ≤ 18 | Resistant (R) |
NOTE: For streptococci (other than β-hemolytic
streptococci and S. pneumoniae), an ampicillin MIC
should be determined.
S. pneumoniae
S. pneumoniae should be tested using a 1 mcg
oxacillin disk. Isolates with oxacillin zone sizes of ≥ 20 mm are susceptible to
amoxicillin. An amoxicillin MIC should be determined on isolates of S. pneumoniae with oxacillin zone sizes of≤ 19 mm.
For Gram-Negative Aerobes:
Enterobacteriaceae
| Zone Diameter
(mm) | Interpretation |
| ≥ 17 | Susceptible (S) |
| 14 to 16 | Intermediate (I) |
| ≤ 13 | Resistant (R) |
H. influenzaeg
| | |
| Zone Diameter
(mm) | Interpretation |
| ≥ 22 | Susceptible (S) |
| 19 to 21 | Intermediate (I) |
| ≤ 18 | Resistant (R) |
f Staphylococci which are susceptible to amoxicillin
but resistant to methicillin/oxacillin should be considered as resistant to
amoxicillin.
g These interpretive standards are applicable only to
disk diffusion susceptibility tests with H. influenzae
using Haemophilus Test Medium (HTM).2
Interpretation should be as stated above for results using dilution
techniques.
As with standard dilution techniques, disk diffusion susceptibility test
procedures require the use of laboratory control microorganisms. The 10 mcg
ampicillin disk should provide the following zone
diameters in these laboratory test quality control strains:
| Microorganism | Zone Diameter (mm) |
| E. coli | ATCC 25922 | 16 to 22 |
| H. influenzae | ATCC 49247h | 13 to 21 |
| S. aureus | ATCC 25923 | 27 to 35 |
Using 1 mcg oxacillin disk:
| Microorganism | Zone Diameter
(mm) |
| S. pneumoniae | ATCC 49619i | 8 to 12 |
h This quality control range is applicable to only
H. influenzae ATCC 49247 tested by a disk diffusion
procedure using HTM.2
i This quality control range is applicable to only S.
pneumoniae ATCC 49619 tested by a disk diffusion procedure using
Mueller-Hinton agar supplemented with 5% sheep blood and incubated in 5% CO2.
Susceptibility Testing for Helicobacter
pylori: In vitro susceptibility testing
methods and diagnostic products currently available for determining minimum
inhibitory concentrations (MICs) and zone sizes have not been standardized,
validated, or approved for testing H. pylori
microorganisms.
Culture and susceptibility testing should be obtained in patients who fail
triple therapy. If clarithromycin resistance is found, a
non-clarithromycin-containing regimen should be used.