Thymoquinone is an effective phytochemical compound in the treatment of various diseases. However, its practical administration has been limited due to poor aqueous solubility and bioavailability. In this work, we developed a novel inclusion complex of thymoquinone and hydroxypropyl-β-cyclodextrin that features improved solubility and bioactivity. The drug solubility was markedly accelerated in the increasing ratio of hydroxypropyl-β-cyclodextrin to thymoquinone amount. The formation of the thymoquinone/hydroxypropyl-β-cyclodextrin inclusion complex was evidenced using X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, Fourier transform infrared, scanning electron microscopy and nuclear magnetic resonance. The release behavior of the complex, as well as of their mixtures, was examined in artificial gastric (pH 1.2) and intestinal (pH 6.8) dissolution media. The formulated complex released the drug rapidly at the initial stage, followed by a slow release. Thermodynamic parameters ΔH, ΔS and ΔG were calculated with temperatures ranging from 20 to 45 °C to evaluate the complexation process. The activity of the inclusion complex was evaluated on IgE-mediated allergic response in rat basophilic leukemia (RBL-2H3) cells by monitoring key allergic mediators. The results revealed that compared with free thymoquinone, the inclusion complex more strongly inhibited the release of histamine, tumor necrosis factor-α, and interleukin-4, and was not cytotoxic at the tested thymoquinone concentrations (0.125-4 μg/mL) indicating the inclusion complex possibly had better antiallergic effects. Our finding suggested that the inclusion complex achieved prolonged action and reduced side-effect of thymoquinone.
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