Drug Interactions between aspirin / calcium carbonate and elvitegravir

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.

ADJUST DOSING INTERVAL: The concomitant use of antacids or oral products containing polyvalent cations (e.g., Mg, Al, Ca, Fe, Zn) may significantly decrease the plasma concentrations of elvitegravir. According to the elvitegravir manufacturer, the mechanism of the interaction may be related to local complexation in the gastrointestinal tract and not to changes in gastric pH. When 20 mL of an antacid containing magnesium and aluminum was given 2 hours before a 50 mg dose of elvitegravir with ritonavir (100 mg pharmacokinetic booster dose) to 11 study subjects, mean elvitegravir peak plasma concentration (Cmax), systemic exposure (AUC) and trough plasma concentration (Cmin) were reduced by 18%, 15%, and 10%, respectively. When the antacid was given 2 hours after elvitegravir to 10 study subjects, the mean elvitegravir Cmax, AUC, and Cmin decreased by approximately 20% each. When the antacid was given simultaneously with elvitegravir to 13 study subjects, the mean elvitegravir Cmax, AUC, and Cmin decreased by approximately 47%, 45%, and 41%, respectively. In contrast, when the antacid was given 4 hours before or 4 hours after elvitegravir, no significant differences were observed in elvitegravir pharmacokinetics compared to the administration of elvitegravir alone. The interaction has not been studied with other polyvalent cation-containing products, but they are expected to decrease elvitegravir plasma concentrations as well.

MANAGEMENT: Antacid and oral products containing polyvalent cations should be administered at least 2 hours before or 2 hours after the elvitegravir dose. Some experts recommend an interval of at least 4 hours. Alternatively, H2-receptor antagonists or proton pump inhibitors may be considered instead of antacids, with no dosage adjustment needed.