Pharmacokinetics, tissue distribution and excretion of luteolin and its major metabolites in rats: Metabolites predominate in blood, tissues and are mainly excreted via bile

To elucidate the in vivo bioactivities of luteolin, an important food-derived flavonoid, its metabolic fate and pharmacokinetic properties in rats were investigated. Total of 8 metabolites were isolated from urine and bile and identified by NMR. Luteolin was firstly metabolized via either glucuronidation or methylation and could subsequently transform into methylated glucuronides. Systematic pharmacokinetic investigations demonstrated luteolin was rapidly and efficiently absorbed in rat intestine and then extensively metabolized, which was responsible of low availability of 17.5% for unchanged luteoin. Luteolin presented mainly as conjugates in systemic circulation, among which luteolin-3′-O-β-d-glucuronide was the most abundant both in plasma and most of tissues. Luteolin and its metabolites preferred to distribute in the gastrointestine, liver, kidney and lung. Biliary excretion dominated the elimination pathways of the conjugated luteolin, especially the metabolites with 7-O-glucuronidation. The current study suggests further investigation on bioactivities of metabolites is crucial to elucidate the functional mechanism of luteolin.