Abstract
Feasibility of microcalorimetry to evaluate the physical stability of amorphous drugs was studied. Amorphous forms of nifedipine and phenobarbital were prepared by melting and subsequent cooling in a differential scanning calorimetry (DSC) sample pan, and their heats of crystallization were monitored by isothermal microcalorimetry. The time required for 10% of the amorphous drug to crystallize ( t 90), a direct measure of the crystallization rate, could be obtained from a single microcalorimetric trace of the amorphous nifedipine or phenobarbital. The t 90 values were also determined by conventional storage studies in which the heat of crystallization was determined by DSC. The t 90 values obtained by microcalorimetry were consistent with those obtained by DSC, within experimental error, indicating that microcalorimetry is a useful method for evaluating the physical stability of amorphous drugs.
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