Use of a physiologic bicarbonate buffer system for dissolution characterization of ionizable drugs.

PURPOSE: The aim of this study was to examine if sparging with CO2(g) could be used to establish stable biorelevant bicarbonate buffers, in aqueous medium, for use in dissolution characterization of low-solubility ionizable drugs.
METHODS: Preparation of the bicarbonate-containing dissolution medium was monitored by use of a commercially available fiberoptic probe to measure the concentration of dissolved CO2(aq). Intrinsic dissolution measurements at 100 rpm, 37 degrees C for indomethacin and dipyridamole were performed using a rotating disk and UV detection at pH 6.8 and 5.0 in a USP dissolution vessel apparatus.
RESULTS: Indomethacin dissolution at pH 6.8 was significantly impacted by the concentration of CO2(g) in the sparging gas. Dipyridamole flux at pH 6.8 was independent of buffer species or buffer concentrations studied. However, dipyridamole dissolution at pH 5 was also a strong function of the concentration of CO2(g) in the sparging gas.
CONCLUSIONS: Stable bicarbonate biorelevant buffers could be established to perform intrinsic dissolution rate determinations for indomethacin and dipyridamole as long a continuous gas sparging of CO2(g) was used. Depending of the pH of the dissolution medium, the intrinsic dissolution rates of both indomethacin and dipyridamole were affected by the bicarbonate concentration. Sparging with CO2(g) to create physiologic buffers has a unique advantage over conventional buffers in that gas sparging serves as a continuous source of bicarbonate buffer species. This advantage was demonstrated by performing dissolution experiments at pH values typically associated with the fed state (pH 5) and applying relatively low CO2(g) pressures, resulting in bicarbonate concentrations less than 0.5 mM. It was demonstrated that CO2(g) sparging at a pH consistent with the fed state created an in-situ bicarbonate buffer at low concentrations, which had a significant impact on the dissolution of a basic drug such as dipyridamole.