Features of carbapenem use: the example of imipenem
Some carbapenem antibiotics are vulnerable to the body's enzymes. In particular, imipenem is rapidly degraded in the renal tubules by a specific enzyme, dehydropeptidase I.
To prevent such inactivation and ensure a pronounced therapeutic effect, the drug is used in a strict fixed combination with a specific inhibitor of this enzyme, cilastatin. In addition, this group is not absorbed from the gastrointestinal tract, so all drugs are administered exclusively via the parenteral route — intravenously or intramuscularly.
Rational combinations: principles of synergism
The main goal of the rational combination of antimicrobial agents is to achieve mutual enhancement of their activity without compromising patient safety.
- Aminoglycosides and penicillins. This is a classic example of bactericidal synergism, which is used in severe infections, including endocarditis.
- The mechanism of interaction is that penicillins disrupt bacterial cell wall synthesis. This damage significantly facilitates the penetration of aminoglycosides inside the microorganism to their direct target — ribosomes.
Irrational combinations: antagonism and toxicity
Certain antibiotic combinations are strictly contraindicated due to the high risk of pharmacodynamic conflict or cumulative dangerous side effects:
- Aminoglycosides + Polymyxins. A dangerous summation of toxic effects occurs. Both groups exhibit marked nephrotoxicity and neurotoxicity, increasing the risk of neuromuscular blockade.
- Tetracyclines + Penicillins. Pharmacodynamic antagonism occurs. Bactericidal penicillins are effective only against actively dividing cells, while bacteriostatic tetracyclines inhibit this process. As a result, the bacteriostatic agent sharply reduces the efficacy of the bactericidal agent.
- Macrolides + Lincosamides. Competitive antagonism develops, as both groups target the same structure — the 50S subunit of the bacterial ribosome. The drugs compete for binding, weakening each other's effect and promoting cross-resistance.
Briefly: answers to topical questions
What other antibiotics, besides tetracyclines, exhibit antagonism with penicillins?
In addition to tetracyclines, drugs of the chloramphenicol group exhibit antagonism with penicillins.
- Chloramphenicol is a bacteriostatic drug.
Entering into a pharmacodynamic interaction, it weakens the therapeutic effect of bactericidal antibiotics (including penicillins and aminoglycosides). This is because bactericidal agents act on actively dividing cells, whereas chloramphenicol suppresses their growth and reproduction.
Which drugs belong to the carbapenem group along with imipenem?
In addition to imipenem, the carbapenem group includes meropenem and ertapenem.
- Meropenem is administered intravenously, does not possess convulsive activity, but is not used for bone and joint infections.
- Ertapenem is characterized by a prolonged duration of action, allowing once-daily administration, but is inferior to other carbapenems in activity against non-fermenting Gram-negative bacteria (P. aeruginosa, Acinetobacter).
Both drugs can be used for community-acquired pneumonia with the risk of Enterobacteriaceae infection.
What other enzyme inhibitors are used to protect beta-lactam antibiotics?
To protect beta-lactam antibiotics from degradation, dehydropeptidase I inhibitors and beta-lactamase inhibitors are used.
- Cilastatin is a specific dehydropeptidase I inhibitor that prevents the inactivation of imipenem in the renal tubules.
- Clavulanic acid is a beta-lactamase inhibitor combined with amoxicillin and ticarcillin.
- Sulbactam is a beta-lactamase inhibitor used in combination with ampicillin or cefoperazone.
- Tazobactam is a beta-lactamase inhibitor used in combination with piperacillin.
Beta-lactamase inhibitors contain a beta-lactam ring, bind to bacterial enzymes, and irreversibly inactivate them.
In which severe infections, other than endocarditis, is the combination of penicillins and aminoglycosides indicated?
Apart from endocarditis, the source directly indicates the combination of gentamicin with antipseudomonal penicillins for Klebsiella infection. Gentamicin is also used systemically for severe infections caused by Gram-negative bacteria, including sepsis, pneumonia, complicated urogenital and abdominal infections. The combination provides bactericidal synergism: penicillins disrupt cell wall synthesis, facilitating the penetration of aminoglycosides into the bacterial cell.
Imipenem without cilastatin is "destroyed" in the kidneys, while tetracycline and penicillin conflict: one stops cell division, and the other requires cell activity.
Class and exam questions
Why is imipenem necessarily prescribed in combination with cilastatin?
Imipenem is rapidly degraded in the renal tubules under the influence of the specific enzyme dehydropeptidase I. Cilastatin acts as an inhibitor of this enzyme, protecting the antibiotic from premature inactivation.
Why is the combination of penicillins and tetracyclines considered irrational?
This is pharmacodynamic antagonism. Penicillins have a bactericidal effect on actively dividing microorganisms, whereas tetracyclines (bacteriostatics) inhibit cell division, causing the efficacy of penicillins to drop sharply.
What is the mechanism of synergism between aminoglycosides and penicillins?
Beta-lactams (penicillins) destroy the bacterial cell wall, which facilitates the penetration of aminoglycosides into the microbial cell toward their intracellular target — the ribosomes.
What else to study on this topic
- Pharmacokinetic and pharmacodynamic types of antimicrobial synergism
- Mechanisms of neuro- and nephrotoxicity of aminoglycosides and polymyxins
- Comparative characteristics of oral and parenteral routes of administration of beta-lactams
- The role of beta-lactamase inhibitors (sulbactam, clavulanic acid, tazobactam, avibactam)
- Causes of cross-resistance during the combined use of macrolides and lincosamides
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