Drug Interactions between Adenoscan and aluminum carbonate

GENERALLY AVOID: The concomitant administration of aluminum-containing products (e.g., antacids and phosphate binders) and citrates may significantly increase serum aluminum concentrations, resulting in toxicity. Citrates or citric acid are contained in numerous soft drinks, citrus fruits, juices, and effervescent and dispersible drug formulations. Citrates enhance the gastrointestinal absorption of aluminum by an unknown mechanism, which may involve the formation of a soluble aluminum-citrate complex. Various studies have reported that citrate increases aluminum absorption by 4.6- to 50-fold in healthy subjects. Patients with renal insufficiency are particularly at risk of developing hyperaluminemia and encephalopathy. Fatalities have been reported. Patients with renal failure or on hemodialysis may also be at risk from soft drinks and effervescent and dispersible drug formulations that contain citrates or citric acid. It is unknown what effect citrus fruits or juices would have on aluminum absorption in healthy patients.

MANAGEMENT: The concomitant use of aluminum- and citrate-containing products and foods should be avoided by renally impaired patients. Hemodialysis patients should especially be cautioned about effervescent and dispersible over-the-counter remedies and soft drinks. Some experts also recommend that healthy patients should separate doses of aluminum-containing antacids and citrates by 2 to 3 hours.

ADJUST DOSING INTERVAL: The administration of aluminum-containing antacids with enteral nutrition may result in precipitation, formation of bezoars, and obstruction of feeding tubes. The proposed mechanism is the formation of an insoluble complex between the aluminum and the protein in the enteral feeding. Several cases of esophageal plugs and nasogastric tube obstructions have been reported in patients receiving high-protein liquids and an aluminum hydroxide-magnesium hydroxide antacid or an aluminum hydroxide antacid.

MANAGEMENT: Some experts recommend that antacids should not be mixed with or given after high protein formulations, that the antacid dose should be separated from the feeding by as much as possible, and that the tube should be thoroughly flushed before administration.

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ADJUST DOSING INTERVAL: Caffeine and other xanthine derivatives (e.g., theophylline) are nonspecific, competitive antagonists of adenosine receptors and may interfere with the hemodynamic effects of adenosine. There have been case reports of patients receiving theophylline who required higher than normal dosages of adenosine for the treatment of paroxysmal supraventricular tachycardia. In studies of healthy volunteers, caffeine and theophylline have been shown to reduce the cardiovascular response to adenosine infusions (i.e., heart rate increases, vasodilation, blood pressure changes), and theophylline has also been shown to attenuate adenosine-induced respiratory effects and chest pain/discomfort.

MANAGEMENT: Clinicians should be aware that adenosine may be less effective in the presence of xanthine derivatives including caffeine. Patients should avoid consumption of caffeine-containing products for at least 12 hours, preferably 24 hours, prior to administration of adenosine for myocardial perfusion imaging.

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ADJUST DOSING INTERVAL: Methylxanthines (e.g., caffeine, theophylline) are nonspecific, competitive antagonists of adenosine receptors. As such, they may interfere with the pharmacologic effects of adenosine and other adenosine receptor agonists such as dipyridamole and regadenoson. There have been case reports of patients receiving theophylline who required higher than normal dosages of adenosine for the treatment of paroxysmal supraventricular tachycardia. In studies of healthy volunteers, caffeine and theophylline have been shown to reduce the cardiovascular response to adenosine infusions (i.e., heart rate increases, vasodilation, blood pressure changes), and theophylline has also been shown to attenuate adenosine-induced respiratory effects and chest pain/discomfort. Similarly, caffeine has been found to reduce the hemodynamic response to dipyridamole, and both caffeine and theophylline have been reported to cause false-negative results in myocardial scintigraphy tests using dipyridamole. In a placebo-controlled study that assessed the effects of oral caffeine on regadenoson-induced increase in coronary flow reserve (CFR), healthy subjects who took caffeine 200 mg orally two hours prior to regadenoson administration exhibited a median CFR that was 92% that of subjects who took placebo. The study was done using positron emission tomography with radiolabeled water.

MANAGEMENT: Clinicians should be aware that adenosine and other adenosine receptor agonists may be less effective in the presence of methylxanthines. Methylxanthines including caffeine should be withheld for 12 to 24 hours (or five half-lives) prior to administration of adenosine receptor agonists for myocardial perfusion imaging. However, parenteral aminophylline should be readily available for treating severe or persistent adverse reactions to adenosine receptor agonists such as bronchospasm or chest pain.

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Nicotine may enhance adenosine-associated tachycardia and chest pain. The mechanism is not known. No special precautions appear to be necessary.

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