Experimental mechanochemical screening of cocrystals with linezolid (LIN) resulted in the formation of six new crystal phases, including three neat cocrystals and three cocrystal hydrates, in addition to seven previously described cocrystals. In an attempt to understand the factors governing the formation of these phases, different experimental conditions of the mechanochemical reactions (polymorphic forms of LIN and presence of different solvents to create liquid-assisted grinding conditions) were tested and the results were compared with the predictions from three commonly used virtual cocrystal screening tools: molecular complementarity, hydrogen bond propensity, and molecular electrostatic potential maps. It is shown that these three methods can be used to help understand a molecule’s preferences to form cocrystals with particular coformers. The influence of molecular conformation on the outcome of the predictions is also evaluated. A comparison between the prediction methods indicates that while considering a set of similar coformers, the approach based on molecular electrostatic potential maps seems to be more consistent with the experimental results than molecular complementarity and hydrogen bond propensity tools. Instead, these two latter approaches are recommended at the early stages of coformer selection. In addition, intermolecular energy contribution (lattice energy) to the total energy of crystal forms of coformers was found to be indicative of the feasibility of cocrystal formation in the case of coformers capable of forming similar supramolecular synthons.

In a search for new crystalline forms of linezolid with modified thermal properties five cocrystals of this wide range antibiotic with aromatic acids were obtained via mechanochemical grinding and analyzed with single crystal X-ray diffraction, solid-state NMR spectroscopy, powder X-ray diffraction and DSC measurements. The coformers used in this study were benzoic acid, p-hydroxybenzoic acid, protocatechuic acid, γ-resorcylic acid and gallic acid. In each of the cocrystals distinct structural features have been found, including a variable amount of water and different heterosynthons, indicating that there is more than one type of intermolecular interaction preferred by the linezolid molecule. Basing on the frequency of the observed supramolecular synthons, the proposed hierarchy of the hydrogen-bond acceptor sites of linezolid (LIN) is C=Oamide > C=Ooxazolidone > C—O—Cmorpholine > C—N—Cmorpholine > C—O—Coxazolidone. In addition, aromatic–aromatic interactions were found to be important in the stabilization of the analyzed structures. The obtained cocrystals show modified thermal properties, with four of them having melting points lower than the temperature of the phase transition from linezolid form II to linezolid form III. Such a change in this physicochemical property allows for the future application of melting-based techniques of introducing linezolid into drug delivery systems. In addition a change in water solubility of linezolid upon cocrystalization was evaluated, but only in the case of the cocrystal with protocatechuic acid was there a significant (43%) improvement in solubility in comparison with linezolid.

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