The effect of media and other variables on the BP solution rate test for slow lithium carbonate tablets

The British Pharmacopoeial test for assessing the solution rate of slow lithium carbonate tablets has been evaluated using ‘controlled release’ tablets containing 400 mg of lithium carbonate. No significant differences in lithium release were found when the volume of media used in the test was reduced from 250 ml to 200 ml, the final stage of the test in pH 6.8 phosphate buffer reduced from 5 to 3 h, the number of tablets in each thimble reduced from three to one, or the prescribed phosphate buffers replaced with phthalate and Tris, respectively. A four-fold increased concentration of phosphate in the phosphate buffers used resulted in a significant retardation in lithium solution rate. This was not attributable to an ionic strength effect but possibly to the formation of trilithium phosphate at the interface. Dissolution studies using the USP Basket Method showed a significantly slower release rate of one tablet into 900 ml of phosphate buffer compared with Tris buffer. This difference was markedly increased when three tablets were investigated in 200 ml of similar media. These differences were considered to be due to the formation of the much less soluble trilithium phosphate in the phosphate buffers.