Solubility and Mass and Nuclear Magnetic Resonance Spectroscopic Studies on Interaction of Cyclosporin A with Dimethyl‐α‐ and ‐β‐Cyclodextrins in Aqueous Solution

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Abstract
The interaction of cyclosporin A (CsA) with dimethyl-alpha- and -beta-cyclodextrins (DM-alpha-CyD and DM-beta-CyD) was investigated by the solubility method, electrospray ionization mass spectrometry (ESI-MS) and 1H-nuclear magnetic resonance spectroscopy (1H NMR). The extremely low solubility (1.9 x 10(-5) M at 25 degreesC) of CsA in water was significantly improved by the complexation with DM-CyDs: for example, the solubility increased 87-fold in the presence of 5.0 x 10(-2) M DM-beta-CyD. The phase solubility diagram of CsA/DM-CyD systems showed an Ap type and the stability constants (1060 M-1 and 1050 M-1, respectively) of the 1:1 CsA/DM-alpha-CyD and CsA/DM-beta-CyD complexes were much higher than those of the 1:2 complexes (15 M-1 and 21 M-1, respectively). In ESI-MS spectra of the CsA/DM-beta-CyD system, a new signal emerged at 1268 which corresponds to the 1:1 adduct of the di-ionized guest molecule with the host molecule. This signal intensity was significantly decreased by the addition of chlorpromazine (CPZ) which has a large stability constant (8800 M-1) of the DM-beta-CyD complex, whereas the signal corresponding to the CPZ/DM-beta-CyD complex was little affected by the addition of CsA, indicating a competitive inclusion of CPZ and CsA within the host cavity. CsA gave many new peaks in the 1H NMR spectrum when the solvent was changed from chloroform to methanol/water, suggesting conformational diversity of CsA in polar solvents. Inspection of 1H-chemical shift changes and the two-dimensional rotating frame nuclear Overhauser effect (ROESY) spectra of the CsA/DM-CyD system suggested that the side chains of amino acids in CsA molecule take part in the inclusion within DM-CyDs, although there is seemingly no preference of particular amino acid residues. All the data obtained here suggested that CsA forms inclusion complexes with DM-alpha- and -beta-CyDs in an aqueous medium and side chains of CsA are mainly involved in the inclusion.

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