Imipenem: Carbapenem Antibiotic with Cilastatin Combination for Nosocomial Infections
Imipenem is a carbapenem antibiotic with the broadest spectrum of activity among all beta lactams. It was approved in 1985 and is always available in fixed combination with cilastatin (imipenem/cilastatin, brand names Zienam and generics). Cilastatin itself is not antibacterially active, but inhibits the renal enzyme dehydropeptidase 1, which inactivates imipenem in the kidney. Without cilastatin, imipenem would be converted to inactive metabolites in the kidney and could also exert nephrotoxic effects.
Imipenem is a reserve antibiotic for severe nosocomial infections, septic disease states, and multidrug resistant pathogens. In the carbapenem spectrum, meropenem (Meronem), ertapenem (Invanz), and doripenem (Doribax) are competitive options. The choice of carbapenem depends on indication, pathogen spectrum, renal insufficiency, and seizure risk. Meropenem is now used more frequently than imipenem due to lower seizure risk and simpler handling.
Mechanism of Action
Imipenem is a beta lactam antibiotic from the carbapenem class. It binds to penicillin binding proteins (PBPs), particularly PBP1, PBP2, and PBP4 in gram-negative and gram-positive bacteria. This binding blocks the cross-linking of the peptidoglycan cell wall and leads to osmotic lysis of the bacteria.
Carbapenems have the broadest spectrum of activity of all beta lactams: gram-positive pathogens including streptococci, methicillin-sensitive staphylococci, Listeria; gram-negative including Pseudomonas aeruginosa and ESBL producers; anaerobes such as Bacteroides. Resistant organisms include methicillin-resistant Staphylococci (MRSA), Stenotrophomonas maltophilia, Burkholderia cepacia, Enterococcus faecium, and carbapenemase producers (KPC, NDM, OXA 48), which increasingly occur in hospital settings and represent a growing problem.
Stability against most beta lactamases, including AmpC and ESBL, is a major advantage. Pharmacokinetically, imipenem is administered exclusively intravenously with a half-life of approximately one hour, necessitating multiple daily doses.
Indications
- Nosocomial pneumonia and ventilator-associated pneumonia: when multidrug-resistant pathogens are suspected
- Complicated intra-abdominal infections: peritonitis, cholangitis, perforated diverticulitis
- Complicated urinary tract infection and pyelonephritis: when other antibiotics fail
- Sepsis of unknown origin in critically ill patients
- Febrile neutropenia: as empiric first-line therapy
- Complicated skin and soft tissue infections: diabetic foot syndrome, necrotizing fasciitis
- Endocarditis: with ampicillin-sensitive enterococci or as reserve
- Bone and joint infections: osteomyelitis, septic arthritis
Dosage and Administration
Standard dose in adults: 500 mg imipenem/500 mg cilastatin every 6 hours intravenously or 1 g/1 g every 8 hours for severe infections or Pseudomonas. Maximum dose: 4 g imipenem daily, typically not exceeded.
Renal insufficiency: dose adjustment is mandatory. At eGFR 60 to 90 ml/min no adjustment is needed, at 30 to 60 ml/min 500 mg every 8 hours, at 15 to 30 ml/min 500 mg every 12 hours, at eGFR below 15 ml/min not recommended except with dialysis.
Infusion duration: 20 to 30 minutes for 500 mg doses, 40 to 60 minutes for 1 g doses. Excessively rapid infusion increases the risk of nausea, vomiting, and seizures.
Adverse Effects
Common: nausea, vomiting, diarrhea, rash, elevated liver transaminases, local phlebitis and irritation at the infusion site, eosinophilia, pruritus.
Serious: seizures (especially with renal insufficiency, high-dose therapy, and pre-existing CNS disease), Clostridioides difficile-associated diarrhea and pseudomembranous colitis, anaphylactoid reactions, hepatotoxicity, bone marrow suppression during long-term therapy, acute kidney injury, Stevens Johnson syndrome.
Important, seizures: Imipenem has the highest seizure risk of all carbapenems (approximately 0.2 to 0.4 percent), especially with renal insufficiency, pre-existing CNS disease (stroke, brain tumor, epilepsy), or concurrent use of other seizure threshold-lowering medications. Meropenem is often preferred in these patients.
Drug Interactions
- Valproic acid: substantial reduction of valproate levels by approximately 60 to 100 percent, with risk of seizure exacerbation; combination should be avoided if possible
- Ganciclovir: additive seizure risk
- Probenecid: increases imipenem levels
- Aminoglycosides (gentamicin, tobramycin): physically incompatible in the same infusion line, administer separately
- Cyclosporine: possibly additive nephrotoxicity and CNS toxicity
- Theophylline: additive seizure risk
Special Precautions
Pregnancy and breast-feeding: only with strict indication as data are limited. In life-threatening infections, therapy is not withheld.
Beta lactam allergy: contraindicated in patients with known severe IgE-mediated penicillin allergy due to cross-reactivity of approximately 10 percent. For mild, non-IgE-mediated reactions, individual risk-benefit assessment is warranted.
Antibiotic stewardship: Carbapenems are valuable reserve antibiotics, and broad use promotes resistance development in carbapenemase producers. De-escalation to targeted antibiotic therapy based on pathogen identification and susceptibility testing is standard practice.
Caution in CNS disease: Patients with brain tumor, stroke, epilepsy, or other pre-existing CNS diseases should receive meropenem instead of imipenem when a carbapenem is indicated.
You May Also Be Interested In
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- Ertapenem, once daily carbapenem
- Piperacillin, aminopenicillin in combination with tazobactam
- Tazobactam, beta lactamase inhibitor
- Vancomycin, glycopeptide for MRSA
Frequently Asked Questions
Why is imipenem always combined with cilastatin?
Imipenem alone is metabolized in the kidney by the enzyme dehydropeptidase 1 to inactive and potentially nephrotoxic metabolites. Cilastatin inhibits this enzyme, protecting imipenem from inactivation and preventing the formation of toxic metabolites. Without cilastatin, imipenem would not be clinically practical.
Why is the seizure risk with imipenem higher?
Imipenem has higher affinity for the GABA receptor and can act as an antagonist there, increasing central nervous system excitability. With renal insufficiency, the substance accumulates, further increasing the risk. In patients with pre-existing CNS disease or seizure disorder, meropenem or a non-carbapenem is therefore preferred.
Does imipenem work against MRSA?
No. MRSA carry an altered PBP2a against which carbapenems are not sufficiently active. With suspected MRSA, vancomycin, linezolid, or daptomycin is used.
Why is imipenem a reserve antibiotic?
Carbapenems cover a very broad pathogen spectrum and should be reserved for severe infections or multidrug-resistant organisms. Broad use promotes selection of carbapenemase-producing pathogens (KPC, NDM, OXA 48), which leave few therapeutic options. Antibiotic stewardship is therefore essential.
Sources
- Gelbe Liste, Imipenem Cilastatin Active Ingredient Profile
- AWMF S3 Guideline Empiric Antibiotic Therapy and Nosocomial Pneumonia
- BfArM, Federal Institute for Drugs and Medical Devices
- EMA Summary of Product Characteristics Imipenem Preparations
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