Drug Interaction Report

MONITOR: Phosphodiesterase-5 (PDE5) inhibitors may potentiate the blood pressure-lowering effect of antihypertensive medications, including vasodilators. These agents inhibit PDE5-mediated degradation of cyclic guanosine monophosphate (cGMP), which in vascular smooth muscles can lead to peripheral vasodilation and thus blood pressure (BP) reduction. For example, when sildenafil (100 mg) was coadministered with amlodipine (5 mg or 10 mg) to hypertensive patients, the mean additional reduction in supine BP was 8/7 mmHg. Likewise, in a group of patients whose hypertension was controlled with nifedipine slow-release (30 mg or 60 mg) once daily, the addition of vardenafil (20 mg) produced a mean additional supine BP reduction of 6/5 mmHg compared to placebo. When coadministered with amlodipine (5 mg) daily or enalapril (20 mg) daily, a single dose of avanafil (200 mg) produced a mean maximum decrease in supine systolic BP of 1.2 mmHg (amlodipine) and supine BP of 1.8/3.5 mmHg (enalapril) compared to placebo.

MANAGEMENT: Caution is advised if PDE5 inhibitors are prescribed in combination with antihypertensive agents. Patients receiving the combination should be advised to avoid rising abruptly from a sitting or recumbent position, especially following treatment initiation or a dosage increase, and to contact their doctor if they experience symptoms of hypotension such as dizziness, lightheadedness, fainting, or tachycardia.

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GENERALLY AVOID: Coadministration with grapefruit juice may slightly increase the oral bioavailability and delay the onset of action of sildenafil. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. In a randomized, crossover study with 24 healthy male volunteers, ingestion of 250 mL of grapefruit juice one hour before and concurrently with a 50 mg dose of sildenafil increased the mean area under the plasma concentration-time curve (AUC) of sildenafil and its pharmacologically active N-desmethyl metabolite by 23% and 24%, respectively, compared to water. Peak plasma concentrations (Cmax) were unaltered, but the time to reach sildenafil Cmax was prolonged by 0.25 hour. The observed increase in sildenafil bioavailability is unlikely to be of clinical significance in most individuals. However, pharmacokinetic interactions involving grapefruit juice are often subject to a high degree of interpatient variability and may be significant in the occasional susceptible patient. Indeed, one subject in the study had a 2.6-fold increase in sildenafil concentrations.

MANAGEMENT: It may be advisable to avoid administration of sildenafil with grapefruit juice to prevent potential toxicity and delay in onset of action.

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MONITOR: Many psychotherapeutic and CNS-active agents (e.g., anxiolytics, sedatives, hypnotics, antidepressants, antipsychotics, opioids, alcohol, muscle relaxants) exhibit hypotensive effects, especially during initiation of therapy and dose escalation. Coadministration with antihypertensives and other hypotensive agents, in particular vasodilators and alpha-blockers, may result in additive effects on blood pressure and orthostasis.

MANAGEMENT: Caution and close monitoring for development of hypotension is advised during coadministration of these agents. Some authorities recommend avoiding alcohol in patients receiving vasodilating antihypertensive drugs. Patients should be advised to avoid rising abruptly from a sitting or recumbent position and to notify their physician if they experience dizziness, lightheadedness, syncope, orthostasis, or tachycardia.

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ADJUST DOSING INTERVAL: Concurrent administration with calcium salts may decrease the oral bioavailability of atenolol and possibly other beta-blockers. The exact mechanism of interaction is unknown. In six healthy subjects, calcium 500 mg (as lactate, carbonate, and gluconate) reduced the mean peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) of atenolol (100 mg) by 51% and 32%, respectively. The elimination half-life increased by 44%. Twelve hours after the combination, beta-blocking activity (as indicated by inhibition of exercise tachycardia) was reduced compared to that with atenolol alone. However, during a 4-week treatment in six hypertensive patients, there was no difference in blood pressure values between treatments. The investigators suggest that prolongation of the elimination half-life induced by calcium coadministration may have led to atenolol cumulation during long-term dosing, which compensated for the reduced bioavailability.

MANAGEMENT: It may help to separate the administration times of beta-blockers and calcium products by at least 2 hours. Patients should be monitored for potentially diminished beta-blocking effects following the addition of calcium therapy.

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