Mechanism of Action and Structural Features
Carbapenems have a unique chemical structure: in their thiazolidine ring, the sulfur atom is replaced by a methyl group. This key difference from classical penicillins provides them with high resistance to the destructive action of $\beta$-lactamases.
The mechanism of action of these drugs is bactericidal. It is implemented in several successive stages:
- Due to structural similarity to D-alanine-D-alanine of the bacterial cell wall, the antibiotic binds to specific penicillin-binding proteins (PBPs).
- Inhibition of a vital enzyme, transpeptidase, occurs.
- The formation of cross-links in peptidoglycan, which serves as the framework of the bacterial wall, is disrupted.
- Autolytic enzymes of the bacterium itself are activated.
- The cell wall sustains irreversible damage, leading to the death of the microorganism.
Clinical positioning
Carbapenems have an ultra-broad spectrum of activity. They destroy gram-positive and gram-negative flora, including aerobes and anaerobes, as well as strains resistant to 3rd- and 4th-generation cephalosporins.
In clinical practice, these are reserve antibiotics. Their main purpose is the therapy of severe, including nosocomial (hospital-acquired) infections caused by multi-resistant strains. However, in conditions directly threatening the patient's life, carbapenems can be used as first-line agents for empirical therapy.
It is important to remember the gaps in the spectrum of activity. Methicillin-resistant staphylococci (MRSA), vancomycin-resistant enterococci (VRE), and Legionella are naturally resistant to carbapenems.
Key representative features
- Imipenem. This is a thienamycin derivative (produced by Streptomyces cattleya). Its main pharmacokinetic problem is rapid destruction in the renal tubules by a specific enzyme, dehydropeptidase I. Because of this, a therapeutic concentration is not established in the urine, and monotherapy is ineffective. The solution is a combined drug with cilastatin (trade names: Tienam, Primaxin). Cilastatin has no antibacterial activity, but it blocks dehydropeptidase I, increasing the concentration of unchanged imipenem in the urine. Administered intravenously 3–4 times a day. In case of meningitis, it crosses the blood-brain barrier (BBB) well.
- Meropenem. Unlike imipenem, it is resistant to renal dehydropeptidase I, and therefore is used as a standalone drug. It is more active against gram-negative flora, but weaker against gram-positive cocci. Administered exclusively intravenously (by infusion or bolus). Not used for bone and joint infections.
- Ertapenem (Invanz). Its main advantage is long-acting efficacy, allowing the drug to be administered just once a day (intravenously or intramuscularly). However, it is inferior to other carbapenems in activity against non-fermenting gram-negative bacteria (P. aeruginosa, Acinetobacter).
Safety profile and interactions
Carbapenems can cause standard allergic hypersensitivity reactions, nausea, vomiting, and local reactions (phlebitis at the injection site). Eosinophilia, leukopenia, and neutropenia are possible hematological adverse effects.
A specific central nervous system adverse effect is seizures. This seizure activity is characteristic of imipenem, but absent in meropenem.
Drug interactions:
- Probenecid is capable of increasing the concentration of meropenem in the blood.
- All three drugs (imipenem, meropenem, ertapenem) decrease the level of valproic acid (sodium valproate). This requires special caution when prescribing carbapenems to patients with epilepsy.
How to easily remember the key differences between the drugs: - Imipenem — Inhibitor needed (used only with cilastatin). - Meropenem — Monotherapy (resistant to renal enzymes on its own). - Ertapenem — Economical regimen (administered just once a day).
Class and exam questions
Why is imipenem not used as a standalone drug?
Imipenem is degraded in the renal tubules by the enzyme dehydropeptidase I, which prevents it from achieving a therapeutic concentration in the urine. It is always combined with cilastatin, a specific inhibitor of this enzyme.
Are carbapenems effective against MRSA?
No, methicillin-resistant staphylococci (MRSA), as well as vancomycin-resistant enterococci (VRE) and Legionella, possess natural resistance to this group of antibiotics.
Which carbapenem is safer for patients at risk of seizures?
Meropenem. Unlike imipenem, it does not exhibit seizure activity and is considered safer for the central nervous system.
How do carbapenems interact with epilepsy medications?
Imipenem, meropenem, and ertapenem decrease blood levels of sodium valproate (valproic acid), which can provoke a seizure in patients with epilepsy.
What else to study on this topic
- The role of penicillin-binding proteins (PBPs) in bacterial cell wall synthesis
- Mechanisms of natural resistance of MRSA and VRE to $\beta$-lactams
- Pharmacokinetics of imipenem in meningitis (BBB permeability)
- Comparative activity of carbapenems against Pseudomonas aeruginosa (P. aeruginosa)
- Clinical significance of dehydropeptidase I inhibitors
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