Drug Interactions between lonafarnib and rifampin

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of lonafarnib. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Inhibition of hepatic CYP450 3A4 may also contribute. The interaction has not been studied with grapefruit juice, but has been reported for other CYP450 3A4 inhibitors. When a single 50 mg oral dose of lonafarnib was administered following pretreatment with the potent CYP450 3A4 inhibitor ketoconazole (200 mg once daily for 5 days) in healthy study subjects, lonafarnib peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 270% and 425%, respectively, compared to lonafarnib administered alone. In general, the effect of grapefruit juice is concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit juice (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition. Increased exposure to lonafarnib may increase the risk and/or severity of adverse effects such as nausea, vomiting, diarrhea, anorexia, electrolyte disturbances, liver enzyme elevations, myelosuppression, infection, and hypertension.

ADJUST DOSING INTERVAL: Food does not have clinically relevant effects on the oral bioavailability of lonafarnib. When a single 75 mg oral dose of lonafarnib was administered with a high-fat meal (952 calories; approximately 43% from fat) in healthy subjects, lonafarnib Cmax and AUC decreased by 55% and 29%, respectively, compared to administration under fasted conditions. When administered with a low-fat meal (421 calories; approximately 12% from fat), lonafarnib Cmax decreased by 25% and AUC decreased by 21% relative to fasting. However, administration with food may help improve gastrointestinal tolerance to lonafarnib, which may commonly cause nausea, vomiting, diarrhea, and abdominal pain.

MANAGEMENT: Lonafarnib should be administered with the morning and evening meals and an adequate amount of water. Patients should avoid consumption of grapefruit or grapefruit juice and Seville oranges (also known as bitter or sour oranges).during treatment with lonafarnib.

GENERALLY AVOID: Concurrent use of rifampin in patients who ingest alcohol daily may result in an increased incidence of hepatotoxicity. The increase in hepatotoxicity may be due to an additive risk as both alcohol and rifampin are individually associated with this adverse reaction. However, the exact mechanism has not been established.

ADJUST DOSING INTERVAL: Administration with food may reduce oral rifampin absorption, increasing the risk of therapeutic failure or resistance. In a randomized, four-period crossover phase I study of 14 healthy male and female volunteers, the pharmacokinetics of single dose rifampin 600 mg were evaluated under fasting conditions and with a high-fat meal. Researchers observed that administration of rifampin with a high-fat meal reduced rifampin peak plasma concentration (Cmax) by 36%, nearly doubled the time to reach peak plasma concentration (Tmax) but reduced overall exposure (AUC) by only 6%.

MANAGEMENT: The manufacturer of oral forms of rifampin recommends administration on an empty stomach, 30 minutes before or 2 hours after meals. Patients should be encouraged to avoid alcohol or strictly limit their intake. Patients who use alcohol and rifampin concurrently or have a history of alcohol use disorder may require additional monitoring of their liver function during treatment with rifampin.