ABSTRACT
Compared with crystalline drugs, their amorphous forms present long-range disordered molecular arrangements, and often exhibit higher apparent solubility and dissolution. However, several small molecule amorphous drugs may exhibit gelation phenomenon during the dissolution process, and show abnormal dissolution behavior with significantly lower dissolution than crystalline drugs. The current study aims to discover the relationship between the gelation of amorphous drugs and their abnormal dissolution, and further explore the internal gelation mechanism. Amorphous simvastatin (SIM), carvedilol (CAR), and irbesartan (IRB) were prepared by melt cooling method and characterized via X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FT-IR). Gel formation causes the dissolution of these three amorphous drugs to be significantly lower than their crystalline state. The formed gels were characterized as three-dimensional dense network structures by scanning electron microscope (SEM). Furthermore, amorphous SIM, CAR and IRB showed the critical gel temperature at 8-15 ℃, 25-30 ℃ and 45-50 ℃, and amorphous CAR and IRB showed the critical gel pH at 1 and 0.25. The mechanism of gel formation was proposed to be closely related to the transformation of amorphous drugs into the supercooled liquid state (as the important driving force) and the protonation induced self-assembling under acidic conditions. In addition, the wettability and properties of amorphous drugs also affect the formation of gelation.
ABSTRACT
The purpose of this study was to investigate the thermodynamics of naringenin (NAR)-isonicotinamide (INT) cocrystal (stoichiometric ratio, 1∶2) formed in different solvents. The dissolution behavior of cocrystal was explored in the water. Solubility of NAR-INT cocrystals under various temperatures were measured, followed by fitting the complexation model to calculate the thermodynamic parameters solubility products (Ksp), complexation constants (K12) and Gibbs energy change (ΔG) of cocrystal during formation progress. Ternary phase diagrams (TPDs) of the NAR-INT-solvent systems under various temperatures were plotted. Based on the non-linear simulation, 1∶2 complexation model was well fitted to the NAR-INT cocrystal formation in ethanol, isopropanol and ethyl acetate, while no complexation model was more suitable for that in methanol. The cocrystallization reaction was exothermic and spontaneous (ΔG H S Ksp increased while K12 decreased when increasing temperature, suggesting that the two components could cocrystallize more easily at the lower temperature. In comparison to TPDs in other solvents, the area of homogeneous liquid phase in ethyl acetate was the smallest, indicating the easiest formation of NAR-INT cocrystal in ethyl acetate. The current study provides a theoretical foundation for preparation and optimization of scale-up NAR-INT cocrystals.