Kinetic analysis of glycyrrhetic acid, an active metabolite of glycyrrhizin, in rats: role of enterohepatic circulation

Glycyrrhizae Radix et Rhizoma: The popular occurrence of herbal medicine applied in classical prescriptions

Glycyrrhizae Radix et Rhizoma is a well-known herbal medicine with a wide range of pharmacological functions that has been used throughout Chinese history. This review presents a comprehensive introduction to this herb and its classical prescriptions. The article discusses the resources and distribution of species, methods of authentication and determination chemical composition, quality control of the original plants and herbal medicines, dosages use, common classical prescriptions, indications, and relevant mechanisms of the active content. Pharmacokinetic parameters, toxicity tests, clinical trials, and patent applications are discussed. The review will provide a good starting point for the research and development of classical prescriptions to develop herbal medicines for clinical use.

18β-Glycyrrhetinic acid monoglucuronide (GAMG) alleviates single-walled carbon nanotubes (SWCNT)-induced lung inflammation and fibrosis in mice through PI3K/AKT/NF-κB signaling pathway

Carbon nanotubes (CNTs) have become far and wide used in a number of technical and merchant applications as a result of substantial advances in nanotechnology, therein single-walled carbon nanotubes (SWCNT) are one of the most promising nanoparticles. Inhaling CNTs has been linked to a variety of health problems, including lung fibrosis. Glycyrrhetinic acid 3-O-mono-β-D-glucuronide (GAMG), a natural sweetener, has anti-inflammatory and antioxidant capacities. The purpose of this study was to evaluate the potential for GAMG to alleviate SWCNT-induced lung inflammation and fibrosis. During days 3-28 after SWCNT intratracheal administration, we observed a remarkable increase of IL-1β, IL-6 and TNF-α in bronchoalveolar lavage fluid (BALF) on day 3 and collagen deposition on day 28. GAMG treatment remarkably ameliorated SWCNT-induced pulmonary fibrosis and attenuated SWCNT-induced inflammation and collagen deposition, and suppressed the activation of PI3K/AKT/NF-κB signaling pathway in the lungs. Therefore, GAMG has a therapeutic potential for the treatment of SWCNT-induced pulmonary fibrosis. Targeting PI3K/AKT/NF-κB signaling pathway may be a potential therapeutic approach to treat pulmonary fibrosis in mice with SWCNT.

Physiology-Based Pharmacokinetic Study on 18β-Glycyrrhetic Acid Mono-Glucuronide (GAMG) Prior to Glycyrrhizin in Rats

To understand that 18β-Glycyrrhetic acid 3-O-mono-β-D-glucuronide (GAMG) showed better pharmacological activity and drug-like properties than 18β-Glycyrrhizin (GL); a rapid and sensitive HPLC-MS/MS method was established for the simultaneous determination of GAMG and its metabolite 18β-Glycyrrhetinic acid (GA) in rat plasma and tissues after oral administration of GAMG or GL. This analytical method was validated by linearity, LLOQ, specificity, recovery rate, matrix effect, etc. After oral administration, GAMG exhibited excellent C_max (2377.57 ng/mL), T_max (5 min) and AUC_0-T (6625.54 mg/L*h), which was much higher than the C_max (346.03 ng/mL), T_max (2.00 h) and AUC_0-T (459.32 mg/L*h) of GL. Moreover, GAMG had wider and higher tissue distribution in the kidney, spleen, live, lung, brain, etc. These results indicated that oral GAMG can be rapidly and efficiently absorbed and be widely distributed in tissues to exert stronger and multiple pharmacological activities. This provided a physiological basis for guiding the pharmacodynamic study and clinical applications of GAMG.

Effect of hot water extract of a glycyrrhizin-deficient strain of Glycyrrhiza uralensis on contact hypersensitivity in mice

To evaluate the medicinal properties of a glycyrrhizin (GL)-deficient strain of Glycyrrhiza uralensis, we investigated the anti-allergic effect of the hot water extract obtained from its roots on contact hypersensitivity in mice, and compared it with that of the hot water extract of a commercial crude drug, Glycyrrhiza Radix. The hot water root extract of the GL-deficient strain contained glucoglycyrrhizin (GGL) and rhaoglucoglycyrrhizin (RGL) instead of GL, and it showed anti-allergic activity against contact hypersensitivity in a fashion similar to that of the crude drug extract. We further confirmed the presence of glycyrrhetinic acid (GA), a major metabolite of GL, in mice serum after oral administration of the hot water root extract of a GL-deficient strain. We demonstrated that GGL underwent hydrolysis by intestinal microflora of mice to form GA. These results suggest that a GL-deficient strain of G. uralensis is a useful medicinal resource since the glycosides of GA work in a fashion similar to that of GL when orally administered.

Evaluation of the safety and efficacy of Glycyrrhiza uralensis root extracts produced using artificial hydroponic-field hybrid cultivation systems II: comparison of serum concentration of glycyrrhetinic acid serum concentration in mice

To evaluate the safety and efficacy of Glycyrrhiza uralensis root produced using artificial hydroponic and artificial hydroponic-field hybrid cultivation systems, we investigated the pharmacokinetics of a major metabolite of glycyrrhizin (GL), glycyrrhetinic acid (GA). Hot water extracts obtained from the roots of the artificial hydroponic-field hybrid cultivated Glycyrrhiza uralensis were orally administered at a dose of 100 mg/kg as GL in mice and, compared with a commercial crude drug, Glycyrrhizae Radix. The temporal changes in serum GA concentration was found to depend on the GL concentration of the hot-water extracts. When hot-water extracts containing relatively high GL were administered, bimodal peaks appeared. In contrast, a broad single peak was detected when a hot-water extract containing relatively low GL content was administered. These tendencies in the serum GA concentration time course were observed for all samples, regardless of their derivation. Moreover, we compared the pharmacokinetic parameters and found that the C_max and AUC_0-48 values after oral administration of the extracts from Glycyrrhiza uralensis roots produced by the artificial cultivation system are within the range of variation for the commercial crude drugs. These results suggest the possibility that roots of Glycyrrhiza uralensis cultivated by the artificial hydroponic-field hybrid cultivation system can be used in addition to currently available commercial crude drugs produced from wild plant resources.

Blocking autophagy enhances the apoptotic effect of 18β-glycyrrhetinic acid on human sarcoma cells via endoplasmic reticulum stress and JNK activation

Sarcoma, a rare form of cancer, is unlike the much more common carcinomas as it occurs in a distinct type of tissue. The potent antitumor effects of 18β-glycyrrhetinic acid (GA), a novel naturally derived agent, have been demonstrated in various cancers. However, the effect of GA on human sarcoma, and the underlying mechanisms, remain to be elucidated. In the current study, we show that GA inhibits sarcoma cell proliferation by inducing G0/G1-phase arrest. Exposure to GA resulted in the activation of caspase-3, -8, and -9, indicating that GA induced apoptosis through both extrinsic and intrinsic pathways. In addition, the autophagy pathway, characterized by the conversion of LC3-I to LC3- II, was activated, resulting in increased Beclin-1 protein levels, decreased p62 expression, and stimulation of autophagic flux. The present findings showed that GA stimulated autophagy by inducing endoplasmic reticulum (ER) stress via the IRE1–JNK pathway. Our data supported the prosurvival role of GA-induced autophagy when the autophagy pathway was blocked with specific chemical inhibitors. Finally, GA markedly reduced sarcoma growth, with little organ-related toxicity, in vivo. The present results suggest that the combination of GA with a specific autophagy inhibitor represents a promising therapeutic approach for the treatment of sarcoma.

Glycyrrhetinic acid induces cytoprotective autophagy via the inositol-requiring enzyme 1α-c-Jun N-terminal kinase cascade in non-small cell lung cancer cells

Glycerrhetinic acid (GA), one of the main bioactive constituents of Glycyrrhiza uralensis Fisch, exerts anti-cancer effects on various cancer cells. We confirmed that GA inhibited cell proliferation and induced apoptosis in non-small cell lung cancer A549 and NCI-H1299 cells. GA also induced expression of autophagy marker phosphatidylethanolamine-modified microtubule-associated protein light-chain 3 (LC3-II) and punta formation of green fluorescent protein microtubule-associated protein light-chain 3. We further proved that expression of GA-increased autophagy marker was attributed to activation instead of suppression of autophagic flux. The c-jun N-terminal kinase (JNK) pathway was activated after incubation with GA. Pretreatment with the JNK inhibitor SP600125 or silencing of the JNK pathway by siRNA of JNK or c-jun decreased GA-induced autophagy. The endoplasmic reticulum (ER) stress responses were also apparently stimulated by GA by triggering the inositol-requiring enzyme 1α (IRE1α) pathway. The GA-induced JNK pathway activation and autophagy were decreased by IRE1α knockdown, and inhibition of autophagy or the JNK cascade increased GA-stimulated IRE1α expression. In addition, GA-induced cell proliferative inhibition and apoptosis were increased by inhibition of autophagy or the JNK pathway. Our study was the first to demonstrate that GA induces cytoprotective autophagy in non-small cell lung cancer cells by activating the IRE1α-JNK/c-jun pathway. The combined treatment of autophagy inhibitors markedly enhances the anti-neoplasmic activity of GA. Such combination shows potential as a strategy for GA or GA-contained prescriptions in cancer therapy.

Role of enterohepatic recirculation in drug disposition: cooperation and complications

Enterohepatic recirculation (EHC) concerns many physiological processes and notably affects pharmacokinetic parameters such as plasma half-life and AUC as well as estimates of bioavailability of drugs. Also, EHC plays a detrimental role as the compounds/drugs are allowed to recycle. An in-depth comprehension of this phenomenon and its consequences on the pharmacological effects of affected drugs is important and decisive in the design and development of new candidate drugs. EHC of a compound/drug occurs by biliary excretion and intestinal reabsorption, sometimes with hepatic conjugation and intestinal deconjugation. EHC leads to prolonged elimination half-life of the drugs, altered pharmacokinetics and pharmacodynamics. Study of the EHC of any drug is complicated due to unavailability of the apposite model, sophisticated procedures and ethical concerns. Different in vitro and in vivo methods for studies in experimental animals and humans have been devised, each having its own merits and demerits. Involvement of the different transporters in biliary excretion, intra- and inter-species, pathological and biochemical variabilities obscure the study of the phenomenon. Modeling of drugs undergoing EHC has always been intricate and exigent models have been exploited to interpret the pharmacokinetic profiles of drugs witnessing multiple peaks due to EHC. Here, we critically appraise the mechanisms of bile formation, factors affecting biliary drug elimination, methods to estimate biliary excretion of drugs, EHC, multiple peak phenomenon and its modeling.

Advances in plant-based inhibitors of P-glycoprotein

Multidrug resistance (MDR) has emerged as the main problem in anti-cancer therapy. Although MDR involves complex factors and processes, the main pivot is the expression of multidrug efflux pumps. P-glycoprotein (P-gp) belongs to the family of adenosine triphosphate (ATP)-binding cassette (ABC) transporters. It functions in cellular detoxification, pumping a wide range of xenobiotic compounds out of the cell. An attractive therapeutic strategy for overcoming MDR is to inhibit the transport function of P-gp and thus, increase intracellular concentration of drugs. Recently, various types of P-gp inhibitors have been found and used in experiments. However, none of them has passed clinical trials due to their high side-effects. Hence, the search for alternatives, such as plant-based P-gp inhibitors have gained attention recently. Therefore, we give an overview of the source, function, structure and mechanism of plant-based P-gp inhibitors and give more attention to cancer-related studies. These products could be the future potential drug candidates for further research as P-gp inhibitors.

A semi-physiologically based pharmacokinetic pharmacodynamic model for glycyrrhizin-induced pseudoaldosteronism and prediction of the dose limit causing hypokalemia in a virtual elderly population

Glycyrrhizin (GL) is a widely used food additive which can cause severe pseudoaldosteronism at high doses or after a long period of consumption. The aim of the present study was to develop a physiologically based pharmacokinetic (PBPK) pharmacodynamic (PD) model for GL-induced pseudoaldosteronism to improve the safe use of GL. Since the major metabolite of GL, glycyrrhetic acid (GA), is largely responsible for pseudoaldosteronism via inhibition of the kidney enzyme 11β-hydroxysteroiddehydrogenase 2 (11β-HSD 2), a semi-PBPK model was first developed in rat to predict the systemic pharmacokinetics of and the kidney exposure to GA. A human PBPK model was then developed using parameters either from the rat model or from in vitro studies in combination with essential scaling factors. Kidney exposure to GA was further linked to an Imax model in the 11β-HSD 2 module of the PD model to predict the urinary excretion of cortisol and cortisone. Subsequently, activation of the mineralocorticoid receptor in the renin-angiotensin-aldosterone-electrolyte system was associated with an increased cortisol level. Experimental data for various scenarios were used to optimize and validate the model which was finally able to predict the plasma levels of angiotensin II, aldosterone, potassium and sodium. The Monte Carlo method was applied to predict the probability distribution of the individual dose limits of GL causing pseudoaldosteronism in the elderly according to the distribution of sensitivity factors using serum potassium as an indicator. The critical value of the dose limit was found to be 101 mg with a probability of 3.07%.

Pharmacokinetics of vitexin-4″-O-glucoside in rats after intravenous application

Vitexin-4″-O-glucoside (VOG), being a main component in the leaves of Crataegus pinnatifida Bge. var. major, was isolated and then three different doses (20, 40, and 60 mg/kg) of VOG were administered intravenously to rats. To study its pharmacokinetics, a simple and rapid HPLC method was developed using hesperidin as internal standard and the relative parameters were calculated by both compartmental and non-compartmental approach. The results showed that VOG fitted a two-compartment open model. The values of AUC increased proportionally within the range of 20-60 mg/kg. Additionally, α half-life, β half-life, (a)CL, MRT(0→t ), MRT(0→∞ ), and terminal half-life of VOG in rats showed significant differences between 20 mg/kg and other doses. Thereby, VOG presented a dose-dependent pharmacokinetics in the range of 20-60 mg/kg and non-linear pharmacokinetics at lower dose.

Glycyrrhetic acid-loaded microparticles: liver-specific delivery and therapeutic potential against carbon tetrachloride-induced hepatitis

The microparticles (MPs) of an anti-hepatotoxic drug, glycyrrhetic acid (GLA), were prepared using poly(dl-lactic acid-co-glycolic acid) as a drug carrier, and their in-vitro properties, biodistribution and therapeutic effects were investigated. The MPs showed a particle diameter distribution of 1.0–1.4 μm and a drug content of approximately 10% (w/w). In the in-vitro release in a mixture of methanol and phosphate-buffered saline pH 7.4 (3:7, v/v), slow release was observed after an initial burst release of approximately 30% (w/w). After i.v. administration of MPs in normal mice, GLA was mainly distributed to the liver. After i.v. administration in normal mice, the MPs maintained a much higher liver concentration than did GLA solution, and the plasma concentration also tended to be higher for MPs than for GLA solution. As to therapeutic effect, the liver was damaged by repeated injection of carbon tetrachloride (CCl4) in mice every 48 h, and the drugs were administered intravenously as a single dose 3 h after the first injection of CCl4. At 10 mg GLA eq. kg−1, the MPs significantly suppressed the plasma level of glutamic pyruvic transaminase for at least 141 h after administration, while GLA solution did not become significantly effective within 45 h post-administration. MPs are suggested as a possible useful system to prolong the therapeutic effect of GLA.

Glucuronide-sulfate diconjugate as a novel metabolite of glycyrrhetic acid in rat bile

To study the metabolites of glycyrrhetic acid (GA) in rat bile, an analytical method was developed to identify GA and its metabolites by liquid chromatography mass spectrometry (LC-MS). Rat bile was collected after i.v. injection of GA. Three major GA-related peaks were detectable in rat bile by high-performance liquid chromatography (HPLC) analysis at 254 nm. LC-MS spectra showed their protonated molecular ions at m/z 727, 647, and 551. Furthermore, the three metabolites were also confirmed to exist in rat bile on LC-MS total ion chromatogram (TIC). Taken together with the susceptible nature to beta-glucuronidase digestion and alkaline conditions, they were identified to be a novel sulfate-glucuronide diconjugate and the known monoglucuronide and sulfate conjugate, respectively.

Inhibition of P-glycoprotein-mediated transport by terpenoids contained in herbal medicines and natural products

Terpenoids form a large and structurally diverse family of natural products and are ingredients of various herbal medicines. We have investigated possible interactions between herbal medicines and conventional medicines, and recently reported that monoterpenoids contained in Zanthoxyli Fructus can be potent inhibitors of P-glycoprotein (P-gp). In the present study, the influence of 70 kinds of terpenoids present in natural products on P-gp-mediated efflux transport was investigated. LLC-GA5-COL150 cells transfected with human MDR1 cDNA encoding P-gp were used to screen the terpenoids. Large increases in the intracellular accumulation of [(3)H]digoxin were observed in the presence of (R)-(+)-citronellal, (S)-(-)-beta-citronellol, alpha-terpinene, terpinolene, (-)-beta-pinene, abietic acid, ophiobolin A, cucurbitacin I, and glycyrrhetic acid. A study of the concentration-dependency revealed that the IC(50) of ophiobolin A, glycyrrhetic acid, (R)-(+)-citronellal, abietic acid, and cucurbitacin I was smaller than that of verapamil. The transcellular transport of [(3)H]digoxin across Caco-2 cell monolayers was then examined in the presence of (R)-(+)-citronellal, abietic acid, and glycyrrhetic acid. Significant increases in the apical-to-basolateral transport and decreases in the basolateral-to-apical transport and efflux ratio were demonstrated. These findings suggest that some natural products containing these terpenoids may inhibit P-gp-mediated transport and interact with P-gp substrates in the intestinal absorption process.

Licorice and cancer

Licorice root is one of the oldest and most frequently employed botanicals in Chinese medicine. In the United States, licorice products are most often used as flavoring and sweetening agents in food products. Constituents of licorice include triterpenoids, such as glycyrrhizin and its aglycone glycyrrhizic acid, various polyphenols, and polysaccharides. A number of pharmaceutical effects of licorice are known or suspected (anti-inflammatory, antivirus, antiulcer, anticarcinogenesis, and others). Licorice and its derivatives may protect against carcinogen-induced DNA damage and may be suppressive agents as well. Glycyrrhizic acid is an inhibitor of lipoxygenase and cyclooxygenase, inhibits protein kinase C, and downregulates the epidermal growth factor receptor. Licorice polyphenols induce apoptosis in cancer cells. These and other activities of licorice are reviewed, and a rationale is suggested for combinations of agents in preventive clinical trials.

Prediction of pharmacokinetics prior to in vivo studies. 1. Mechanism-based prediction of volume of distribution

In drug discovery and nonclinical development the volume of distribution at steady state (V(ss)) of each novel drug candidate is commonly determined under in vivo conditions. Therefore, it is of interest to predict V(ss) without conducting in vivo studies. The traditional description of V(ss) corresponds to the sum of the products of each tissue:plasma partition coefficient (P(t:p)) and the respective tissue volume in addition to the plasma volume. Because data on volumes of tissues and plasma are available in the literature for mammals, the other input parameters needed to estimate V(ss) are the P(t:p)'s, which can potentially be predicted with established tissue composition-based equations. In vitro data on drug lipophilicity and plasma protein binding are the input parameters used in these equations. Such a mechanism-based approach would be particularly useful to provide first-cut estimates of V(ss) prior to any in vivo studies and to explore potential unexpected deviations between sets of predicted and in vivo V(ss) data, when the in vivo data become available during the drug development process. The objective of the present study was to use tissue composition-based equations to predict rat and human V(ss) prior to in vivo studies for 123 structurally unrelated compounds (acids, bases, and neutrals). The predicted data were compared with in vivo data obtained from the literature or at Roche. Overall, the average ratio of predicted-to-experimental rat and human V(ss) values was 1.06 (SD = 0.817, r = 0.78, n = 147). In fact, 80% of all predicted values were within a factor of two of the corresponding experimental values. The drugs can therefore be separated into two groups. The first group contains 98 drugs for which the predicted V(ss) were within a factor of two of those experimentally determined (average ratio of 1.01, SD = 0.39, r = 0.93, n = 118), and the second group includes 25 other drugs for which the predicted and experimental V(ss) differ by a factor larger than two (average ratio of 1.32, SD = 1.74, r = 0.42, n = 29). Thus, additional relevant distribution processes were neglected in predicting V(ss) of drugs of the second group. This was true especially in the case of some cationic-amphiphilic bases. The present study is the first attempt to develop and validate a mechanistic distribution model for predicting rat and human V(ss) of drugs prior to in vivo studies.

The pharmacokinetics of glycyrrhizic acid evaluated by physiologically based pharmacokinetic modeling

Glycyrrhizic acid is widely applied as a sweetener in food products and chewing tobacco. In addition, it is of clinical interest for possible treatment of chronic hepatitis C. In some highly exposed subjects, side effects such as hypertension and symptoms associated with electrolyte disturbances have been reported. To analyze the relationship between the pharmacokinetics of glycyrrhizic acid in its toxicity, the kinetics of glycyrrhizic acid and its biologically active metabolite glycyrrhetic acid were evaluated. Glycyrrhizic acid is mainly absorbed after presystemic hydrolysis as glycyrrhetic acid. Because glycyrrhetic acid is a 200-1000 times more potent inhibitor of 11-beta-hydroxysteroid dehydrogenase compared to glycyrrhizic acid, the kinetics of glycyrrhetic acid are relevant in a toxicological perspective. Once absorbed, glycyrrhetic acid is transported, mainly taken up into the liver by capacity-limited carriers, where it is metabolized into glucuronide and sulfate conjugates. These conjugates are transported efficiently into the bile. After outflow of the bile into the duodenum, the conjugates are hydrolyzed to glycyrrhetic acid by commensal bacteria; glycyrrhetic acid is subsequently reabsorbed, causing a pronounced delay in the terminal plasma clearance. Physiologically based pharmacokinetic modeling indicated that, in humans, the transit rate of gastrointestinal contents through the small and large intestines predominantly determines to what extent glycyrrhetic acid conjugates will be reabsorbed. This parameter, which can be estimated noninvasively, may serve as a useful risk estimator for glycyrrhizic-acid-induced adverse effects, because in subjects with prolonged gastrointestinal transit times, glycyrrhetic acid might accumulate after repeated intake.

Physiologically based pharmacokinetic modeling of glycyrrhizic acid, a compound subject to presystemic metabolism and enterohepatic cycling

Glycyrrhizic acid is currently of clinical interest for treatment of chronic hepatitis. It is also applied as a sweetener in food products and chewing tobacco. In some highly exposed subgroups of the population, serious side effects such as hypertension and electrolyte disturbances have been reported. In order to analyze the health risks of exposure to this compound, the kinetics of glycyrrhizic acid and its active metabolites were evaluated quantitatively. Glycyrrhizic acid and its metabolites are subject to complex kinetic processes, including enterohepatic cycling and presystemic metabolism. In humans, detailed information on these processes is often difficult to obtain. Therefore, a model was developed that describes the systemic and gastrointestinal tract kinetics of glycyrrhizic acid and its active metabolite glycyrrhetic acid in rats. Due to the physiologically based structure of the model, data from earlier in vitro and in vivo studies on absorption, enterohepatic cycling, and presystemic metabolism could be incorporated directly. The model demonstrates that glycyrrhizic acid and metabolites are transported efficiently from plasma to the bile, possibly by the hepatic transfer protein 3-alpha-hydroxysteroid dehydrogenase. Bacterial hydrolysis of the biliary excreted metabolites following reuptake of glycyrrhetic acid causes the observed delay in the terminal plasma clearance of glycyrrhetic acid. These mechanistic findings, derived from analysis of experimental data through physiologically based pharmacokinetic modeling, can eventually be used for a quantitative health risk assessment of human exposure to glycyrrhizic acid containing products.

Analysis in the rat of 4-hydroxynonenal metabolites excreted in bile: evidence of enterohepatic circulation of these byproducts of lipid peroxidation

4-Hydroxynonenal (HNE) is a cytotoxic product resulting from the lipid peroxidation of membrane polyunsaturated fatty acids. In vitro, metabolism mainly leads to the corresponding alcohol (DHN), carboxylic acid (HNA), and the glutathione conjugate, whereas in vivo, mercapturic acid conjugates of HNE, DHN, HNA, and HNA-lactone and, more recently, dicarboxylic acids and related mercapturate conjugates were identified in urine of rats. In the study presented here, the identity of the HNE biotransformation products in the bile of rats following a single iv administration of [4-(3)H]HNE and the potential for enterohepatic recycling of HNE metabolites were investigated. The identity of metabolites was assessed by comparison of their HPLC retention times with those of the corresponding synthesized standards and by mass spectrometry analysis. Five metabolites were present in the bile; two of them corresponded to HNE- and DHN-glutathione conjugates. Two others metabolites were identified as DHN- and HNA-lactone mercapturic acid conjugates. The fifth metabolite was isolated but remained unidentified. As previously observed for urinary elimination, the kinetic excretion of biliary metabolites exhibited a rapid metabolism of HNE in rats. Within 4 h of injection, the bile accounted for 19.5% (+/-2.8%) of the injected radioactivity, whereas only 3% was found in the feces within 48 h [Alary, J., et al. (1995) Chem. Res. Toxicol. 8, 34-39]. The extent of HNE enterohepatic recycling was estimated utilizing a modified version of the linked rat model in three animals. All rat recipients were found to have measurable levels of HNE metabolites in bile, confirming that HNE is likely to undergo enterohepatic recirculation in the rat. The extent of recycling was approximatly 7. 7% of the total dose in this model. Two unknown metabolites were present in the bile of recipient rats and not found in the bile of donors rats, suggesting that intestinal microflora and/or intestinal mucosa could biotransform HNE-related compounds before or during the reabsorption process.

First-pass metabolism and biliary recirculation of droloxifene in the female Sprague-Dawley rat

1. Utilizing a validated ultrasensitive hplc assay (lower limit of quantitation 25 pg/ml), we characterized the disposition profile of droloxifene in the female Sprague-Dawley rat following intravenous, oral and intraportal administration. 2. The site and extent of first-pass metabolism and the extent of enterohepatic recirculation were investigated. 3. Our findings suggest that the intestine is neither a metabolic nor an absorptive barrier to the bioavailability of droloxifene in the female Sprague-Dawley rat and that first-pass hepatic extraction is approximately 70-80% following an oral dose of 1 mg/kg. 4. Employment of a modified linked-rat model revealed that droloxifene is subject to enterohepatic recirculation (approximately 5%) in the rat.