Studies of dissolution behavior of highly soluble drugs using a rotating disk

Abstract The dissolution behavior of highly soluble drugs having varying diffusion coefficients was investigated using the rotating disk methodology. The effect of solution viscosity as a result of high drug concentration causes the drug diffusion coefficient to be variable in the boundary layer. A modified Levich equation was obtained by an iterative numerical algorithm to solve the non-linear diffusion equation. The resulting equation predicts that a plot of dissolution rate vs the square root of the rotation speed of the disk is linear. Moreover, the slope is dependent upon the profile of diffusion coefficient vs drug concentration. To demonstrate the theory, the dissolution rates of the compressed disks of the model drugs, sodium ampicillin and sodium salicylate, were determined quantitatively at various rotation speeds, and compared with those predicted by the modified Levich equation. It shows that the modified Levich equation gives a close agreement with the experimental data. For comparison, the conventional Levich equation, which was derived for constant diffusion coefficients, was also employed to predict the dissolution rate of these drugs. The rates predicted by the Levich equation exhibit significant discrepancy from the experimental data.