What is the mechanism of action of first-line anti-tuberculosis (TB) drugs, including isoniazid (INH), rifampicin (RIF), pyrazinamide (PZA), and ethambutol (EMB), in a standard adult patient with no significant medical history?

Mechanisms of Action of First-Line Anti-Tuberculosis Drugs

The four first-line anti-TB drugs work through distinct molecular mechanisms: isoniazid inhibits mycolic acid synthesis in the bacterial cell wall, rifampicin blocks bacterial RNA polymerase, pyrazinamide disrupts membrane energetics in acidic environments, and ethambutol interferes with arabinogalactan synthesis in the cell wall. 1, 2, 3, 4

Isoniazid (INH)

Molecular Mechanism

• Isoniazid inhibits the synthesis of mycolic acids, which are essential components of the mycobacterial cell wall. 2
• The drug acts as a prodrug that requires activation by the bacterial catalase-peroxidase enzyme KatG to form the active metabolite. 2
• At therapeutic levels, isoniazid is bactericidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms. 2

Pharmacological Properties

• Isoniazid produces peak blood levels within 1-2 hours after oral administration, which decline to 50% or less within 6 hours. 2
• It diffuses readily into all body fluids including cerebrospinal fluid, pleural fluid, and ascitic fluid, as well as tissues and organs. 2
• The drug passes through the placental barrier and into breast milk in concentrations comparable to plasma levels. 2
• Isoniazid has profound early bactericidal activity against rapidly dividing mycobacterial cells. 1

Resistance Mechanism

• Resistance develops rapidly when isoniazid is used as monotherapy, occurring through single-step mutations in existing genes (primarily in the katG gene encoding catalase-peroxidase or inhA gene encoding enoyl-ACP reductase), not by acquisition of new genetic material. 2, 5

Rifampicin (Rifampin/RIF)

Molecular Mechanism

• Rifampicin inhibits DNA-dependent RNA polymerase activity in susceptible M. tuberculosis organisms. 3
• Specifically, it interacts with bacterial RNA polymerase but does not inhibit the mammalian enzyme, providing selective toxicity. 3
• Rifampicin has bactericidal activity against slow and intermittently growing M. tuberculosis organisms both in vitro and in vivo. 3

Pharmacological Properties

• After intravenous administration of 600 mg infused over 30 minutes, mean peak plasma concentrations reach 17.5 ± 5.0 mcg/mL. 3
• Rifampicin is widely distributed throughout the body and is present in effective concentrations in many organs and body fluids, including cerebrospinal fluid. 3
• The drug is approximately 80% protein bound, with most of the unbound fraction not ionized, allowing free diffusion into tissues. 3
• Rifampicin is rapidly eliminated in bile and undergoes progressive enterohepatic circulation and deacetylation to 25-desacetyl-rifampin, which retains microbiological activity. 3

Resistance Mechanism

• Organisms resistant to rifampicin are likely resistant to other rifamycins due to single-step mutations of the DNA-dependent RNA polymerase (primarily in the rpoB gene). 3, 5
• Resistance can emerge rapidly, particularly when small numbers of resistant cells within large populations of susceptible cells become predominant during inadequate therapy. 3

Pyrazinamide (PZA)

Molecular Mechanism

• The exact mechanism of action of pyrazinamide is unknown, but the drug is active only at slightly acidic pH both in vitro and in vivo. 4
• Pyrazinamide may be bacteriostatic or bactericidal against M. tuberculosis depending on the concentration achieved at the site of infection. 4
• The drug is hydrolyzed in the liver to its major active metabolite, pyrazinoic acid, which is believed to be responsible for its antimycobacterial activity. 4

Pharmacological Properties

• Pyrazinamide is well absorbed from the gastrointestinal tract and attains peak plasma concentrations within 2 hours. 4
• Plasma concentrations generally range from 30 to 50 mcg/mL with doses of 20 to 25 mg/kg. 4
• It is widely distributed in body tissues and fluids including liver, lungs, and cerebrospinal fluid. 4
• The CSF concentration is approximately equal to concurrent steady-state plasma concentrations in patients with inflamed meninges. 4
• The half-life is 9-10 hours in patients with normal renal and hepatic function. 4

Resistance Mechanism

• Resistance to pyrazinamide occurs through mutations in existing genes (primarily the pncA gene encoding pyrazinamidase), not by acquisition of new genetic material. 5

Ethambutol (EMB)

Molecular Mechanism

• Ethambutol interferes with the synthesis of arabinogalactan, a critical component of the mycobacterial cell wall. 1
• The drug specifically inhibits arabinosyl transferases, enzymes involved in the polymerization of arabinogalactan and lipoarabinomannan. 1
• Ethambutol is primarily bacteriostatic but can be bactericidal at higher concentrations. 1

Pharmacological Properties

• Ethambutol is included in initial regimens to prevent the emergence of resistance while awaiting drug susceptibility testing results. 6
• The drug penetrates adequately into cerebrospinal fluid only when meninges are inflamed in the early stages of treatment. 7

Resistance Mechanism

• Resistance to ethambutol develops through mutations in the embB gene encoding arabinosyl transferase, though resistance is more difficult to measure reliably compared to isoniazid and rifampicin. 5, 8

Clinical Implications of Mechanisms

Synergistic Activity

• The four-drug regimen (isoniazid, rifampicin, pyrazinamide, and ethambutol) is effective even when the infecting organism is resistant to isoniazid, because rifampicin and pyrazinamide provide sufficient bactericidal activity. 9, 10
• The combination prevents the emergence of resistance by ensuring at least two active drugs are always present. 1, 5

pH-Dependent Activity

• Pyrazinamide's requirement for acidic pH explains its particular effectiveness against bacilli in the acidic environment of macrophages and in caseous necrotic lesions. 4
• This unique property makes pyrazinamide essential for sterilizing activity and shortening treatment duration from 9-12 months to 6 months. 9

Penetration into Special Sites

• The excellent CSF penetration of isoniazid and pyrazinamide throughout treatment, combined with adequate rifampicin penetration, makes these three drugs the backbone of CNS tuberculosis therapy. 7, 4
• Ethambutol and streptomycin only achieve therapeutic CSF levels when meninges are inflamed, limiting their utility in later stages of CNS TB treatment. 7

Common Pitfalls Related to Mechanisms

• Never use isoniazid as monotherapy, as resistance develops rapidly through single-step mutations when only one drug is present. 2, 5
• Never assume β-lactamase production affects rifampicin activity—rifampicin's mechanism is completely independent of β-lactam pathways. 3
• Never expect pyrazinamide to work in alkaline environments or in vitro at neutral pH—its mechanism requires acidic conditions. 4
• Never discontinue ethambutol before drug susceptibility results confirm full susceptibility to isoniazid and rifampicin, as its mechanism provides critical protection against resistance emergence. 1, 11