Drug Interactions between pirfenidone and thioguanine

ADJUST DOSING INTERVAL: Food significantly slows the rate but only modestly reduces the extent of absorption of pirfenidone. In healthy, older adult volunteers aged 50 to 66 years, administration of a single 801 mg oral dose of pirfenidone in the fed state resulted in an approximately 50% reduction in peak plasma concentration (Cmax) and a 15% to 20% reduction in systemic exposure (AUC) compared to administration in the fasted state. Median time to reach peak concentration (Tmax) increased from 0.5 hours to 3 hours with food. Less nausea and dizziness were observed in fed subjects compared to fasted subjects.

GENERALLY AVOID: Consumption of grapefruit juice is associated with inhibition of CYP450 1A2 and may increase the plasma concentrations of pirfenidone, which is primarily metabolized by the isoenzyme.

GENERALLY AVOID: Cigarette smoking may reduce pirfenidone exposure due to induction of CYP450 1A2, the isoenzyme primarily responsible for the metabolic clearance of pirfenidone. Following a single 801 mg oral dose of pirfenidone in 25 smokers and 25 healthy nonsmokers, the Cmax and AUC of pirfenidone in smokers were 68% and 46% of those in nonsmokers, respectively.

MANAGEMENT: Pirfenidone should be administered with food to reduce the likelihood of dizziness and gastrointestinal side effects such as nausea, diarrhea, dyspepsia, and vomiting. Patients who experience intolerance to therapy due to these adverse events should be reminded to take pirfenidone with food. If symptoms do not improve, or they worsen in severity, a dosage reduction or discontinuation of therapy may be warranted. Patients should be advised to avoid consumption of grapefruit and grapefruit juice during treatment with pirfenidone. Cigarette smoking should also be avoided during therapy to prevent reduced exposure to pirfenidone.

MONITOR: The concomitant or sequential use of other agents known to induce hepatotoxicity may potentiate the risk of liver injury associated with thioguanine. A high risk of liver toxicity characterized by vascular endothelial damage has been reported with long-term continuous use of thioguanine, particularly in children receiving the drug as part of maintenance therapy for acute lymphoblastic leukemia and in other conditions associated with continuous use. Liver toxicity usually presents as the clinical syndrome of hepatic veno-occlusive disease (hyperbilirubinemia, tender hepatomegaly, weight gain due to fluid retention, and ascites) or with signs of portal hypertension (splenomegaly, thrombocytopenia, and esophageal varices). Histopathological features include hepatoportal sclerosis, nodular regenerative hyperplasia, peliosis hepatitis, and periportal fibrosis.

MANAGEMENT: The risk of hepatic injury should be considered when thioguanine is used with other potentially hepatotoxic agents (e.g., acetaminophen; alcohol; androgens and anabolic steroids; antituberculous agents; azole antifungal agents; ACE inhibitors; cyclosporine (high dosages); disulfiram; endothelin receptor antagonists; interferons; ketolide and macrolide antibiotics; kinase inhibitors; minocycline; nonsteroidal anti-inflammatory agents; nucleoside reverse transcriptase inhibitors; proteasome inhibitors; retinoids; sulfonamides; tamoxifen; thiazolidinediones; tolvaptan; vincristine; zileuton; anticonvulsants such as carbamazepine, hydantoins, felbamate, and valproic acid; lipid-lowering medications such as fenofibrate, lomitapide, mipomersen, niacin, and statins; herbals and nutritional supplements such as black cohosh, chaparral, comfrey, DHEA, kava, pennyroyal oil, and red yeast rice). Patients should be advised to seek medical attention if they experience potential signs and symptoms of hepatotoxicity such as fever, rash, itching, anorexia, nausea, vomiting, fatigue, malaise, right upper quadrant pain, dark urine, pale stools, and jaundice. Baseline and regular monitoring of hepatic function is recommended. Thioguanine therapy should be discontinued if there is evidence of toxic hepatitis or biliary stasis, as reversal of signs and symptoms of liver toxicity have been reported upon withdrawal. Early indications of liver toxicity are signs associated with portal hypertension such as thrombocytopenia out of proportion with neutropenia and splenomegaly. Elevations of liver enzymes have also been reported, but do not always occur.