Lesinurad,
a uric acid reabsorption inhibitor that received Food and Drug Administration
approval in 2015, is known to crystallize in three unsolvated crystal
forms and in a few solvated phases. The structures of the former have
been determined by state-of-the-art powder diffraction methods, highlighting
significant conformational as well as supramolecular differences,
resulting in hydrogen bonded centrosymmetric dimers (Form 1) or helical
chains (Form 2). In the complex crystal packing in Form 3, additional
one-dimensional (1D) ribbons held together by unexpected CO···Br
interactions of the halogen-bond type are found. Thermal analyses
and variable-temperature powder diffraction measurements (including
high-temperature synchrotron X-ray diffraction experiments) provided
evidence for the reversible formation of a new phase, Form 2hT, obtained
upon heating above 100 °C powders of Form 2. Structure solution
and refinement of the high-temperature phase made it possible to attribute
the structural change to a 60° rotation of the cyclopropyl residue,
leaving unaffected the conformation of the (longer) polar branch and
the supramolecular 1D helical chain arrangement found in the RT phase.
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