Abstract
Two polymorphs of the free acid of MK571 have been characterized unambiguously by differential scanning calorimetry (DSC), thermal gravimetric analysis (TG), X-ray powder diffraction (XRPD) and solubility measurements. These two species are designated forms I and II, respectively. The solubilities of the two crystal forms of MK571 were determined at several temperatures (5–55° C) in isopropyl alcohol (IPA) and methyl ethyl ketone (MEK). In all cases, Form II shows higher solubility than Form I which indicates that the latter is the thermodynamically stable form in this temperature range. The melting point of Form I (164° C) is higher than that of Form II (152° C), indicating that Form I is the more stable polymorph in the high temperature range. These data suggest that the two forms are monotropic polymorphs.
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