Effect of antacid magaldrate oral suspension on in-vitro and in-vivo availability of indomethacin in dogs

The adsorption of indomethacin onto Riopan and Rioplus (Magaldrate antacid oral suspension U.S.P.) was determined at 37 °C in phosphate buffer pH 7.4 and in simulated intestinal fluid U.S.P. pH 7.5. The effect of 20 ml of Riopan, given 1 h after oral administration of Indocid capsules (MSD, 25 mg) to fed dogs, on the bioavailability of indomethacin was also studied. The in-vitro study showed that the logarithm of amount of indomethacin adsorbed was linearly related to the logarithm of free drug concentration in conformity to Freundlich adsorption isotherm. The adsorptive capacities followed the sequence: Riopan (intestinal fluid) = Rioplus(intestinal fluid)>Riopan(phosphate buffer) = Rioplus (phosphate buffer). In all cases the adsorbed drug was eluted almost completely by washing with different volumes of different elution media indicating competitive physical adsorption via weak Van der Waal's attractive forces. The in-vivo study showed that the oral administration of Riopan 1 h after administration of Indocid capsules to fed dogs resulted in a significant (p < 0.05) reduction in plasma concentrations of indomethacin, a non-significant (p > 0.05) change in C_max, significantly (p < 0.05) shorter T_max and significantly reduced AUC where it decreased from 15.06 ± 5.65 to 9.52 ± 3.69 μg h/ml resulted in a relative bioavailability of 63.21% after Riopan administration.