Isothermal heat conduction microcalorimetry was used to evaluate chemical stability of a solid drug in tablets. A variety of mixtures were compressed to flat faced tablets of 300 mg weight and 10 mm diameter. The content of drug amounted to 10%. Besides drug containing tablets, also placebo tablets as well as the non compressed mixtures were examined by microcalorimetry at 80 degrees C. The excipient Emcompress exhibited a substantially high exothermic heat flow that was due to a change in crystallinity. For Emcompress containing tablets this interfering signal resulted in such a way that the calorimetric data did not reflect the drug decomposition with sufficient accuracy. In the case of the other preparations the heat flow of the excipients were low, and the calorimetric data did reflect the drug decomposition. The stability increased with increasing content of CaHPO4, respectively, with decreasing content of water.
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