Molecular Simulation Approaches for the Prediction of Unknown Crystal Structures and Solubilities of (<i>R</i>)- and (<i>R</i>,<i>S</i>)-Crizotinib in Organic Solvents

Abstract

In this work, the solubilities of (R)- and (R,S)-crizotinib in six solvents including methanol, ethanol, 1-propanol, 1-butanol, ethyl acetate, and n-hexane were first measured by a static method in the temperature range from 273.15 to 323.15 K at atmospheric pressure. Then, molecular simulations were performed to predict the crystal structures of (R)-crizotinib and (R,S)-crizotinib from their powder X-ray diffractograms and the solubilities in the above organic solvents using the Gibbs free energy calculation approach via the sublimation cycle. The results showed that the solubilities of (R)- and (R,S)-crizotinib in the selected solvents generally increased with the increasing temperature, and at room temperature the molar fraction solubility order was methanol > 1-butanol > 1-propanol > ethanol > ethyl acetate > n-hexane. The (R)-crizotinib crystal had a monoclinic unit cell, while the (R,S)-crizotinib crystal had a triclinic one. In particular, though it was first applied to the solubility estimation for water-insoluble drug compounds in organic solvents, the Gibbs energy calculation approach developed gave a desirable solubility prediction performance with a root-mean-square error (RMSE) of 0.7638 log S units for (R)-crizotinib and 1.1577 log S units for (R,S)-crizotinib in the selected solvents, respectively. Furthermore, the difference in the solubilities of (R)-crizotinib in different solvents mainly resulted from the different Gibbs free energies for solvation between solute and solvent other than those for sublimation. However, as to the difference in the solubilities of (R)- and (R,S)-crizotinib in the same solvent, the Gibbs free energy for sublimation made more of a contribution than that for solvation. Finally, on the basis of the solid-state characterization, the nature of crystalline (R,S)-crizotinib was confirmed to be a racemic compound. All these results shall provide pharmaceutical industry with a better understanding of this chiral system for crystallization resolution.