Thermodynamics and preliminary pharmaceutical characterization of a melatonin–pimelic acid cocrystal prepared by a melt crystallization method

Pharmaceutical cocrystals have been extensively investigated as a promising approach to improve drugs' physicochemical properties. Cocrystals are usually prepared using solution-mediated methods and grinding methods. In this study, a cocrystal of pimelic acid with poorly soluble melatonin was produced using a melt crystallization method by controlling the crystallization within a specific temperature range. The cocrystal was characterized by infrared spectroscopy, powder and single crystal X-ray diffraction. The binary phase diagram and the formation enthalpy established by differential scanning calorimetry provided the rationale for this spontaneous cocrystallization system. The cocrystal exhibited higher apparent solubility, a faster dissolution rate and acceptable stability as compared to the original melatonin. The study has shown that the melt crystallization method can be considered as a competitive route for producing pharmaceutical cocrystals with improved properties and the thermodynamic investigation can provide in depth understanding and guidance on the melt crystallization of cocrystals.