AMOXICILLIN (Page 3 of 4)

OVERDOSAGE SECTION

In case of overdosage, discontinue medication, treat symptomatically, and institute supportive measures as required. A prospective study of 51 pediatric patients at a poison-control center suggested that overdosages of less than 250 mg/kg of amoxicillin are not associated with significant clinical symptoms. Interstitial nephritis resulting in oliguric renal failure has been reported in a small number of patients after overdosage with amoxicillin1.

Crystalluria, in some cases leading to renal failure, has also been reported after amoxicillin overdosage in adult and pediatric patients. In case of overdosage, adequate fluid intake and diuresis should be maintained to reduce the risk of amoxicillin crystalluria.

Renal impairment appears to be reversible with cessation of drug administration. High blood levels may occur more readily in patients with impaired renal function because of decreased renal clearance of amoxicillin. Amoxicillin may be removed from circulation by hemodialysis.

DESCRIPTION SECTION

Formulationof amoxicillin for oral suspension, USP contains amoxicillin, a semisynthetic antibiotic, an analog of ampicillin, with a broad spectrum of bactericidal activity against many Gram-positive and Gram-negative microorganisms. Chemically, it is (2S,5R,6R)-6-[(R)-(-)-2-amino-2-(p-hydroxyphenyl)acetamido]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid trihydrate. It may be represented structurally as:

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The amoxicillin molecular formula is C16H19N3O5S•3H2O, and the molecular weight is 419.45.

Each 5 mL of reconstituted suspension contains amoxicillin trihydrate equivalent to 200 mg or 400 mg anhydrous amoxicillin. Each 5 mL of the 200 mg and 400 mg reconstituted suspension contains 0.16 mEq (3.61 mg) of sodium; Inactive ingredients: sucrose, sodium citrate, sodium benzoate, edetate disodium, FD&C Red No. 3, xanthan gum, bubble-gum flavor, and colloidal silicon dioxide.

CLINICAL PHARMACOLOGY SECTION

12.1 Mechanism of Action

Amoxicillin is an antibacterial drug [see Clinical Pharmacology (12.4)].12.3 Pharmacokinetics

Absorption: 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 amoxicillin has been partially investigated; 400 mg and 875 mg formulations have been studied only when administered at the start of a light meal.

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 mg 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 chewable tablets and 400 mg/5 mL suspension of amoxicillin to 24 adult volunteers yielded comparable pharmacokinetic data:

Table 3: Mean Pharmacokinetic Parameters of Amoxicillin (400 mg chewable tablets and 400 mg/5 mL suspension) in Healthy Adults

* Administered at the start of a light meal. † Mean values of 24 normal volunteers. Peak concentrations occurred approximately 1 hour after the dose.
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)

Distribution: Amoxicillin diffuses readily into most body tissues and fluids, with the exception of brain and spinal fluid, except when meninges are inflamed. In blood serum, amoxicillin is approximately 20% protein-bound. 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.

Metabolism and Excretion: The half-life of amoxicillin is 61.3 minutes. Approximately 60% of an orally administered dose of amoxicillin is excreted in the urine within 6 to 8 hours. Detectable serum levels are observed up to 8 hours after an orally administered dose of amoxicillin. Since most of the amoxicillin is excreted unchanged in the urine, its excretion can be delayed by concurrent administration of probenecid [see Drug Interactions (7.1)].12.4 Microbiology

Mechanism of Action

Amoxicillin is similar to penicillin in its bactericidal action against susceptible bacteria during the stage of active multiplication. It acts through the inhibition of cell wall biosynthesis that leads to the death of the bacteria.

Method of Resistance

Resistance to amoxicillin is mediated primarily through enzymes called beta-lactamases that cleave the beta-lactam ring of amoxicillin, rendering it inactive.

Amoxicillin has been shown to be active against most isolates of the bacteria listed below, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Gram-Positive Bacteria Gram-Negative Bacteria
Enterococcus faecalis Staphylococcusspp. Streptococcus pneumoniae Alpha and β-hemolytic streptococci. Escherichia coli Haemophilus influenzae Neisseria gonorrhoeae Proteus mirabilis Helicobacter pylori

Susceptibility Test Methods: (susceptibility to amoxicillin can be determined using ampicillin powder and a 10 mcg ampicillin disk)

When available, clinical microbiology should provide the results of in vitro susceptibility test results for antimicrobial drugs used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antimicrobial drug product for treatment.

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 dilution methods (broth or agar)2,3 or equivalent with standardized inoculum concentrations and standardized concentrations of ampicillin powder. The MIC values should be interpreted according to the criteria in Table 4.

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 procedure3 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 10 mcg ampicillin to test the susceptibility of bacteria to ampicillin. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for amoxicillin. 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 criteria listed in Table 4.

Table 4. Susceptibility Test Interpretive Criteria for Amoxicillin
Minimum Inhibitory Concentration (mcg/mL) Disk Diffusion (zone diameter in mm)
Susceptible Intermediate Resistant Susceptible Intermediate Resistant
* 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. ** A positive beta lactamase test indicates resistance to amoxicillin. Isolates that are resistant to penicillin by MIC testing are also expected to be resistant to amoxicillin.
Enterococcus spp. ≤ 8 ≥ 16 ≥ 17 ≤ 16
Staphylococcus spp. ≤ 0.25 ≥ 0.5 ≥ 29 ≤ 28
Streptococci, viridians group (alpha-hemolytic streptococci) ≤ 0.25 0.5 to 4 ≥ 8
β-hemolytic streptococci ≤ 0.25 ≥ 24
Streptococcus pneumoniae(non-meningitis isolates)* ≤ 2 4 ≥ 8
Enterobacteriaceae ≤ 8 16 ≥ 32 ≥ 17 14 to 16 ≤ 13
Haemophilus influenzae ≤ 1 2 ≥ 4 ≥ 22 19 to 21 ≤ 18
Neisseria gonorrhoeae**

A report of “Susceptible” indicates the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches concentrations that are usually achievable. A report of “Intermediate” indicates that result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. The intermediate 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. The intermediate category also provides a buffer zone, which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches concentrations that are usually achievable and other therapy(ies) are likely to be preferred.

Quality Control

Susceptibility techniques require use of laboratory control microorganisms to control the technical aspects of the laboratory standardized procedures.2,3,4 Standard ampicillin powder should provide the MIC values described below. For the diffusion technique using the 10 mcg ampicillin disk, the criteria are provided in Table 5.

Table 5. Acceptable QualityControlRanges for Amoxicillin
Bacteria ATCC# MICRange (mcg/mL) DiskDiffusionZoneRange (mm)
# ATCC = American Type Culture Collection
Escherichia coli 25922 2 to 8 16 to 22
Enterococcus faecalis 29212 0.5 to 2
Haemophilus influenzae 49247 2 to 8 13 to 21
Staphylococcus aureus 29213 0.5 to 2
25923 27 to 35
Streptococcus pneumoniae 49619 0.06 to 0.25

Susceptibility Testing for Helicobacter pylori: Amoxicillin in vitro susceptibility testing methods for determining minimum inhibitory concentrations (MICs) and zone sizes have not been standardized, validated, or approved for testing H. pylori. Specimens for H. pylori and clarithromycin susceptibility test results should be obtained on isolates from patients who fail triple therapy. If clarithromycin resistance is found, a non-clarithromycin-containing regimen should be used.

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