Carbapenems
On this page
Direct answer
Carbapenems — imipenem, meropenem, ertapenem and doripenem — are beta-lactams with the broadest antibacterial spectrum in current use, covering gram-positive cocci, gram-negative bacilli including extended-spectrum beta-lactamase producers, and anaerobes. They are reserved for serious hospital infections and multiply resistant organisms precisely to preserve their activity. Imipenem must be paired with cilastatin to prevent renal enzymatic degradation, and it carries the highest seizure risk of the group.
What you must remember
- Mechanism: bind penicillin-binding proteins, especially PBP2, inhibit cell-wall synthesis; stable to most beta-lactamases including extended-spectrum beta-lactamases, though metallo-beta-lactamases such as NDM-1 destroy them.
- Imipenem is degraded by renal dehydropeptidase-1 in the proximal tubule, yielding a nephrotoxic metabolite, so it is always combined with the dehydropeptidase inhibitor cilastatin.
- Meropenem is not degraded by the renal enzyme, is less emetogenic and less epileptogenic than imipenem, penetrates inflamed meninges and is preferred for nosocomial meningitis of gram-negative origin.
- Ertapenem lacks Pseudomonas and Acinetobacter activity but offers convenient once-daily dosing for complicated urinary, intra-abdominal, skin and diabetic foot infections; faropenem is an oral member used in India for respiratory and urinary infections.
- Prime indications: febrile neutropenia with vancomycin, hospital-acquired and ventilator-associated pneumonia, complicated intra-abdominal infection with anaerobe cover, and infections caused by ESBL-producing Klebsiella and Escherichia coli.
- Adverse effects: nausea, infusion-site phlebitis, rash and, characteristically, dose- and accumulation-related seizures with imipenem in renal impairment or epilepsy; reduce doses when creatinine clearance falls.
- Cross-allergenicity with penicillins is low but real, and carbapenems are best avoided where the penicillin reaction was anaphylaxis.
Common confusion
Candidates blur carbapenem breadth with universality — ertapenem glaringly omits Pseudomonas and Acinetobacter while the others cover them, so it must never be empirical therapy where these are likely. Equally confused are imipenem and meropenem: the cilastatin partnership and seizure liability belong to imipenem, while meropenem is the safer neurosurgical and paediatric option.
Exam-focused takeaway
Theory answers should place carbapenems within the beta-lactam family, explain the cilastatin combination, list organisms covered and excluded, and end with toxicity. Viva examiners probe NDM-1 and why carbapenems must be conserved as last-line agents. MCQs favour the imipenem-cilastatin pairing, seizures with imipenem, ertapenem's absent pseudomonal cover and meropenem for hospital-acquired meningitis.
Frequently asked questions
Why is cilastatin always given with imipenem?
Cilastatin inhibits renal dehydropeptidase-1, which otherwise destroys imipenem in the proximal tubule and liberates a nephrotoxic product.
Which patients are at risk of imipenem-induced seizures?
Those with renal impairment without dose reduction, underlying epilepsy, or central nervous system lesions; meropenem is preferred in these settings.
When is ertapenem an inappropriate choice?
Whenever Pseudomonas aeruginosa or Acinetobacter is a likely pathogen — hospital-acquired pneumonia or neutropenic sepsis — because ertapenem has no activity against them.
What makes carbapenems stable to most beta-lactamases?
The trans-hydroxyethyl side chain at position six of the beta-lactam ring sterically shields the ring from beta-lactamase attack, except against metallo-enzymes.
What is the significance of NDM-1?
New Delhi metallo-beta-lactamase hydrolyses all carbapenems, creating pan-resistant gram-negative infections treatable only by colistin or newer inhibitor combinations.