Macrolides

Classification and Chemical Structure

The foundation of the macrolide molecule is a macrocyclic lactone ring attached to sugars. In pharmacology, these drugs are typically classified according to two main principles:

1. By source of production: natural and semisynthetic antibiotics.
2. By lactone ring size: depending on the number of carbon atoms in the molecular core, macrolides are divided into 14-, 15-, and 16-membered rings.

Mechanism of antimicrobial action

Macrolides exhibit a bacteriostatic type of action—they halt bacterial growth and reproduction by interfering with protein synthesis.

• Molecular target: the antibiotic binds to a specific ligand within the ribosomal tunnel on the 50S subunit of the bacterial ribosome.
• Site of action: inhibition occurs at the translocation stage.
• Pathogenesis: the macrolide physically blocks the movement of the formed peptide from the A-site to the P-site of the ribosome. This disrupts the export of synthesized peptide chains, halting protein assembly.

Important: despite their basic bacteriostatic effect, at high concentrations macrolides are capable of acting bactericidally against a number of pathogens: pneumococci, group A $\beta$-hemolytic streptococcus, as well as bacteria causing pertussis and diphtheria.

Spectrum of activity and clinical use

The spectrum of activity of macrolides is largely similar to benzylpenicillin. They are active against gram-positive (streptococci, staphylococci) and gram-negative cocci (gonococci, meningococci), as well as Haemophilus influenzae, Borrelia, Treponema pallidum, and Helicobacter pylori.

The primary advantage of the group is their ability to readily penetrate host cells. This makes them a potent weapon against intracellular pathogens: chlamydiae, legionellae, ureaplasmas, and mycoplasmas.

Main indications for prescription:

• Respiratory tract and ENT organs: streptococcal tonsillopharyngitis, pertussis, diphtheria, and "atypical" pneumonias.
• Skin and soft tissues: erysipelas, scarlet fever.
• Specific infections: chlamydiosis, mycoplasma infection, osteomyelitis, oral cavity infections.
• Gastroenterology: inclusion in H. pylori eradication regimens for the treatment of gastric and duodenal ulcers.
• Mycobacteriosis: treatment and prevention of infections caused by Mycobacterium avium (clarithromycin and azithromycin are used, presumably inhibiting protein synthesis in the organelles of these bacteria).
• Prophylaxis: year-round prophylaxis of rheumatic fever in patients with penicillin allergies.

Resistance and Environmental Influence

The efficacy of macrolides directly depends on tissue acidity. In an inflammatory focus, pH typically drops. In such an acidic environment, drug ionization increases, leading to inactivation. Optimal conditions for antibiotic activity require a pH above 5.

Bacteria actively develop resistance to macrolides, with resistance genes frequently transmitted via plasmids. Four microbial defense mechanisms are distinguished:

1. Enzymatic inactivation: production of esterases that hydrolyze the drug. This pathway is particularly characteristic of enterococci.
2. Target modification: alteration of the ribosomal binding site structure. This may occur via chromosomal mutations or methylation. The production of methylase enzymes is the primary resistance mechanism in gram-positive flora.
3. Impaired transport: the bacterium either decreases cell membrane permeability or activates active efflux of the antibiotic.
4. Cross-resistance (MLS phenotype): if a bacterium constitutively produces methylase, it becomes invulnerable not only to macrolides but also to structurally distinct yet mechanistically similar drugs — lincosamides (clindamycin) and group B streptogramins.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Differences in the properties of 14-, 15-, and 16-membered macrolides
• Efflux mechanisms: how bacteria "pump out" the antibiotic
• The role of clarithromycin in Helicobacter pylori eradication regimens
• Specifics of treating infections caused by Mycobacterium avium
• Resistance genetics: the role of plasmids and chromosomal mutations

Other topics in the section "Antibacterial Chemotherapeutic Agents"

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