Hydrates
are common in pharmaceutical development, and the formation
of hydrates affects the performance of the final product. However,
the role that water plays in crystal packing remains unclear. In this
study, Avibactam sodium, which has one dihydrate (Form E), one monohydrate
(Form A), and two anhydrous forms (Form B and D), was chosen as the
model compound to understand this subject. Single crystal structures
of four solid forms were obtained and characterized by single X-ray
diffraction. The dynamic vapor sorption experiments revealed the moisture-dependent
stability increased in the order: Form B < Form D < Form A <
Form E. It can be envisaged that the integration of water molecules
could noticeably compensate the potential intermolecular interactions,
thereby significantly improving the crystal stabilities of hydrates.
Furthermore, the hydration of Form B was investigated to understand
the integration of water molecules by measuring the critical hydration
water activities (a
w). The results indicated
that both water activities and temperature are vital factors to determine
the amount of water molecules existing in crystal lattice. Moreover,
to probe the disintegration of water molecules, the dehydration of
dihydrate was investigated in detail by solid-state transformation
and solvent-mediated transformation experiments. Finally, two-step
dehydration and one-step dehydration + recrystallization mechanisms
of these different pathways were proposed by analyzing the transformation
experiment results and the crystal structure of various solid forms.