Characterization and evaluation of inclusion complexes between astaxanthin esters with different molecular structures and hydroxypropyl-β-cyclodextrin

Abstract

In this study, we focused on the characterization and evaluation of the inclusion complexes between astaxanthin esters (Asta-E) and hydroxypropyl-β-cyclodextrin (HPβ-CD). Asta-E of varying chain lengths (C4, C8, C18, and C22) and unsaturation degrees (C22:0 and C22:6) of the fatty acids were purified, and the inclusion complexes of HPβ-CD with Asta-E were characterized by various methods (phase solubility analysis, differential scanning calorimetry, ultraviolet–visible absorption spectroscopy, Fourier-transform infrared spectroscopy, and 1H nuclear magnetic resonance spectrometry). Thermodynamic analyses showed that astaxanthin docosahexaenoic acid monoester (Asta-C22:6) had the best binding capacity among the HPβ-CD–Asta-E inclusion complexes (complexation efficiency was 14.42 × 10⁻3 at 310 K). Moreover, binding site analysis of Asta-E with HPβ-CD indicated that the six-membered Asta-E ring was bound in the cavity of HPβ-CD by hydrogen bonds. The fatty acid chain had a positive influence on both the thermal stability and binding capacity of the inclusion complexes. Asta-C22:6 was selected to investigate the enhancing effect HPβ-CD has on its anticancer activity, oral bioavailability, and storage stability. HPβ-CD–Asta-C22:6 inclusion complex exhibited higher antiproliferative activity on HepG2 cells (IC50 = 160 μg/mL for 48 h) and arrested the cells in their S and G2/M phases. Bioavailability experiments showed that the area under curve (AUC0–t) after oral gavage of the HPβ-CD−Asta-C22:6 inclusion complex (9.62 ± 0.21 h μg/mL) was 2.68-times higher than that after oral gavage of Asta-C22:6 (3.59 ± 0.33 h μg/mL). Thus, the formation of an inclusion complex with HPβ-CD is an effective method to solubilize and stabilize Asta-E for potential future applications.