Factors limiting the oral bioavailability of N-acetylglucosaminyl-N-acetylmuramyl dipeptide (GMDP) and enhancement of absorption in rats by delivery in a water-in-oil microemulsion.

The bioavailability (BA) of radio-labelled N-acetylglucosaminyl-N-acetylmuramyl dipeptide (GMDP) was low when administered by oral gavage as an aqueous solution to conscious male Sprague-Dawley rats (8.3+/-4.4% (mean+/-S.D., n=3)). To assess the likely factors contributing to the poor BA of GMDP, the stability of GMDP in the lumen of the gastrointestinal (GI) tract was examined in vitro, using ex vivo GI contents. GMDP was degraded by the contents of the small intestine, caecum and large intestine but was more stable in stomach contents. The permeability coefficient (p(app)) of GMDP in isolated sections of rabbit ileum was 1.67x10(-6) cm/s in the mucosal to serosal direction and was not significantly different in the serosal to mucosal direction, indicating that GMDP is poorly permeable and passively transported across the intestinal wall. First pass metabolism was considered to be unlikely to be the primary limitation to the oral bioavailability of GMDP and therefore, that the oral bioavailability of GMDP was likely limited by instability in the lumen of the gastrointestinal tract and low intestinal permeability. A water-in-oil (w/o) microemulsion formulation subsequently developed to address these problems was trialed in a preliminary bioavailability study in rats and enhanced the bioavailability of GMDP ten-fold when administered intraduodenally, indicating that w/o microemulsions may represent a viable mechanism for enhancing the bioavailability of poorly GI-stable and poorly permeable peptide-based molecules.