Novel Pharmaceutical Cocrystals and Solvate Crystals of Nobiletin, a Citrus Flavonoid with Potent Pharmacological Activity

Nobiletin (NOB) is a flavonoid with attractive pharmaceutical characteristics, including anti-Alzheimer’s, anti-inflammation, and anti-cancer properties, but it has low solubility in water, resulting in reduced bioavailability. Its solubility must be improved to develop NOB as a drug. Cocrystal engineering can change the physicochemical properties of an active pharmaceutical ingredient and generate remarkable drug candidates that are superior in drug formulation. In this report, extensive co-crystal screening of NOBs with 31 cocrystal formers (coformers) with various functional groups was carried out by the liquid-assisted grinding method. As a result, four cocrystals (NOB with urea (URE), oxalic acid, gallic acid and salicylic acid) and one solvate crystal (NOB with formic acid (FOR)) were found. Powder X-ray diffraction and thermal analysis revealed the unique crystal morphology of all the obtained samples. In addition, the crystal structures of two of them (NOB-URE and NOB-FOR) were determined by single crystal X-ray diffraction. The results revealed that NOB-URE and NOB-FOR are new cocrystals or solvate crystals consisting of molar ratios of 1 : 2 and 1 : 0.73, respectively. In NOB-URE, we could observe a transient increase in solubility due to supersaturation, suggesting that URE is one of the better coformers of NOB.