Polymorphs and Solvates of Molnupiravir: Crystal Structures and Solid Forms Transformation Analysis

Molnupiravir (MPV) is a widely used oral anti-COVID-19 drug. Owing to the rotatable bonds and the large number of acceptors and donors of hydrogen bonds in the molecular structure of MPV, three pure polymorphs (forms I, II, and III) and four solvates (ethanol, isopropanol, isobutanol, and tetrahydrofuran) of MPV (MPV-EtOH, MPV-IPA, MPV-IBA, and MPV-THF) were obtained. Importantly, the crystal structures of form I and the four solvates were successfully determined by single-crystal X-ray diffraction for the first time. Interestingly, all solvates are isostructural and have very similar unit cell parameters. Meanwhile, the solvent molecules in their structures are all in the voids surrounded by MPV molecules, forming a channel structure that facilitates desolvation. The formation mechanism of the solvates was also investigated based on the molecular electrostatic potential and hydrogen-bond energy of possible synthons. Moreover, a detailed stability study of MPV was conducted by using thermal analysis and dynamic vapor sorption. Finally, the phase transformations of the different solid forms were summarized. Our study provides a comprehensive understanding of the crystal landscape of the MPV.