Elucidating an Amorphous Form Stabilization Mechanism for Tenapanor Hydrochloride: Crystal Structure Analysis Using X‑ray Diffraction, NMR Crystallography, and Molecular Modeling

By the combined use of powder and single-crystal X-ray diffraction, solid-state NMR, and molecular modeling, the crystal structures of two systems containing the unusually large tenapanor drug molecule have been determined: the free form, <b>ANHY</b>, and a dihydrochloride salt form, <b>2HCl</b>. Dynamic nuclear polarization (DNP) assisted solid-state NMR (SSNMR) crystallography investigations were found essential for the final assignment and were used to validate the crystal structure of <b>ANHY</b>. From a structural informatics analysis of <b>ANHY</b> and <b>2HCl</b>, conformational ring differences in one part of the molecule were observed which influence the relative orientation of a methyl group on a ring nitrogen and thereby impact the crystallizability of the dihydrochloride salt. From quantum chemistry calculations, the dynamics between different ring conformations in tenapanor is predicted to be fast. Addition of HCl to tenapanor results in general in a mixture of protonated ring conformers and hence a statistical mix of diastereoisomers which builds up the amorphous form, <b>a-2HCl</b>. This was qualitatively verified by ¹³C CP/MAS NMR investigations of the amorphous form. Thus, to form any significant amount of the crystalline material <b>2HCl</b>, which originates from the minor (i.e., energetically less stable) ring conformations, one needs to involve nitrogen deprotonation to allow exchange between the minor and major conformations of <b>ANHY</b> in solution. Thus, by controlling the solution pH value to well below the p<i>K</i>_a of <b>ANHY</b>, the equilibrium between <b>ANHY</b> and <b>2HCl</b> can be controlled and by this mechanism the crystallization of <b>2HCl</b> can be avoided and the amorphous form of the dichloride salt can therefore be stabilized.