A comparison with stirred beaker methods

The earliest dissolution apparata included Pernarowski’s basket dissolution assembly2 and Poole’s paddle design.3 Until recently, official dissolution testing methods were based entirely on modifications of these static models. However, these methods suffer from a number of disadvantages4 and the need for a new official dissolution test was recognised, resulting in the introduction of the flow-through dissolution apparatus in the United States Pharmacopeia5 and the British Pharmaceopoeia.6 Basic Design of Dissolution Apparata The traditional dissolution apparata are based on a stirred-tank static system model, where the drug is dissolved in a relatively large, fixed volume of dissolution medium contained in a cylindrical vessel with a hemispherical bottom. The vessel is partially immersed in a suitable water bath to maintain the temperature of the medium at 37OC. A forced convection type of agitation is accomplished by means of a stirring, rotating or oscillating device, generally a motor-driven paddle or wire-mesh basket. Discrete samples are periodically withdrawn from the dissolution medium and analysed. In flow-through methods, the assembly consists of a reservoir and pump for the dissolution medium, a thermostatically-controlled flowthrough cell and a water bath that maintains the dissolution medium at 37OC (Figure I). The pump is separated from the dissolution unit in order to shield the latter against vibrations originating from the pump. The cell is made of transparent and inert material, and is mounted vertically with a screen and filter system that prevents escape of undissolved particles from the top of the cell. The bottom cone is usually filled with small 1-mm glass beads with one 5-mm bead positioned at the apex to protect the fluid entry tube. The dosage form under investigation is placed on the beads or on a wire carrier inside the cell and a continuous flow of the dissolution medium from the reservoir is forced upwards through the cell by the pump. The dissolution fluid is usually collected in a separate reservoir as it leaves the dissolution cell; fractions are removed at specified intervals and analysed.