Drug Interaction Report

MONITOR: Additive reductions in heart rate, cardiac conduction, and cardiac contractility may occur when calcium channel blockers are used concomitantly with beta blockers, particularly in patients with ventricular or conduction abnormalities. While this combination may be useful and effective in some situations, potentially serious cardiovascular adverse effects such as congestive heart failure, severe hypotension, and/or exacerbation of angina may occur. The proposed mechanisms include additive slowing in AV conduction, reduced cardiac contractility secondary to beta-blockade, and decreased peripheral vascular resistance secondary to calcium channel blockade. In addition, some calcium channel blockers may inhibit the CYP450 metabolism of hepatically metabolized beta blockers, resulting in increased serum concentrations.

MANAGEMENT: Close clinical monitoring of patient hemodynamic response and tolerance is recommended if a calcium channel blocker is prescribed with a beta blocker, and the dosage of one or both agents adjusted as necessary. The same precaution should be observed when beta blocker ophthalmic solutions are used, since they are systemically absorbed and can produce clinically significant systemic effects even at low or undetectable plasma levels.

MONITOR: Limited clinical data suggest that selective serotonin reuptake inhibitors (SSRIs) may potentiate the pharmacologic effects of some beta-blockers. The proposed mechanism is SSRI inhibition (competitive and/or noncompetitive) of CYP450 2D6, the isoenzyme responsible for the metabolic clearance of beta-blockers such as carvedilol, labetalol, metoprolol, nebivolol, propranolol, and timolol. Paroxetine and norfluoxetine (the active metabolite of fluoxetine), in particular, are potent inhibitors of CYP450 2D6 and may be more likely than other SSRIs to cause the interaction. There have been case reports of patients stabilized on beta-blocker therapy who developed bradycardia, hypotension, and complete heart block following the addition of a SSRI, subsequently requiring discontinuation of one or both agents and/or institution of a permanent pacemaker. The interaction is also corroborated by data from in vitro and clinical studies involving paroxetine and metoprolol conducted by one group of investigators. Coadministration of nebivolol 5 mg and paroxetine 20 mg in 23 healthy subjects increased nebivolol Cmax and AUC by 5.7-fold and 6.1-fold, respectively, with no increase in pharmacodynamic effects on heart rate or blood pressure. Likewise, coadministration f nebivolol 10 mg with fluoxetine 20 mg in 10 subjects increased nebivolol Cmax and AUC by 2.3-fold and 6-fold, respectively, without reports of vital sign-related adverse effects. Additionally, coadministration of nebivolol 5 mg with fluvoxamine 100 mg, a mild CYP2D6 inhibitor, resulted in a 1.41-fold and 1.44-fold increase in fluvoxamine's Cmax and AUC, respectively, without enhancing nebivolol's pharmacodynamic effects. Conversely, fluvoxamine is a potent inhibitor of CYP450 1A2 and may significantly interact with propranolol, which is a substrate of both CYP450 2D6 and 1A2.

MANAGEMENT: During concomitant therapy with SSRIs, a lower initial dosage and more cautious titration of the beta-blocker may be appropriate. Cardiac function should be closely monitored and the beta-blocker dosage adjusted accordingly, particularly following initiation, discontinuation or change of dosage of SSRI in patients who are stabilized on their beta-blocker regimen. Due to the long half-life of fluoxetine and its active metabolite, norfluoxetine, the risk of an interaction may exist for an extended period (up to several weeks) after discontinuation of fluoxetine. To avoid the interaction, use of beta-blockers that are primarily eliminated by the kidney such as atenolol, acebutolol, betaxolol, carteolol, and nadolol may be considered.

Calcium channel blockers and angiotensin converting enzyme (ACE) inhibitors may have additive hypotensive effects. While these drugs are often safely used together, careful monitoring of the systemic blood pressure is recommended during coadministration, especially during the first one to three weeks of therapy.

GENERALLY AVOID: Moderate-to-high dietary intake of potassium can cause hyperkalemia in some patients who are using angiotensin converting enzyme (ACE) inhibitors. In some cases, affected patients were using a potassium-rich salt substitute. ACE inhibitors can promote hyperkalemia through inhibition of the renin-aldosterone-angiotensin (RAA) system.

MANAGEMENT: It is recommended that patients who are taking ACE inhibitors be advised to avoid moderately high or high potassium dietary intake. Particular attention should be paid to the potassium content of salt substitutes.

GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of CNS-active agents. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

MANAGEMENT: Patients receiving CNS-active agents should be warned of this interaction and advised to avoid or limit consumption of alcohol. Ambulatory patients should be counseled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

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. Patients should also avoid driving or operating hazardous machinery until they know how the medications affect them.

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. Patients should also avoid driving or operating hazardous machinery until they know how the medications affect them.

MONITOR: Calcium-containing products may decrease the effectiveness of calcium channel blockers by saturating calcium channels with calcium. Calcium chloride has been used to manage acute severe verapamil toxicity.

MANAGEMENT: Management consists of monitoring the effectiveness of calcium channel blocker therapy during coadministration with calcium products.

The consumption of grapefruit juice may slightly increase plasma concentrations of amlodipine. The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. Data have been conflicting and the clinical significance is unknown. Monitoring for calcium channel blocker adverse effects (e.g., headache, hypotension, syncope, tachycardia, edema) is recommended.

The recommended maximum number of medicines in the 'antihypertensives' category to be taken concurrently is usually two. Your list includes three medicines belonging to the 'antihypertensives' category:

• amlodipine
• enalapril
• nebivolol

Note: In certain circumstances, the benefits of taking this combination of drugs may outweigh any risks. Always consult your healthcare provider before making changes to your medications or dosage.