Polymorphism and pseudopolymorphism of clarithromycin
De Jager, Mari-Alet
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Clarithromycin is a 14-membered-ring , macrolide antibiotic, a derivative of erythromycin, that is commonly used for the treatment of chronic, obstructive upper - and lower respiratory -, skin - and soft tissue -, as well as for gastric (duodenal ulcers caused by H. pylon] infections. During this study, polymorphic and pseudopolymorphic forms were evaluated, identified and characterised, using normal spectral and thermal methods, such as x-ray powder diffractometry, infrared spectroscopy, differential scanning calorimetry, thermogravimetrical analysis and thermal microscopy. Literature described Form 0, a solvate; Form I, a metastable polymorph; Form II, the thermodynamically more stable polymorph; Form Ill, a solvate (from acetonitrile) and Form IV, an amorph. Various solvents were used for slow recrystallisation of clarithromycin. The thermodynamically more stable Form II was prepared from acetone, methanol, benzene, dimethylformamide and tetrahydrofuran. Recrystallisations from acetonitrile produced Form II, and not Form Ill as reported in the literature. Two new forms, i.e. an ethyl acetate (Form V) and a chloroform solvate (Form VI) were prepared. Recrystallisation from both ethanol and iso-propanol produced Forms 0 and II. Although Form 0 desolvated prior to analysis, thus no longer contained the recrystallisation solvent, these crystals retained the structure of the solvated Form 0. Form 0 was recrystallised from n-propanol, n-butanol, dioxane and dichloromethane. Forms 0, 1, Ill, V and VI all transformed into the thermodynamically more stable Form II, after storage at room conditions. The dissolution results, relating to chloroform recrystallisation, showed that desolvation can improve the dissolution rate, since the chloroform solvate had a poor dissolution performance (61% after 30 minutes), whilst the desolvated form was almost completely dissolved within 30 minutes (96%), in the 0.1 M sodium acetate buffer.
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