The use of MTDSC to assess the amorphous phase content of a micronized drug substance

Mechanical treatments such as grinding, milling or micronization applied to crystalline drug substances may induce changes such as the occurrence of crystal defects and/or amorphous regions. These changes are likely to affect the chemical and physical properties of the material as well as the corresponding drug product performances. Various analytical techniques such as standard differential scanning calorimetry, isothermal and solution microcalorimetry as well as dynamic vapour sorption can be used to characterise and possibly quantify the amorphous phase content of these materials. These techniques have been applied for the development of analytical methods based on temperature- or solvent-induced (including water) recrystallization of the amorphous phase in semi-crystalline drug substances and excipients and have sometimes allowed for detecting low amounts of amorphous phase. We have developed an alternative MTDSC method for the quantitation of the amorphous content in samples of a micronized drug substance co-crystal (form A), an antibiotic drug substance which does not recrystallize even when exposed to temperature or solvent vapours. This is performed through measurement of the heat capacity jump associated with the amorphous phase glass transition. The MTDSC parameters and experimental conditions were optimised for this system. The amorphous content calibration curve was established using pure crystalline and amorphous drug substance samples and their known mixtures. Limits of detection and quantification of 0.9 and 3.0% (w/w) respectively were obtained for specimen mass less than 5 mg.