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 the tablets and suspension of AMOXIL 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 5.0 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 AMOXIL with
875 mg of AUGMENTIN® (amoxicillin/clavulanate potassium) showed that the
875-mg tablet of AMOXIL 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 3.0 mcg/mL and 3.5 mcg/mL to 5.0 mcg/mL, respectively.
Oral administration of single doses of 400-mg chewable tablets and 400 mg/5
mL suspension of AMOXIL 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 sources.
(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 2-5% 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) |
α-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.
Clinical Studies
H. pylori Eradication to Reduce the Risk of Duodenal Ulcer Recurrence:
Randomized, double-blind clinical studies performed in the United States
in patients with H. pylori and duodenal ulcer disease (defined as an
active ulcer or history of an ulcer within 1 year) evaluated the efficacy of
lansoprazole in combination with amoxicillin capsules and clarithromycin tablets
as triple 14-day therapy, or in combination with amoxicillin capsules as dual
14-day therapy, for the eradication of H. pylori. Based on the results
of these studies, the safety and efficacy of 2 different eradication regimens
were established:
Triple Therapy: Amoxicillin 1 gram twice daily/clarithromycin
500 mg twice daily/lansoprazole 30 mg twice daily.
Dual Therapy: Amoxicillin 1 gram three times daily/lansoprazole
30 mg three times daily.
All treatments were for 14 days. H. pylori eradication was defined as
2 negative tests (culture and histology) at 4 to 6 weeks following the end of
treatment.
Triple therapy was shown to be more effective than all possible dual therapy
combinations. Dual therapy was shown to be more effective than both monotherapies.
Eradication of H. pylori has been shown to reduce the risk of duodenal
ulcer recurrence.
H. pylori Eradication Rates - Triple Therapy (amoxicillin/clarithromycin/lansoprazole)
Percent of Patients Cured [95% Confidence Interval] (Number of Patients)
| Study |
Triple Therapy |
Triple Therapy |
| |
Evaluable Analysis* |
Intent-to-Treat Analysis† |
| Study 1 |
92‡ |
86‡ |
| [80.0 - 97.7] |
[73.3 - 93.5] |
| (n = 48) |
(n = 55) |
| Study 2 |
86§ |
83§ |
| [75.7 - 93.6] |
[72.0 - 90.8] |
| (n = 66) |
(n = 70) |
* This analysis was based on evaluable patients with confirmed
duodenal ulcer (active or within 1 year) and H. pylori infection
at baseline defined as at least 2 of 3 positive endoscopic tests from
CLOtest®, (Delta West Ltd., Bentley, Australia), histology, and/or
culture. Patients were included in the analysis if they completed the
study. Additionally, if patients dropped out of the study due to an
adverse event related to the study drug, they were included in the analysis
as failures of therapy.
† Patients were included in the analysis if they
had documented H. pylori infection at baseline as defined above
and had a confirmed duodenal ulcer (active or within 1 year). All dropouts
were included as failures of therapy.
‡ (p<0.05) versus lansoprazole/amoxicillin and lansoprazole/clarithromycin
dual therapy.
§(p<0.05) versus clarithromycin/amoxicillin dual therapy.
|
H. pylori Eradication Rates - Dual Therapy (amoxicillin/lansoprazole)
Percent of Patients Cured [95% Confidence Interval] (Number of Patients)
| Study |
Dual Therapy |
Dual Therapy |
| |
Evaluable Analysis* |
Intent-to-Treat Analysis† |
| Study 1 |
77‡ |
70‡ |
| [62.5 - 87.2] |
[56.8 - 81.2] |
| (n = 51) |
(n = 60) |
| Study 2 |
66§ |
61§ |
| [51.9 - 77.5] |
[48.5 - 72.9] |
| (n = 58) |
(n = 67) |
* This analysis was based on
evaluable patients with confirmed duodenal ulcer (active or within 1 year)
and H. pylori infection at baseline defined as at least 2 of 3
positive endoscopic tests from CLOtest®, histology, and/or culture.
Patients were included in the analysis if they completed the study. Additionally,
if patients dropped out of the study due to an adverse event related to
the study drug, they were included in the analysis as failures of therapy.
† Patients were included in the analysis if they had
documented H. pylori infection at baseline as defined above and
had a confirmed duodenal ulcer (active or within 1 year). All dropouts
were included as failures of therapy.
‡(p<0.05) versus lansoprazole alone.
§(p<0.05) versus lansoprazole alone or amoxicillin alone. |
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: 7/10/2008