Microsomal quercetin glucuronidation in rat small intestine depends on age and segment

Age-and Region-Dependent Disposition of Raloxifene in Rats

PURPOSE

Raloxifene undergoes extensive glucuronidation in the gastrointestinal (GI) tract and the liver. However, the impact of age on raloxifene disposition has never been studied. The purpose of this paper is to determine glucuronidation and Pharmacokinetics (PK) profiles of raloxifene in rats at different ages.

METHODS

Raloxifene glucuronidation was characterized using S9 fractions prepared from different intestinal segments and the liver of F344 rats at 4-, 11-, and 28-week. PK studies were conducted to determine raloxifene oral bioavailability at different ages. Raloxifene and its glucuronides were quantified using LC-MS/MS.

RESULTS

Raloxifene-6-glucuronide and raloxifene-4'-glucuronide were detected as the major metabolites and the ratio of these two glucuronides were different ranging from 2.1 to 4.9 folds in the ileum, jejunum, liver, and duodenum, and from 14.5 to 50 folds in the colon. The clearances in the duodenum at 4-week for both two glucuronides were significantly lower than those at the other two ages. PK studies showed that the oral bioavailability of raloxifene is age dependent. The absolute oral bioavailability of raloxifene was 3.5-folds higher at 4-week compared to that at 11-weeks. When raloxifene was administered through IV bolus, its half-life was 5.9 ± 1.16 h and 3.7 ± 0.68 h at 11-and 4-week, respectively.

CONCLUSION

These findings suggested that raloxifene metabolism in the duodenum was significantly slower at young age in rats, which increased the oral bioavailability of raloxifene. At 11-week, enterohepatic recycling efficiency was higher than that of 4-week. Raloxifene's dose at different ages should be carefully considered.

Hepatoenteric recycling is a new disposition mechanism for orally administered phenolic drugs and phytochemicals in rats

Many orally administered phenolic drugs undergo enterohepatic recycling (EHR), presumably mediated by the hepatic phase II enzymes. However, the disposition of extrahepatically generated phase II metabolites is unclear. This paper aims to determine the new roles of liver and intestine in the disposition of oral phenolics. Sixteen representative phenolics were tested using direct portal vein infusion and/or intestinal perfusion. The results showed that certain glucuronides were efficiently recycled by liver. OATP1B1/1B3/2B1 were the responsible uptake transporters. Hepatic uptake is the rate-limiting step in hepatic recycling. Our findings showed that the disposition of many oral phenolics is mediated by intestinal glucuronidation and hepatic recycling. A new disposition mechanism 'Hepatoenteric Recycling (HER)", where intestine is the metabolic organ and liver is the recycling organ, was revealed. Further investigations focusing on HER should help interpret how intestinal aliments or co-administered drugs that alter gut enzymes (e.g. UGTs) expression/activities will impact the disposition of phenolics.

Inhibitory effects of quercetin and its major metabolite quercetin-3-O-β-D-glucoside on human UDP-glucuronosyltransferase 1A isoforms by liquid chromatography-tandem mass spectrometry

Quercetin is a flavonoid that is widely present in plant-derived food. Quercetin-3-O-β-D-glucoside (Q3GA) is a predominant metabolite of quercetin in animal and human plasma. The inhibitory effects of the UDP-glucuronosyl transferases (UGTs) caused by herbal components may be a key factor for the clinical assessment of herb-drug interactions (HDIs). The present study aimed to investigate the inhibitory profile of quercetin and Q3GA on recombinant UGT1A isoforms in vitro. The metabolism of the nonspecific substrate 4-methylumbelliferone (4-MU) by the UGT1A isoforms was assessed by liquid chromatography-tandem mass spectrometry. Preliminary screening experiments indicated that quercetin exhibited stronger inhibitory effects on UGT1A1, UGT1A3, UGT1A6 and UGT1A9 enzymes than Q3GA. Kinetic experiments were performed to characterize the type of inhibition caused by quercetin and Q3GA towards these UGT isoforms. Quercetin exerted non-competitive inhibition on UGT1A1 and UGT1A6, with half maximal inhibitory concentration (IC₅₀) values of 7.47 and 7.07 µM and inhibition kinetic parameter (K_i) values of 2.18 and 28.87 µM, respectively. Quercetin also exhibited competitive inhibition on UGT1A3 and UGT1A9, with IC₅₀ values of 10.58 and 2.81 µM and K_i values of 1.60 and 0.51 µM, respectively. However, Q3GA displayed weak inhibition on UGT1A1, UGT1A3 and UGT1A6 enzymes with IC₅₀ values of 45.21, 106.5 and 51.37 µM, respectively. In the present study, quercetin was a moderate inhibitor of UGT1A1 and UGT1A3, a weak inhibitor of UGT1A6, and a strong inhibitor on UGT1A9. The results of the present study suggested potential HDIs that may occur following quercetin co-administration with drugs that are mainly metabolized by UGT1A1, UGT1A3 and UGT1A9 enzymes.

Glucuronidation as a metabolic barrier against zearalenone in rat everted intestine

Zearalenone (ZON), produced by Fusarium fungi, exhibits estrogenic activity. Livestock can be exposed to ZON orally through contaminating feeds such as cereals, leading to reproductive disorders such as infertility and miscarriage via endocrine system disruption. However, the details of ZON metabolism remain unclear, and the mechanism of its toxicity has not been fully elucidated. In this study, we investigated the kinetics of ZON absorption and metabolism in rat segmented everted intestines. ZON absorption was confirmed in each intestine segment 60 min after application to the mucosal buffer at 10 µM. Approximately half of the absorbed ZON was metabolized to α-zearalenol, which tended to be mainly glucuronidated in intestinal cells. In the proximal intestine, most of the glucuronide metabolized by intestinal cells was excreted to the mucosal side, suggesting that the intestine plays an important role as a first drug metabolism barrier for ZON. However, in the distal intestine, ZON metabolites tended to be transported to the serosal side. Glucuronide transported to the serosal side could be carried via the systemic circulation to the local tissues, where it could be reactivated by deconjugation. These results are important with regard to the mechanism of endocrine disruption caused by ZON.

Plasma quercetin metabolites are affected by intestinal microbiota of human microbiota-associated mice fed with a quercetin-containing diet

Protective effect of quercetin on high-fat diet-induced non-alcoholic fatty liver disease in mice has been reported. Recent research has revealed that several intestinal bacteria metabolize quercetin. We hypothesize that the difference in composition of intestinal microbiota affects quercetin absorption from the intestine. Germ-free BALB/cA female mice (18 weeks old) were randomly divided into four groups and orally administered with fecal suspension from four human individuals (HF1, HF2, HF3, HF4) to produce the human microbiota-associated mice. All mice were fed the 0.05% quercetin-containing pelleted diet for four weeks. Significant differences were observed in plasma total cholesterol and cecal microbiota among the four groups. Plasma quercetin concentration was significantly higher in the HF3 group than in the HF1 group. The plasma isorhamnetin/quercetin ratio showed significant negative correlation with visceral fat levels (r = -0.544, p = 0.013). Positive correlation was observed between the Log₁₀ Enterobacteriaceae count and the plasma quercetin metabolites. Principal component analysis showed that all groups were distributed in different regions by using the correlation diagram with the second and third principal component. This study indicates that intestinal microbiota of human microbiota-associated mice inoculated with different fecal suspensions react to dietary quercetin in different ways.

Dietary fat increases quercetin bioavailability in overweight adults

SCOPE

Epidemiologic evidence supports that dietary quercetin reduces cardiovascular disease (CVD) risk, but its oral bioavailability is paradoxically low. The aim of this study was to determine whether dietary fat would improve quercetin bioavailability in adults at high risk for CVD and to assess lipid-mediated micellarization of quercetin in vitro.

METHODS AND RESULTS

In a randomized, cross-over study, overweight/obese men and postmenopausal women (n = 4 M/5 F; 55.9 ± 2.1 years; 30.8 ± 1.4 kg/m(2) ) ingested 1095 mg of quercetin aglycone with a standardized breakfast that was fat-free (<0.5 g), low-fat (4.0 g), or high-fat (15.4 g). Plasma was obtained at timed intervals for 24 h to measure quercetin and its methylated metabolites isorhamnetin and tamarixetin. Compared to the fat-free trial, plasma quercetin maximum concentration (Cmax ), and area under curve (AUC0-24 h ) increased (p < 0.05) by 45 and 32%, respectively, during the high-fat trial. During the high-fat trial, isorhamnetin Cmax and AUC0-24 h also increased by 40 and 19%, respectively, whereas Cmax and AUC0-24 h of tamarixetin increased by 46 and 43%, respectively. Dietary fat dose-dependently increased micellarization efficiency of quercetin aglycone in vitro.

CONCLUSION

Dietary fat improves quercetin bioavailability by increasing its absorption, likely by enhancing its micellarization at the small intestine.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.