Drug Interactions between Caltrate 600 with Iron and Vitamin D and UroAv-B

MONITOR CLOSELY: The following interaction applies only to products containing sodium biphosphate that are used for bowel cleansing. It does not apply to products containing sodium biphosphate that are used for other, non-laxative related purposes.

Coadministration with agents that affect renal function or perfusion such as diuretics, ACE inhibitors, angiotensin receptor blockers, and nonsteroidal anti-inflammatory drugs (NSAIDs) may increase the risk of acute phosphate nephropathy associated with the use of bowel-cleansing phosphate solutions. The risk and/or severity of fluid and electrolyte disturbances may also be increased, which can lead to serious adverse events including cardiac arrhythmias, seizures, and renal impairment. Acute phosphate nephropathy is a rare adverse event that presents as acute renal failure with minimal proteinuria and a bland urine sediment. Renal biopsy findings are consistent with nephrocalcinosis and include acute and/or chronic renal tubular injury, calcium-phosphate crystal deposition in the distal tubules and collecting ducts, and no other pattern of histological injury. The risk of acute phosphate nephropathy stems from the large phosphate load, fluid shifts, and decreased intravascular volume, which can be exacerbated in the presence of medications that affect renal perfusion or function. In reported cases, acute renal failure was typically diagnosed within two to five months of colonoscopy. These cases often resulted in permanent impairment of renal function, some requiring long-term dialysis.

MANAGEMENT: Caution is advised when bowel-cleansing phosphate preparations are prescribed in patients treated with agents that affect renal function or perfusion, particularly if they are frail or elderly. Bowel-cleansing phosphate preparations should not be used in patients who have impaired renal function or perfusion, dehydration, or uncorrected electrolyte abnormalities. In patients at risk for acute phosphate nephropathy, baseline and postprocedure labs including serum electrolytes, calcium, phosphate, BUN, and creatinine should be performed. Patients should be advised not to exceed the recommended dosage of their bowel-cleansing preparation and to drink sufficient quantities of clear fluids during before, during, and after bowel cleansing. Limited data suggest that administration of an electrolyte rehydration solution may attenuate the electrolyte abnormalities and hypovolemia. Hospitalization and intravenous fluid hydration may be appropriate for frail or elderly patients who may be unable to drink an adequate volume of fluid.

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GENERALLY AVOID: Agents that can alkalinize the urine such as thiazide diuretics, carbonic anhydrase inhibitors, and antacids may decrease the antibacterial effectiveness of methenamine by inhibiting its conversion to formaldehyde. Methenamine is most effectively converted in an acidic milieu of pH less than 5.5.

MANAGEMENT: Concomitant use of methenamine-containing preparations with thiazide diuretics, carbonic anhydrase inhibitors, or large doses of antacids should be avoided if possible. Otherwise, frequent urine pH testing may be considered. Some methenamine products may be used with antacids if dosing times are separated by at least one hour. Consult the manufacturer's product labeling for specific recommendations.

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MONITOR: Chronic administration of antacids may reduce serum salicylate concentrations in patients receiving large doses of aspirin or other salicylates. The mechanism involves reduction in salicylate renal tubular reabsorption due to urinary alkalinization by antacids, resulting in increased renal salicylate clearance. In three children treated with large doses of aspirin for rheumatic fever, serum salicylate levels declined 30% to 70% during coadministration with a magnesium and aluminum hydroxide antacid. Other studies have found similar, albeit less dramatic results. Antacids reportedly have no effect on the oral bioavailability of aspirin in healthy adults. However, administration of antacids containing either aluminum and magnesium hydroxide or calcium carbonate two hours before aspirin dosing led to reduced absorption of aspirin in uremic patients.

MANAGEMENT: Patients treated chronically with antacids (or oral medications that contain antacids such as didanosine buffered tablets or pediatric oral solution) and large doses of salicylates (i.e. 3 g/day or more) should be monitored for potentially diminished or inadequate analgesic and anti-inflammatory effects, and the salicylate dosage adjusted if necessary.

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ADJUST DOSING INTERVAL: The bioavailability of orally administered iron may be reduced by concomitant administration of antacids or other agents with acid-neutralizing effects. The exact mechanism is unknown but may involve reduced iron solubility due to increase in gastric pH and/or reduced absorption due to complexation or precipitation of the iron. Based on existing data, sodium bicarbonate and calcium carbonate appear to have greater effects than antacids containing magnesium and aluminum hydroxides. In a study of patients with mild iron deficiency anemia, coadministration of ferrous sulfate with sodium bicarbonate 1 gram and calcium carbonate 500 mg reduced iron absorption by 50% and 67%, respectively, while 5 mL of an antacid containing magnesium and aluminum hydroxides had little effect. Another study also found no effect on iron absorption when ferrous sulfate (equivalent to 10 mg/kg of elemental iron) was coadministered with magnesium hydroxide (1 mg for every 5 mg of elemental iron ingested) in a group of healthy, fasting male subjects. In contrast, absorption of iron from ferrous sulfate and ferrous fumarate tablets was reduced by 37% and 31%, respectively, following administration of an antacid containing magnesium carbonate, magnesium hydroxide, and aluminum hydroxide in a study of healthy, iron-replete volunteers. Similarly, in a study of nine patients, coadministration of 5 mg of ferrous sulfate with a 35 gram dose of magnesium trisilicate was found to reduce iron absorption by an average of more than 70%. The interaction reportedly does not occur in the presence of ascorbic acid, which may competitively bind with iron and prevent the interference with iron absorption.

MANAGEMENT: To minimize the potential for interaction, it may be appropriate to administer oral iron preparations at least two hours apart from antacids or other agents with acid-neutralizing effects.

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