Placebo granules as cores for timed release drug delivery systems.

A drug delivery system is proposed constituted of spherical placebo granules as cores with polymeric surface films containing drug. This timed release dosage form has been prepared by means of a fluidized bed coating technique using ethyl cellulose as the polymeric film and caffeine and salicylic acid as model drugs. The release of the drugs from the dosage form (a) at different drug concentrations and (b) into solutions of different pH showed that drug release was linearly related to the square root of time. Good agreement was found between the theoretical release rate of caffeine, calculated according to Higuchi's equation for a homogenous matrix using membrane permeation parameters measured on linear films, and the experimental results in the case of low drug concentrations. Deviation of the release rate from the homogenous model at high drug concentrations could be explained by crystallization of the drug from the film.