Can I combine aztreonam with ceftazidime-avibactam (Ceftazidime-Avibactam) for a patient with a severe or resistant Pseudomonas aeruginosa infection?

Aztreonam Combined with Ceftazidime-Avibactam for Resistant Pseudomonas

Yes, you should combine aztreonam with ceftazidime-avibactam for severe Pseudomonas aeruginosa infections when the organism carries metallo-β-lactamases (MBLs) or is resistant to new antibiotic monotherapies. 1

When This Combination Is Indicated

For carbapenem-resistant Enterobacterales (CRE) with MBLs: The ESCMID guidelines conditionally recommend aztreonam plus ceftazidime-avibactam combination therapy for severe infections caused by CRE carrying metallo-β-lactamases and/or resistant to new antibiotic monotherapies (moderate quality evidence). 1

For extensively drug-resistant (XDR) Pseudomonas aeruginosa: This combination demonstrates synergistic bactericidal activity in hollow-fiber models against carbapenemase-producing P. aeruginosa (including ST175, ST111, ST235 high-risk clones), achieving 4-5 log10 CFU/mL bacterial reduction even when isolates are nonsusceptible to each agent alone. 2

Clinical success data: In a Spanish cohort of 8 patients with VIM-producing P. aeruginosa infections treated with this combination, clinical success and cure were achieved in 7/8 patients (87.5%), with 28-day mortality of 2/8 (25%), no treatment-related adverse events, and no infection relapses. 3

Mechanism of Synergy

• Avibactam inhibits serine β-lactamases (including KPC, OXA-48) but does NOT inhibit metallo-β-lactamases. 2
• Aztreonam remains stable against MBLs but is hydrolyzed by serine β-lactamases. 2
• When combined, avibactam protects aztreonam from serine β-lactamases while aztreonam bypasses MBL-mediated resistance, creating synergistic coverage against organisms producing both enzyme types. 2, 4

When NOT to Use This Combination

For standard carbapenem-resistant P. aeruginosa (CRPA) without MBLs: ESCMID guidelines state there is insufficient evidence to recommend for or against combination therapy with ceftazidime-avibactam for CRPA infections. 1

When monotherapy is adequate: For CRE infections susceptible to and treated with ceftazidime-avibactam, meropenem-vaborbactam, or cefiderocol, ESCMID strongly recommends AGAINST combination therapy (low quality evidence). 1

For P. aeruginosa producing only VIM or NDM without serine β-lactamases: P. aeruginosa isolates producing NDM or VIM may remain resistant to the aztreonam-ceftazidime-avibactam combination due to non-β-lactamase resistance mechanisms (efflux pumps, porin loss). 4

Practical Dosing Regimen

• Ceftazidime-avibactam: 2.5 grams (ceftazidime 2g + avibactam 0.5g) IV every 8 hours over 2-hour infusion 5
• Aztreonam: 2g IV every 8 hours 6, 7
• Duration: 7-14 days depending on infection site and severity 6, 7

Critical Pitfalls to Avoid

• Do not assume susceptibility without testing: All CRE and Enterobacterales producing MBL and GES that were resistant to aztreonam and ceftazidime-avibactam alone became susceptible to the combination, but P. aeruginosa with MBLs may remain resistant. 4
• Obtain susceptibility testing: Use disk elution, strip stacking, or strip crossing methods to assess combination activity in the laboratory (100% sensitivity and specificity). 4
• Do not use aztreonam monotherapy for MBL-producers: Aztreonam monotherapy carries risk of selecting resistant mutants during treatment. 3
• Consider infectious disease consultation: All multidrug-resistant organism infections warrant ID consultation for optimal management. 6

Alternative Options When This Combination Fails

• Cefiderocol: Preferred agent when metallo-β-lactamases are present and the combination is not available or has failed. 1
• Polymyxins combined with meropenem: Limited evidence option for MBL-producing organisms. 8
• Aztreonam-avibactam: Emerging option but limited clinical data. 8