Metabolic Activation of Rhein: Insights into the Potential Toxicity Induced by Rhein-Containing Herbs

The phytochemistry and pharmacology of three Rheum species: A comprehensive review with future perspectives

BACKGROUND

Rheum palmatum, R. tanguticum, and R. officinale, integral species of the genus Rheum, are widely used across global temperate and subtropical regions. These species are incorporated in functional foods, medicines, and cosmetics, recognized for their substantial bioactive components.

PURPOSE

This review aims to synthesize developments from 2014 to 2023 concerning the botanical characteristics, ethnopharmacology, nutritional values, chemical compositions, pharmacological activities, mechanisms of action, and toxicity of these species.

METHODS

Data on the three Rheum species were gathered from a comprehensive review of peer-reviewed articles, patents, and clinical trials accessed through PubMed, Google Scholar, Web of Science, and CNKI.

RESULTS

The aerial parts are nutritionally rich, providing essential amino acids, fatty acids, and minerals, suitable for use as health foods or supplements. Studies have identified 143 chemical compounds, including anthraquinones, anthrones, flavonoids, and chromones, which contribute to their broad pharmacological properties such as laxative, anti-diarrheal, neuroprotective, hepatoprotective, cardiovascular, antidiabetic, antitumor, anti-inflammatory, antiviral, and antibacterial effects. Notably, the materials science approach has enhanced understanding of their medicinal capabilities through the evaluation of bioactive compounds in different therapeutic contexts.

CONCLUSION

As medicinal and economically significant herb species, Rheum species provide both edible aerial parts and medicinal underground components that offer substantial health benefits. These characteristics present new opportunities for developing nutritional ingredients and therapeutic products, bolstering the food and pharmaceutical industries.

Fe(III)/peroxymonosulfate oxidation system for the degradation of rhein, a toxic component abundance in rhubarb residue

Rhubarb is widely used in health care, but causing a great amount of rhein-containing herbal residue. Rhein with several toxicities might pollute environment, damage ecology and even hazard human health if left untreated. In this study, the degradation effects of bisulfite- (BS) and peroxymonosulfate- (PMS) based oxidation systems on rhein in rhubarb residue were compared and investigated. The effects of BS and PMS with two valence states of ferric ion (Fe) on the degradation of rhein in rhubarb residue were optimized for the selection of optimal oxidation system. The influences of reaction temperature, reaction time and initial pH on the removal of rhein under the optimal oxidation system were evaluated. The chemical profiles of rhubarb residue with and without oxidation process were compared by UPLC-QTOF-MS/MS, and the degradation effects were investigated by PLS-DA and S plot/OPLS-DA analysis. The results manifested that PMS showed relative higher efficiency than BS on the degradation of rhein. Moreover, Fe(III) promoted the degradation effect of PMS, demonstrated that Fe(III)/PMS is the optimal oxidation system to degrade rhein in rhubarb residue. Further studies indicated that the degradation of rhein by the Fe(III)/PMS oxidation system was accelerated with the prolong of reaction time and the elevation of reaction temperature, and also affected by the initial pH. More importantly, Fe(III)/PMS oxidation system could degrade rhein in rhubarb residue completely under the optimal conditions. In conclusion, Fe(III)/PMS oxidation system is a feasible method to treat rhein in rhubarb residue.

Antiviral, antioxidant, and anti-inflammatory activities of rhein against white spot syndrome virus infection in red swamp crayfish (Procambarus clarkii)

IMPORTANCE

Aquaculture is essential for ensuring global food security by providing a significant source of animal protein. However, the spread of the white spot syndrome virus (WSSV) has resulted in considerable economic losses in crustacean industries. In this study, we evaluated the antiviral activity of rhein, the primary bioactive component of Rheum palmatum L., against WSSV infection, and many pathological aspects of WSSV were also described for the first time. Our mechanistic studies indicated that rhein effectively arrested the replication of WSSV in crayfish by modulating innate immunity to inhibit viral gene transcription. Furthermore, we observed that rhein attenuated WSSV-induced oxidative and inflammatory stresses by regulating the expression of antioxidant and anti-inflammatory-related genes while enhancing innate immunity by reducing total protein levels and increasing phosphatase activity. Our findings suggest that rhein holds great promise as a potent antiviral agent for the prevention and treatment of WSSV in aquaculture.

Photo- and Sono-Active Food Colorants Inactivating Bacteria

Food colorants are commonly used as excipients in pharmaceutical and nutraceutical fields, but they have a wide range of other potential applications, for instance, as cytotoxic drugs or mediators of physical antimicrobial treatments. The photodynamic antibacterial activity of several edible food colorants is reported here, including E127, E129, E124, E122, E133, and E150a, alongside Rhein, a natural lipophilic antibacterial and anticancer compound found in medicinal plants. Minimal inhibitory concentration (MIC) values for S. aureus and E. coli showed that E127 and Rhein were effective against both bacteria, while other colorants exhibited low activity against E. coli. In some cases, dark pre-incubation of the colorants with Gram-positive S. aureus increased their photodynamic activity. Adding Rhein to E127 increased the photodynamic activity of the latter in a supportive mode. Optional sensing mechanism pathways of combined E127/Rhein action were suggested. The antibacterial activity of the studied colorants can be ranged as follows: E127/Rhein >> E127 >> E150a > E122 > E124 >> E129 ≈ E133. E127 was also found to exhibit photodynamic properties. Short ultrasonic treatment before illumination caused intensification of E127 photodynamic activity against E. coli when applied alone and especially in combination with Rhein. Food colorants exhibiting photo- and sonodynamic properties may have good potential in food preservation.

Potential Medicinal Value of Rhein for Diabetic Kidney Disease

Diabetic kidney disease (DKD) is the primary cause of mortality among diabetic patients. With the increasing prevalence of diabetes, it has become a major concern around the world. The therapeutic effect of clinical use of drugs is far from expected, and therapy choices to slow the progression of DKD remain restricted. Therefore, research on new drugs and treatments for DKD has been a hot topic in the medical field. It has been found that rhein has the potential to target the pathogenesis of DKD and has a wide range of pharmacological effects on DKD, such as anti-nephritis, decreasing blood glucose, controlling blood lipids and renal protection. In recent years, the medical value of rhein in the treatment of diabetes, DKD and renal disease has gradually attracted worldwide attention, especially its potential in the treatment of DKD. Currently, DKD can only be treated with medications from a single symptom and are accompanied by adverse effects, while rhein improves DKD with a multi-pathway and multi-target approach. Therefore, this paper reviews the therapeutic effects of rhein on DKD, and proposes solutions to the limitations of rhein itself, in order to provide valuable references for the clinical application of rhein in DKD and the development of new drugs.

Therapeutic Potential of Diacerein in Management of Pain

Diacerein (DCN), an analogue of rhein (a glycosidal compound of natural origin), is currently used in the treatment of osteoarthritis and is given a fast-track designation for development to treat epidermolysis bullosa (EB). It is a nonsteroidal anti-inflammatory drug having disease-modifying properties in osteoarthritis and anti-inflammatory effects for the treatment of EB. Diacerein has a beneficial effect on pain relief and demonstrated antioxidant and anti-apoptotic effects, which are useful in renal disease, diabetes, and other disorders. This review discusses the possible mechanism of diacerein in the management of pain. The potential role of rhein and diacerein in the treatment of neuropathic, inflammatory and nociceptive pain is also reviewed. The effect of diacerein and rhein on mediators of pain, such as transient receptor potential cation channel subfamily V (TRPV1), Substance P, glutamate, inflammatory cytokines, nitric oxide, matrix metalloproteinases, histamine, palmitoylethanolamide, nuclear factor-kappa B (NFkB), and prostaglandin, has also been discussed. The data highlights the role of diacerein in neuropathic, nociceptive and inflammatory pain. Clinical trials and mechanism of action studies are needed to ascertain the role of diacerein, rhein or their analogues in the management of pain, alone or in combination with other approved therapies.

Metabolic Activation of the Toxic Natural Products From Herbal and Dietary Supplements Leading to Toxicities

Currently, herbal and dietary supplements have been widely applied to prevent and treat various diseases. However, the potential toxicities and adverse reactions of herbal and dietary supplements have been increasingly reported, and have gradually attracted widespread attention from clinical pharmacists and physicians. Metabolic activation of specific natural products from herbal and dietary supplements is mediated by hepatic cytochrome P450 or intestinal bacteria, and generates chemical reactive/toxic metabolites that bind to cellular reduced glutathione or macromolecules, and form reactive metabolites-glutathione/protein/DNA adducts, and these protein/DNA adducts can result in toxicities. The present review focuses on the relation between metabolic activation and toxicities of natural products, and provides updated, comprehensive and critical comment on the toxic mechanisms of reactive metabolites. The key inductive role of metabolic activation in toxicity is highlighted, and frequently toxic functional groups of toxic natural products were summarized. The biotransformation of drug cytochrome P450 or intestinal bacteria involved in metabolic activation were clarified, the reactive metabolites-protein adducts were selected as biomarkers for predicting toxicity. And finally, further perspectives between metabolic activation and toxicities of natural products from herbal and dietary supplements are discussed, to provide a reference for the reasonable and safe usage of herbal and dietary supplements.

Anthraquinones as Potential Antibiofilm Agents Against Methicillin-Resistant Staphylococcus aureus

Biofilms formed by methicillin-resistant Staphylococcus aureus (MRSA) are one of the contributing factors to recurrent nosocomial infection in humans. There is currently no specific treatment targeting on biofilms in clinical trials approved by FDA, and antibiotics remain the primary therapeutic strategy. In this study, two anthraquinone compounds isolated from a rare actinobacterial strain Kitasatospora albolonga R62, 3,8-dihydroxy-l-methylanthraquinon-2-carboxylic acid (1) and 3,6,8-trihydroxy-1-methylanthraquinone-2-carboxylic acid (2), together with their 10 commercial analogs 3-12 were evaluated for antibacterial and antibiofilm activities against MRSA, which led to the discovery of two potential antibiofilm anthraquinone compounds anthraquinone-2-carboxlic acid (6) and rhein (12). The structure-activity relationship analysis of these anthraquinones indicated that the hydroxyl group at the C-2 position of the anthraquinone skeleton played an important role in inhibiting biofilm formation at high concentrations, while the carboxyl group at the same C-2 position had a great influence on the antibacterial activity and biofilm eradication activity. The results of crystal violet and methyl thiazolyl tetrazolium staining assays, as well as scanning electron microscope and confocal scanning laser microscopy imaging of compounds 6 and 12 treatment groups showed that both compounds could disrupt preformed MRSA biofilms possibly by killing or dispersing biofilm cells. RNA-Seq was subsequently used for the preliminary elucidation of the mechanism of biofilm eradication, and the results showed upregulation of phosphate transport-related genes in the overlapping differentially expressed genes of both compound treatment groups. Herein, we propose that anthraquinone compounds 6 and 12 could be considered promising candidates for the development of antibiofilm agents.

Calcium pectinate and hyaluronic acid modified lactoferrin nanoparticles loaded rhein with dual-targeting for ulcerative colitis treatment

Herein, dual-bioresponsive of Rhein (RH) in promoting colonic mucous damage repair and controlling inflammatory reactions were combined by the dual-targeting (intestinal epithelial cells and macrophages) oral nano delivery strategy for effective therapy of ulcerative colitis (UC). Briefly, two carbohydrates, calcium pectinate (CP) and hyaluronic acid (HA) were used to modify lactoferrin (LF) nanoparticles (NPs) to encapsulate RH (CP/HA/RH-NPs). CP layer make CP/HA/RH-NPs more stable and protect against the destructive effects of the gastrointestinal environment and then release HA/RH-NPs to colon lesion site. Cellular uptake evaluation confirmed that NPs could specifically target and enhance the uptake rate via LF and HA ligands. in vivo experiments revealed that CP/HA/RH-NPs significantly alleviated inflammation by inhibiting the TLR4/MyD88/NF-κB signaling pathway and accelerated colonic healing. Importantly, with the help of CP, this study was the first to attempt for LF as a targeting nanomaterial in UC treatment and offers a promising food-based nanodrug in anti-UC.

Role of anthraquinones in Cassia occidentalis induced hepato-myo-encephalopathy

ETHNOPHARMACOLOGICAL RELEVANCE

The different plant parts of Cassia occidentalis Linn, (CO) such as root, leaves, seeds and pods have traditionally been used in multifarious medicines for the treatment of dysentery, diarrhea, constipation, fever, eczema, cancer and venereal diseases.

MATERIALS AND METHODS

A systematic search of literature has been done in books and scientific databases like Science Direct, Pubmed, Google Scholar and Scopus etc. These sources were used to compile, analyze and review the information regarding the phytochemistry, toxicology and mechanism of toxicity of CO. The various references on this subject are cited in our review ranging from 1956 to 2019.

RESULTS

Unintentional exposure of CO causes serious pathological condition in children, known as hepato-myo-encephalopathy (HME). The toxicity after CO consumption is associated with the presence of anthraquinones (AQs), a class of secondary plant metabolites. These AQs at high concentrations are known to cause detrimental effects on essential vital organs such as liver, kidney, spleen, brain, muscle and reproductive organs. The animal studies in rodent models as well as clinical investigations have clearly revealed that CO toxicity is associated with enhanced hepatotoxicity serum markers (ALT, AST, and LDH) and presence of necrotic lesions in liver. Furthermore, CO also causes vacuolization in muscle tissue and increases the level of CPK which is a prominent muscle damage marker. Apart from these target organs, CO consumption also causes neuronal damage via disturbing the levels of different proteins such as (GFAP and b-tubulin III). The mechanistic studies show that AQs present in CO have the potential to disturb the cellular homeostasis via binding to DNA, increasing the production ROS and showing inhibitory effects on essential enzymes etc. Therefore, AQs have been observed to be the primary culprit agents contributing to the toxicity of CO in children and animals.

CONCLUSION

Despite its therapeutic potential, CO consumption can be detrimental if consumed in high amounts. A thorough analysis of literature reveals that AQs are the primary factors contributing to toxicity of CO seeds. Exposure to CO seeds causes HME, which is a serious life threatening condition for the malnourished children from lower strata. Multiple mechanisms are involved in the CO induced HME in patients. Lack of appropriate diagnostic measures and a poor understanding of the CO toxicity mechanism in humans and animals complicate the clinical management of CO poisoning subjects. Therefore, development of point of care diagnostic kits shall help in early diagnosis & suitable management of CO poisoning.

The Metabolism and Pharmacokinetics of Rhein and Aurantio-Obtusin

BACKGROUND

Anthraquinones, rhein and aurantio-obtusin were isolated from the herb Duhaldea nervosa for the first time by our group, which were also found in plants that belong to the plant family Compositae. Anthraquinone compounds have a range of pharmacological activities such as anti-inflammatory, anti-cancer, antioxidation, anti-diabetes, etc. and can be used as a laxative, for liver protection, treatment of chronic renal failure, etc. However, in recent years, anthraquinones have been reported to be cytotoxic to the liver and kidneys. Therefore, it is very important to study the pharmacokinetics and metabolism of rhein and aurantio-obtusin, which are common ingredients in many traditional Chinese medicines (TCM). According to our research, the pharmacokinetics and metabolism of rhein and aurantio-obtusin are comprehensively summarized in the paper for the first time.

OBJECTIVE

The study provides comprehensive information on pharmacokinetics and metabolism of rhein and aurantio- obtusin in different Species; meanwhile, the aim of this review is also to provide a reference for a reasonable application of TCM enriched with these two ingredients.

METHODS

The metabolism and pharmacokinetics of rhein and aurantio-obtusin were searched by the Web of Science, PubMed, Google scholar and some Chinese literature databases.

RESULTS

Rhein and aurantio-obtusin exist mainly in the form of metabolites in the body. Rhein and aurantio-obtusin and its metabolites might be responsible for pharmacological effects in the body. Therefore, the significance of studying the in vivo metabolites of rhein and aurantio-obtusin is not only essential to clarify their pharmacological mechanism, but also to find new active compound ingredients. The metabolism of rhein is different in different species, so the toxicity effects of rhein may also be different after oral administration in different species; however, the metabolic profiles of aurantio-obtusin in the liver microsomes of different species are similar.

CONCLUSION

This paper not only provides detail regarding the pharmacokinetics of rhein and aurantio-obtusin, but it is anticipated that it will also facilitate further study on the metabolism of rhein and aurantio-obtusin.

Role of gut microbiota in identification of novel TCM-derived active metabolites

Traditional Chinese Medicine (TCM) has been extensively used to ameliorate diseases in Asia for over thousands of years. However, owing to a lack of formal scientific validation, the absence of information regarding the mechanisms underlying TCMs restricts their application. After oral administration, TCM herbal ingredients frequently are not directly absorbed by the host, but rather enter the intestine to be transformed by gut microbiota. The gut microbiota is a microbial community living in animal intestines, and functions to maintain host homeostasis and health. Increasing evidences indicate that TCM herbs closely affect gut microbiota composition, which is associated with the conversion of herbal components into active metabolites. These may significantly affect the therapeutic activity of TCMs. Microbiota analyses, in conjunction with modern multiomics platforms, can together identify novel functional metabolites and form the basis of future TCM research.

Development of an Enzyme-Linked Immunosorbent Assay Method for the Detection of Rhein in Rheum officinale

Rhein is an important quality-control marker of Rheum officinale. The aim of this study was to develop an indirect competitive enzyme-linked immunosorbent assay (icELISA) for rhein detection, which acts as a powerful tool for quality control and proper usage of Rheum officinale. First, a specific and sensitive monoclonal antibody (mAb) against rhein was produced from a stable hybridoma cell line, 1F8, generated by the fusion of mouse myeloma sp2/0 with spleen cells obtained from a Bal b/c mouse immunized with rhein-BSA. Then, an icELISA method was developed with an IC₅₀ value and working range of 0.05 μg L⁻¹ and 0.02–0.11 μg L⁻¹, respectively. The icELISA revealed high assay specificity, since it only had a relatively high cross reactivity with aloe-emodin (27%) and almost no cross reactivity with any other anthraquinones (<1%). When spiked with 0.2–2 mg kg⁻¹ of rhein, the recoveries ranged from 84.19% to 102.90%. Finally, icELISA was used to detect rhein contents of Rheum officinale collected from different regions, and the results corresponded well with those of HPLC. Overall, the developed icELISA with high specificity and sensitivity provided a rapid and simple method for rhein detection, and it may be a powerful tool for quality control and proper usage of Rheum officinale.

Elevated system exposures of baicalin after combinatory oral administration of rhein and baicalin: Mainly related to breast cancer resistance protein (ABCG2), not UDP-glucuronosyltransferases

ETHNOPHARMACOLOGICAL RELEVANCE

A traditional Chinese medicine (TCM) prescription follows the principle of compatibility (peiwu) to achieve the fundamental purpose: to increase efficacy and reduce toxicity. Rhei rhizoma, commonly known as Chinese rhubarb, is the most frequently used herb with Radix Scutellariaee. This classic fixed compatibility is considered for heat-clearing, qi regulation and detoxifying to gain better efficacy and reduce cytotoxicity with respect to unilateral medicine. With this in mind, we propose it is highly promising to find ingredients in rhubarb to increase the bioavailability of baicalin.

AIM OF STUDY

In the present study, effect of rhien on pharmacokinetic profile of baicalin in rat plasma was investigated, and the underlying mechanisms were partly dissected through intestinal absorption, metabolism and biliary excretion with in vivo, in vitro and in situ assays.

MATERIALS AND METHODS

Pharmacokinetic analysis in rats was first performed to provide a general overview of the in vivo exposure of baicalin and rhein after co-administration, while the biliary excretion study provided insight to the effect of rhein on the transport of baicalin from hepatocytes to bile. In vitro incubation and inhibition studies in human/rat liver microsome and human/rat intestinal S9 fraction were conducted to elucidate the role of uridine diphosphate-glucuronosyltransferases (UGTs) on the hepatic and intestinal metabolism of baicalein (the aglycone of baicalin), and to determine whether rhein can affect the UGT-mediated glucuronidation of baicalein. In situ intestinal perfusion study was designed to investigate the effect of rhein on intestinal absorption of baicalin, and breast cancer resistance protein (BCRP) inhibitor was co-perfused as positive control to demonstrate the role of the efflux transporter, while BCRP-MDCK II cell(Madin-Daby canine kidney cell) model was used as an in vitro approach to further confirm the conclusion.

RESULTS

The AUC and C_max of baicalin were increased to 189.93% and 305.73%, respectively, and the clearance of baicalin was significantly decreased from 4.17 ± 2.40 to 1.65 ± 0.79 L/h/kg following oral co-administration of rhein. The AUC of baicalin was markedly increased and the biliary clearance was significantly decreased when baicalin and rhein were co-administered intravenously. The effect of rhein on the glucuronidation of baicalein in various subcellular fractions was examined, and it was found that rhein did not affect the UGT-mediated glucuronidation of baicalein. Results of in situ intestinal perfusion revealed that co-perfusion with Ko143 (a potent BCRP inhibitor) or rhein significantly reduced the cumulative excretion amount of baicalin, from 9.27 ± 2.79 to 2.80 ± 0.97 or 4.84 ± 0.60 nM, respectively. Additionally, the efflux ratio P_app(BL-AP)/P_app(AP-BL) of baicalin in BCRP-MDCK II was decreased significantly in the presence of rhein or Ko143, which meant rhein could inhibit the BCRP-mediated efflux transport of baicalin.

CONCLUSIONS

These results indicated that rhein can increase the bioavailability of baicalin by inhibiting BCRP-mediated efflux transport of baicalin in enterocytes and hepatocytes rather than by affecting the activity of UGT enzyme.

Synthesis and Evaluation of Diindole-Based MRI Contrast Agent for In Vivo Visualization of Necrosis

PURPOSE

Noninvasive imaging of cell necrosis can provide an early evaluation of tumor response to treatments. Here, we aimed to design and synthesize a novel diindole-based magnetic resonance imaging (MRI) contrast agent (Gd-bis-DOTA-diindolylmethane, Gd-DIM) for assessment of tumor response to therapy at an early stage.

PROCEDURES

The oil-water partition coefficient (Log P) and relaxivity of Gd-DIM were determined in vitro. Then, its necrosis avidity was examined in necrotic cells in vitro and in rat models with microwave ablation-induced muscle necrosis (MAMN) and ischemia reperfusion-induced liver necrosis (IRLN) by MRI. Visualization of tumor necrosis induced by combretastatin A-4 disodium phosphate (CA4P) was evaluated in rats bearing W256 orthotopic liver tumor by MRI. Finally, DNA binding assay was performed to explore the possible necrosis-avidity mechanism of Gd-DIM.

RESULTS

The Log P value and T1 relaxivity of Gd-DIM is - 2.15 ± 0.01 and 6.61 mM^-1 s^-1, respectively. Gd-DIM showed predominant necrosis avidity in vitro and in vivo. Clear visualization of the tumor necrosis induced by CA4P was achieved at 60 min after administration of Gd-DIM. DNA binding study indicated that the necrosis-avidity mechanism of Gd-DIM may be due to its binding to exposed DNA in necrotic cells.

CONCLUSION

Gd-DIM may serve as a promising necrosis-avid MRI contrast agent for early assessment of tumor response to therapy.

Rhein Induces Cell Death in HepaRG Cells through Cell Cycle Arrest and Apoptotic Pathway

Rhein, a naturally occurring active anthraquinone found abundantly in various medicinal and nutritional herbs, possesses a wide spectrum of pharmacological effects. Furthermore, previous studies have reported that rhein could induce hepatotoxicity in rats. However, its cytotoxicity and potential molecular mechanisms remain unclear. Therefore, the present study aimed to investigate the cytotoxicity of rhein on HepaRG cells and the underlying mechanisms of its cytotoxicity. Our results demonstrate, by 3-(4,5-dimethyl thiazol-2-yl-)-2,5-diphenyl tetrazolium bromide (MTT) and Annexin V-fluoresce isothiocyanate (FITC)/propidium iodide (PI) double-staining assays, that rhein significantly inhibited cell viability and induced apoptosis in HepaRG cells. Moreover, rhein treatment resulted in the generation of reactive oxygen species (ROS), loss of mitochondrial membrane potential (MMP), and S phase cell cycle arrest. The results of Western blotting showed that rhein treatment resulted in a significant increase in the protein levels of Fas, p53, p21, Bax, cleaved caspases-3, -8, -9, and poly(ADP-ribose)polymerase (PARP). The protein expression of Bcl-2, cyclin A, and cyclin-dependent kinase 2 (CDK 2) was decreased. In conclusion, these results suggest that rhein treatment could inhibit cell viability of HepaRG cells and induce cell death through cell cycle arrest in the S phase and activation of Fas- and mitochondrial-mediated pathways of apoptosis. These findings emphasize the need to assess the risk of exposure for humans to rhein.

In vitro and in vivo metabolic activation of rhein and characterization of glutathione conjugates derived from rhein

Rhein (RH), 4,5-dihydroxyanthrauinone-2-carboxylic acid, is found in rhubarb (Dahuang), a traditional herbal medicine. RH has reportedly demonstrated multiple pharmacologic properties. Previous studies have also shown that RH induced hepatotoxicity, but the mechanisms of the adverse effect remain unknown. The major objective of the present study was to study the metabolic pathways of RH in order to identify potential reactive metabolites. One mono-hydroxylation metabolite (M1) was detected in urine and bile of rats given RH. M1 was also observed in rat and human liver microsomal incubations after exposure to RH. A total of three (GSH) conjugates (M2, M3 and M5) were detected in bile of rats treated with RH. We concluded that M2-M3 were directly derived from parent compound RH through spontaneous reaction with GSH. M5 was derived from M1 by reaction with GSH, which required cytoslic GSTs. M5 was further metabolized to the corresponding NAC conjugate (mercapturic acid) and was excreted in urine. P450 2C9 was mainly involved in the oxidation of RH.