Improvement of chemical instability of digitoxin in aqueous solution by complexation with .BETA.-cyclodextrin derivatives.

Abstract

Inclusion complexation of digitoxin with .beta.-cyclodextrin (.beta.-CyD) derivatives, such as 2,6-di-O-methyl-.beta.-CyD (DM-.beta.-Cyd), 2,3,6-tri-O-methyl-.beta.-CyD (TM-.beta.-CyD), hydroxypropyl-.beta.-CyD (HP-.beta.-CyD, substitution degree 4.3) and hydroxyethyl-.beta.-CyD (HE-.beta.-CyD, substitution degree 4.0) was studied by the solubility method. The apparent 1:1 stability constant (Kc) of the complexes in water at 25.degree. C was in the order of DM-.beta.-CyD (84000 M-1) > HP-.beta.-CyD (84000 M-1) .apprxeq. HE-.beta.-CyD(17000 M-1) .apprxeq. .beta.-CyD (17000 M-1) > TM-.beta.-CyD (5600 M-1). From the high-performance liquid chromatographic tracing of each of the four components (digitoxin, bisdigitoxoside, monodigitoxoside, digitoxigenin) in the reaction mixtures, the individual hydrolysis rate constants (k1-6) were determined by the non-linear least squares method. .beta.-CyDs suppressed the hydrolysis rates of digitoxin species in an acidic medium (pH 1.2), particularly the appearance of digitoxigenin, the final hydrolysis product, and the inhibitory effect was generally in the order of DM-.beta.-CyD > .beta.-CyD .apprxeq. HP-.beta.-CyD .apprxeq. HE-.beta.-CyD > TM-.beta.-CyD. By analyzing the concentration dependency of the hydrolysis rate, DM-.beta.-CyD was found to decelerate the appearance rate of digitoxigenin more than 2400 times.