Chemical formula: C₁₃H₁₇N₅O₈S₂ Molecular mass: 435.433 g/mol PubChem compound: 5742832
Aztreonam exhibits activity in vitro against gram-negative aerobic pathogens, including P. aeruginosa. Aztreonam binds to penicillin-binding proteins of susceptible bacteria, which leads to inhibition of bacterial cell wall synthesis, followed by filamentation and cell lysis.
Loss of susceptibility to aztreonam in CF patients with P. aeruginosa occurs either through selection of strains with mutations located on the chromosome or rarely through acquisition of plasmid/integron mediated genes.
Known mechanisms of resistance to aztreonam mediated by mutation of chromosomal genes include: hyperexpression of the Class C beta-lactamase AmpC and up-regulation of the efflux pump MexAB-OprM. The known mechanism of resistance to aztreonam mediated by acquisition of genes involves acquisition of extended spectrum beta-lactam enzymes (ESBLs) that hydrolyse the fourmember, nitrogen-containing ring of aztreonam.
ESBLs from Class A, B and D beta-lactamases may have activity against aztreonam. Class A betalactamases reported to hydrolyse aztreonam include the VEB type (primarily Southeast Asia), PER type (Turkey), and GES and IBC types (France, Greece, and S. Africa). There are rare reports of organisms with metallo-beta-lactamases (MBLs), Class B, that are resistant to aztreonam, VIM-5 (K. pneumoniae and P. aeruginosa - Turkey), VIM-6 (P. putida - Singapore) and VIM-7 (P. aeruginosa - United States), however, it is possible that these organisms were expressing multiple resistance mechanisms and thus a MBL was not responsible for the observed resistance to aztreonam. There are rare reports of Class D beta-lactamases from clinical isolates of P. aeruginosa, OXA-11 (Turkey) and OXA-45 (United States) that hydrolyse aztreonam.
Individual patients' sputum aztreonam concentrations exhibited considerable variability. For the combined Phase 3 placebo-controlled studies, ten minutes following a single dose of 75 mg inhaled aztreonamon Days 0, 14, and 28, the mean sputum concentrations in 195 patients with CF were 726 µg/g, 711 µg/g, and 715 µg/g, respectively, indicating no increased accumulation of aztreonam following repeated dosing.
Individual patients' plasma aztreonam concentrations exhibited considerable variability.
One hour following a single dose of 75 mg inhaled aztreonam (at approximately peak plasma concentration), the mean plasma level in patients with CF was 0.59 µg/ml. Mean peak plasma levels at Days 0, 14, and 28 of a course with 75 mg inhaled aztreonam 3 times a day were 0.55 µg/ml, 0.67 µg/ml, and 0.65 µg/ml, respectively, indicating no systemic accumulation of aztreonam following 3 times a day dosing. In contrast, the serum concentration of aztreonam following administration of aztreonam for injection (500 mg) is approximately 54 µg/ml.
Plasma aztreonam concentrations in paediatric patients aged 3 months to <6 years are comparable to those observed for children >6 years, adolescents and adults.
The protein binding of aztreonam in plasma is approximately 77% at clinically relevant plasma concentrations.
Aztreonam is not extensively metabolised. The principal metabolite (SQ26,992) is inactive and is formed by opening of the beta-lactam ring due to hydrolysis. Recovery data indicate that about 10% of the dose is excreted as this metabolite.
The elimination half-life of aztreonam from serum is approximately 2.1 hours for inhalation administration, similar to what has been reported for aztreonam for injection. Approximately 10% of the total inhaled aztreonamdose is excreted in the urine as unchanged drug, as compared to 60-65% following intravenous administration of aztreonam for injection. Systemically absorbed aztreonam is eliminated about equally by active tubular secretion and glomerular filtration.
There was no clinically relevant effect of age or sex on the pharmacokinetics of aztreonam.
Pharmacokinetic studies have not been performed in patients with renal or hepatic impairment.
Peak levels of aztreonam are achieved at about one hour after i.m. administration. After identical single i.m. or i.v. doses, the serum concentrations are comparable at 1 hour (1.5 hours from the start of i.v. infusion), with similar slopes of serum concentrations thereafter. The serum half-life of aztreonam averaged 1.7 hours in subjects with normal renal function, independent of the dose and route. In healthy subjects 60-70% of a single i.m. or i.v. dose was recovered in the urine by 8 hours, and urinary excretion was essentially complete by 12 hours.
The Phase 2 and 3 placebo-controlled, registrational studies permitted comparison of plasma concentrations 1 hour post dose of Cayston by age (6 to 12 years, 13 to 17 years, and ≥18 years). Data from these studies revealed minimal differences in mean plasma aztreonam concentrations between age groups in patients receiving Cayston 3 times a day.
Pooled sputum concentration data from the Phase 2 and 3 registrational studies revealed some evidence of lower mean sputum concentrations in patients aged 13 to 17 years following one dose of Cayston 3 times a day. However, all mean sputum concentration values were associated with relatively large standard deviations.
Aztreonam was well tolerated in a comprehensive series of preclinical toxicity and safety studies.
A 104-week rat inhalation toxicology study to assess the carcinogenic potential of ascending doses of aztreonamdemonstrated no drug-related increase in malignant tumours.
Genotoxicity (Chromosomal aberration and mouse lymphoma mutation assay) studies with aztreonam were negative.
Fertility, teratology, perinatal and postnatal studies were conducted with aztreonam for i.v. injection in rats at daily doses up to 750 mg/kg without adverse effects. The survival rate during the lactation period was slightly reduced in the offspring of rats that received the highest dose.
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