Per-6-Thiolated Cyclodextrins: A Novel Type of Permeation Enhancing Excipients for BCS Class IV Drugs

The purpose of the study was to develop a per-6-thiolated alpha-cyclodextrin (alpha-CD) by substituting all primary hydroxyl groups of alpha-CD with thiol groups and to assess its solubility-improving and permeation-enhancing properties for a BCS Class IV drug in vitro as well as in vivo. The primary hydroxyl groups of alpha-CD were replaced by iodine, followed by substitution with -SH groups. The structure of per-6-thiolated alpha-CD was approved by FT-IR and H-1 NMR spectroscopy. The per-6-thiolated was characterized for thiol content, -SH stability, cytotoxicity, and solubility-improving properties by using the model BCS Class IV drug furosemide (FUR). The mucoadhesive properties of the thiolated oligomer were investigated via viscoelastic measurements with porcine mucus, whereas permeation-enhancing features were evaluated on the Caco-2 cell monolayer and rat gut mucosa. Furthermore, oral bioavailability studies were performed in rats. The per-6-thiolated alpha-CD oligomer displayed 4244 +/- 402 mu mol/g thiol groups. These -SH groups were stable at pH <= 4, exhibiting a pK(a) value of 8.1, but subject to oxidation at higher pH. Per-6-thiolated alpha-CD was not cytotoxic to Caco-2 cells in 0.5% (m/v) concentration within 24 h. It improved the solubility of FUR in the same manner as unmodified alpha-CD. The addition of per-6-thiolated alpha-CD (0.5% m/v) increased the mucus viscosity up to 5.8-fold at 37 degrees C within 4 h. Because of the incorporation in per-6-thiolated alpha-CD, the apparent permeability coefficient (P-app) of FUR was 6.87-fold improved on the Caco-2 cell monolayer and 6.55-fold on the intestinal mucosa. Moreover, in vivo studies showed a 4.9-fold improved oral bioavailability of FUR due to the incorporation in per-6-thiolated alpha-CD. These results indicate that per-6-thiolated alpha-CD would be a promising auxiliary agent for the mucosal delivery of, in particular, BCS Class IV drugs.