Preparation and Investigation of Solvatomorphs of Anti-Tumor Candidate RA-XII

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RA-XII, a unique natural cyclopeptide, is a potential anti-colorectal cancer candidate by mainly inhibiting protective autophagy, SREBP-1-dependent lipid biogenesis and suppressing activation of the Akt-mTOR pathway in HCT-116 cells. However, there have been no investigations into its solid forms, and its poor solubility restricts its application in the development of clinical anticancer agents. Background/Objectives: To investigate and develop advantageous medicinal solid forms of RA-XII and further improve its solubility characteristics. Methods: Various recrystallization methods were used to screen crystal forms of RA-XII, and single crystals (Forms I, II and V) were obtained by the vapor diffusion method. The forms were characterized by powder X-ray diffraction (PXRD), spectroscopy (FTIR, Raman), thermal analyses (DSC, TGA) and scanning electron microscopy (SEM). Structural parameters were determined by single crystal X-ray diffraction (SCXRD). The solubility and intrinsic dissolution rate were determined in a pH 7.4 pharmaceutical buffer solution with water. Hirshfeld surface analyses, molecular electrostatic potential surface (MEPs) analyses and molecular dynamics (MD) simulations were employed to reveal differences in hydrogen bond interactions of the crystal structure. Results: Five solvatomorphs and an amorphous form of RA-XII were successfully obtained. Then, three single crystals of Forms I, II and V were obtained, and their conformations were investigated. Hirshfeld surface analyses and crystal structures revealed significant differences in hydrogen bonding interactions and molecular packing. Moreover, MEPs analyses and MD suggested the formation mechanisms of RA-XII solvatomorphs, in which the hydrogen bonding strengths increase between RA-XII and solvent molecules. Conclusions: Form I is physically more stable and exhibits great solubility in water. Form I possesses significant medicinal advantages. These findings provide valuable insights into understanding the solvatomorph mechanisms and the quality control of RA-XII.