Current Drug Metabolism - Volume 13, Issue 5, 2012

Botanical products are gaining growing popularity and increasing use worldwide, but they are very diverse with regard to product nature. Some products are identical to medicines, while others come close to food. When a botanical product at a defined dosage can be proven to have a clear medicinal effect for a defined medicinal purpose and presented with a medicinal claim, the product should be regarded and registered as an herbal medicine [1]. Such medicinal botanical products are expected to meet comparable standards for synthetic drugs with respect to efficacy, safety, and pharmaceutical quality. For the herbal medicine, an important scientific question from pharmaceutical scientists deals with the absorption, disposition (including distribution, metabolism, and excretion), and pharmacokinetics (ADME/PK) of their bioactive constituents after dosing, which is critically important in identifying the medicinal principles responsible for the medicine’s therapeutic effects and in establishing the risk-benefit relationship [2-4]. The ADME/PK information is also crucial to establishment of a therapeutic regimen providing optimal efficacy with minimal toxicity for an herbal medicine. In this issue, we hope to illustrate the current state of ADME/PK science relating to herbal medicines and the associated phytochemicals. Unlike synthetic drugs, herbal medicines are complex chemical mixtures. The effect of an herbal therapy is not necessarily the result of a single mechanism induced by a single ingredient, but a range of activities of multiple compounds working together to produce a medicinal benefit. Accordingly, the ADME/PK study of an herbal medicine should involve elucidating the systemic exposure to the multiple herbal compounds from the administered medicine and understanding their fates in the body. The aim of study is to bridge the gap between the complex chemical composition of the herbal medicine and its medicinal effects. Recently, a methodology has been established to implement multi-compound ADME/PK studies of herbal medicines [5,6], which is referred to as “ADME/PK-bridged pharmacology-to-chemistry” methodology. The study is performed to identify, from a certain group of pharmacologically active constituents present in an herbal medicine, the compounds that have favorable PK properties and substantial, medicine dose-dependent systemic exposure levels after administration of the medicine. In this issue, Li et al. describes another methodology for performing multi-compound ADME/PK studies of herbal medicines [7], which is designated as “ADME/PK-bridged chemistry-to-pharmacology” methodology. These authors identified, from a broad range of chemical constituents present in a standardized extract of Ginkgo biloba leaves (GBE50 extract; including 72 terpene lactones, flavonoids, and carboxylic acids), the ginkgo compounds that could traverse the enterohepatic barrier after p.o. administration of the extract to rats, as well as the chemical forms in which the ginkgo compounds entered considerably the systemic circulation. In addition, they also revealed the relevant mechanisms governing the intestinal absorption and presystemic elimination for the tested ginkgo compounds. With such research information, pharmacological and toxicological scientists can be aware which herbal compounds are worth their testing. These multicompound studies provide guidance for further systematic investigation into chemical basis responsible for the in vivo biological effects produced by herbal medicines....

The nature and level of systemic exposure to the active herbal constituents will profoundly affect their effects at action sites, which is fundamental in understanding their roles in the overall beneficial effects of an herbal medicine. The objective of this study is to gain a full picture of the systemic exposure to various putatively active ginkgo constituents after p.o. administration of GBE50 extract, a standardized extract of Ginkgo biloba leaves, to rats and understanding of the relevant mechanisms governing the intestinal absorption and presystemic elimination. To define the ginkgo compounds to be studied, literature informatics-guided chemical profiling revealed that GBE50 extract contained 72 ginkgo constituents, including terpene lactones, flavonols, flavones, an isoflavone, biflavones, flavanols, and carboxylic acids, at levels ranging from 0.01 to 55.3 mg/g. Among the ginkgo constituent groups were the terpene lactones and the flavonols that were significantly measurable in plasma after p.o. administration of GBE50 extract to rats. The intestinal absorption of terpene lactones appeared to be dictated by their intermediate membrane permeability, while the influences of MDR-1- and MRP-2- mediated intestinal efflux and the presystemic metabolism and biliary excretion might be relatively limited. Because of their deglycosylation absent in the small intestine and relatively slow presystemic elimination, many intact flavonol glycosides appeared in the rat plasma albeit with a limited extent of absorption. Colonic deglycosylation of the flavonol glycosides occurred and the glucuronides of flavonol aglycones were also measured in the plasma. Although some biflavones also had relatively high abundance in GBE50 extract, these ginkgo constituents were not measured in the rat plasma because of their poor solubility and poor permeability that hindered the intestinal absorption. The levels of the remaining ginkgo constituents in GBE50 extract were too low to be measured in the rat plasma. The current study enabled us to better understand the nature of systemic exposure to ginkgo compounds after p.o. administration of GBE50 extract and to more precisely implement multicomponent PK study of the extract.

Qishen yiqi pills (QY pills) are a type of standardized cardiovascular herbal medicine, which contain four component herbs, i.e., Astragalus membranaceus (Huangqi), Salvia miltiorrhiza (Danshen), Panax notoginseng (Sanqi), and Dalbergia odorifera (Jiangxiang). After oral administration of QY pills, the in vivo exposure types of each component herb in rats were first uncovered and identified according to a target-directed strategy based on hyphenated chromatography techniques. The dominated metabolites in urine, blood and bile were originated from flavonoids of Huangqi and monomer phenolic acids of Danshen; no metabolites but parent drugs of Sanqi ginsenosides, namely ginsenosides Rb1, Rd, Re and Rg1, notoginsenoside R1 and gypenoside XVII, were detected in rat urine and blood, and the 20(S)-protopanaxatriol type ginsenosides (NR1, GRe, GRg1) could also be excreted to bile; the high liposolubility of volatile oils from Jiangxiang restricted them to small intestine, liver and adipose tissues. The identification of metabolites in bio-samples was achieved by exact mass measurement and detailed fragmentation pathway analyses. In specific conditions, not only the types of phase II metabolism but also their conjugation positions could be determined by our established cleavage pathways, which lead to discriminate the phase II metabolites of protocatechualdehyde for the first time. Based on the metabolite study in rats, the 4 main compounds (tanshinol, astragaloside IV, GRb1 and GRg1) in QY pills were selected as pharmacokinetic markers. The PK results showed that their maximal concentrations in blood were obtained within one hour, much shorter than the reported values in single herbs. The rat exposure was proximately linear under the studied dosages from 1 to 6 g/kg.

Pharmacokinetic (PK) study of medicinal herbs is a great challenge, because which component(s) is(are) the bioactive ingredients is largely unknown. Most of the reported PK studies of herbs focused on the major ingredients regardless of their in vivo bioactivities, while PK of components with low content in herbs is often ignored. The present study demonstrates how PK study can reveal potential importance of a low content ingredient to the herbal bioactivities using Z-butylidenephthalide (BuPh), a bioactive phthalide present in a significantly low quantity in medicinal herb Chuanxiong Rhizoma, as an example. PK of BuPh was investigated in rats using Chuanxiong extract, fraction containing BuPh and ligustilide, and pure BuPh, respectively. The results demonstrated that remarkable blood concentrations of BuPh were observed after administration of the herbal extract and its systemic exposure was significantly different between BuPh given in pure and mixed forms. More interestingly, AUC of BuPh via intake of fraction (9.3-fold) and extract (4.5-fold) was significantly greater than that obtained from pure BuPh, which was further evidenced to be mainly due to metabolic conversion from ligustilide, a major component in Chuanxiong. Our findings revealed that although it naturally occurred in low amount, BuPh reached significant systemic concentrations via metabolic conversion from ligustilide. Moreover, our results demonstrated that PK study is one of crucial and inevitable steps for revealing in vivo bioactive ingredients of herbal medicines, and such studies should be more appropriate to focus on in vivo profile of the ingredients co-existing in herbs rather than only studying them individually.

Metabolisms of herbal remedies and their natural components, which play a critical role in support of medicine development, and clinical medication, are drastically different from those of designed drugs. The separation, isolation and identification of drug metabolites from complex endogenous matrices like urine, plasma and tissue extracts are extremely challenging. For herbal medicine studies, it is even more difficult due to the complex chemical composition. Usually, a combination of high performance liquid chromatography (HPLC) and mass spectrometry (MS) is proven to be a powerful analytical tool for screening and identifying drug metabolites. For suitable instruments, the quadrupole time-of-flight (Q-TOF), hybrid ion trap time-of-flight (IT-TOF), and orbitrap mass spectrometry could clearly enhance the efficiency in metabolite profiling compared to general triquadrupole (QQQ) and ion trap (IT) mass spectrometry technique. Due to the ability for unambiguous structure determination, nuclear magnetic resonance spectroscopy (NMR) is also coupled to HPLC for on-line analysis of metabolites. Capillary electrophoresis and gas chromatography are also optional methods. These techniques could provide abundant information from a wide variety of samples. However, in many cases, preparations of metabolites are critical for further pharmacokinetics, pharmacologic, and toxic evaluation of the remedy. Therefore, accumulations of metabolites from the in vivo biological samples are essential. Biotransformation models are considered to be important complementary sources for preparation of drug metabolites. Fungi, plant cells, and a variety of enzymes were used to provide information for further in vivo testing. This review focuses on the screening and identification of drug metabolites of herbal medicines.

Recently, there is a global trend of using herbal medicines to treat various chronic diseases and promote health. But the controversy over the safety and efficacy of herbal medicines is a focus of attention, primarily because of the many unknown and unrevealed natures of herbal medicines, which strongly restricts their application and development. Pharmacokinetics is a bridge linking the herbal medicines and their pharmacological responses. It is assumed in traditional pharmacokinetics that an excellent drug should have appropriate pharmacokinetic behaviours and its pharmacological effect is related with plasma drug concentrations. However, most herbal medicines exhibit excellent pharmacological responses despite poor pharmacokinetic behaviours. As most drugs are intracellulartargeted, we put forward cellular pharmacokinetic-pharmacodynamic strategy, which is focused on the intracellular fate of drugs. This strategy could partially explain the marked pharmacological activities of herbal medicines from their intracellular pharmacokinetic behaviours, rather than their plasma concentrations. It is a helpful complementarity to traditional pharmacokinetics, and takes a potential role in the research and development of new herb-origined drugs. In this review, the pharmacokinetics-pharmacology disconnections of herbal medicines (such as ginseng, berberine and danshen) are retrospected. Then our proposed cellular pharmacokineticpharmacodynamic strategy, its characteristics, as well as its research procedures are described, followed by the subcellular distributions of drug transporters and metabolic enzymes which are the determinants of cellular pharmacokinetics-pharmacodynamics. Finally, our successful applications of cellular pharmacokinetic-pharmacodynamic strategy in elucidating ginsenoside Rh2 as an adjuvant agent and tanshinone IIA as an anticancer agent are illustrated.

Saponins are a group of amphiphilic glycosides containing one or more sugar chains linked to a nonpolar triterpene or steroid aglycone skeleton, which are believed to be responsible for the pharmacological activities of many Chinese medicinal herbs. The purpose of this paper is to summarize the contemporary knowledge of the absorption, disposition, and pharmacokinetics of some important saponins, including ginsenosides, licorice saponins, dioscorea saponins, astragalosides, and saikosaponins. Poor intestinal absorption of saponins is mainly due to their unfavorable physicochemical traits, such as large molecular mass (>500 Da), high hydrogen-bonding capacity (>12), and high molecular flexibility (>10), that underlie poor membrane permeability. Rapid and extensive biliary excretion is another primary factor that limits the oral bioavailability of most saponins. However, several saponins, including ginsenosides Ra3, Rb1, Rc, and Rd, and dioscin, are excreted slowly into the bile and in turn have significantly long elimination half lives (7-25 h in rats). These longcirculating saponins may be used as pharmacokinetic markers to substantiate systemic exposure to the ingested herb extracts. In addition to biliary excretion for elimination of most saponins unchanged, renal excretion may also be important for certain saponins. Saponins can be hydrolyzed by the colonic microflora. After absorption, the deglycosylated aglycones undergo phase I and/or II metabolism by the host. In line with the poor permeability, saponin concentrations in most rat tissues are lower than the concurrent plasma level and the brain level is usually very low. However, the liver concentrations of many saponins, as well as the kidney levels of certain saponins, can be quite high, which involves transporter-mediated uptake mechanisms. Repeated p.o. ingestion of glycyrrhizin appears to be able to induce CYP3A in rodents and humans, while several deglycosylated products of ginsenosides can moderately inhibit CYP activities in vitro with IC50 values of 10-50 μM. More research is required for elucidation of the absorption, disposition, and pharmacokinetics of multiple saponins to enhance understanding which saponins are most likely to exert pharmacological effects in vivo, as well as influence of complex herb matrix. In addition, research is also needed to characterize the microbiotal deglycosylation and the subsequent aglycone metabolism by the host for a broader range of saponins, as well as the hepatobiliary transporter phenotyping for and the interaction with saponins. Furthermore, in vitro and in vivo studies of saponin-based herb-drug interactions are also warranted.

Traditional Chinese medicine (TCM) formulas with fixed combinations rely on "sovereign, minister, assistant and guide" and fuzzy mathematical quantitative law, leading to greater challenges for the identification of active ingredients. Transformation and metabolic studies involving the Phase I drug-metabolizing enzyme cytochrome P450 (CYP) might potentially solve some of these challenges. The pharmacological effects can not be attributed to one active ingredient in TCMs, but integrated effects resulting from the combined actions of multiple ingredients. However, it is only after long-term administration that most ingredients exert their actions, which can result in prolonged exposure to herbs in vivo. Therefore, interactions between herbal compounds and CYPs appear to be inevitable. Yet unlike Western drugs, experimental determination of the absorption and disposition properties is not commonly carried out for TCMs. Moreover, the use of TCM as injections is an innovation aimed to improve efficiency in extensive clinical use in Mainland China. Therefore, in recent years, cases of adverse drug reactions (ADR) mainly concerning allergic reactions involving TCMs such as ShenMai injection and QingKaiLing injection have been reported, which have attracted attention with regard to the legal responsibilities for TCM approval. The lack of information on the ADME characteristics, especially the metabolic stability and interaction potential between CYPs and herbs, increases ADR occurrence due to TCMs. In this article, we review the most common herbs used in TCM prescriptions and fixed combinations of their usable frequency, and summarize the current understanding of the ability of phytochemical ingredients to act as substrates, inhibitors or inducers of human CYP enzymes, through which the key role of CYP enzymes on the herb disposition and toxicity is highlighted. The potential interaction between herbal phytochemicals and CYP enzymes dominates the target exposure, which further helps to elucidate the herbal pharmacological basis, assess the individual toxic risk of herbal remedies and gain mechanistic insight into herb–drug interactions (HDIs).

UDP-glucuronosyltransferases (UGTs) play important roles in the disposition of many drugs and xenobiotics. Herbal medicine, an important group of multicomponent therapeutics, is widely and increasingly used. Drug metabolism of herbal medicine mediated by cytochrome P450s has been extensively studied; however, herbal medicine metabolism mediated by UGTs has not been adequately investigated. Thus, it is necessary to evaluate current evidence on the glucuronidation of herbal medicines by UGTs. In this review, the research advances of the potential for commonly used herbal medicines as UGT substrate and modulator are summarized. In addition, the herb-drug interactions associated with UGTs are also discussed.

There is accumulating evidence that many compounds, known as phytochemicals (PCs), which are derived from dietary plants and herbs, may have a role in combating a number of chronic diseases. Despite many in vitro studies elucidating the mechanism(s) of action of various PCs, there are still reservations with regard to their health benefits in vivo, particularly as there is a paucity of research on their oral bioavailability, their pharmacokinetics, and the concentrations achieved at their site(s) of action. Recently various transporters, including the ATP-binding cassette (ABC) and the solute carrier (SLC) transporters, have been cloned and functional analyses have suggested that they play significant roles in the absorption and disposition of most drugs and PCs. While some SLC transporters facilitate absorption of PCs into the systemic circulation, various efflux pumps, including the ABC transporters, actively transport the PC back into the gastro-intestinal (GI) lumen, thus preventing further penetration into the body. Some ABC transporters also act in concert with Phase 1 and 2 metabolizing enzymes as a defensive barrier in the intestines and liver. If the PC overcomes the defence mechanisms of the gut and the liver, it will enter the systemic circulation and be distributed to the other organs of the body and possible site(s) of action. PCs can usually pass with ease through the pores of the capillaries of organs such as the heart and lungs, but with difficulty into pharmacological sanctuaries, such as the brain, testis, or foetus. Such sanctuaries contain a number of efflux transporters in their protective membrane, which restrict the penetration of xenobiotics, including PCs. The ABC and SLC transporters are also abundantly expressed in the liver and kidney and regulate the excretion of many compounds, including PCs and their metabolites. It is also becoming apparent that there is a complex interplay between various PCs and their ability to modulate the activity of these transporters involved in the processes of absorption, metabolism, distribution and excretion, which control the extent of xenobiotic exposure in the body. This review describes the importance of the ABC and SLC transporters in the pharmacokinetics of dietary and herbal PCs, and their interactions with other xenobiotics.

Herbal medicines are often used in combination with conventional drugs, and this may give rise to the potential of harmful herb-drug interactions. This paper updates our knowledge on clinical herb-drug interactions with an emphasis of the mechanistic and clinical consideration. In silico, in vitro, animal and human studies are often used to predict and/or identify drug interactions with herbal remedies. To date, a number of clinically important herb-drug interactions have been reported, but many of them are from case reports and limited clinical observations. Common herbal medicines that interact with drugs include St John’s wort (Hypericum perforatum), ginkgo (Ginkgo biloba), ginger (Zingiber officinale), ginseng (Panax ginseng), and garlic (Allium sativum). For example, St John's wort significantly reduced the area under the plasma concentration-time curve (AUC) and blood concentrations of cyclosporine, midazolam, tacrolimus, amitriptyline, digoxin, indinavir, warfarin, phenprocoumon and theophylline. The common drugs that interact with herbal medicines include warfarin, midazolam, digoxin, amitriptyline, indinavir, cyclosporine, tacrolimus and irinotecan. Herbal medicines may interact with drugs at the intestine, liver, kidneys, and targets of action. Importantly, many of these drugs have very narrow therapeutic indices. Most of them are substrates for cytochrome P450s (CYPs) and/or P-glycoprotein (P-gp). The underlying mechanisms for most reported herb-drug interactions are not fully understood, and pharmacokinetic and/or pharmacodynamic mechanisms are implicated in many of these interactions. In particular, enzyme induction and inhibition may play an important role in the occurrence of some herbdrug interactions. Because herb-drug interactions can significantly affect circulating levels of drug and, hence, alter the clinical outcome, the identification of herb-drug interactions has important implications.

The number of new drugs approved for clinical use per year is falling in the last decade. One approach to reduce the high rate of attrition during early drug development is to systematically determine the toxic metabolites on the mechanism basis. Traditional Chinese herbal medicines (CHM) have been used extensively for disease treatment and health care. Recently, they have also been used as raw materials for preparation of herbal dietary supplements and nutraceuticals worldwide. However, problems arise due to the adverse effects caused by CHM and their derived products. Similar to synthetic drugs, among the diverse mechanisms the metabolism-induced adverse effect/toxicity is an important safety issue of CHM. For safe use of CHM and herbal products, it is also necessary to study herbinduced toxicities using the mechanism-based approach. CHM consist of multi-ingredients, which makes the study of toxic metabolites more difficult and challenging than that of synthetic drug-induced toxicity. In this mini-review, using hepatotoxicity and nephrotoxicity induced respectively by metabolic activation of pyrrolizidine alkaloids and aristolochic acid present in CHM as examples, we address the significance of metabolic study of CHM and how it contributes to the delineation of the toxic mechanisms, development of mechanismbased biomarkers for the diagnosis and assessment of adverse effect/toxicity of CHM, prediction of toxic dosage, and reduction and prevention of toxicity of CHM.

Over the past few years, nanoscale Chinese medicine has become one of focuses in modern Chinese medicine research. There is an increasing need for a more systematic study on the basic issues involved in traditional Chinese medicine and a more active participation of researchers in the application area of nanoscale traditional Chinese drugs. In this review, author analyzed the current applications of nanotechnology in research and development of drugs from natural products and herbal medicines involving traditional Chinese medicines, and also discussed the bio-medicinal evaluation issues on ADME including bio-distribution and metabolism of nanodrugs. Author noted that great challenges faced in nanodrugs from herb drugs and natural products are the follows: (1) the first challenge is to prepare nanodrug delivery system and quantitatively evaluate the therapeutic effects and safety; (2) the second challenge is to clarify the concrete metabolism course; and (3) the third challenge is to study the pharmacokinetics of nanodrugs.

The drug metabolising enzyme CYP1A2 contributes to the metabolism of a number of medicines including clozapine, olanzapine and theophylline. These medicines display a high degree of inter-individual variability in pharmacokinetics and response. Measuring CYP1A2 activity in vivo can be an important tool to identify the factors that influence variability in drug pharmacokinetics and inform dose selection. Caffeine is the only currently accepted probe to conduct in vivo phenotyping of CYP1A2. Despite the number of proposed matrices (biological fluid containing the drug and/or metabolite/s of interest) and metrics (mathematical formula relating the drug and/or metabolite/s to enzyme activity) proposed to measure CYP1A2 activity using caffeine, many of these are compromised by factors related to the specific metabolic pathway studied or pharmacokinetic characteristics of caffeine and its metabolites. Furthermore, questions regarding the appropriate study design and methodology to conduct studies to evaluate CYP1A2 activity have often been overlooked. These issues include the potential influence of a methylxanthine abstinence period prior to caffeine CYP1A2 phenotyping and the impact of caffeine formulation on determining CYP1A2 activity. This review aims to discuss the various CYP1A2 matrices and metrics with a particular focus on unresolved methodological issues.

The genetically variable CYP450 isozymes are responsible for the metabolism of up to 80% of commonly used drugs, many of which are detected in cases of unexpected or suspicious death in Australia. The aim of this study was to examine the genetic profiles of individuals in a cohort of Australian deceased individuals dying of drug toxicity (219), natural disease (150), external injury (109) or unascertained (8) causes, to determine if there was an over-representation of individuals with a genetic predisposition to altered drug metabolism in cases attributed to drug toxicity compared with other causes. Single nucleotide polymorphisms (SNP) of CYP1A2, 2C9, 2C19, 2D6, 3A4 and 3A5 were analyzed. There were 27 cases (6.1%) that were CYP2D6 poor metabolizers (PM) and an additional 8 cases (1.7 %) that were CYP2C19 PMs. Around 31% of the cases were CYP2D6 intermediate-poor metabolizers, with a number of cases exhibiting drug combinations that were likely to have caused pharmacokinetic or pharmacodynamic interactions. There was no correlation between cause of death type and CYP2D6 metabolizer status. Increased enzyme activity was also indicated by the presence of hyperinducible variants such as CYP1A2*1F, which was observed at a frequency of 48%.