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Mechanism of Action
MOXATAG is an extended-release formulation of amoxicillin intended to provide once-daily dosing. Following the administration of MOXATAG with a low-fat meal in healthy subjects, mean amoxicillin AUC0-∞, Cmax, and Tmax values were 29.8 µg•h/mL, 6.6 ug/mL and 3.1 hours, respectively. The mean plasma concentration-time curve is shown below in Figure 1.
Figure 1. Mean Amoxicillin Plasma Concentrations Following
a Single Oral Dose of MOXATAG (amoxicillin extended-release tablets) With a Low-Fat Meal in Healthy Subjects (N=20)
Administration of MOXATAG with food decreases the rate, but not the extent of amoxicillin absorption. Compared to immediate-release amoxicillin suspension, the rate of amoxicillin absorption following administration of MOXATAG was slower, resulting in a lower Cmax and longer Tmax. Total amoxicillin exposure (AUC) achieved with MOXATAG (amoxicillin extended-release tablets) is similar to that observed after oral administration of a comparable dose of immediate-release amoxicillin suspension.
Amoxicillin diffuses readily into most body tissues and fluids, with the exception of brain and spinal fluid, except when meninges are inflamed. Amoxicillin is approximately 20% protein bound in human serum.
Amoxicillin is primarily cleared by renal excretion. Approximately 60% of an oral dose of immediate-release amoxicillin is eliminated unchanged in urine. The half-life of amoxicillin after oral administration of MOXATAG is approximately 1.5 hours, similar to that of immediate-release amoxicillin. No accumulation of amoxicillin was observed after once-daily dosing of 775 mg of MOXATAG (amoxicillin extended-release tablets) for 7 days.
In a study of healthy adult subjects, amoxicillin AUC was similar whereas Cmax increased approximately 35% following the administration of lansoprazole with MOXATAG (amoxicillin extended-release tablets) given with food.
Probenecid decreases the renal tubular secretion of amoxicillin. Concurrent use of MOXATAG (amoxicillin extended-release tablets) and probenecid may result in increased and prolonged blood levels of amoxicillin. The clinical relevance of this finding has not been evaluated.
Mechanism of Action
Mechanism of Resistance
To date there are no known mechanisms of resistance to penicillin or amoxicillin in Streptococcus pyogenes.
MOXATAG (amoxicillin extended-release tablets) has been shown to be active in vitro against isolates of the microorganism S. pyogenes and in clinical infections as described in the INDICATIONS section.
Facultative Gram-Positive Bacteria:
The following in vitro data are available, but their clinical significance is unknown. At least 90% of the following microorganisms exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptibility breakpoint of amoxicillin (as determined by susceptibility tests using the class representative agents penicillin or ampicillin).
Facultative Gram Positive Bacteria:
Streptococcus spp. (Group B, C, and G; β-hemolytic)
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide cumulative results of the in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas 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 the most effective antimicrobial.
Susceptibility testing of penicillins (such as amoxicillin) and other β-lactams approved by FDA for the treatment of Group A streptococcus (S. pyogenes) is not routinely necessary for clinical purposes. Isolates of Group A streptococcus resistant to amoxicillin have not been recognized and therefore all isolates can be considered susceptible to amoxicillin However, susceptibility tests can be conducted using dilution or diffusion techniques employing penicillin or ampicillin to predict susceptibility to amoxicillin.
Quantitative methods are used to determine antimicrobial MICs. These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure1,3. Standardized procedures are based on a dilution method (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of penicillin or ampicillin. The susceptibility of Group A streptococcus to penicillin or amoxicillin should be interpreted according to the criteria in Table 2.
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 a standardized inoculum concentration. This procedure uses paper disks impregnated with 10 units penicillin or 10 mcg ampicillin to test the susceptibility of S. pyogenes to penicillin or amoxicillin. Reports from the laboratory providing results of the standard single-disk susceptibility test should be interpreted according to the criteria in Table 2
Table 2: Susceptibility Test Results Interpretive Criteria
for Beta-Hemolytic Streptococci including Streptococcus pyogenes
|Antimicrobial||Minimum Inhibitory Concentration (mcg/mL)||Disk Diffusion (zone diameter in mm)|
|Penicillin||≤ 0.12 = Susceptible||≥ 24 = Susceptible|
|Ampicillin||≤ 0.25 = Susceptible||≥ 24 = Susceptible|
The current absence of data on resistant isolates precludes defining any categories other than "Susceptible". Isolates yielding results suggestive of a "nonsusceptible" category should be retested, and if the result is confirmed, the isolate should be submitted to a reference laboratory for further testing.
Isolates yielding results suggestive of a "nonsusceptible" category should be retested, and if the result is confirmed, the isolate should be submitted to a reference laboratory for further testing.
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test. Standard penicillin or ampicillin powders should provide the MIC ranges provided in Table 3. For the disk diffusion technique using the 10 unit penicillin disk or 10 µg ampicillin disk the criteria in Table 3 should be achieved.
Table 3: Acceptable Quality Control Ranges for Susceptibility
|QC Organism||Antimicrobial|| Minimum Inhibitory Concentration
|Disk Diffusion (zone diameter in mm)|
|Streptococcus pneumoniae||Penicillin||0.25 - 1||24 - 30|
|ATCC 49619||Ampicillin||0.06 - 0.25||30 - 36|
|ATCC = American Type Culture Collection|
In a randomized, parallel-group, multi-center, double-blind, double- dummy study in adults and pediatrics (age ≥ 12 years) with tonsillitis and/or pharyngitis secondary to S. pyogenes, MOXATAG (amoxicillin extended-release tablets) 775 mg QD for 10 days was non-inferior to penicillin VK 250 mg QID for 10 days.
Using strict evaluability and microbiologic response criteria 4-8 days post-therapy, the following bacteriological eradication rates and statistical outcomes in the per-protocol (PPb) and modified intent-to-treat (mITT) populations were obtained (Table 4). The mITT population included all randomized patients with a positive throat culture for S. pyogenes at baseline. The PPb population included mITT patients who had post-therapy cultures, were compliant with treatment, and didn't have major protocol violations.
Table 4: Bacteriological Eradication Rates in Patients with
Tonsillitis and/or Pharyngitis
|MOXATAG||Penicillin VK|| Rate Difference
95% CI (%)
1.) Clinical and Laboratory Standards Institute (CLSI). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard-7th ed Clinical and Laboratory Standards Institute document M7-A7. Clinical and Laboratory Standards Institute, 940 West Valley Road, Suite 1400, Wayne, Pennsylvania 19087-1898 USA, 2006.
2.) CLSI. Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard-9th ed. CLSI document M2-A9 CLSI, Wayne, PA 19087-1898 2006.
3.) CLSI. Performance Standards for Antimicrobial Susceptibility Testing; 17th Informational Supplement. CLSI document M100-S17 CLSI, Wayne, PA 19087-1898, 2007.
Last reviewed on RxList: 2/8/2008
This monograph has been modified to include the generic and brand name in many instances.
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