Advances in physical chemistry and pharmaceutical applications of cyclodextrins

Cyclodextrins (CDs) attract much attention for industrial applications and academic research. A few experimental methods for determination of the binding constant between CD and a guest molecule were reviewed critically. A hydrophile–hydrophobe matching model for host–guest docking was proposed for estimation of the binding constant and the solution structure of the complex. Rather detailed solution structures of CD complexes were determined by proton NMR spectroscopy, aided by calculations of molecular mechanics and surface areas, and were used to analyze the binding constants. The binding constants of CDs with multi-site guests were analyzed on the basis of their solution structures. The working mechanisms and physicochemical predictions in a few pharmaceutical applications of CDs were proposed on the basis of detailed solution structures and accurate binding constants.

Masking Mechanisms of Bitter Taste of Drugs Studied with Ion Selective Electrodes

The masking mechanisms of the bitter taste of propantheline bromide (PB) and oxyphenonium (OB) bromide by native and modified cyclodextrins, saccharides, surfactants, organic acids, nonionic and anionic polymers, and other compounds were investigated with ion selective electrodes. The intensity of the bitter taste for a mixed solution of cyclodextrin with PB or OB was quantitatively explained from the observed electromotive force with the following assumptions: the complex and the masking agent do not have any tastes and the bitter taste is independent of other tastes. Sodium dodecyl sulfate reduced the bitter taste remarkably, and this reduction was also explicable on the basis of the same mechanism. Sodium taurodeoxycholate enhanced the bitter taste, because of its strong bitterness, although it formed 1 : 1 complexes with PB and OB. The masking mechanism of saccharides was ascribed to overcoming the weak bitterness of the drug by the strong sweetness. Lambda-carrageenan suppressed the bitter taste remarkably. This suppression was ascribed to the binding of PB and OB to lambda-carrageenan, the effect of the solution viscosity on the bitter taste, and the covering of the bitter taste receptor by lambda-carrageenan. It was suggested that the moderate masking by other polymers was attributable to the effect of the solution viscosity or the receptor covering. Native and modified beta-cyclodextrins, sodium dodecyl sulfate, lambda-carrageenan, Tween 20, and sodium carboxymethyl cellulose are good masking agents for the bitter tastes of PB and OB. The drug ion selective electrode is a useful tool for understanding of the masking mechanism of the bitter taste, screening of masking agents, and estimation of appropriate concentrations of the masking agents.

Solution Structures of 1:1 Complexes of Oxyphenonium Bromide with β- and γ-Cyclodextrins

The solution structures of complexes of oxyphenonium bromide (OB) with beta- and gamma-cyclodextrins (beta- and gamma-CDs, respectively) in deuterium oxide have been investigated by 500 MHz proton NMR spectroscopy and molecular mechanics calculations. The chemical shifts induced by complex formation provide the 1:1 binding constants and the chemical shift variations, DeltadeltaOB-CD, with complexation for the protons of OB and the CDs. The observed binding constants are very close to those obtained by other methods and are in the following order: beta-CD > gamma-CD > alpha-CD. Initial structures of the complexes are constructed on the basis of the ROESY spectra and the DeltadeltaOB-CD values and are optimized by molecular mechanics calculations. The intermolecular distances between the protons of OB and CD calculated for these structures are well-correlated with the observed ROESY intensities. The cyclohexyl group of OB penetrates deeply into a beta-CD cavity, and the phenyl group is close to the wide rim of the cavity. The phenyl and cyclohexyl groups of OB are both incorporated into a gamma-CD cavity. Furthermore, these structures of the complexes are consistent with the suppression of bitter taste and basic hydrolysis of OB by CDs and the polarity of binding sites of OB.