Pseudopolymorphs of LB30870, a Direct Thrombin Inhibitor: One-Dimensional Solvent Channel Structures Explain Reversible Hydration/Dehydration

Abstract

Three different hydrates of LB30870, a new direct thrombin inhibitor, were identified during the screening of solid form, and their interconversion relationship and relative thermodynamic stabilities were investigated. Form I (hexahydrate) changes to Form II (dihydrate) or Form III (tetrahydrate) by dehydration, while Form II becomes Form I by hydration, and both Form II and Form III change to Form I in the aqueous slurry. Single crystals obtained from two different crystallization conditions, wet or air-dried, were found to be isostructural with a difference in the solvent channels, and based on the simulated and experimental powder X-ray diffraction patterns, the air-dried crystals are assigned as Form I and Form III, respectively. In all crystal structures, LB30870 is in a folded conformation forced by the presence of strong hydrogen bonds by two structural water molecules. The solvent channel formed can hold up to six additional hydration water sites per each LB30870, and the one-dimensional solvent channel facilitates the interconversion among the hydrates and rapid conversion to Form I in water. Although all hydrate forms would not differ in oral bioavailability as Form I predominates in the aqueous phase, considering the stable water content at 40-75% relative humidity Form III would be the most suitable for further development.