First-Line Treatment of Tuberculosis

Overview

Standard 6-Month Regimen (Drug-Susceptible TB)

"For drug-susceptible disease, a standardized regimen is used with an intensive phase consisting of four drugs - isoniazid (H), rifampin (R), pyrazinamide (Z), and ethambutol (E) - given for 2 months, which is followed by a continuation phase of isoniazid and rifampin for 4 months, for a total treatment duration of 6 months."

• Harrison's Principles of Internal Medicine 22E (2025)

The Four First-Line Drugs

1. Isoniazid (INH / H)

• Dose: 5 mg/kg/day orally (max 300 mg/day)
• Mechanism: Inhibits mycolic acid synthesis (cell wall); bactericidal against rapidly dividing bacilli
• Key side effects: Peripheral neuropathy, hepatotoxicity, hypersensitivity
• Note: Always co-administer pyridoxine (Vitamin B6) 25-50 mg/day to prevent peripheral neuropathy, especially in elderly, malnourished, diabetic, or pregnant patients

2. Rifampicin (RIF / R)

• Dose: 10 mg/kg/day orally (max 600 mg/day)
• Mechanism: Inhibits bacterial RNA polymerase - bactericidal for multiple bacterial populations; uniquely prevents relapse (sterilizing activity)
• Key side effects: Orange-red discoloration of body fluids, hepatotoxicity, flu-like syndrome (with intermittent use), thrombocytopenia
• Important: Potent CYP450 inducer - interacts with antiretrovirals, anticoagulants, anticonvulsants, oral contraceptives, antifungals, calcineurin inhibitors

3. Pyrazinamide (PZA / Z)

• Dose: 25 mg/kg/day orally (max 2 g/day)
• Mechanism: Bactericidal against semi-dormant intracellular bacilli in acidic (lysosomal) environments - active only in the first 2 months
• Key side effects: Hepatotoxicity, hyperuricemia (can precipitate gout), photosensitivity
• Monitor: Liver function tests and uric acid levels

4. Ethambutol (EMB / E)

• Dose: 15-25 mg/kg/day orally (max 2.5 g/day)
• Mechanism: Inhibits arabinosyl transferase, disrupting arabinogalactan cell wall synthesis; primarily bacteriostatic (bactericidal at high doses)
• Key side effects: Optic neuritis - reduced visual acuity and red-green color discrimination
• Note: Ethambutol is included to protect against undetected isoniazid resistance; can be stopped once susceptibility to H and R is confirmed
• Monitoring: Baseline and follow-up color vision testing

Dosing Schedules (from Fishman's Pulmonary Diseases)

When to Extend Treatment to 9 Months

• Cavitary pulmonary disease
• Pyrazinamide not completed during the intensive phase
• Delayed sputum culture conversion (positive cultures beyond 2 months of treatment)

Special Situations

TB Meningitis

• Four-drug regimen (HRZE) for intensive phase, same principles apply
• Total duration typically 9-12 months
• Corticosteroids (dexamethasone) added to reduce mortality

HIV Co-infection

• Standard 2HRZE/4HR is used
• Avoid 2x/week or 3x/week dosing (risk of acquired rifampicin resistance)
• Manage drug interactions between rifampicin and antiretrovirals (consider rifabutin as substitute)
• CD4 count guides timing of ART initiation

Renal Impairment (from Comprehensive Clinical Nephrology)

• Isoniazid: No dose reduction if GFR 10-50; 66% dose if GFR <10
• Rifampicin: No dose reduction needed
• Pyrazinamide: No reduction if GFR 10-50; 50% dose if GFR <10
• Ethambutol: 75% dose if GFR 10-50; avoid or use 50% dose if GFR <10

Pregnancy

• Rifampicin, isoniazid, and ethambutol are safe; pyrazinamide is generally used (WHO recommends it)
• Add pyridoxine throughout

Newer 4-Month Regimen (2025 Update)

• Shown noninferior to the standard 6-month regimen
• Now carries a conditional recommendation from ATS/CDC/ERS/IDSA (2025 guidelines)
• HIV-positive patients with CD4 >100 were included in the trial
• This is the first approved shortening of DS-TB therapy in decades

Pre-Treatment Baseline Assessments

1. HIV testing and other blood-borne virus screening
2. Baseline liver function tests (LFTs) - rifampicin, isoniazid, and pyrazinamide are all potentially hepatotoxic
3. Baseline renal function
4. Baseline visual acuity and color vision (if using ethambutol)
5. Baseline audiometry (if injectable agents are used, e.g., in MDR-TB)
6. Drug susceptibility testing (DST)
7. Check for drug interactions (especially rifampicin)

Drug Resistance Classification

Summary Mnemonics

• Rifampicin - Red urine, Resistance induction (CYP450)
• Isoniazid - Inhibits B6 (give pyridoxine), Injury to liver
• Pyrazinamide - Pain (gout/hyperuricemia), Photosensitivity, liver
• Ethambutol - Eye (optic neuritis), check color vision