Influence of phase transformation on indomethacin release from microemulsions.

The release rates of indomethacin from microemulsions containing water, isopropyl myristate, lecithin, lysolecithin and alcohol are reported. Depending on the composition, the microemulsions transformed on contact with the release medium into emulsions, liquid crystals, or remained as microemulsions. The release rates, measured by monitoring the change in pH of an acceptor phase as the release system was diluted into it, were consistent with an interfacial barrier being the rate-limiting step for solute release. The release rates were found to be dependent on the size of the disperse phase after dilution with the release medium, and on the alcohol used in the formulation. Microemulsions that remained transparent after dilution produced rate constants lower than those that transformed into emulsions. The rate constant for the microemulsion that transforms into liquid crystals was not calculated because of the broad dispersion, but the percentage of drug released was much lower than those from the others systems. The release rates were too rapid to allow sustained drug release, while the release observed for the microemulsion that transformed into liquid crystals was consistent with the slow release required for drug delivery applications.