Icariin is a bioactive herbal ingredient isolated from Herba epimedii, which has been widely used for the treatment of osteoporosis and male sexual dysfunction in traditional Chinese medicine. The major objective of this work is to investigate the different enhancing effects of β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) on the intestinal absorption of icariin, and to identify the molecular mechanisms of this action. Host-guest-type interactions of icariin with cyclodextrins nanocavities were unambiguously demonstrated by the phase-solubility diagram, ultraviolet spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, X-ray powder diffractometry, and two dimensional proton nuclear magnetic resonance rotating-frame Overhauser effect spectroscopy. These results were further supported using molecular modeling studies. The rat single-pass intestinal perfusion model showed that the absorption of icariin was affected by P-glycoprotein (Pgp). The icariin/HP-β-CD inclusion complex provided greater enhancement in the intestinal absorption than the icariin/β-CD inclusion complex. Therefore, the enhancing effect was involved in a solubilizing effect and/or Pgp inhibitory effect. Finally, fluorescence anisotropy measurements and Pgp adenosine triphosphatase (ATPase) assay demonstrated that β-CD exhibited no effect on Pgp, while HP-β-CD showed inhibition by restraining the Pgp ATPase activity rather than changing the fluidity of cell membrane.
Keywords: P-glycoprotein (Pgp); cyclodextrin; icariin; inclusion complex; intestinal absorption; molecular modeling.