Relationship Between Phase Solubility Diagrams and Crystalline Structures During Dissolution of Cimetidine/Cyclodextrin Complex Crystals.

Cyclodextrins (CDs) form complex crystals with drugs and improve physicochemical properties of drugs. However, only few reports have summarized relationships between crystal structures of drug/CD and dissolution behavior. In this study, we developed cimetidine (CIM)/CD complex crystals to achieve sustained drug release and investigated the relationship between the dissolution behavior of CIM/CD complexes and their crystal structures. CIM and 3 types of CDs (α-, β-, and γ-CD) formed a complex crystal when subjected to solvent mixing. The CIM/CD complexes had a highly reduced dissolution rate compared to that of the physical mixture of CIM and CD. β-CD improved the solubility of CIM, whereas γ-CD decreased its solubility. Based on the phase solubility diagram, CIM and α-, β-, and γ-CD indicated A-type positive (A) and A deviation, and B-type limited solubility (B) profiles, respectively. In γ-CD, the saturated concentration of CIM decreased owing to the formation of a low-solubility complex with CIM. CIM/α-CD formed cage-type crystals, and CIM/β-CD and CIM/γ-CD formed channel-type crystals. The dissolution rate constant (k) of CIM/α-CD and CIM/β-CD were 0.045 and 0.04 h, respectively. CIM/γ-CD and CIM/β-CD displayed channel-type crystals; however, the channel-type crystals of CIM/γ-CD were stabilized by the presence of additional water molecules.