Effects of Molecular Weight on Phase Separated Coatings for Controlled Release of Drugs

Phase separated films with controlled porosity were made from ethyl cellulose (EC) and 30% w/w hydroxypropyl cellulose (HPC). The molecular weight of EC can be used to modify the mass transfer rate through coatings by effects on microstructure of the film. Processing conditions are, however, affected by the solution rheology, which could influence the film quality when using different molecular weights. INTRODUCTION Oral controlled release formulations often consist of drug pellets coated with insoluble membranes, which are intended to extend drug release in the gastrointestinal tract. Ethyl cellulose (EC) is a waterinsoluble cellulose derivative commonly used for such coatings, but its permeability for most drugs is low1. In order to modify the release profile through EC coatings a water-soluble component, such as hydroxypropyl cellulose (HPC), can be added to the coating mixture. If the coating is exposed to aqueous liquids HPC could dissolve, leak out from the coating and cause pore formation in the coating through which the drug can be released2. The mechanical properties of the coating is important for drug release, i.e. for resistance of crack formation during the handling and tableting of pellets3,4, as well as for withstanding stresses on the coating from water accumulation within the pellets5. However, the latter effect is more likely to affect drug release when the content of HPC in the EC-coating is low. Marucci et al.6,7 found that above an HPC-content of 22% w/w drug release occurs predominantly though pores from HPC-leaching and this process reduces the pressure within the coating. Another parameter that should be taken into account for films consisting of two polymers is the phase separation. During the coating of pellets a solution of EC, HPC and ethanol is sprayed onto drug cores in a fluidized bed7. Solvent evaporation during the coating process initiates phase Effects of Molecular Weight on Phase Separated Coatings for Controlled