Drug Interaction Report

MONITOR: Coadministration of medicines targeting the prostacyclin pathway (e.g., prostacyclin (PGI2), PGI2 analogs, or selective non-prostanoid prostacyclin IP receptor agonists) with diuretics, antihypertensive agents, or other vasodilators may potentiate the hypotensive effects of these agents. The mechanism involves the additive reduction in blood pressure due to the vasodilatory effects of PGI2, PGI2 analogs or selective non-prostanoid prostacyclin IP receptor agonists on the prostacyclin pathway.

MANAGEMENT: While therapies that target the prostacyclin pathway have been used in combination with diuretics, antihypertensives, or other vasodilators in the management of pulmonary arterial hypertension, caution is recommended if they must be administered concurrently. If these drugs are used together, it is generally recommended that blood pressure be measured more frequently until a stable blood pressure pattern is observed. Patients should be advised to avoid rising abruptly from a sitting or recumbent position and to notify their doctor if they experience dizziness, lightheadedness, syncope, orthostatic hypotension, or tachycardia. Treatment guidelines should be consulted for the optimum management of pulmonary arterial hypertension.

GENERALLY AVOID: Grapefruit juice may increase the plasma concentrations of guanfacine. The risk of adverse reactions such as hypotension, bradycardia, and sedation may increase. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Ketoconazole, a potent CYP450 3A4 inhibitor, has been reported to increase guanfacine peak plasma concentration (Cmax) and systemic exposure (AUC) by approximately 2- and 3-fold, respectively. A computer simulation suggests that fluconazole, a moderate CYP450 3A4 inhibitor, would increase guanfacine Cmax and AUC by about 1.5- and 2-fold, respectively. 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.

GENERALLY AVOID: Alcohol may enhance the sedative and hypotensive effects of guanfacine.

GENERALLY AVOID: Administration of extended-release guanfacine with a high-fat meal may increase the bioavailability of guanfacine. When a single 4 mg dose of extended-release guanfacine was administered to adult volunteers with a high-fat breakfast, mean guanfacine peak plasma concentration (Cmax) and systemic exposure (AUC) increased by approximately 75% and 40%, respectively, compared to dosing in a fasted state.

MANAGEMENT: Patients treated with guanfacine should avoid consumption of grapefruit and grapefruit juice. In addition, it is preferable to avoid or limit the use of alcohol during treatment. Patients should be advised against driving or operating hazardous machinery until they know how the medication affects them. The extended-release formulation of guanfacine should not be taken together with a high-fat meal.