Drug Interactions between amprenavir and HalfLytely and Bisacodyl

ADJUST DOSING INTERVAL: By increasing gastric pH, antacids may reduce the resistance of the enteric coating of bisacodyl tablets, resulting in earlier release of bisacodyl and gastric irritation and dyspepsia.

MANAGEMENT: The administration of antacids and bisacodyl should be separated by at least one hour.

ADJUST DOSING INTERVAL: Concurrent administration of antacids or other agents with acid-neutralizing effects may decrease the oral bioavailability of amprenavir and reduce its concentrations in plasma. Amprenavir solubility decreases with increasing pH, thus reduction in gastric acidity may interfere with dissolution of the drug. Subtherapeutic antiretroviral drug levels may lead to reduced viral susceptibility and development of resistance. In contrast, the prodrug fosamprenavir appears to be less affected by antacids. Administration of a single 1400 mg dose with 30 mL of antacid (Maalox TC) resulted in decreased peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) of amprenavir by 35% and 18%, respectively. However, trough plasma concentration (Cmin) increased by 14%.

MANAGEMENT: Amprenavir should be administered at least 1 hour before or after antacids or other oral preparations containing antacids (e.g., didanosine buffered tablets or pediatric oral solution).

GENERALLY AVOID: Concomitant use of stimulant laxatives (e.g., bisacodyl, sodium picosulfate) may increase the risk of serious gastrointestinal adverse effects associated with certain osmotic laxatives (e.g., polyethylene glycol (PEG), oral sulfate solution), such as colonic mucosal ulcerations or ischemic colitis. There have been isolated case reports of ischemic colitis occurring with the use of PEG-based bowel cleansing products in combination with higher dosages of bisacodyl (usually greater than 10 mg). Bisacodyl can cause colonic ischemia due to transient reduction in splanchnic blood flow. When administered in conjunction with an osmotic laxative such as PEG, increased intramural pressure secondary to increased peristalsis may lead to ischemic colitis and perforation.

MANAGEMENT: The manufacturers for some osmotic bowel cleansing products recommend avoiding the concurrent use of stimulant laxatives. However, stimulant laxatives, in particular bisacodyl and sodium picosulfate, are sometimes used with PEG in certain bowel cleansing regimens to help reduce dose volume and improve patient tolerability and acceptance. Please consult individual product labeling for specific recommendations and guidance. Patients using osmotic bowel cleansing products and stimulant laxatives who present with sudden abdominal pain, rectal bleeding, or other symptoms of ischemic colitis should be evaluated promptly.