Origins of the unusual hygroscopicity observed in LY297802 tartrate.

Hygroscopicity is an important physical property of drug materials, which can significantly impact physicochemical stability and bulk processing and handling characteristics. Although moisture sorption behavior is routinely investigated for pharmaceutical compounds, the nature of water-solid interactions is generally not well-understood at the molecular level for nonstoichiometric or variable hydrates. The unusual hygroscopic behavior of the muscarinic agonist, LY297802 tartrate, has been investigated using moisture sorption analysis, solid-state NMR (SSNMR) spectroscopy, and X-ray crystallography. LY297802 tartrate shows a unique tendency to form nonstoichiometric hydrates of variable, but specific, composition, with water content varying continuously from 0 to 0.5 mol (hemihydrate). The propensity of this crystal form to rapidly equilibrate with the humidity in the environment may be rationalized in terms of a sequential migration of the weakly bound water of hydration in to and out of readily accessible hydrogen-bonding sites in the lattice.