Why was it necessary to modify penicillin?
Natural penicillins revolutionized medicine but had several significant drawbacks. Pharmacologists solved this problem by taking 6-aminopenicillanic acid as a base and attaching various chemical radicals to its amino group.
This modification pursued three main goals:
- Protection against enzymes. Bacteria learned to produce beta-lactamases (e.g., penicillinases), which destroy the antibiotic. The new radicals were designed to act as a "shield" for the beta-lactam ring.
- Acid resistance. Natural drugs were destroyed by hydrochloric acid in the stomach. Modification made it possible to create forms for oral administration (per os).
- Broadening the spectrum. It was necessary to make the antibiotic effective not only against gram-positive but also against gram-negative flora.
Classification of semisynthetic penicillins
Globally, the drugs are divided into groups depending on their spectrum of action and ability to resist bacterial enzymes.
- Narrow-spectrum, beta-lactamase-resistant: isoxazolyl (antistaphylococcal) penicillins. Representatives: oxacillin, nafcillin, cloxacillin, dicloxacillin, flucloxacillin.
- Broad-spectrum, NOT beta-lactamase-resistant: aminopenicillins. Representatives: ampicillin, amoxicillin.
- Carboxypenicillins: carbenicillin, carfecillin, ticarcillin.
- Ureidopenicillins: azlocillin, piperacillin, mezlocillin.
Antistaphylococcal penicillins and the MRSA problem
The main feature of this group is the presence of the isoxazolyl radical. It sterically blocks the access of staphylococcal beta-lactamases (penicillinases) to the beta-lactam ring of the antibiotic, preventing its hydrolysis.
Key pharmacodynamic nuances:
- The drugs are highly effective against penicillinase-producing staphylococcal strains.
- However, their activity against susceptible flora (which do not produce enzymes) is significantly lower than that of natural penicillins. Therefore, they are prescribed strictly according to indications.
- There are differences within the group: nafcillin is more active than oxacillin against strains resistant to benzylpenicillin, while cloxacillin and flucloxacillin achieve higher serum concentrations.
The MRSA problem: There are hospital-acquired and community-acquired strains of Staphylococcus aureus that have developed resistance to oxacillin and methicillin (MRSA — methicillin-resistant Staphylococcus aureus). Methicillin is unique in that it acts on only one transpeptidase in S. aureus. In infections caused by MRSA, penicillins are powerless — vancomycin or linezolid become the therapy of choice.
Aminopenicillins: ampicillin vs amoxicillin
Aminopenicillins feature an expanded spectrum of action. The presence of a positively charged amino group in the side chain helps them penetrate through porin channels of the outer membrane of gram-negative bacteria (E. coli, salmonellae, shigellae, Haemophilus influenzae).
Important: this amino group does not protect the drug from beta-lactamases. Aminopenicillins are hydrolyzed by all types of these enzymes and have absolutely no effect on Pseudomonas aeruginosa, Klebsiella, and Serratia.
Despite belonging to the same group, the two main representatives differ radically in pharmacokinetics:
| Characteristic | Ampicillin | Amoxicillin |
|---|---|---|
| Absorption | Incomplete (30–40%), significantly reduced by food intake. | High (90–95%), independent of food. |
| Routes of administration | Oral, intravenous, intramuscular. | Oral only (per os). |
| Intestinal concentration | High (remains in the lumen due to poor absorption). | Low (almost completely absorbed in the upper GI tract). |
| Specific indications | Intestinal infections, listeriosis (drug of choice). | ENT infections, eradication of H. pylori, endocarditis prophylaxis. |
Protected penicillins
Since aminopenicillins are vulnerable to bacterial enzymes, a strategy to protect them was developed. The drug formulation includes not only the antibiotic itself but also a $\beta$-lactamase inhibitor.
The inhibitor takes the hit, binding to bacterial enzymes, which allows the antibiotic to reach transpeptidases unhindered and destroy the cell wall. Classical examples of such combinations are amoxicillin + clavulanic acid and ampicillin + sulbactam.
Briefly: answers to topical questions
What beta-lactamase inhibitors are used in combination with penicillins?
Three main beta-lactamase inhibitors are used in combination with penicillins to irreversibly inactivate bacterial enzymes.
- Clavulanic acid — used in combination products together with amoxicillin.
- Sulbactam — used in combination with ampicillin or amoxicillin.
- Tazobactam — mentioned in the classification as one of the representatives of beta-lactamase inhibitors.
Which groups of semisynthetic penicillins possess antipseudomonal activity?
Antipseudomonal activity is possessed by two groups of semisynthetic penicillins, the main advantage of which is their action against Pseudomonas aeruginosa.
- Carboxypenicillins — this group includes carbenicillin, carfecillin, and ticarcillin.
- Ureidopenicillins — include azlocillin, piperacillin, and mezlocillin; they are 4–8 times more active against Pseudomonas aeruginosa than carboxypenicillins.
What are the main side effects of semisynthetic penicillins?
The following side effects of penicillins are described in the sources:
- Allergic reactions — anaphylactic shock, urticaria, angioedema, and various rashes.
- Acute interstitial nephritis — a possible rare complication.
- Candidomycosis — occurs due to the suppression of normal flora by broad-spectrum antibiotics; localization in the oral cavity, intestines, and urogenital tract is possible.
- Pseudomembranous colitis — associated with the proliferation of Clostridium difficile following the suppression of competitive flora; may manifest as bloody diarrhea.
What is the molecular mechanism of action of semisynthetic penicillins?
The molecular mechanism of action of semisynthetic penicillins involves structural mimicry and disruption of bacterial cell wall synthesis.
- Transpeptidase inhibition — the antibiotic covalently binds to penicillin-binding proteins, blocking the formation of cross-links between peptidoglycan chains.
- Autolysis activation — β-lactams inhibit the endogenous autolysin inhibitor, leading to peptidoglycan degradation and bactericidal autocytolysis.
AMPicillin — Actively Mashes Pathogens in the gut. It is poorly absorbed into the bloodstream, remaining in the GI lumen, which makes it effective for intestinal infections. Amoxicillin is extremely well absorbed, so almost none of it remains in the intestine.
Class and exam questions
Why are aminopenicillins not used to treat Pseudomonas infections?
Pseudomonas aeruginosa (P. aeruginosa), as well as Klebsiella and Serratia species, have intrinsic resistance to aminopenicillins. Furthermore, these drugs are readily degraded by any beta-lactamases frequently produced by nosocomial strains.
Can salmonellosis or shigellosis be treated with amoxicillin?
No, this is not appropriate. Amoxicillin has a bioavailability of 90–95% and is almost completely absorbed in the upper GI tract. Its concentration is far too low in the lower parts of the intestine where the pathogens reside. Ampicillin is much better suited for these purposes.
What is the rationale for prescribing oxacillin when broad-spectrum drugs are available?
Oxacillin and other isoxazolyl penicillins possess a unique side chain that protects them from staphylococcal penicillinases. Broad-spectrum agents (without inhibitors) are destroyed by these enzymes.
What else to study on this topic
- Structural features of bacterial transpeptidases in cocci versus rod-shaped bacteria.
- Mechanism of antibiotic penetration through porin channels of Gram-negative flora.
- Combination therapy regimens for peptic ulcer disease (Helicobacter pylori eradication).
- Spectrum of activity and clinical use of carboxy- and ureidopenicillins.
- Prophylaxis protocols for infective endocarditis during dental procedures.
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