Drug Interactions between isoniazid / pyrazinamide / rifampin and Lignospan

MONITOR CLOSELY: The risk of hepatotoxicity is greater when rifampin and isoniazid (INH) are given concomitantly, than when either drug is given alone. The proposed mechanism is rifampin's induction of isoniazid hydrolase, an enzyme involved in the conversion of INH to isonicotinic acid and hydrazine. Hydrazine is the proposed toxic metabolite of INH, which has been shown in animal studies to cause steatosis, hepatocyte vacuolation and glutathione depletion. Some studies have also shown that slow acetylators have a two-fold increased risk of developing antituberculosis drug-induced hepatotoxicity (ATDH) as compared with fast acetylators due to more available INH for direct hydrolysis to hydrazine. Theoretically, a similar reaction may occur with rifabutin and isoniazid. Additional risk factors for developing hepatotoxicity include patients with advanced age, malnutrition, existing hepatic impairment, daily alcohol consumption, female gender, HIV infection, extra-pulmonary tuberculosis and/or patients who are taking other potent CYP450-inducing agents.

MANAGEMENT: Caution and close monitoring should be considered if isoniazid (INH) is coadministered with rifampin or rifabutin. In cases where coadministration is required, careful monitoring of liver function, especially ALT and AST, should be done at baseline and then every 2 to 4 weeks during therapy, or in accordance with individual product labeling. Some manufacturers of INH recommend strongly considering its discontinuation if serum aminotransferase concentrations (AST or SGOT, ALT or SGPT) exceed 3 to 5 times the upper limit of normal. Product labeling for rifampin also recommends the immediate discontinuation of therapy if hepatic damage is suspected. INH product labeling suggests alternate drugs be used if hepatitis is attributed to INH in patients with tuberculosis. However, if INH must be used, it should only be resumed after the patient's symptoms and laboratory abnormalities have cleared. It should also be restarted in very small, gradually increasing doses and immediately withdrawn if there is any indication of recurrent liver involvement. Patients should be counseled to immediately report signs or symptoms consistent with liver damage and notified that prodromal symptoms usually consist of fatigue, weakness, malaise, anorexia, nausea, and/or vomiting.

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GENERALLY AVOID: A two-month regimen consisting of rifampin (RIF) and pyrazinamide (PZA) for the treatment of latent tuberculosis infection (LTBI) has been associated with liver injury resulting in high rates of hospitalization and death. The exact mechanism of interaction is unknown, although both agents are individually hepatotoxic and may have additive effects on the liver during coadministration. In one prospective cohort study of 224 patients in a community setting between 1999 and 2001, investigators found that the risk of hepatotoxicity in patients receiving the RIF-PZA regimen was increased threefold compared to patients receiving six months of isoniazid (INH). When patients were monitored more intensively, severe hepatotoxicity did not develop, but the difference did not reach statistical significance.

MANAGEMENT: The American Thoracic Society and the Centers for Disease Control and Prevention recommend that the two-month RIF-PZA regimen generally not be offered to patients with LTBI (Note: This recommendation does not apply to the appropriate use of RIF and PZA in multidrug regimens for the treatment of persons with active TB disease). A nine-month course of daily INH remains the preferred treatment for LTBI in both HIV-negative and HIV-positive patients. Other acceptable options include nine months of twice-weekly INH, or six months of either daily or twice-weekly INH. Twice-weekly therapy must be administered under direct observed therapy (DOT), and the six-month regimens should generally not be used in HIV-infected individuals, those with fibrotic lesions on chest radiographs, and children. Four months of daily RIF may be considered for persons who are contacts of patients with INH-resistant, RIF-susceptible TB. The RIF-PZA regimen should never be offered to patients who are taking concomitant medications associated with liver injury; patients who drink alcohol excessively (even if alcohol use is discontinued during treatment); patients with underlying liver disease; and patients with a history of INH-associated liver injury. RIF-PZA may be considered in carefully selected patients if there is reason to believe they are not likely to complete the preferred six- or nine-month regimens. If RIF-PZA is prescribed, the PZA dosage should be no more than 20 mg/kg/day (up to a maximum of 2 g/day) or 50 mg/kg twice weekly (up to a maximum of 4 g twice weekly), and no more than a two-week supply of the medications should be dispensed at any given time. Patients should be evaluated in person by a healthcare provider at 2, 4, and 6 weeks of treatment for adherence, tolerance and adverse effects, and at 8 weeks to document treatment completion. Patients should also be instructed to discontinue the drugs promptly and seek medical attention if signs and symptoms of hepatic injury develop, including fever, rash, anorexia, nausea, vomiting, fatigue, right upper quadrant pain, dark urine, and jaundice. Serum transaminases and bilirubin should be measured at baseline and at 2, 4, 6, and 8 weeks of treatment in patients taking RIF-PZA. Therapy should be withdrawn and not resumed if transaminase levels exceed five times the upper limit of normal or are anywhere above the normal range when accompanied by symptoms of hepatitis, or if serum bilirubin is greater than the normal range. U.S. healthcare providers should report possible cases of RIF-PZA hepatotoxicity to CDC's Division of Tuberculosis Elimination, telephone 404-639-8442.

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MONITOR: Coadministration of isoniazid (INH) with other agents known to induce hepatotoxicity may potentiate the risk of liver injury. INH-associated hepatotoxicity is believed to be due to an accumulation of toxic metabolites and may also be partly immune mediated, though the exact mechanisms are not universally agreed upon. INH is metabolized by N-acetyltransferase and CYP450 2E1. The rate of INH's acetylation is genetically determined and generally classified as slow or rapid, with slow acetylators characterized by a relative lack of N-acetyltransferase. While the rate of acetylation does not significantly alter INH's effectiveness, it can lead to higher blood levels of INH and an increase of adverse reactions. In addition, INH is an in vitro inhibitor of several CYP450 isoenzymes (2C9, 2C19, 2E1, and 3A4). Coadministration of hepatotoxic drugs eliminated by one or more of these pathways may lead to elevated concentrations of the concomitant drug and increase the risk of hepatotoxicity. Most of the INH-associated hepatitis cases occur during the first 3 months of treatment, but may occur at any time and have been reported to be severe or even fatal. INH is reported in medical literature to cause clinically apparent acute liver injury with jaundice in 0.5% to 1% and fatality in 0.05% to 0.1% of recipients. A United States Public Health Service Surveillance Study of 13,838 people taking INH reported 8 deaths among 174 cases of hepatitis. Risk factors for INH related liver injury may include: age > 35 years, female gender, postpartum period, daily consumption of alcohol, injection drug user, slow acetylator phenotype, malnutrition, HIV infection, pre-existing liver disease, extra-pulmonary tuberculosis, and concomitant use of hepatotoxic medications. Clinical data have been reported with concurrent use of acetaminophen, alcohol, carbamazepine, phenobarbital, phenytoin, and rifampin.

MANAGEMENT: Coadministration of isoniazid (INH) with other hepatotoxic medications should be done with caution and close clinical monitoring. Some authorities recommend avoiding concurrent use when possible. If coadministration is needed, baseline and monthly liver function testing as well as monthly interviewing of the patient to check for signs and symptoms of adverse effects is recommended. More frequent testing may be advisable in patients at increased risk of INH-associated liver injury. Some manufacturers of INH recommend strongly considering its discontinuation if serum aminotransferase concentrations (AST or SGOT, ALT or SGPT) exceed 3 to 5 times the upper limit of normal. Patients should be counseled to immediately report signs or symptoms consistent with liver damage and notified that prodromal symptoms usually consist of fatigue, weakness, malaise, anorexia, nausea, and/or vomiting. If hepatic damage is suspected, INH should be immediately discontinued as continuation may lead to more severe damage. If hepatitis is attributed to INH in patients with tuberculosis, alternative drugs should be used. However, if INH must be used, it should only be resumed after the patient's symptoms and laboratory abnormalities have cleared. It should also be restarted in very small, gradually increasing doses and immediately withdrawn if there is any indication of recurrent liver involvement. Consultation with product labeling and relevant guidelines is advisable.

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MONITOR: While monoamine oxidase inhibitors (MAOIs) reportedly do not significantly enhance the hypertensive effects of direct-acting sympathomimetic amines, caution is nevertheless advised when these agents are used together. Some cases of increased heart rate and blood pressure have been reported.

MANAGEMENT: Blood pressure and heart rate should be monitored during coadministration of direct-acting sympathomimetic agents with MAOIs or other agents that possess MAOI activity (e.g., furazolidone, methylene blue, procarbazine).

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Coadministration with inducers of CYP450 1A2 and/or 3A4 may decrease the plasma concentrations of lidocaine, which is primarily metabolized by these isoenzymes. In four healthy volunteers (2 smokers and 2 nonsmokers), administration of a single 400 mg oral dose of lidocaine following pretreatment with the CYP450 inducer phenobarbital (15 mg/day for 4 weeks, followed by 30 mg/day for 4 weeks) decreased lidocaine systemic exposure (AUC) by 37% and increased its oral clearance by 56% compared to administration of lidocaine alone. In another study, the mean bioavailability of a single 750 mg oral dose of lidocaine in six patients receiving chronic antiepileptic drug therapy (consisting of one or more of the following enzyme-inducing anticonvulsants: phenobarbital, primidone, phenytoin, carbamazepine) was approximately 2.5-fold lower than that reported for six healthy control subjects, while intrinsic clearance was nearly threefold higher. By contrast, the interaction was modest for lidocaine administered intravenously, suggesting induction of primarily hepatic first-pass rather than systemic metabolism of lidocaine. When a single 100 mg dose of lidocaine was given intravenously, mean lidocaine AUC was reduced by less than 10% and serum clearance increased by just 17% in the epileptic patients compared to controls. These changes were not statistically significant. Likewise, mean lidocaine AUC decreased by approximately 11% and plasma clearance increased by 15% when a single 50 mg intravenous dose of lidocaine was administered following pretreatment with the potent CYP450 inducer rifampin (600 mg/day for six days) in ten healthy, nonsmoking male volunteers. Another pharmacokinetic study found that cigarette smoke, an inducer of CYP450 1A2, reduced the bioavailability of lidocaine when administered orally, but had only minor effects on lidocaine administered intravenously. When 4 smokers and 5 non-smokers received 2 doses of lidocaine (100 mg IV followed by 100 mg orally after a 2-day washout period), the smoker's systemic exposure (AUC) of oral lidocaine was 68% lower than non-smokers. The AUC of IV lidocaine was only 9% lower in smokers compared with non-smokers. The clinical impact of smoking on lidocaine has not been studied, however, a loss of efficacy may occur.

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