Antimicrobials · Monobactams
The facts most likely to be tested
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Aztreonam is a monobactam that functions by binding exclusively to penicillin-binding protein 3 (PBP3) in aerobic gram-negative rods.
The primary clinical utility of aztreonam is treating severe gram-negative infections in patients with a history of severe penicillin or cephalosporin allergy.
Aztreonam exhibits a lack of cross-reactivity with most beta-lactams, with the notable exception of ceftazidime due to identical side-chain structures.
The spectrum of activity for aztreonam is limited to aerobic gram-negative bacteria, including Pseudomonas aeruginosa.
Aztreonam possesses no activity against gram-positive organisms or anaerobes.
The drug is administered via parenteral routes (IV or IM) because it is not absorbed from the gastrointestinal tract.
Aztreonam is considered safe for use in patients with a history of IgE-mediated anaphylaxis to other beta-lactam antibiotics.
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A 62-year-old male is hospitalized for a complicated urinary tract infection. His medical history is significant for anaphylaxis following the administration of piperacillin-tazobactam three years ago. Urine culture reveals Pseudomonas aeruginosa susceptible to multiple agents. The patient is currently hemodynamically stable with no signs of sepsis. The team decides to initiate targeted antibiotic therapy that minimizes the risk of an allergic reaction.
Which of the following is the most appropriate antibiotic for this patient?
Aztreonam
Aztreonam is the only beta-lactam that does not exhibit cross-reactivity with penicillins (except ceftazidime), making it the drug of choice for patients with a history of severe beta-lactam allergy requiring coverage for aerobic gram-negative rods like Pseudomonas.
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Classification
Synthetic monobactam antibiotic.
Indications
Gram-negative infections in patients with penicillin allergy.
Mechanism of Action
Binds PBP-3 to inhibit cell wall synthesis.
Side Effects
GI upset, elevated LFTs, anaphylaxis.
Contraindications / Monitoring
Hypersensitivity to aztreonam. Renal function.
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Mechanism of Action
Aztreonam binds with high affinity to PBP-3 (penicillin-binding protein 3) in aerobic Gram-negative bacteria. This binding inhibits the third stage of bacterial cell wall synthesis, leading to cell lysis. It is highly resistant to beta-lactamases, maintaining efficacy where other agents fail.
Unique Properties
It is the only commercially available monobactam. It lacks activity against Gram-positive organisms and anaerobes, making it a narrow-spectrum agent. It is structurally distinct, allowing use in patients with severe IgE-mediated penicillin allergies.
Indications
Indicated for serious Gram-negative infections including pyelonephritis, pneumonia, and sepsis. It is the drug of choice for patients with a history of anaphylaxis to other beta-lactams. It is also used for cystic fibrosis patients with chronic Pseudomonas aeruginosa colonization.
Pharmacokinetics
Excreted primarily via the kidneys as unchanged drug. Dosage adjustments are required in patients with renal impairment to prevent accumulation. It does not undergo significant hepatic metabolism.
Side Effects & Adverse Events
Common effects include diarrhea, nausea, and elevated transaminases. Superinfection with Gram-positive organisms can occur due to narrow spectrum. Anaphylaxis is rare but possible; cross-reactivity with ceftazidime is a known risk due to identical side-chain structures.
Contraindications
Hypersensitivity to aztreonam is the only absolute contraindication. Use with caution in patients with a history of ceftazidime allergy due to potential cross-reactivity.
Monitoring
Monitor serum creatinine and BUN to adjust dosing in renal failure. Periodically check liver function tests if therapy is prolonged. Monitor for signs of superinfection.
Clinical Pearls
Think of Aztreonam as the 'safe' beta-lactam for the penicillin-allergic patient. It is the classic board answer for treating Pseudomonas in a patient who cannot tolerate piperacillin-tazobactam or carbapenems.