Drug Interaction Report

MONITOR: Coadministration with human growth hormone may decrease the plasma concentrations of drugs that are metabolized by CYP450 3A4. Data from an interaction study performed in growth hormone deficient adults suggest that growth hormone (somatropin) administration can increase the clearance of compounds known to be metabolized by cytochrome P450 isoenzymes, especially CYP450 3A4. The clinical significance is unknown.

MANAGEMENT: Caution is advised when human growth hormone is used concurrently with drugs that are known CYP450 3A4 substrates, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever human growth hormone is added to or withdrawn from therapy.

ADJUST DOSE: Smoking tobacco may decrease the steady-state systemic exposure of istradefylline by 38% to 54%.

MANAGEMENT: The possibility of reduced therapeutic effects of istradefylline should be considered in smokers. The manufacturer recommends an istradefylline dosage of 40 mg once daily in patients who smoke 20 or more cigarettes (or the equivalent amount of another tobacco product) per day.