Assembly of beta-cyclodextrin with 3S-tetrahydro-beta-carboline-3-carboxylic acid and self-assembly of 6-(3'S-carboline-3'-carboxylaminoethylamino)-6-deoxy-beta-cyclodextrin: approaches to enhance anti-oxidation stability and anti-thrombotic potency.

3 S-1,2,3,4-Tetrahydro-beta-carboline-3-carboxylic acid (THCA) isolated from Bulbus allii macrostemi was identified as the active antiplatelet aggregation ingredient. However, the very poor water solubility and the shortcoming of being oxidized easily in vivo seriously limit the clinical application of THCA. In the present study, two strategies were used to reduce this tendency. First, the inclusion complex of THCA with beta-cyclodextrin (beta-CD) was prepared. Spectral studies identified that the inclusion complex (beta-CD1,2/THCA) was in equilibrium between beta-CD/THCA and beta-CD2/THCA, and the proportion of two isomers was beta-CD concentration dependent; it was 89% vs 11% in our study. The oxidation of both THCA and beta-CD1,2/THCA by H2O2 followed first-order kinetics, and 35% of THCA and 33% of beta-CD1,2/THCA were oxidized during the monitoring period. In vitro antiplatelet aggregation and in vivo oral administration antithrombotic activity of THCA was largely increased via inclusion complexation with beta-CD. Second, a novel conjugate 6-(3' S-carboline-3'-carboxyamino-ethylamino)-6-deoxy-beta-CD (5-monomer) was prepared. Spectral characterizations demonstrated that 5-monomer was able to self-assemble into 5-dimer, which was coexisting with the monomer with a ratio of 79% vs 21% in solution. The in vitro oxidation of 5-monomer/5-dimer by H2O2 did not occur during the monitoring period. The in vitro antiplatelet aggregation and in vivo antithrombotic assays of 5-monomer /5-dimer demonstrated that the bioactivity of THCA was remarkably increased via conjugation with 6-ethylamino-6-deoxy-beta-CD and produced greater in vitro and in vivo effectiveness than that of the inclusion complex beta-CD1,2/THCA at the same dose. The significant improvement of the bioactivity and stability of THCA indicates that inclusion complexation and conjugation with beta-CD provide promising approaches to improve the practical use of THCA in clinical applications.