Characterization of cyclodextrin complexes of camostat mesylate by ESI mass spectrometry and NMR spectroscopy

Supramolecular interactions between camostat mesylate, a serine protease inhibitor (1), with α-, β-, and γ-cyclodextrin (CD) in water were investigated using electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. ESI mass spectral analysis revealed that the 1:1 stoichiometry in all the complexes was formed. The binding constants (Kst) calculated by linear equations constructed from the ESI mass spectra of all the complexes indicated that γ-CD was most favorable complexing agent for the binding with 1 among the CDs. Pronounced changes in the 1H chemical shift upon complex formation with γ-CD were observed for the protons of the two aromatic rings of 1, with much larger chemical shift changes observed for the protons of the guanidinyl group-linked aromatic ring of 1. These results suggest that the cavity of γ-CD rather than that of α- or β-CD is large enough to accommodate the guanidine group of 1. Spatial geometry of 1 within the cavity of γ-CD was further identified with two-dimensional rotating frame nuclear Overhauser effect spectroscopy (2D ROESY) experiment. The observed ROESY cross peaks indicated intermolecular dipolar interactions between the two aromatic ring protons of 1 and the protons within the cavity of γ-CD. Based on the 1:1 stoichiometry of the complex, ROESY cross peaks suggest that two types of 1:1 complexes of γ-CD with 1 exist simultaneously in solution with different geometries.