Co-delivery of honokiol, a constituent of Magnolia species, in a self-microemulsifying drug delivery system for improved oral transport of lipophilic sirolimus.

Sirolimus is recognized as a P-glycoprotein (P-gp) substrate with poor water-solubility. To improve its solubility and bioabsorption, self-microemulsifying drug delivery systems (SMEDDS) containing a novel P-gp inhibitor, honokiol, were prepared. The aim of this work was to evaluate the enhanced transport of sirolimus SMEDDS as well as the roles of honokiol. In situ single-pass intestinal perfusion and in vitro human colon adenocarcinoma (Caco-2) cell models were applied to study the effects of honokiol within SMEDDS on the transport of sirolimus. The results indicated that a combination of honokiol with sirolimus in SMEDDS did not significantly alter the particle size, polydispersity index and release of drugs. In addition, the absorption rate constant (Ka) as well as the effective permeability coefficients (Peff) of sirolimus in situ intestinal absorption, and the apparent permeability coefficients (Papp) of sirolimus in caco-2 cells were significantly enhanced by cremophor EL-based SMEDDS with honokiol as compared with those of SMEDDS without honokiol. Rhodamine123 uptake rate in caco-2 cells and in vitro cytotoxicity of sirolimus were enhanced by honokiol in SMEDDS indicating a substantial P-gp inhibition of honokiol. In conclusion, coadministration of honokiol with poor soluble P-gp substrate in SMEDDS, could serve as a favorable approach for oral delivery.