Structural and thermal analyses of metaxalone cocrystals with succinic, adipic and salicylic acids

Metaxalone (MTX) is a skeletal muscle relaxant drug, crystallized with carboxylic acid coformers like succinic acid (SUC), adipic acid (ADP) and salicylic acid (SAL). Their molecular structure, crystal packing and thermal analysis were studied. In the crystallization screening, two polymorphic forms of MTX-SAL were discovered. Form I of MTX-SAL was obtained from ethanol while form II was obtained concomitantly along with form I in ethyl acetate. All four structures form MTX homodimer of R22(8) motif. In MTX-SUC, the N-H⋅⋅⋅O, O-H⋅⋅⋅N and C-H⋅⋅⋅O hydrogen bonds are involved in the crystal packing and generate a three-dimensional hydrogen-bonded network. In MTX-ADP, the coformer bridges the adjacent MTX homodimers and forms an extended chain. In both the polymorphs of MTX-SAL, the MTX molecule forms a homodimer, which is further connected by SAL coformer. Solvent assisted grinding and slurry techniques were employed to obtain phase pure bulk material. The results obtained in different trials were compared using powder X-ray diffraction (PXRD) and the process was optimized for bulk solid form preparation of MTX-SUC, MTX-ADP and MTX-SAL form I. MTX-SUC exhibits a higher melting point than the other two cocrystals. The close packing indices, crystal densities and melting temperatures between both the MTX-SAL cocrystal polymorphs resulted in the concomitant existence of form II with form I.