Characterization Of Absorption And Metabolism Of Asiaticoside And Its Analogs

Objective: Asiaticoside is a compound which showed new pharmacological activity and good potency but poor bioavailability in rats. The purpose of this study was to explore the properties of absorption and metabolism of asiaticoside and its analogs, so as to elucidate the reasons for the poor bioavailability of asiaticoside. Methods: Solubility and LogD of asiaticoside were determined by HPLC assay. LogP and pK_a were predicted by computational method. The cytotoxic effect of compounds on Caco-2 cells, MDCK cells and rat hepatocytes was assessed using the MTT or LDH test. Intestinal transport of compounds was measured using the single pass intestinal perfusion technique in rats and MDCK cell monolayers. The metabolism of asiaticoside was studied in vitro by incubation with rat liver microsomes, hepatocytes and enzymes of intestinal bacteria. Additionally, recovery of asiaticoside from feces was quantified by performing in vivo experiments with the rat as animal model. TLC and LC-MS were employed to identify asiaticoside and its metabolites.Results: Only LogP of asiaticoside was in the range of 'rule of 5'. The computational method was able to predict LogP and LogD of asiaticoside within 0.6 log units for the experimental data. Asiaticoside and madecassoside were shown to be non-cytotoxic in Caco-2 cells, MDCK cells and rat hepatocytes, whereas the cytotoxic effect of asiatic acid and madecassic acid was obvious. Asiaticoside exhibited low in situ and in vitro rat intestinal permeability and asiatic acid appeared to be a higher permeability compared to asiaticoside. Passive, transcellular diffusion dominated the intestinal transport mechanism of asiaticoside, with no evidence of P-glycoprotein involvement. Asiaticoside was highly stable in rat liver microsomes and hepatocytes. After oral administration, asiaticoside could be degraded by intestinal bacteria to asiatic acid and other two metabolites, and then most amounts of asiaticoside were excreted in the form of asiatic acid. Asiaticoside and all its metabolites could be aborbed into the body. Conclusion: The above results indicate that asiaticoside has low absorption, which may be the outcome of its undesirable physical and chemical properties. Asiaticoside is poorly metabolized by P450, but can be hydrolyzed by intestinal microflora with the release of the corresponding aglycone. The primary methods of absorption and metabolism in early phase of drug discovery were established.