[Ion exchange resins].

Ion exchange resins have been used to help formulate pharmaceuticals since the late 1950's. During that time they have proved to be safe and effective excipients and are now used in many commercial formulations throughout the world. In this article we will look at some of the common problems faced by formulators and how using ion exchange resins may be able to solve them. Ion exchange resins are insoluble polymers that contain acidic or basic functional groups and have the ability to exchange counter-ions with aqueous solutions surrounding them. The equation in Figure I shows a representative reaction when drugs are loaded onto or released from the resins. A drug ion and an inorganic ion are exchanged. The reaction is an equilibrium, the position of which will depend on many factors including salt concentration in the aqueous phase. This property allows drugs to be loaded onto resins (forming drug resinates) and then released in vivo by the salts present in GI fluids. The resinates possess physical properties similar to the resin. These two properties drug release and physical properties can be manipulated to create many variations of use to the formulator.