Vioxx - Clinical Pharmacology

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Food and Antacid Effects

Food had no significant effect on either the peak plasma concentration (Cmax) or extent of absorption (AUC) of rofecoxib when VIOXX Tablets were taken with a high fat meal. The time to peak plasma concentration (Tmax), however, was delayed by 1 to 2 hours. The food effect on the suspension formulation has not been studied. VIOXX tablets can be administered without regard to timing of meals. There was a 13% and 8% decrease in AUC when VIOXX was administered with calcium carbonate antacid and magnesium/ aluminum antacid to elderly subjects, respectively. There was an approximate 20% decrease in Cmax of rofecoxib with either antacid.

Distribution

Rofecoxib is approximately 87% bound to human plasma protein over the range of concentrations of 0.05 to 25 mcg/ mL. The apparent volume of distribution at steady state (Vdss) is approximately 91 L following a 12.5-mg dose and 86 L following a 25-mg dose. Rofecoxib has been shown to cross the placenta in rats and rabbits, and the blood-brain barrier in rats. Metabolism

Metabolism of rofecoxib is primarily mediated through reduction by cytosolic enzymes. The principal metabolic products are the cis-dihydro and trans-dihydro derivatives of rofecoxib, which account for nearly 56% of recovered radioactivity in the urine. An additional 8.8% of the dose was recovered as the glucuronide of the hydroxy derivative, a product of oxidative metabolism. The biotransformation of rofecoxib and this metabolite is reversible in humans to a limited extent (< 5%). These metabolites are inactive as COX-1 or COX-2 inhibitors.

Cytochrome P450 plays a minor role in metabolism of rofecoxib. Inhibition of CYP 3A activity by administration of ketoconazole 400 mg daily does not affect rofecoxib disposition. However, induction of general hepatic metabolic activity by administration of the non-specific inducer rifampin 600 mg daily produces a 50% decrease in rofecoxib plasma concentrations. (Also see Drug Interactions.)