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Elucidating an amorphous form stabilization mechanism of tenapanor hydrochloride: crystal structure analysis using Xray diffraction, NMR crystallography and molecular modelling

Nillson Lill, S. O., Widdifield, C. M., Pettersen, A., Ankarberg, A. S., Lindkvist, M., Aldred, P., Gracin, S., Shankland, N., Shankland, K., Schantz, S. and Emsley, L. (2018) Elucidating an amorphous form stabilization mechanism of tenapanor hydrochloride: crystal structure analysis using Xray diffraction, NMR crystallography and molecular modelling. Molecular Pharmaceutics, 15 (4). pp. 1476-1487. ISSN 1543-8392

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To link to this item DOI: 10.1021/acs.molpharmaceut.7b01047

Abstract/Summary

By the combined use of powder and single crystal X-ray diffraction, solid-state NMR, and molecular modelling, the crystal structures of two systems containing the unusually large tenapanor drug molecule have been determined: the free form, ANHY and a dihydrochloride salt form, 2HCl. 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 ANHY. From the structural informatics analysis of ANHY and 2HCl, conformational ring differences in one part of the molecule were observed which influences the relative orientation of a methyl group on a ring nitrogen and thereby impacts 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, a-2HCl. This was qualitatively verified by 13C CP/MAS NMR investigations of the amorphous form. Thus, to form any significant amount of the crystalline material 2HCl, which originates from the minor (i.e., energetically less stable) ring conformations, one needs to involve nitrogen deprotonation to allow exchange between minor and major conformations of ANHY in solution. Thus, by controlling the solution pH value to well below the pKa of ANHY, the equilibrium between ANHY and 2HCl can be controlled and by this mechanism the crystallization of 2HCl can be avoided and the amorphous form of the dichloride salt can therefore be stabilized.

Item Type:Article
Refereed:Yes
Divisions:Faculty of Life Sciences > School of Chemistry, Food and Pharmacy > School of Pharmacy > Pharmaceutics Research Group
ID Code:75888
Publisher:American Chemical Society

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