Microbiology

Mechanism of Action

Aztreonam is a bactericidal agent that acts by inhibition of bacterial cell wall synthesis.  Aztreonam has activity in the presence of some beta-lactamases, both penicillinases and cephalosporinases, of Gram-negative and Gram-positive bacteria.

Mechanism of Resistance

Resistance to aztreonam is primarily through hydrolysis by beta-lactamase, alteration of penicillin-binding proteins (PBPs), and decreased permeability.

Interaction with Other Antimicrobials

Aztreonam and aminoglycosides have been shown to be synergistic in vitro against most strains of P. aeruginosa, many strains of Enterobacteriaceae, and other Gram-negative aerobic bacilli.

Aztreonam 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 USAGE5 section.

Aerobic Gram-negative microorganisms:

Citrobacter species

Enterobacter species

Escherichia coli

Haemophilus influenzae (including ampicillin-resistant and other penicillinase-producing strains)

Klebsiella oxytoca

Klebsiella pneumoniae

Proteus mirabilis

Pseudomonas aeruginosa

Serratia species 

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 susceptible breakpoint for aztreonam.  However, the efficacy of aztreonam in treating clinical infections due to these microorganisms has not been established in adequate and well-controlled clinical trials.

Aerobic Gram-negative microorganisms:

Aeromonas hydrophila

Morganella morganii

Neisseria gonorrhoeae (including penicillinase-producing strains)

Pasteurella multocida

Proteus vulgaris

Providencia stuartii

Providencia rettgeri

Yersinia enterocolitica 

Aztreonam and aminoglycosides have been shown to be synergistic in vitro against most strains of P. aeruginosa, many strains of Enterobacteriaceae, and other Gram-negative aerobic bacilli.

Alterations of the anaerobic intestinal flora by broad-spectrum antibiotics may decrease colonization resistance, thus permitting overgrowth of potential pathogens, e.g., Candida and Clostridium species.  Aztreonam has little effect on the anaerobic intestinal microflora in in vitro studies.  Clostridium difficile and its cytotoxin were not found in animal models following administration of aztreonam (see ADVERSE REACTIONS ,Gastrointestinal).

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drug products 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 antibacterial drug product for treatment.

Dilution Techniques

Quantitative methods are used to determine antimicrobial MICs. 1-3  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 method 6 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of aztreonam powder.  The MIC values should be interpreted according to the criteria in Table 2.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds.  The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds.  The zone size should be determined using a standardized test method. 6,7  This procedure uses paper disks impregnated with 30 mcg aztreonam to test the susceptibility of microorganisms to aztreonam.  The disk diffusion interpretive criteria are provided in Table 2. 

                                                                        Table 2:  Susceptibility Test Interpretive Criteria for Aztreonam

a The current absence of data on resistant isolates precludes defining any category other than “Susceptible”.  If isolates yield MIC results other than susceptible, they should be submitted to a reference laboratory for additional testing.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individual performing the test. 6-8  Standard aztreonam powder should provide the following range of MIC values noted in Table 3.  For the diffusion technique using the 30 mcg disk, the criteria in Table 3 should be achieved.  

                                                                       Table 3:  Acceptable Quality Control Ranges for Aztreonam  

Concurrent Therapy

Concurrent initial therapy with other antimicrobial agents and aztreonam for injection is recommended before the causative organism(s) is known in seriously ill patients who are also at risk of having an infection due to Gram-positive aerobic pathogens.  If anaerobic organisms are also suspected as etiologic agents, therapy should be initiated using an anti-anaerobic agent concurrently with aztreonam for injection (see DOSAGE AND ADMINISTRATION).  Certain antibiotics (e.g., cefoxitin, imipenem) may induce high levels of beta-lactamase in vitro in some Gram-negative aerobes such as Enterobacter and Pseudomonas species, resulting in antagonism to many beta-lactam antibiotics including aztreonam.  These in vitro findings suggest that such beta-lactamase-inducing antibiotics not be used concurrently with aztreonam.  Following identification and susceptibility testing of the causative organism(s), appropriate antibiotic therapy should be continued.

General

Prescribing aztreonam for injection in the absence of a proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.

In patients with impaired hepatic or renal function, appropriate monitoring is recommended during therapy.

If an aminoglycoside is used concurrently with aztreonam, especially if high dosages of the former are used or if therapy is prolonged, renal function should be monitored because of the potential nephrotoxicity and ototoxicity of aminoglycoside antibiotics.

The use of antibiotics may promote the overgrowth of nonsusceptible organisms, including Gram-positive organisms ( Staphylococcus aureus and Streptococcus faecalis) and fungi.  Should superinfection occur during therapy, appropriate measures should be taken.

Information for Patients

Patients should be counseled that antibacterial drugs including aztreonam for injection should only be used to treat bacterial infections.  They do not treat viral infections (e.g., the common cold).  When aztreonam for injection is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed.  Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by aztreonam for injection or other antibacterial drugs in the future.

Diarrhea is a common problem caused by antibiotics which usually ends when the antibiotic is discontinued. Sometimes after starting treatment with antibiotics, patients can develop watery and bloody stools (with or without stomach cramps and fever) even as late as 2 or more months after having taken the last dose of the antibiotic.  If this occurs, patients should contact their physician as soon as possible.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenicity studies with aztreonam have not been conducted using an intravenous route of administration.  A 104-week rat inhalation toxicology study to assess the carcinogenic potential of aztreonam demonstrated no drug-related increase in the incidence of tumors.  Rats were exposed to aerosolized aztreonam for up to 4 hours per day.  Peak plasma levels of aztreonam averaging approximately 6.8 mcg/mL were measured in rats at the highest dose level.

Genetic toxicology studies performed with aztreonam in vitro (Ames test, mouse lymphoma forward mutation assay, gene conversion assay, chromosome aberration assay in human lymphocytes) and in vivo (mouse bone marrow cytogenetic assay) did not reveal evidence of mutagenic or clastogenic potential.

A two-generation reproduction study in rats at daily doses of 150, 600, or 2,400 mg/kg given prior to and during gestation and lactation, revealed no evidence of impaired fertility.  Based on body surface area, the high dose is 2.9-fold greater than the maximum recommended human dose (MRHD) for adults of 8 g per day.  There was a slightly reduced survival rate during the lactation period in the offspring of rats that received the highest dose, but not in offspring of rats that received lower doses of aztreonam.

Pregnancy

Pregnancy Category B

In pregnant women, aztreonam crosses the placenta and enters the fetal circulation. 

Developmental toxicity studies in pregnant rats and rabbits with daily doses of aztreonam up to 1,800 and 1,200 mg/kg, respectively, revealed no evidence of embryotoxicity or fetotoxicity or teratogenicity.  These doses, based on body surface area, are 2.2- and 2.9-fold greater than the MRHD for adults of 8 g per day.  A peri/postnatal study in rats revealed no drug-induced changes in any maternal, fetal, or neonatal parameters.  The highest dose used in this study, 1,800 mg/kg/day, is 2.2 times the MRHD based on body surface area.

There are no adequate and well-controlled studies of aztreonam on human pregnancy outcomes.  Because animal reproduction studies are not always predictive of human response, aztreonam should be used during pregnancy only if clearly needed.

Nursing Mothers

Aztreonam is excreted in human milk in concentrations that are less than 1% of concentrations determined in simultaneously obtained maternal serum; consideration should be given to temporary discontinuation of nursing and use of formula feedings.

Pediatric Use

The safety and effectiveness of intravenous aztreonam for injection have been established in the age groups 9 months to 16 years.  Use of aztreonam for injection in these age groups is supported by evidence from adequate and well-controlled studies of aztreonam for injection in adults with additional efficacy, safety, and pharmacokinetic data from non-comparative clinical studies in pediatric patients.  Sufficient data are not available for pediatric patients under 9 months of age or for the following treatment indications/pathogens:  septicemia and skin and skin-structure infections (where the skin infection is believed or known to be due to H. influenzae type b).  In pediatric patients with cystic fibrosis, higher doses of aztreonam for injection may be warranted (see CLINICAL PHARMACOLOGY, DOSAGE AND ADMINISTRATION, and CLINICAL STUDIES).

Geriatric Use

Clinical studies of aztreonam for injection did not include sufficient numbers of subjects aged 65 years and over to determine whether they respond differently from younger subjects.  Other reported clinical experience has not identified differences in responses between the elderly and younger patients. 9-12  In general, dose selection for an elderly patient should be cautious, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

In elderly patients, the mean serum half-life of aztreonam increased and the renal clearance decreased, consistent with the age-related decrease in creatinine clearance. 1-4  Since aztreonam is known to be substantially excreted by the kidney, the risk of toxic reactions to this drug may be greater in patients with impaired renal function.  Because elderly patients are more likely to have decreased renal function, renal function should be monitored and dosage adjustments made accordingly (see DOSAGE AND ADMINISTRATION, Renal Impairment in Adult Patients and Dosage in the Elderly).

Aztreonam for injection contains no sodium.

Pediatric Adverse Reactions

Of the 612 pediatric patients who were treated with aztreonam for injection in clinical trials, less than 1% required discontinuation of therapy due to adverse events.  The following systemic adverse events, regardless of drug relationship, occurred in at least 1% of treated patients in domestic clinical trials: rash (4.3%), diarrhea (1.4%), and fever (1%).  These adverse events were comparable to those observed in adult clinical trials.

In 343 pediatric patients receiving intravenous therapy, the following local reactions were noted: pain (12%), erythema (2.9%), induration (0.9%), and phlebitis (2.1%).  In the US patient population, pain occurred in 1.5% of patients, while each of the remaining 3 local reactions had an incidence of 0.5%.

The following laboratory adverse events, regardless of drug relationship, occurred in at least 1% of treated patients: increased eosinophils (6.3%), increased platelets (3.6%), neutropenia (3.2%), increased AST (3.8%), increased ALT (6.5%), and increased serum creatinine (5.8%).

In US pediatric clinical trials, neutropenia (absolute neutrophil count less than 1,000/mm 3) occurred in 11.3% of patients (8/71) younger than 2 years receiving 30 mg/kg every 6 hours.  AST and ALT elevations to greater than 3 times the upper limit of normal were noted in 15% to 20% of patients aged 2 years or above receiving 50 mg/kg every 6 hours.  The increased frequency of these reported laboratory adverse events may be due to either increased severity of illness treated or higher doses of aztreonam for injection administered.

Adverse Laboratory Changes

Adverse laboratory changes without regard to drug relationship that were reported during clinical trials were:

Hepatic —elevations of AST (SGOT), ALT (SGPT), and alkaline phosphatase; signs or symptoms of hepatobiliary dysfunction occurred in less than 1% of recipients (see above).

Hematologic —increases in prothrombin and partial thromboplastin times, positive Coombs’ test.

Renal —increases in serum creatinine.

Dosage in Adult Patients

Aztreonam for injection may be administered intravenously or by intramuscular injection.  Dosage and route of administration should be determined by susceptibility of the causative organisms, severity and site of infection, and the condition of the patient.

                                                               Table 4:  Aztreonam for Injection Dosage Guidelines for Adults*

* Maximum recommended dose is 8 g per day.

Because of the serious nature of infections due to Pseudomonas aeruginosa, dosage of 2 g every six or eight hours is recommended, at least upon initiation of therapy, in systemic infections caused by this organism.

The intravenous route is recommended for patients requiring single doses greater than 1 g or those with bacterial septicemia, localized parenchymal abscess (e.g., intra-abdominal abscess), peritonitis or other severe systemic or life-threatening infections.

The duration of therapy depends on the severity of infection.  Generally, aztreonam for injection should be continued for at least 48 hours after the patient becomes asymptomatic or evidence of bacterial eradication has been obtained.  Persistent infections may require treatment for several weeks.  Doses smaller than those indicated should not be used.

Renal Impairment in Adult Patients

Prolonged serum levels of aztreonam may occur in patients with transient or persistent renal insufficiency.  Therefore, the dosage of aztreonam for injection should be halved in patients with estimated creatinine clearances between 10 and 30 mL/min/1.73 m 2 after an initial loading dose of 1 or 2 g.

When only the serum creatinine concentration is available, the following formula (based on sex, weight, and age of the patient) may be used to approximate the creatinine clearance (Clcr).  The serum creatinine should represent a steady state of renal function.

Males: Clcr =        weight (kg) x (140 – age)

                       72 x serum creatinine (mg/dL)

                    Females: 0.85 x above value

In patients with severe renal failure (creatinine clearance less than 10 mL/min/1.73 m 2), such as those supported by hemodialysis, the usual dose of 500 mg, 1 g or 2 g should be given initially.  The maintenance dose should be one-fourth of the usual initial dose given at the usual fixed interval of 6, 8 or 12 hours.  For serious or life-threatening infections, in addition to the maintenance doses, one-eighth of the initial dose should be given after each hemodialysis session.

Dosage in the Elderly

Renal status is a major determinant of dosage in the elderly; these patients in particular may have diminished renal function.  Serum creatinine may not be an accurate determinant of renal status.  Therefore, as with all antibiotics eliminated by the kidneys, estimates of creatinine clearance should be obtained and appropriate dosage modifications made if necessary.

Dosage in Pediatric Patients

Aztreonam for injection should be administered intravenously to pediatric patients with normal renal function.  There are insufficient data regarding intramuscular administration to pediatric patients or dosing in pediatric patients with renal impairment (see PRECAUTIONS, Pediatric Use). 

                                                              Table 5: Aztreonam for Injection Dosage  Guidelines for Pediatric Patients* 

*Maximum recommended dose is 120 mg/kg/day.

Preparation of Parenteral Solutions

General

Upon the addition of the diluent to the container, contents should be shaken immediately  and vigorously.   Constituted solutions are not for multiple-dose use; should the entire volume in the container not be used for a single dose, the unused solution must be discarded.

Depending upon the concentration of aztreonam and diluent used, constituted aztreonam for injection yields a colorless to light straw yellow solution which may develop a slight pink tint on standing (potency is not affected).  Parenteral drug products should be inspected visually for particulate matter and discoloration whenever solution and container permit.

Admixtures with Other Antibiotics

Intravenous infusion solutions of aztreonam not exceeding 2% w/v prepared with Sodium Chloride Injection, USP 0.9% or Dextrose Injection USP 5%, to which clindamycin phosphate, gentamicin sulfate, tobramycin sulfate, or cefazolin sodium have been added at concentrations usually used clinically, are stable for up to 48 hours at room temperature or 7 days under refrigeration.  Ampicillin sodium admixtures with aztreonam in Sodium Chloride Injection, USP 0.9% are stable for 24 hours at room temperature and 48 hours under refrigeration; stability in Dextrose Injection, USP 5% is 2 hours at room temperature and 8 hours under refrigeration.

Aztreonam-cloxacillin sodium and aztreonam-vancomycin hydrochloride admixtures are stable in Dianeal 137 (Peritoneal Dialysis Solution) with 4.25% Dextrose for up to 24 hours at room temperature.

Aztreonam is incompatible with nafcillin sodium, cephradine, and metronidazole.

Other admixtures are not recommended since compatibility data are not available.

Intravenous Solutions

For Bolus Injection

The contents of an aztreonam for injection vial should be constituted with 6 to 10 mL Sterile Water for Injection, USP.

For Infusion

If the contents of a vial are to be transferred to an appropriate infusion solution, each gram of aztreonam should be initially constituted with at least 3 mL Sterile Water for Injection, USP.   Further dilution may be obtained with one of the following intravenous infusion solutions:

Sodium Chloride Injection, USP, 0.9%

Ringer’s Injection, USP

Lactated Ringer’s Injection, USP

Dextrose Injection, USP, 5% or 10%

Dextrose and Sodium Chloride Injection, USP, 5%:0.9%,

5%:0.45% or 5%:0.2%

Sodium Lactate Injection, USP (M/6 Sodium Lactate)

Ionosol ® B and 5% Dextrose

Isolyte ® E

Isolyte ® E with 5% Dextrose

Isolyte ® M with 5% Dextrose

Normosol ®-R

Normosol ®-R and 5% Dextrose

Normosol ®-M and 5% Dextrose

Mannitol Injection, USP, 5% or 10%

Lactated Ringer’s and 5% Dextrose Injection

Plasma-Lyte ® M and 5% Dextrose

Intramuscular Solutions

The contents of an aztreonam for injection vial should be constituted with at least 3 mL of an appropriate diluent per gram aztreonam.  The following diluents may be used:

Sterile Water for Injection, USP

Sterile Bacteriostatic Water for Injection, USP (with benzyl alcohol or with methyl- and propylparabens)

Sodium Chloride Injection, USP, 0.9%

Bacteriostatic Sodium Chloride Injection, USP (with benzyl alcohol)

Stability of Intravenous and Intramuscular Solutions

Aztreonam solutions for intravenous infusion at concentrations not exceeding 2% w/v must be used within 48 hours following constitution if kept at controlled room temperature (20º to 25ºC/68º to 77ºF, see USP) or within 7 days if refrigerated (2º to 8ºC/ 36° to 46°F).

Aztreonam for Injection, USP, solutions at concentrations exceeding 2% w/v, except those prepared with Sterile Water for Injection, USP or Sodium Chloride Injection, USP, should be used promptly after preparation; the 2 excepted solutions must be used within 48 hours if stored at controlled room temperature or within 7 days if refrigerated.

Intravenous Administration

Bolus Injection

A bolus injection may be used to initiate therapy.  The dose should be slowly injected directly into a vein, or the tubing of a suitable administration set, over a period of 3 to 5 minutes (see next paragraph regarding flushing of tubing).

Infusion

With any intermittent infusion of aztreonam and another drug with which it is not pharmaceutically compatible, the common delivery tube should be flushed before and after delivery of aztreonam with any appropriate infusion solution compatible with both drug solutions; the drugs should not be delivered simultaneously.  Any aztreonam for injection infusion should be completed within a 20 to 60 minute period.  With use of a Y-type administration set, careful attention should be given to the calculated volume of aztreonam solution required so that the entire dose will be infused.  A volume control administration set may be used to deliver an initial dilution of aztreonam for injection (see Preparation of Parenteral Solutions, Intravenous Solutions , For Infusion) into a compatible infusion solution during administration; in this case, the final dilution of aztreonam should provide a concentration not exceeding 2% w/v.

Intramuscular Administration

The dose should be given by deep injection into a large muscle mass (such as the upper outer quadrant of the gluteus maximus or lateral part of the thigh).  Aztreonam is well tolerated and should not be admixed with any local anesthetic agent.

Storage

Store in original packages at 20º to 25ºC (68º to 77ºF) [see USP Controlled Room Temperature]; avoid excessive heat.

The container closure is not made with natural rubber latex.