Polymorphs and polymorphic cocrystals of temozolomide

Crystal polymorphism in the antitumor drug temozolomide (TMZ), cocrystals of TMZ with 4,4'-bipyridine-N,N'-dioxide (BPNO), and solid-state stability were studied. Apart from a known X-ray crystal structure of TMZ (form 1), two new crystalline modifications, forms 2 and 3, were obtained during attempted cocrystallization with carbamazepine and 3-hydroxypyridine-N-oxide. Conformers A and B of the drug molecule are stabilized by intramolecular amide N-H center dot center dot center dot N-imidazole, and N-H center dot center dot center dot N-tetrazine, interactions. The stable conformer A is present in forms 1 and 2, whereas both conformers crystallized in form 3. Preparation of polymorphic cocrystals I and II (TMZ.BPNO 1:0.5 and 2:1) were optimized by using solution crystallization and grinding methods. The metastable nature of polymorph 2 and cocrystal 11 is ascribed to unused hydrogen-bond donors/acceptors in the crystal structure. The intramolecularly bonded amide N-H donor in the less stable structure makes additional intermolecular bonds with the tetrazine C=O group and the imidazole N atom in stable polymorph 1 and cocrystal 1, respectively. All available hydrogen-bond donors and acceptors are used to make intermolecular hydrogen bonds in the stable crystalline form. Synthon polymorphism and crystal stability are discussed in terms of hydrogen-bond reorganization.