Enhanced oral paclitaxel absorption with vitamin E-TPGS: Effect on solubility and permeability in vitro, in situ and in vivo

Solubility and permeability being important determinants of oral drug absorption, this study was aimed to investigate the effect of d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) on the solubility and intestinal permeability of paclitaxel in vitro, in situ and in vivo, in order to estimate the absorption enhancement ability of TPGS. Aqueous solubility of paclitaxel is significantly enhanced by TPGS, where a linear increase was demonstrated above a TPGS concentration of 0.1 mg/ml. Paclitaxel demonstrated asymmetric transport across rat ileum with significantly greater (26-fold) basolateral-to-apical (B-A) permeability than that in apical-to-basolateral (A-B) direction. Presence of P-glycoprotein (P-gp) inhibitor, verapamil (200 μM), diminished asymmetric transport of paclitaxel suggesting the role of P-gp-mediated efflux. TPGS showed a concentration-dependent increase in A-B permeability and decreased B-A permeability. The maximum efflux inhibition activity was found at a minimum TPGS concentration of 0.1 mg/ml, however, further increase in TPGS concentration resulted in decreased A-B permeability with no change in B-A permeability. Thus, the maximum paclitaxel permeability attained with 0.1 mg/ml TPGS was attributed to the interplay between TPGS concentration dependent P-gp inhibition activity and miceller formation. In situ permeability studies in rats also demonstrated the role of efflux in limiting permeability of paclitaxel and inhibitory efficiency of TPGS. The plasma concentration of [14C]paclitaxel following oral administration (25 mg/kg) was significantly increased by coadministration of TPGS at a dose of 50 mg/kg in rats. Bioavailability is enhanced about 4.2- and 6.3-fold when [14C]paclitaxel was administrated with verapamil (25 mg/kg) and TPGS, respectively, as compared to [14C]paclitaxel administered alone. The effect of verapamil on oral bioavailability of [14C]paclitaxel was limited relative to the TPGS, consistent with the in vitro solubility and permeability enhancement ability of TPGS. In conclusion, the current data suggests that the coadministration of TPGS may improve the bioavailability of BCS class II-IV drugs with low solubility and/or less permeable as a result of significant P-gp-mediated efflux.