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  • Title: Thermally-prepared polymorphic forms of cilostazol.
    Author: Stowell GW, Behme RJ, Denton SM, Pfeiffer I, Sancilio FD, Whittall LB, Whittle RR.
    Journal: J Pharm Sci; 2002 Dec; 91(12):2481-8. PubMed ID: 12434391.
    Abstract:
    Prior to this study, cilostazol, an antithrombotic drug, was thought to exist as a single crystalline phase with a melting point of approximately 159 degrees C (Form A). On cooling, melts often form a glass that, when heated, may crystallize as additional crystalline polymorphic forms. Cilostazol, when reheated, subsequently forms polymorphs that melt at approximately 136 degrees C (Form B) and 146 degrees C (Form C). Free-energy temperature diagrams estimated from calorimetry data reveal that each pair of the cilostazol polymorphs (A-B, B-C, and A-C) is monotropic. Essentially pure samples of suitable crystalline shape and size permitted single crystal structural analysis of Forms A and C. Theoretical solubility ratios calculated using calorimetry data indicate that at 37 degrees C, Form B should be more than four times more soluble and Form C should be more than two times more soluble than Form A. Forms B and C could not be crystallized from solvents. Metastable forms from super cooled melts analyzed by intrinsic dissolution and Fourier transform-Raman experiments demonstrated that Forms B and C undergo a rapid, solvent-mediated recrystallization to Form A, making dissolution rate measurements difficult.
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