Effect of ionic interaction on the entrapping of drug into porous microspheres and drug release characteristics.

Much research has been done on means to minimize harmful side effects and to maintain drug levels in a therapeutically desirable range by employing porous structures as an inert matrix to provide controlled drug release.1-3) We have studied the employment of porous microspheres in the preparation of capsules containing a drug, and the drug release profiles from such porous microspheres. In previous papers,4,5) it was found that the amount of drug entrapped in porous microspheres was increased significantly by graft polymerization of methyl methacrylate into the microspheres, although the original microspheres had no ability to absorb the drug. Furthermore, pretreatment of the sphere surfaces and/or the internal parts of spheres with metal oxide before irradiation led to an increase in the amount of drug entrapped in the spheres and a decrease of the drug release rate. It was considered that interaction between the substrate and the drug was an important factor in entrapment of the drug. In this report, polymer-coated porous microspheres containing salicylic acid as a model drug were prepared by graft polymerization of methyl methacrylate (MMA) and MMA containing amino esters such as dimethylaminoethyl methacrylate (DM) and diethylaminoethyl methacrylate (DE) by a pre-irradiation method. The effects of adding amino esters on the entrapment of salicylic acid into porous microspheres and on the release profile of salicylic acid are discussed.