Contrasting Polymorphism of Related Small Molecule Drugs Correlated and Guided by the Computed Crystal Energy Landscape

Solid form screening and crystal structure prediction (CSP) calculations were carried out on two related molecules, 3-(4-(benzo[d]isoxazole-3-yl)piperazin-1-yl)-2,2-dimethylpropanoic acid (B5) and 3-(4-dibenzo[b,f][1,4]oxepin-11-yl-piperazin-1-yl)-2,2-dimethylpropanoic acid (DB7). Only one anhydrate form was crystallized for B5, whereas multiple solid forms, including three neat polymorphs, were found for DB7. The crystal structure of B5 is P21/n Z′ = 1 with intramolecular hydrogen bonding, whereas Forms I and II of DB7 are conformational polymorphs with distinct Z′ = 1 P1̅ structures and intermolecular hydrogen bonds. A disordered structure for Form III of DB7 is proposed, based on CSP-generated structures which gave a promising match to the X-ray powder diffraction and solid state NMR data for this metastable form. The differences in the hydrogen bonding and experimental solid form landscapes of the two molecules appear to arise from the dominance of the self-assembly of the benzoisoxazolepiperazinyl and dibenzoxepinylpiperazinyl fragments and the consequent inability to produce amorphous or solvate forms as intermediates for B5. There is a subtle balance between the intramolecular conformational energy and the intermolecular dispersion, electrostatic and polarization interactions apparent in the analysis of the computationally generated thermodynamically competitive structures, which makes their relative stability quite sensitive to the computational method used. The value of simultaneously exploring the computationally and experimentally generated solid form landscapes of molecules in pharmaceutical development is discussed