Mechanism of Action and Selectivity
Rifamycins (specifically rifampin) have a complex structure featuring a macrocyclic ring and belong to the ansamycin group. They exert a bactericidal effect by irreversibly binding to the beta-subunit of bacterial DNA-dependent RNA polymerase.
- The drug does not interfere with the initiation of transcription, but blocks the elongation phase once the RNA chain reaches a length of 2–3 nucleotides.
- The antibiotic molecule is thought to physically block the channel through which the growing RNA chain exits.
Selectivity is very high: mammalian RNA polymerases do not respond to therapeutic doses of the drug, and mitochondrial enzymes are inhibited only at extremely high concentrations that are clinically unreachable.
Spectrum of Activity and Clinical Use
These drugs possess a broad spectrum of activity. Their unique feature is the ability to eradicate bacteria located inside phagosomes (acting on both extracellular and intracellular forms). Rapidly multiplying microorganisms are the most sensitive to rifamycins:
- Mycobacteria: causative agents of tuberculosis (Mycobacterium tuberculosis) and leprosy (M. leprae).
- Intracellular pathogens: Brucella, Chlamydia, Legionella, Rickettsia.
- Gram-positive cocci: Staphylococci, Streptococci.
- Gram-negative cocci: Meningococci, Gonococci.
- Gram-negative bacilli: Escherichia coli.
In clinical practice, rifampin is used to treat tuberculosis, leprosy, brucellosis, legionellosis, and for the chemoprophylaxis of meningococcal disease. The drug can significantly slow down the reactivation of latent tuberculosis.
Resistance and Pharmacokinetics
The golden rule of using rifampin for tuberculosis is a strict prohibition on monotherapy. Bacterial resistance develops rapidly due to mutations in the drug-binding region of the enzyme (at a frequency of 1 in 10⁶–10⁸ bacilli). Therefore, the drug is prescribed exclusively as part of combination therapy (for example, it exhibits synergy with isoniazid in vitro).
The elimination half-life of the drug is 2–5 hours. It is administered once daily or 3 times a week, either orally (per os) or intravenously.
Rifampin is a potent inducer of hepatic microsomal enzymes. It accelerates metabolism and decreases the duration of action of many co-administered drugs, including:
- Coumarin anticoagulants (warfarin).
- Glucocorticoids.
- Oral contraceptives.
- Oral hypoglycemic agents.
Side Effects
The drugs are generally well tolerated, but adverse reactions from various organ systems can occur:
- Specific: Red-orange discoloration of body fluids (sweat, tears, urine).
- Liver: Elevated transaminases and bilirubin, jaundice, drug-induced hepatitis.
- Gastrointestinal tract: Dyspepsia (nausea, vomiting, diarrhea, abdominal pain), oral candidiasis, pseudomembranous colitis.
- Hematologic: Thrombocytopenic purpura, thrombocytopenia, leukopenia, acute hemolytic anemia.
- Kidneys: Tubular necrosis, interstitial nephritis, acute renal failure.
- Allergic reactions: Skin rash, pruritus, urticaria, angioedema, eosinophilia, drug fever (flu-like syndrome).
- Nervous system and special senses: Visual disturbances, arthralgia.
- Intravenous administration: Phlebitis, hypotension (if infused too rapidly).
Rifabutin — A Second-Line Agent
Rifabutin is a semi-synthetic derivative used as a second-line agent for tuberculosis. Its main differences from rifampin include:
- Retains efficacy against certain M. tuberculosis strains already resistant to rifampin.
- Exhibits higher activity against the Mycobacterium avium complex (MAC), which is particularly relevant in HIV infection.
- Gastrointestinal absorption is independent of food intake (although overall bioavailability is lower).
- Lower plasma protein binding.
- Significantly longer elimination half-life (32–67 hours).
- A weaker inducer of hepatic enzymes (CYP3A), thus causing fewer drug-drug interactions.