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Classification of Antitubercular Drugs
Antitubercular drugs are categorized into two main classes:
- First-Line Drugs: High efficacy and lower toxicity. Includes Isoniazid, Rifampicin, Pyrazinamide, Ethambutol, and Streptomycin.
- Second-Line Drugs: Used in cases of resistance or multidrug-resistant TB. Includes Ethionamide, Cycloserine, Capreomycin, Kanamycin, Amikacin, and Fluoroquinolones (e.g., Levofloxacin, Moxifloxacin).
Mechanisms of Action, Adverse Effects, and Contraindications
First-Line Drugs
Isoniazid (INH)
- Mechanism of Action: Inhibits mycolic acid synthesis, affecting the mycobacterial cell wall.
- Adverse Effects: Hepatotoxicity, peripheral neuropathy, hypersensitivity.
- Contraindications: Liver disease, hypersensitivity to INH.
Rifampicin
- Mechanism of Action: Inhibits RNA synthesis by blocking DNA-dependent RNA polymerase.
- Adverse Effects: Hepatotoxicity, orange discoloration of body fluids, gastrointestinal disturbances.
- Contraindications: Liver disease, certain antiretroviral drug interactions.
Pyrazinamide
- Mechanism of Action: Lowers pH inside bacterial cells, disrupting metabolism.
- Adverse Effects: Hepatotoxicity, hyperuricemia (may cause gout), gastrointestinal disturbances.
- Contraindications: Severe liver disease, acute gout.
Ethambutol
- Mechanism of Action: Inhibits arabinosyl transferase, affecting cell wall synthesis.
- Adverse Effects: Optic neuritis, peripheral neuropathy.
- Contraindications: Optic neuritis, young children.
Streptomycin
- Mechanism of Action: Binds to the 30S ribosomal subunit, inhibiting protein synthesis.
- Adverse Effects: Ototoxicity, nephrotoxicity, hypersensitivity.
- Contraindications: Renal impairment, pregnancy, pre-existing hearing loss.
Second-Line Drugs
Ethionamide
- Mechanism of Action: Similar to INH; inhibits mycolic acid synthesis.
- Adverse Effects: Gastrointestinal disturbances, hepatotoxicity, peripheral neuropathy.
- Contraindications: Liver disease, severe gastrointestinal disorders.
Cycloserine
- Mechanism of Action: Inhibits cell wall synthesis by affecting D-alanine incorporation.
- Adverse Effects: Neurological effects (e.g., anxiety, psychosis, seizures).
- Contraindications: Epilepsy, severe mental disorders.
Capreomycin
- Mechanism of Action: Disrupts protein synthesis by binding to bacterial ribosomes.
- Adverse Effects: Nephrotoxicity, ototoxicity.
- Contraindications: Renal impairment, hearing impairment.
Fluoroquinolones (e.g., Levofloxacin, Moxifloxacin)
- Mechanism of Action: Inhibit DNA gyrase and topoisomerase IV, hindering DNA replication.
- Adverse Effects: Tendonitis, gastrointestinal disturbances, QT prolongation.
- Contraindications: Pregnancy, children, history of tendon disorders.
