CLINICAL PHARMACOLOGY
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 conventional 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 conventional tablets of amoxicillin with 875 mg conventional tablets of amoxicillin/clavulanate potassium showed that the 875 mg conventional 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 conventional 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 conventional amoxicillin chewable tablets and 400 mg/5 mL conventional suspension to 24 adult volunteers yielded comparable pharmacokinetic data:
| Dose † | AUC0-∞(mcg•hr/mL) | Cmax (mcg/mL) ‡ |
| amoxicillin | amoxicillin | amoxicillin |
| 400 mg (5 mL of suspension) | 17.1 (3.1) | 5.92 (1.62) |
| 400 mg (1 chewable tablets) | 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.
The following pharmacokinetic data is from Ranbaxy’s study of DisperMox tablets and conventional amoxicillin oral suspension, 400 mg/5 mL. A dispersed mixture of DisperMox tablets, 400 mg, produced blood levels similar to those achieved with the corresponding doses of conventional amoxicillin oral suspensions. Orally administered doses of conventional amoxicillin suspension, 400 mg/5 mL, result in average peak blood levels 1 to 2 hours after administration in the range of 3.3 mcg/mL to 11.5 mcg/mL. Orally administered doses of 400 mg DisperMox tablets result in average peak blood levels 1 to 2 hours after administration in the range of 3.2 mcg/mL to 11.5 mcg/mL.
Oral administration of single doses of 400 mg DisperMox tablets and 400 mg/5 mL conventional suspension to 24 adult volunteers yielded comparable pharmacokinetic data.
| Dose¶ | In AUC0-∞ (mcg•hr/mL) | Cmax (mcg/mL)†† |
| amoxicillin | amoxicillin | amoxicillin |
| 400 mg (5 mL of suspension) | 18.5 | 8.4 |
| 400 mg (1 tablet for oral suspension) | 17.9 | 7.5 |
¶ Dosing was following an overnight fast.
†† Mean values of 24 normal volunteers. Peak concentrations occurred approximately 1 hour after the dose.
A dispersed mixture of DisperMox tablets, 600 mg, produced blood levels similar to those achieved with the corresponding dose (7.5 mL of 400 mg/5 mL) of conventional amoxicillin suspension. Orally administered doses of conventional suspension resulted in average peak blood levels 1 to 2 hours after administration in the range of 5.1 mcg/mL to 19.3 mcg/mL. Orally administered doses of 600 mg DisperMox tablets result in average peak blood levels 1 to 2 hours after oral administration in the range of 5 mcg/mL to 18.9 mcg/mL.
Oral administration of single doses of 600 mg DisperMox tablets and 600 mg (7.5 mL of 400 mg/5 mL) of conventional suspension to 25 adult volunteers yielded comparable pharmacokinetic data.
| Dose¶ | In AUC0-∞ (mcg•hr/mL) | Cmax(mcg/mL)†† |
| amoxicillin | amoxicillin | amoxicillin |
| 600 mg (7.5 mL of 400 mg/5 mL suspension) | 31.4 | 12.3 |
| 600 mg (1 tablet for oral suspension) | 29 | 11.9 |
¶Dosing was following an overnight fast.
†† Mean values of 25 normal volunteers. Peak concentrations occurred approximately 1 hour after the dose.
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) |
| Staphylococcus a | |
| MIC (mcg/mL) | Interpretation |
| ≤ 0.25 | Susceptible (S) |
| ≥ 0.5 | Resistant (R) |
| Strepto coccus (excep t S. pneumoniae) |
| MIC (mcg/mL) | Interpretation |
| ≤ 0.25 | Susceptible (S) |
| 0.5 to 4 | Intermediate (I) |
| ≥ 8 | Resistant (R) |
| S. pneumoniae b from non –meningitis sources. (Amoxicilline 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.influenzae c |
| 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 to 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) |
| Staphylococcus f |
| 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.
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