Effect of polymers on dissolution from drug suspensions.

The effect of three viscosity grades of methylcellulose on the dissolution-dialysis rate of nitrofurantoin suspensions was investigated using a cell designed to provide a large surface area for dialysis. Apparent dialytic rate constants of drug dispersions and solutions were measured in 0.1 N HC1 and in pH 7.4 buffer. Samples containing methylcellulose had lower rates of dialysis, with the lowest rate being observed for samples in which the polymer was used as the suspending agent. The reduced rate of dialysis of the drug suspension containing methylcellulose is thought to be due to complexation of the drug in solution with the polymer as well as formation of microscopic regions of high viscosity surrounding the undissolved drug particles leading to a reduction in the dissolution rate of the drug. An empirical relationship was obtained to enable the estimation of the effective drug concentration in the dissolution chamber for drug dispersions. The method is based on utilizing dialysis rate data of drug solutions. This relationship could be used for comparing suspension formulations in terms of the amount of drug available for dialysis.