Design of a dissolution system for the evaluation of the release rate characteristics of artemether and dihydroartemisinin from tablets.

As none of the pharmacopoeial dissolution methods are suitable to evaluate the release rate of artemether and dihydroartemisinin from tablets, a 'two-phase partition-dissolution' method, based on the one of [J. Pharm. Sci. 85 (1996) 1060] was developed. It consists of an organic solvent in the upper part and the aqueous phase, in which the dissolution test was executed. The main requirements for the selection of the solvent are: the density should be lower than 1; the analyte should dissolve in the organic part as much as required for 'sink' conditions; if possible, the cut off should be near 200 nm, which allows direct HPLC measurement at 215 nm. The most suitable solvent for artemether is isooctane in a ratio of 100/150 ml aqueous phase. Samples could be analysed without further treatment. For dihydroartemisinin, chlorobutane was selected in a ratio 150/150 ml water. In the latter method, the solvent disturbed in the HPLC analysis and therefore samples were evaporated and then reconstituted in methanol. Repeatability of the test was satisfactory and discrimination ability tests on Artenam tablet batches and self-made dihydroartemisinin tablets, respectively, showed good results, confirmed via calculation of the similarity factor f2 (value <50). Dissolution determination of Cotecxin tablets was proven not to be conform as immediate-release tablet.