Graft tapioca starch copolymers as novel excipients for controlled-release matrix tablets

This paper studies new graft tapioca copolymers as an alternative for the formulation of controlled-release matrix tablets, using anhydrous theophylline as model drug. Copolymers were synthesised by free radical copolymerisation of ethyl methacrylate with tapioca starch (TS) or hydroxypropyl starch (THS), being alternatively dried by two methods: oven or freeze-drying. The influence of carbohydrate nature, drying process and breaking force on drug release was evaluated. Radial drug release and fronts movement were also studied using special devices consisting of two Plexiglass® discs. The paper demonstrates the use of these new copolymers as excipients for controlled drug release. All tablets behave as inert matrices controlling drug release mainly by diffusion. However, TSEMA matrices demonstrated to have better binding properties with lower release than THSEMA tablets. Drying process and breaking force had a significant influence on dissolution behaviour only in THSEMA matrices. Porosity and tortuosity values explained the higher drug release observed for THSEMA matrices with low crushing force and for freeze-dried copolymer tablets.