Pharmacology · Antibacterial Agents

Carbapenems, Monobactams & CRE

Spectrum, individual agents, aztreonam, and resistance therapy

Carbapenems are the broadest-spectrum beta-lactams — covering most gram-positives, gram-negatives (including ESBL producers), and anaerobes — but none covers MRSA, and ertapenem specifically lacks Pseudomonas activity.
Imipenem requires co-administration with cilastatin to block renal DHP-I hydrolysis; it carries the highest seizure risk (1–3%) among carbapenems and should be avoided in CNS disease — meropenem is preferred for gram-negative meningitis.
Aztreonam is the only monobactam in clinical use: gram-negative spectrum only, no gram-positive or anaerobic activity, safe in penicillin-allergic patients — except for cross-reactivity with ceftazidime due to a shared side chain.
No single agent covers all carbapenem-resistant organisms — KPC, NDM, OXA-48, and OXA-23/58 each require distinct therapies; genotypic identification of the resistance mechanism is mandatory before selecting definitive treatment.
Aztreonam-avibactam is the key agent for NDM-producing organisms: aztreonam resists metallo-beta-lactamase hydrolysis, and avibactam neutralizes co-produced serine beta-lactamases that would otherwise destroy aztreonam.

Abbreviations: CRE = carbapenem-resistant Enterobacteriaceae · KPC = Klebsiella pneumoniae carbapenemase · NDM = New Delhi metallo-beta-lactamase · VIM = Verona integron-encoded metallo-beta-lactamase · IMP = imipenemase · OXA = oxacillinase carbapenemase · ESBL = extended-spectrum beta-lactamase · MSSA = methicillin-susceptible Staphylococcus aureus · MRSA = methicillin-resistant Staphylococcus aureus · DHP-I = dehydropeptidase I · PBP = penicillin-binding protein · CRAB = carbapenem-resistant Acinetobacter baumannii

Carbapenem Comparison

Agent	Pseudomonas	Formulation Note	Seizure Risk	Key Clinical Niche
Imipenem-Cilastatin	Yes	Cilastatin required — blocks renal DHP-I hydrolysis	Highest (1–3%) — avoid in CNS disease	Broad gram-positive + gram-negative coverage; avoid for meningitis; Acinetobacter activity
Meropenem	Yes	No cilastatin needed; DHP-I–stable	Low	Gram-negative meningitis; ESBL bacteremia; extended infusion for resistant Pseudomonas
Ertapenem	No	Once-daily dosing (long half-life ~4 hr)	Low	ESBL infections without Pseudomonas concern; outpatient parenteral therapy; step-down
Doripenem	Yes	Standard infusion	Low	Complicated UTIs, complicated intra-abdominal infections; limited current use vs. meropenem

Aztreonam — The Monobactam

Monocyclic Beta-Lactam

Aztreonam: Gram-Negative Only, Metallo-Beta-Lactamase–Resistant

Spectrum

Gram-negative aerobes only — Enterobacteriaceae, Pseudomonas aeruginosa, H. influenzae. No gram-positive, anaerobic, or MRSA activity. Binds PBP3 selectively.

Penicillin Allergy

Safe in penicillin-allergic patients — monocyclic ring does not share the penicillin ring system. Important exception: cross-reacts with ceftazidime due to identical 7-position side chain — avoid the combination in ceftazidime-allergic patients.

NDM Role

Aztreonam is not hydrolyzed by metallo-beta-lactamases. Aztreonam + avibactam (co-formulated or administered alongside ceftazidime-avibactam) is the treatment of choice for NDM-producing organisms — avibactam blocks co-produced serine beta-lactamases that would destroy aztreonam.

CRE Therapy by Resistance Mechanism

Class A Carbapenemase

KPC Producers

Ceftazidime-avibactam (first-line)
Meropenem-vaborbactam
Imipenem-relebactam
Cefiderocol (salvage / combination)
Not covered by: aztreonam alone, polymyxins unreliable

Class B Metallo-Beta-Lactamase

NDM / VIM / IMP Producers

Aztreonam-avibactam (co-formulated or aztreonam + ceftazidime-avibactam)
Cefiderocol (alternative / salvage)
Not covered by: ceftazidime-avibactam alone, meropenem-vaborbactam, imipenem-relebactam

Class D Serine Carbapenemase

OXA-48 Producers

Ceftazidime-avibactam (covers OXA-48)
Confirm OXA-48 vs. metallo-beta-lactamase genotype before selecting therapy
Meropenem-vaborbactam does not cover OXA-48

OXA-23/58 — Acinetobacter

Carbapenem-Resistant Acinetobacter baumannii

Sulbactam-durlobactam (first FDA-approved agent for CRAB)
Cefiderocol — activity demonstrated; use with caution (mortality signal vs. best available therapy in one trial)
Polymyxins + combination regimens for salvage
Infectious disease consultation required

Clinical Rule: Genotype Before Therapy — No Agent Covers All CRE

No single novel agent covers all carbapenem-resistant organisms. Ceftazidime-avibactam covers KPC and OXA-48 but fails against NDM and VIM. Aztreonam-avibactam is required for NDM. Meropenem-vaborbactam covers KPC but not OXA-48 or NDM. Phenotypic or genotypic resistance testing must be completed before committing to definitive therapy — empiric treatment with any single novel agent risks failure if the mechanism is wrong.

Ertapenem's lack of Pseudomonas coverage is a useful property, not just a limitation: it can be used for ESBL-producing infections where Pseudomonas has been excluded, preserving broader carbapenems for situations that actually require them. Avoid using meropenem empirically when ertapenem would suffice — carbapenem stewardship reduces selection pressure for carbapenemase-producing organisms.

Suggested References

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Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 50: Penicillins, Cephalosporins, and Other Beta-Lactam Antibiotics	McGraw-Hill, 2023
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Kaye KS, Bhowmick T, Metallidis S, et al.	Effect of meropenem-vaborbactam vs piperacillin-tazobactam on clinical cure or improvement and microbial eradication in complicated urinary tract infection: the TANGO I randomized clinical trial	JAMA. 2018;319(8):788–799
Bassetti M, Echols R, Matsunaga Y, et al.	Efficacy and safety of cefiderocol or best available therapy for the treatment of serious infections caused by carbapenem-resistant gram-negative bacteria (CREDIBLE-CR)	Lancet Infect Dis. 2021;21(2):226–240
Tamma PD, Aitken SL, Bonomo RA, Mathers AJ, van Duin D, Clancy CJ	Infectious Diseases Society of America 2022 guidance on the treatment of extended-spectrum beta-lactamase producing Enterobacterales, carbapenem-resistant Enterobacterales, and Pseudomonas aeruginosa with difficult-to-treat resistance	Clin Infect Dis. 2022;75(2):187–212