Drug Interactions between isoniazid / rifampin and olanzapine

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 It should also be restarted in very small, gradually increasing doses and immediately withdrawn if there is any indication of recurrent liver involvement.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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MONITOR: Coadministration with inducers of CYP450 1A2 may decrease the plasma concentrations of olanzapine. Data from available studies indicate that olanzapine is primarily metabolized by CYP450 1A2 and, to a lesser extent, by CYP450 2D6 and uridine diphosphate glucuronosyltransferase (UGT) 1A4. When coadministered with rifampin, a moderate CYP450 1A2 inducer that also induces UGT 1A4, olanzapine peak plasma concentration (Cmax) and systemic exposure (AUC) decreased by 11% and 48%, respectively. Coadministration with 200 mg twice daily of carbamazepine, another CYP450 1A2 inducer, caused an approximately 50% increase in the clearance of olanzapine. Higher daily dosages of carbamazepine may cause an even greater increase in olanzapine clearance.

MANAGEMENT: The potential for diminished pharmacologic effects of olanzapine should be considered during coadministration with CYP450 1A2 inducers. Alternative treatments may be required if an interaction is suspected.

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MONITOR: Coadministration with inhibitors of CYP450 1A2 may increase the plasma concentrations of olanzapine. Data from available studies indicate that olanzapine is primarily metabolized by CYP450 1A2 and, to a lesser extent, by CYP450 2D6. When coadministered with fluvoxamine, a potent CYP450 1A2 inhibitor that also inhibits CYP450 2D6, olanzapine peak plasma concentration (Cmax) increased by an average of 54% in female nonsmokers and 77% in male smokers, while systemic exposure (AUC) increased by an average of 52% and 108%, respectively. The greater degree of interaction in smokers is likely due to induction of CYP450 1A2 by polycyclic aromatic hydrocarbons in cigarette smoke, resulting in increased expression of the isoenzyme. Similar results have been reported in several other pharmacokinetic studies. In 12 healthy male volunteers, administration of a single 10 mg dose of olanzapine during treatment with fluvoxamine 100 mg/day increased mean olanzapine Cmax, AUC and elimination half-life (T1/2) by 49%, 76% and 40%, respectively, compared to administration of olanzapine alone. In 10 male smokers with schizophrenia, olanzapine Cmax, AUC, and T1/2 increased by 12% to 64%, 30% to 55%, and 25% to 32%, respectively, when a single 10 mg dose of olanzapine was administered on day 10 of treatment with fluvoxamine 50 mg/day and 100 mg/day, each for 2 weeks. In 8 patients with schizophrenia who had been treated with olanzapine 10 to 20 mg/day for at least 3 months, the addition of fluvoxamine 100 mg/day for 8 weeks increased olanzapine plasma concentrations by 12% to 112%, with a mean of 81%, from baseline. In an analysis of data from a therapeutic drug monitoring service, patients treated concomitantly with fluvoxamine had olanzapine plasma concentration to daily dose (C/D) ratios that were on average 2.3-fold higher than those of patients receiving olanzapine alone. The difference was as high as 4.2-fold in some patients. In contrast, coadministration with sertraline was not associated with increased C/D ratios compared to olanzapine alone, and a pharmacokinetic study involving 15 healthy volunteers also demonstrated no significant interaction with fluoxetine. Another similar study conducted in a group of 250 patients receiving olanzapine daily doses ranging from 2.5 to 30 mg found that coadministration with fluvoxamine increased median C/D ratios by 74%. In an investigation to test the hypothesis that coadministration of a low subclinical dose of fluvoxamine (25 mg/day) can help reduce olanzapine therapeutic dose requirements, a 26% reduction in the mean olanzapine dosage taken by 10 male smokers with stable psychotic illness resulted in no significant changes in olanzapine plasma concentration, antipsychotic response, or metabolic indices (e.g., serum glucose, lipids) during treatment with fluvoxamine for up to 6 weeks. Clinical toxicity has been cited in a case report of a patient treated with fluvoxamine 150 mg/day and olanzapine 15 mg/day for several months. The patient had mydriasis, hand tremors, and muscle rigidity in association with toxic olanzapine plasma levels. Subsequent reduction of the olanzapine dosage to 5 mg/day resolved the toxicity but did not produce adequate therapeutic response, and the patient was switched to paroxetine with no further problems. The interaction has also been reported with ciprofloxacin, another CYP450 1A2 inhibitor. Doubling of olanzapine concentrations, akathisia, and QT prolongation have been described in various case reports.

MANAGEMENT: Pharmacologic response and olanzapine plasma levels should be monitored more closely whenever CYP450 1A2 inhibitors are added to or withdrawn from therapy in patients stabilized on their antipsychotic regimen, and the dosage adjusted as necessary.

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