Hepatotoxicity of antitubercular treatments - Rationale for monitoring liver status

The standard antitubercular regimen currently includes a combination of 3 antitubercular agents: isoniazid, rifampicin (rifampin) and pyrazinamide. Administration of a fourth agent, ethambutol, is recommended when isoniazid resistance is suspected. Two of these 4 agents (isoniazid and pyrazinamide) are major hepatotoxins. The remaining 2 agents (rifampicin and ethambutol) are rarely or not hepatotoxic. However, rifampicin, which is a powerful enzyme inducer, may enhance the hepatotoxicity of isoniazid. In patients receiving a combination of isoniazid, rifampicin and pyrazinamide, 2 patterns of fulminant liver injury can be observed. The first pattern is characterised by an increase in serum transaminase activity that occurs soon (usually within the first 15 days) after initiation of treatment. This pattern is likely to be caused by rifampicin-induced isoniazid hepatotoxicity. The prognosis is good in most cases. The second pattern is characterised by an increase in serum transaminase activity that occurs late (usually more than 1 month) after the initiation of treatment. It has been suggested that this pattern may be related to pyrazinamide hepatotoxicity. The prognosis of this type of hepatitis is generally poor. In order to reduce the risk of severe hepatic adverse effects during antitubercular treatment, several measures are proposed. First, patients with underlying liver test abnormalities should not be given pyrazinamide. Second, isoniazid and pyrazinamide should be administered at the lowest dosage within their respective therapeutic ranges. Third, serum transaminase levels should be determined twice weekly during the first 2 weeks of treatment, every 2 weeks during the rest of the first 2 months, and every month thereafter. When serum transaminase levels increase to greater than 3 times the upper limit of normal, therapy with isoniazid, rifampicin and pyrazinamide should be stopped. After serum transaminase levels have returned to normal, isoniazid can be re-introduced at a low daily dose, without rifampicin. Pyrazinamide may not be re-introduced because of the risk of recurrence and the poor prognosis of pyrazinamide-induced hepatitis. Although it is nephrotoxic, streptomycin is an alternative in patients with liver test abnormalities during antitubercular treatment.