Reduced tetracycline bioavailability caused by magnesium aluminum silicate in liquid formulations of bismuth subsalicylate

RATIONALE: Bismuth subsalicylate, tetracycline hydrochloride, and metronidazole are widely used in combination for the treatment of Helicobacter pylori infections. As a result, there is renewed interest in the interaction between tetracycline and bismuth subsalicylate. OBJECTIVE: To determine whether the observed decrease in tetracycline bioavailability is due to the active drug bismuth subsalicylate via complexation, or to magnesium aluminum silicate (Veegum), an inactive excipient present only in the liquid formulation of bismuth subsalicylate, which might adsorb the tetracycline, rendering it unavailable for systemic absorption. METHODS: Eleven healthy volunteers participated in a randomized three-period, three-treatment complete crossover study with a 7-day washout interval between treatments. After an overnight fast, subjects received a 500-mg capsule of tetracycline hydrochloride with either tap water, 30 mL of bismuth subsalicylate (525 mg) liquid containing Veegum (Pepto-Bismol), or 30 mL of a specially formulated bismuth subsalicylate (525 mg) liquid without Veegum. Blood was collected for 24 hours after each dose of tetracycline. Serum was assayed for tetracycline concentration by HPLC. In addition, standard in vitro ultraviolet spectrophotometric methods were used to determine the capacity for complexation of bismuth with tetracycline and for adsorption of tetracycline to Veegum. RESULTS: Compared with the reference treatment of tetracycline hydrochloride with water, the liquid formulation of bismuth subsalicylate containing Veegum decreased the maximum serum concentration (Cmax) of tetracycline by 21% and the serum tetracycline AUC by 27% (p < 0.001). The bismuth subsalicylate formulation without Veegum resulted in decreases in Cmax and AUC of 11% and 13%, respectively (p > 0.05 vs. tetracycline hydrochloride with water). Multiple linear regression analysis of the spectral absorbance data demonstrated a calculated recovery of tetracycline of 100.9% and, therefore, a lack of in vitro complexation with bismuth. At pH 1.2, the amount of tetracycline adsorbed to Veegum ranged from 91.5% to 97.2% over the concentration range of 0.25 to 2 mg/mL. At pH 7.0, the values ranged from 82.9% to 83.9% over the concentration range of 0.25 to 1 mg/mL. CONCLUSIONS: In vitro and in vivo data from this study indicate that Veegum, a suspending agent, and not the active agent bismuth subsalicylate, is the primary ingredient in liquid formulations of bismuth subsalicylate responsible for a decrease in tetracycline bioavailability. In addition, the mechanism of interaction is not likely due to complexation between tetracycline and bismuth subsalicylate, as previously postulated, but rather is caused by adsorption of tetracycline to the excipient Veegum, which is present only in the liquid formulation of bismuth subsalicylate. The clinical relevance of this interaction has not been determined.

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