Rifampicin (Rifabon)

Active Ingredient & Mechanism of Action

• Active Ingredient: Rifampicin (also referred to as Rifampin).
• Mechanism: Rifampicin inhibits bacterial growth by selectively binding to the $\beta$-subunit of bacterial DNA-dependent RNA polymerase.
  1. Transcription Inhibition: By binding to this enzyme, it blocks the initiation of RNA transcription and halts bacterial messenger RNA (mRNA) synthesis.
  2. Bactericidal Activity: This disruption leads directly to bacterial cell death. Because human RNA polymerase has a distinctly different subunit structure, rifampicin targets bacterial cells while sparing host mammalian RNA synthesis.

Dosage Forms & Available Strengths

• Oral Capsules / Tablets: Standard individual strengths include 150 mg, 300 mg, and 600 mg.
• Oral Suspension: Liquid formulations (e.g., 20 mg/mL) designed for pediatric patients or individuals with difficulty swallowing capsules.
• Intravenous (IV) Injection: Powder for reconstitution (typically 600 mg vials) used in acute, severe inpatient settings where oral administration is impossible.
• Fixed-Dose Combinations (FDCs): Frequently co-formulated with other first-line anti-TB medications (such as Isoniazid, Pyrazinamide, and Ethambutol) to prevent drug resistance and simplify daily treatment schedules.

Clinical Indications

• Tuberculosis (TB): A core component of first-line combination regimens for treating all forms of active Mycobacterium tuberculosis infection.
• Leprosy (Hansen’s Disease): Used as part of World Health Organization (WHO) multi-drug therapy (MDT) alongside dapsone and clofazimine.
• Meningococcal Prophylaxis: Short-term administration to clear asymptomatic nasopharyngeal carriage of Neisseria meningitidis in close contacts of infected patients.
• Haemophilus influenzae Type b (Hib) Prophylaxis: Prevention of invasive Hib disease in close household contacts.
• Serious Staphylococcal Infections: Used as adjunctive/combination therapy (never as monotherapy) alongside other agents (e.g., vancomycin or flucloxacillin) to treat severe Staphylococcus aureus infections, including prosthetic valve endocarditis or osteomyelitis.

Key Pharmacokinetics

• Absorption: Well absorbed from the gastrointestinal tract when taken on an empty stomach.
• Food Effect: Food delays absorption and reduces peak plasma concentrations ($C_{max}$).
• Distribution: Highly lipophilic; distributes widely throughout body tissues and fluids, including lung tissue, liver, bone, pleural fluid, and cerebrospinal fluid (CSF).
• Metabolism & Elimination: Metabolized primarily by the liver into an active deacetylated metabolite. It undergoes enterohepatic circulation and is excreted in bile/feces and urine.
• Half-Life: Approximately 3 to 5 hours, shortening over early weeks of continuous therapy due to auto-induction of hepatic enzymes.

Administration Guidelines

• Timing: Administered orally on an empty stomach—either 1 hour before or 2 hours after meals—with a full glass of water to ensure maximal absorption.
• Consistency: Daily compliance and adherence to the full prescribed duration are vital to avoid treatment failure and the emergence of drug-resistant strain variants (MDR-TB).

Safety Profile & Considerations

• Benign Body Fluid Discoloration (Key Patient Warning): Rifampicin causes a reddish-orange discoloration of body fluids, including urine, sweat, saliva, tears, sputum, and feces. This is harmless, but patients should be warned that soft contact lenses may become permanently stained.
• Common Side Effects: Gastrointestinal upset (nausea, vomiting, abdominal pain), rash, fever, fatigue, and transient elevations in liver enzymes.
• Hepatotoxicity: Can cause liver damage, particularly when co-administered with other hepatotoxic drugs like isoniazid or pyrazinamide. Regular monitoring of liver function tests (LFTs) is essential.
• Potent Enzyme Induction (Major Drug-Drug Interactions): Rifampicin is a potent inducer of hepatic CYP450 enzymes (especially CYP3A4 and CYP2C9) and P-glycoprotein (P-gp). It dramatically accelerates the clearance and lowers the blood concentrations of many concurrent medications, including:
  • Oral Contraceptives: Renders hormonal birth control ineffective; non-hormonal barrier methods must be used.
  • Anticoagulants (e.g., Warfarin, DOACs): Significantly reduces blood-thinning effectiveness.
  • Antiretrovirals (e.g., Protease Inhibitors, Integrase Inhibitors): Alters HIV treatment levels.
  • Antidiabetics, Anticonvulsants, & Immunosuppressants (e.g., Cyclosporine, Tacrolimus): Substantially lowers therapeutic drug levels.
• Contraindications: Severe hypersensitivity to rifamycins or severe active hepatic dysfunction. Concomitant use with specific protease inhibitors or combination antivirals (such as saquinavir/ritonavir) is contraindicated.

Note: Rifampicin (Rifabon) is a strictly regulated prescription antibiotic. Liver function monitoring, evaluation for drug interactions, and regimented dosing must be overseen directly by a qualified healthcare professional.