Disposition mechanisms of raloxifene in the human intestinal Caco-2 model

Novel Treatment Strategies for Hormone Receptor (HR)-Positive, HER2-Negative Metastatic Breast Cancer

Estrogen receptor (ER)-positive breast cancer (BC) is the most common BC subtype. Endocrine therapy (ET) targeting ER signaling still remains the mainstay treatment option for hormone receptor (HR)-positive BC either in the early or in advanced setting, including different strategies, such as the suppression of estrogen production or directly blocking the ER pathway through SERMs-selective estrogen receptor modulators-or SERDs-selective estrogen receptor degraders. Nevertheless, the development of de novo or acquired endocrine resistance still remains challenging for oncologists. The use of novel ET combined with targeted drugs, such as cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, has significantly improved long-term outcome rates, thus changing the therapeutic algorithm for metastatic BC (MBC) and recently the therapeutic strategy in the adjuvant setting for early high-risk BC. Eluding the resistance to CDK4/6 inhibitors combined with ET is currently an unmet medical need, and there is disagreement concerning the best course of action for patients who continue to progress after this combination approach. Genetic changes in the tumor along its growth uncovered by genomic profiling of recurrent and/or metastatic lesions through tumor and/or liquid biopsies may predict the response or resistance to specific agents, suggesting the best therapeutic strategy for each patient by targeting the altered ER-dependent pathway (novel oral SERDs and a new generation of anti-estrogen agents) or alternative ER-independent signaling pathways such as PI3K/AKT/mTOR or tyrosine kinase receptors (HER2 mutations or HER2 low status) or by inhibiting pathways weakened through germline BRCA1/2 mutations. These agents are being investigated as single molecules and in combination with other target therapies, offering promising weapons to overcome or avoid treatment failure and propose increasingly more personalized treatment approaches. This review presents novel insights into ET and other targeted therapies for managing metastatic HR+/HER2- BC by exploring potential strategies based on clinical evidence and genomic profiling following the failure of the CDK4/6i and ET combination.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Interaction between apigenin and sodium deoxycholate with raloxifene: A potential risk for clinical practice

Apigenin (API) and sodium deoxycholate (NaDC) have different pharmacodynamic properties and can affect pharmacokinetics of drugs without causing significant toxicity. The aim of our study was to investigate the effect of API and NaDC on raloxifene pharmacokinetics in rats as well as on hemostasis parameters after applying the raloxifene therapeutic dose. Rats were treated daily with oral single dose of saline solution (1 ml/kg), API (10 mg/kg) and/or NaDC (4 mg/kg) for 7 days. Raloxifene was given orally or intravenously in a single dose (6 mg/kg) and during period of 24 h blood samples, feces and urine samples were collected. Blood samples were collected at the 15th, 30th, 45th, 60th, 90th minute and 2, 3, 4, 6, 8, 10, 12 and 24 h after raloxifene administration. Urine and feces samples were collected in the 3th, 6^h, 12th and 24th hour of the experiment. Rats were divided into 10 groups each of which contained 6 animals. Differences were considered statistically significant if p<0.05. Pretreatment with NaDC and API affected raloxifene pharmacokinetic profile after intravenous application. NaDC lead to statistically significant decrease in raloxifene serum concentration and increased volume of distribution and clearance as well as halftime of elimination, while API has also decreased also raloxifene serum concentrations and increased volume of distribution but not as profoundly as NaDC alone. Difference was also noticed in clearance where it was significantly increased in group pretreated with NaDC and slightly decreased in group pretreated with API. NaDC and API increased raloxifene amount in feces, both after peroral (p<0.05) and intravenous application. However, peroral application of raloxifene did not produce measurable raloxifene serum concentration in neither of investigated groups. NaDC shortened activated partial thromboplastin time (aPTT) and prothrombin time (PT). API reduced aPTT, PT and d-dimer level. Fibrinogen level was significantly increased in all experimental groups. Both NaDC and apigenin had significant influence on raloxifene pharmacokinetics and can potentiate the raloxifene effects on hemostasis parameters, by increasing its bioavailability. These substances may be the subject of further investigation into the formulation of raloxifene and other medicines as depot preparations, which could prolong the dosing interval and thus improve patient compliance and quality of life.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

In-Depth Characterization of EpiIntestinal Microtissue as a Model for Intestinal Drug Absorption and Metabolism in Human

The Caco-2 model is a well-accepted in vitro model for the estimation of fraction absorbed in human intestine. Due to the lack of cytochrome P450 3A4 (CYP3A4) activities, Caco-2 model is not suitable for the investigation of intestinal first-pass metabolism. The purpose of this study is to evaluate a new human intestine model, EpiIntestinal microtissues, as a tool for the prediction of oral absorption and metabolism of drugs in human intestine. The activities of relevant drug transporters and drug metabolizing enzymes, including MDR1 P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), CYP3A4, CYP2J2, UDP-glucuronosyltransferases (UGT), carboxylesterases (CES), etc., were detected in functional assays with selective substrates and inhibitors. Compared to Caco-2, EpiIntestinal microtissues proved to be a more holistic model for the investigation of drug absorption and metabolism in human gastrointestinal tract.

Usefulness of the MRP2 promoter to overcome the chemoresistance of gastrointestinal and liver tumors by enhancing the expression of the drug transporter OATP1B1

Tumor response to chemotherapy is often limited by drug export through ABC proteins. To overcome this problem, here we have investigated the usefulness of inducing the expression of the multidrug uptake transporter OATP1B1 under the control of the MRP2 promoter (MRP2pr). Human hepatoma cells (Alexander) were transfected with MRP2pr fragments of different length fused to the firefly luciferase ORF in order to select the shortest fragment with the highest response to dexamethasone, which was subsequently used to generate the chimeric construct MRP2pr-OATP1B1-V5. Hepatoma cells transduced with MRP2pr-OATP1B1-V5 resulted in dexamethasone-sensitive inducible OATP1B1 expression and enhanced selective antitumor response to OATP1B1 substrates (paclitaxel, Bamet-R2 and Bamet-UD2). In human colon cancer cells LS174T/R, used as a model of endogenous chemoresistance due to MRP2 overexpression, MRP2pr-OATP1B1 induced OATP1B1 expression together with chemosensitivity to OATP1B1 substrates. In nude mice, xenografted tumors formed by LS174T/R cells transduced with MRP2pr-OATP1B1 plus treatment with dexamethasone were markedly sensitized to Bamet-UD2. In conclusion, the induced expression of anticancer drug uptake transporters, under the control of promoters of ABC proteins involved in chemoresistance, constitutes an interesting approach to overcome the poor response of cancer to chemotherapy due to reduced drug uptake and/or enhanced drug export.

Enhanced solubility, oral bioavailability and anti-osteoporotic effects of raloxifene HCl in ovariectomized rats by Igepal CO-890 nanomicelles

The purpose of the present study was to enhance the bioavailability and anti-osteoporotic effects of raloxifene HCl (RH) by increasing its solubility and inhibition of the p-glycoprotein pump using surfactant micelles of Igepal CO-890. The micelles were prepared by the direct method and their critical micellar concentration, drug dissolution rate, saturated solubility, drug loading and surface morphology were defined. The cytotoxicity of Igepal CO-890 and its ability to inhibit the p-glycoprotein pump were studied on Caco-2 cells. The pharmacokinetic parameters were analyzed by oral administration of a single dose of 15 mg/kg in Wistar rats. Anti-osteoporotic effects were studied by measuring the calcium, phosphorous, and uterus weight of rats after one month of oral administration of 6 mg/kg/day of RH in ovariectomized rats. Igepal CO-890 micelles enhanced the RH solubility by about two-fold. The FT-IR and DSC studies indicated no interaction between the drug and the surfactant. XRD spectrum showed an amorphous state of RH in the micelles. The p-glycoprotein pump was inhibited by Igepal CO-890 in Caco-2 cells comparable to verapamil. Micelles increased the uterine weight and decreased the serum calcium and phosphorus significantly compared to the untreated drug. Oral bioavailability of RH increased about four-fold by nanomicelles.

Investigation of Need of Natural Bioenhancer for a Metabolism Susceptible Drug-Raloxifene, in a Designed Self-Emulsifying Drug Delivery System

Bioenhancers can increase the bioavailability of metabolism susceptible drugs. The present study was designed to understand the impact of bioenhancer on permeability and bioavailability of a biopharmaceutical drug disposition classification system (BDDCS) class II drug raloxifene (RLX). RLX undergoes extensive first pass metabolism by UGT enzymes in gastrointestinal tract (GIT) and has an oral bioavailability of about 2%. Self-emulsifying drug delivery system (SEDDS) of RLX was developed using a designed approach and this formulation was loaded with reported bioenhancers: quercetin and piperine. These formulations were tested for improvement in permeability and bioavailability of the RLX. The apparent permeability using everted gut sac (P _app) for SEDDS (5.26 ± 1.10 × 10^-8 cm/s) was found to be similar to that of SEDDS with bioenhancers (5.11 ± 1.05 × 10^-8 cm/s). In oral bioavailability study in rat, SEDDS demonstrated a 4-fold and 2.5-fold higher AUC_0-∞ than RLX suspension (control) and marketed product, respectively. No additional improvement in permeability and bioavailability was offered by inclusion of piperine and quercetin (bioenhancers) in the SEDDS.

The effects of dietary and herbal phytochemicals on drug transporters

Membrane transporter proteins (the ABC transporters and SLC transporters) play pivotal roles in drug absorption and disposition, and thus determine their efficacy and safety. Accumulating evidence suggests that the expression and activity of these transporters may be modulated by various phytochemicals (PCs) found in diets rich in plants and herbs. PC absorption and disposition are also subject to the function of membrane transporter and drug metabolizing enzymes. PC-drug interactions may involve multiple major drug transporters (and metabolizing enzymes) in the body, leading to alterations in the pharmacokinetics of substrate drugs, and thus their efficacy and toxicity. This review summarizes the reported in vitro and in vivo interactions between common dietary PCs and the major drug transporters. The oral absorption, distribution into pharmacological sanctuaries and excretion of substrate drugs and PCs are considered, along with their possible interactions with the ABC and SLC transporters which influence these processes.

Raloxifene nanomicelles reduce the growth of castrate-resistant prostate cancer

BACKGROUND

Castrate-resistant prostate cancer (CRPC) patients are characterised by a 5-year relative survival rate of ∼25-33%. Recently, our laboratory encapsulated a selective oestrogen receptor modulator, raloxifene, into poly(styrene-co-maleic acid) (SMA-raloxifene), which demonstrated superior in vitro cytotoxicity compared with free drug against several CRPC cell lines.

PURPOSE

To validate SMA-raloxifene for the management of CRPC using a mouse xenograft model.

METHODS

The internalisation and retention of micellar and free raloxifene in vitro were measured by HPLC. A PC3-CRPC xenograft model was used to compare the biodistribution of both raloxifene formulations, as well as their effect on tumour progression where mice received free raloxifene (1 or 5 mg/kg, i.v.) or SMA-raloxifene (1 mg/kg, i.v.) weekly for 4 weeks.

RESULTS

SMA-raloxifene exhibited 75% higher intracellular content compared to free drug after 48 h in PC3 cells. Biodistribution of raloxifene was 69% higher in tumours following SMA-raloxifene compared with free raloxifene. Weekly administration of 1 mg/kg free raloxifene reduced tumour progression by 20% after 4 weeks, whereas 1 mg/kg SMA-raloxifene and 5 mg/kg free raloxifene reduced progression by 40%.

CONCLUSION

Encapsulation of raloxifene increased its therapeutic potential for the management of CRPC.

Milk Thistle Constituents Inhibit Raloxifene Intestinal Glucuronidation: A Potential Clinically Relevant Natural Product-Drug Interaction

Women at high risk of developing breast cancer are prescribed selective estrogen response modulators, including raloxifene, as chemoprevention. Patients often seek complementary and alternative treatment modalities, including herbal products, to supplement prescribed medications. Milk thistle preparations, including silibinin and silymarin, are top-selling herbal products that may be consumed by women taking raloxifene, which undergoes extensive first-pass glucuronidation in the intestine. Key constituents in milk thistle, flavonolignans, were previously shown to be potent inhibitors of intestinal UDP-glucuronosyl transferases (UGTs), with IC50s ≤ 10 μM. Taken together, milk thistle preparations may perpetrate unwanted interactions with raloxifene. The objective of this work was to evaluate the inhibitory effects of individual milk thistle constituents on the intestinal glucuronidation of raloxifene using human intestinal microsomes and human embryonic kidney cell lysates overexpressing UGT1A1, UGT1A8, and UGT1A10, isoforms highly expressed in the intestine that are critical to raloxifene clearance. The flavonolignans silybin A and silybin B were potent inhibitors of both raloxifene 4'- and 6-glucuronidation in all enzyme systems. The Kis (human intestinal microsomes, 27-66 µM; UGT1A1, 3.2-8.3 µM; UGT1A8, 19-73 µM; and UGT1A10, 65-120 µM) encompassed reported intestinal tissue concentrations (20-310 µM), prompting prediction of clinical interaction risk using a mechanistic static model. Silibinin and silymarin were predicted to increase raloxifene systemic exposure by 4- to 5-fold, indicating high interaction risk that merits further evaluation. This systematic investigation of the potential interaction between a widely used herbal product and chemopreventive agent underscores the importance of understanding natural product-drug interactions in the context of cancer prevention.

Membrane transport of nobilin conjugation products and use of the extract of Chamomillae romanae flos influence absorption of nobilin in the Caco-2 model

The purpose of this work was to investigate the role of bioconjugation and carrier mediated efflux of conjugation products in the absorption mechanism of the sesquiterpene lactone nobilin in the Caco-2 model in vitro and to elucidate the impact of the extract of Chamomillae romanae flos and its ingredients on absorption. Transport experiments with inhibitors of P-gp, BCRP, and MRPs were performed to detect efflux and its connection to bioconversion and the effect of different ingredients of the plant extract on absorption processes was determined. Permeability, transport and bioconversion parameter values were deduced by kinetic multi-compartment modeling. Nobilin exhibited high permeability, low relative absorption and fast bioconversion producing glucuronide, cysteine conjugate, and glutathione conjugate that were transported by P-gp (the first two), apical MRP2 and basal MRP3 and possibly MRP1 out of the cell. Inhibition of efflux resulted in diminished bioconjugation and improved absorption. The extract increased the relative fraction absorbed primarily by directly inhibiting bioconversion, and by reducing efflux. Individual extract ingredients could only partly explain this effect. Extensive bioconversion, hence, limited absorption of nobilin in the Caco-2 model and the interplay between conjugation and efflux was shown to provide a possible mechanism for absorption increase. Plant extract increased absorption by this mechanism in addition to metabolic enzyme inhibition.

A novel role for raloxifene nanomicelles in management of castrate resistant prostate cancer

Of patients with castrate resistant prostate cancer (CRPC), less than 25-33% survive more than five years. Recent studies have implicated estrogen, acting either alone or synergistically with androgens in the development of castrate resistant prostate cancer. Several in vitro and in vivo studies, as well as a limited number of clinical trials, have highlighted the potential of selective estrogen receptor modulators, such as raloxifene (Ral) for the treatment of castrate resistant prostate cancer. However, the poor oral bioavailability and metabolism of selective estrogen receptor modulators limit their efficiency in clinical application. To overcome these limitations, we have used styrene co-maleic acid (SMA) micelle to encapsulate raloxifene. Compared to free drug, SMA-Ral micelles had 132 and 140% higher cytotoxicity against PC3 and DU 145 prostate cell lines, respectively. SMA-Ral effectively inhibits cell cycle progression, increases apoptosis, and alters the integrity of tumor spheroid models. In addition, the micellar system induced changes in expression and localization of estrogen receptors, epidermal growth factor receptor (EGFR), and downstream effectors associated with cell proliferation and survival. Finally, SMA-Ral treatment decreased migration and invasion of castrate resistant prostate cancer cell lines. In conclusion, SMA-Ral micelles can potentially benefit new strategies for clinical management of castrate resistant prostate cancer.

Measuring selective estrogen receptor modulator (SERM)-membrane interactions with second harmonic generation

The interaction of selective estrogen receptor modulators (SERMs) with lipid membranes has been measured at clinically relevant serum concentrations using the label-free technique of second harmonic generation (SHG). The SERMs investigated in this study include raloxifene, tamoxifen, and the tamoxifen metabolites 4-hydroxytamoxifen, N-desmethyltamoxifen, and endoxifen. Equilibrium association constants (Ka) were measured for SERMs using varying lipid compositions to examine how lipid phase, packing density, and cholesterol content impact SERM-membrane interactions. Membrane-binding properties of tamoxifen and its metabolites were compared on the basis of hydroxyl group substitution and amine ionization to elucidate how the degree of drug ionization impacts membrane partitioning. SERM-membrane interactions were probed under multiple pH conditions, and drug adsorption was observed to vary with the concentration of soluble neutral species. The agreement between Ka values derived from SHG measurements of the interactions between SERMs and artificial cell membranes and independent observations of the SERMs efficacy from clinical studies suggests that quantifying membrane adsorption properties may be important for understanding SERM action in vivo.

The discovery and development of selective estrogen receptor modulators (SERMs) for clinical practice

Selective estrogen receptor modulators (SERMs) are structurally different compounds that interact with intracellular estrogen receptors in target organs as estrogen receptor agonists or antagonists. These drugs have been intensively studied over the past decade and have proven to be a highly versatile group for the treatment of different conditions associated with postmenopausal women's health, including hormone responsive cancer and osteoporosis. Tamoxifen, a failed contraceptive is currently used to treat all stages of breast cancer, chemoprevention in women at high risk for breast cancer and also has beneficial effects on bone mineral density and serum lipids in postmenopausal women. Raloxifene, a failed breast cancer drug, is the only SERM approved internationally for the prevention and treatment of postmenopausal osteoporosis and vertebral fractures. However, although these SERMs have many benefits, they also have some potentially serious adverse effects, such as thromboembolic disorders and, in the case of tamoxifen, uterine cancer. These adverse effects represent a major concern given that long-term therapy is required to prevent osteoporosis or prevent and treat breast cancer. The search for the 'ideal' SERM, which would have estrogenic effects on bone and serum lipids, neutral effects on the uterus, and antiestrogenic effects on breast tissue, but none of the adverse effects associated with current therapies, is currently under way. Ospemifene, lasofoxifene, bazedoxifene and arzoxifene, which are new SERM molecules with potentially greater efficacy and potency than previous SERMs, have been investigated for use in the treatment and prevention of osteoporosis. These drugs have been shown to be comparably effective to conventional hormone replacement therapy in animal models, with potential indications for an improved safety profile. Clinical efficacy data from ongoing phase III trials are available or are awaited for each SERM so that a true understanding of the therapeutic potential of these compounds can be obtained. In this article, we describe the discovery and development of the group of medicines called SERMs. The newer SERMs in late development: ospemifene, lasofoxifene, bazedoxifene, are arzoxifene are described in detail.

Plasma membrane transporters in modern liver pharmacology

The liver plays a crucial role in the detoxification of drugs used in the treatment of many diseases. The liver itself is the target for drugs aimed to modify its function or to treat infections and tumours affecting this organ. Both detoxification and pharmacological processes occurring in the liver require the uptake of the drug by hepatic cells and, in some cases, the elimination into bile. These steps have been classified as detoxification phase 0 and phase III, respectively. Since most drugs cannot cross the plasma membrane by simple diffusion, the involvement of transporters is mandatory. Several members of the superfamilies of solute carriers (SLC) and ATP-binding cassette (ABC) proteins, with a minor participation of other families of transporters, account for the uptake and efflux, respectively, of endobiotic and xenobiotic compounds across the basolateral and apical membranes of hepatocytes and cholangiocytes. These transporters are also involved in the sensitivity and refractoriness to the pharmacological treatment of liver tumours. An additional interesting aspect of the role of plasma membrane transporters in liver pharmacology regards the promiscuity of many of these carriers, which accounts for a variety of drug-drug, endogenous substances-drug and food components-drug interactions with clinical relevance.

Influence of hepatic and intestinal efflux transporters and their genetic variants on the pharmacokinetics and pharmacodynamics of raloxifene in osteoporosis treatment

Raloxifene exhibits a large and unexplained interindividual variability in its pharmacokinetics and pharmacodynamics. The aim of our study was to identify transporters involved in the efflux of raloxifene and its glucuronide metabolites by various in vitro models and by an in vivo study to explore the possible involvement of P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP)1, MRP2, MRP3, and breast cancer resistance protein in the observed high interindividual variability. Experiments with the parallel artificial membrane permeability assay showed the highest passive permeability for raloxifene, followed by raloxifene-6-β-glucuronide (M1), raloxifene-4'-β-glucuronide (M2), and raloxifene-6,4'-diglucuronide (M3). Caco-2 cell monolayer experiments indicated an interaction of raloxifene with Pgp. The ATPase assay confirmed the raloxifene interaction with Pgp and indicated interactions of all raloxifene species with MRP1, MRP2, MRP3, and breast cancer resistance protein, except for M1, which did not show any interactions with MRP2. Furthermore, the vesicular experiments confirmed the interaction of M2 and M3 with MRP2. Although the in vivo study on osteoporotic postmenopausal women on raloxifene could not confirm a significant influence of ABCB1 and ABCC2 genetic polymorphisms on its pharmacokinetics, a clear trend toward higher total raloxifene concentrations was observed in carriers of at least 1 ABCB1 c.3435T allele. Moreover, the same polymorphism effect was also observed as a significant increase in total hip bone mineral density after 1 year of treatment. The results of our study support the involvement of efflux transporters in disposition of raloxifene and its metabolites and may partially explain the observed raloxifene variability by the influence of the ABCB1 c.3435C>T polymorphism.

Coupling of UDP-glucuronosyltransferases and multidrug resistance-associated proteins is responsible for the intestinal disposition and poor bioavailability of emodin

Emodin is a poorly bioavailable but promising plant-derived anticancer drug candidate. The low oral bioavailability of emodin is due to its extensive glucuronidation in the intestine and liver. Caco-2 cell culture model was used to investigate the interplay between UDP-glucuronosyltransferases (UGTs) and efflux transporters in the intestinal disposition of emodin. Bidirectional transport assays of emodin at different concentrations were performed in the Caco-2 monolayers with or without multidrug resistance-associated protein (MRP) and breast cancer resistance protein (BCRP) efflux transporter chemical inhibitors. The bidirectional permeability of emodin and its glucuronide in the Caco-2 monolayers was determined. Emodin was rapidly metabolized to emodin glucuronide in Caco-2 cells. LTC4, a potent inhibitor of MRP2, decreased the efflux of emodin glucuronide and also substantially increased the intracellular glucuronide level in the basolateral-to-apical (B-A) direction. MK-571, chemical inhibitor of MRP2, MRP3, and MRP4, significantly reduced the efflux of glucuronide in the apical-to-basolateral (A-B) and B-A directions in a dose-dependent manner. However, dipyridamole, a BCRP chemical inhibitor demonstrated no effect on formation and efflux of emodin glucuronide in Caco-2 cells. In conclusion, UGT is a main metabolic pathway for emodin in the intestine, and the MRP family is composed of major efflux transporters responsible for the excretion of emodin glucuronide in the intestine. The coupling of UGTs and MRP efflux transporters causes the extensive metabolism, excretion, and low bioavailability of emodin.

Nanoemulsion liquid preconcentrates for raloxifene hydrochloride: optimization and in vivo appraisal

Raloxifene hydrochloride (RLX) is a selective estrogen-receptor modulator for treatment of osteoporosis and prevention of breast and endometrial cancer. By virtue of extensive presystemic clearance, RLX bioavailability is only 2%. The current study aimed to tailor and characterize RLX-loaded self-nanoemulsifying drug-delivery systems (SNEDDS) using bioactive excipients affecting drug metabolism. The potential of oral nanocarriers to enhance RLX delivery to endocrine target organs was assessed in fasted and fed female Wistar rats using high-performance liquid chromatography. RLX was loaded in the dissolved and dispersed status in the alkalinized (A-SNEDDS) and nonalkalinized (NA-SNEDDS) systems, respectively. Optimization and assessment relied on solubility studies, emulsification efficiency, phase diagrams, dilution robustness, cloud point, particle size, zeta potential (ZP), polydispersity index (PDI), and transmission electron microscopy. In vitro release was assessed using dialysis bag versus dissolution cup methods. NA-SNEDDS were developed with suitable globule size (38.49 ± 4.30 nm), ZP (31.70 ± 3.58 mV), PDI (0.31 ± 0.02), and cloud point (85°C). A-SNEDDS exhibited good globule size (35 ± 2.80 nm), adequate PDI (0.28 ± 0.06), and lower ZP magnitude (−21.20 ± 3.46 mV). Transmission electron microscopy revealed spherical globules and contended data of size analysis. Release studies demonstrated a nonsignificant enhancement of RLX release from NA-SNEDDS compared to drug suspension with the lowest release shown by A-SNEDDS. A conflicting result was elucidated from in vivo trial. A significant enhancement in RLX uptake by endocrine organs was observed after nanocarrier administration compared to RLX suspension. In vivo studies reflected a poor in vitro/in vivo correlation, recommended nanocarrier administration before meals, and did not reveal any advantage for drug loading in the solubilized form (A-SNEDDS). To conclude, NA-SNEDDS possessed superior in vitro characteristics to A-SNEDDS, with equal in vivo potential. NA-SNEDDS elaborated in this work could successfully double RLX delivery to endocrine target organs, with promising consequences of lower dose and side effects of the drug.

Systematic studies of sulfation and glucuronidation of 12 flavonoids in the mouse liver S9 fraction reveal both unique and shared positional preferences

Sulfation and glucuronidation are the principal metabolic pathways of flavonoids, and extensive phase II metabolism is the main reason for their poor bioavailabilities. The purpose of this study was to compare the similarities and differences in the positional preference of glucuronidation versus sulfation in the mouse liver S9 fraction. The conjugating rates of seven monohydroxyflavones (HFs) (i.e., 2'-, 3'-, 4'-, 3-, 5-, 6-, and 7-HF), and five dihydroxyflavones (diHFs) (i.e., 6,7-, 4',7-, 3,7-, 5,7-, and 3,4'-diHF) were determined in three separate enzymatic reaction systems: (A) sulfation only, (B) glucuronidation only, or (C) simultaneous sulfation and glucuronidation (i.e., Sult-Ugt coreaction). In general, glucuronidation rates were much faster than sulfation rates. Among the HFs, 7-HF was the best substrate for both conjugation reactions, whereas 3-HF was rapidly glucuronidated but was not sulfated. As a result, the rank order of sulfation was very different from that of glucuronidation. Among the diHFs, regiospecific glucuronidation was limited to 7-OH and 3-OH positions, whereas regiospecific sulfation was limited to 7-OH and 4'-OH positions. Other positions (i.e., 6-OH and 5-OH) in diHFs were not conjugated. The positional preferences were essentially maintained in a Sult-Ugt coreaction system, although sulfation was surprisingly enhanced. Lastly, sulfation and glucuronidation displayed different regiospecific- and substrate-dependent characteristics. In conclusion, glucuronidation and sulfation shared the same preference for 7-OH position (of flavonoids) but displayed unique preference in other positions in that glucuronidation preferred the 3-OH position whereas sulfation preferred the 4'-OH position.

Sulfation of selected mono-hydroxyflavones by sulfotransferases in vitro: a species and gender comparison

OBJECTIVES

Sulfation via sulfotransferases is an important metabolic pathway contributing to the low bioavailability of flavonoids. This study aims to characterize the sulfation of mono-hydroxyflavones (MHFs) to obtain useful information on structure-metabolizing relationships in animal species and gender differences.

METHODS

Three representative MHFs, namely, 7-, 6- and 4'-MHF, were studied by incubating each MHF at different concentrations with various liver S9 fractions (mouse, rat, dog and human).

KEY FINDINGS

One mono-sulfate was identified for each MHF. 7-MHF and 4'-MHF usually have greater sulfations than 6-MHF. Regardless of whether the S9 fraction came from a male or female, there was a difference in sulfation in the species observed for all MHFs; the highest activity of sulfotransferases was in dog S9. Furthermore, gender differences affect sulfation of MHFs significantly. In rats, all sulfations for the three MHFs were higher in males than that in females while the opposite was observed in mice.

CONCLUSIONS

Regiospecific, species and gender dependence exist in the sulfonation of all selected MHFs.

Impact of intestinal glucuronidation on the pharmacokinetics of raloxifene

Raloxifene is extensively glucuronidated in humans, effectively reducing its oral bioavailability (2%). It was also reported to be glucuronidated in preclinical animals, but its effects on the oral bioavailability have not been fully elucidated. In the present study, raloxifene and its glucuronides in the portal and systemic blood were monitored in Gunn rats deficient in UDP-glucuronosyltransferase (UGT) 1A, Eisai hyperbilirubinemic rats (EHBRs), which hereditarily lack multidrug resistance-associated protein (MRP) 2, and wild-type rats after oral administration. The in vitro-in vivo correlation (IVIVC) of four UGT substrates (raloxifene, biochanin A, gemfibrozil, and mycophenolic acid) in rats was also evaluated. In Gunn rats, the product of fraction absorbed and intestinal availability and hepatic availability of raloxifene were 0.63 and 0.43, respectively; these values were twice those observed in wild-type Wistar rats, indicating that raloxifene was glucuronidated in both the liver and intestine. The ratio of glucuronides to unchanged drug in systemic blood was substantially higher in EHBRs (129-fold) than in the wild-type Sprague-Dawley rats (10-fold), suggesting the excretion of raloxifene glucuronides caused by MRP2. The IVIVC of the other UGT substrates in rats displayed a good relationship, but the oral clearance values of raloxifene and biochanin A, which were extensively glucuronidated by rat intestinal microsomes, were higher than the predicted clearances using rat liver microsomes, suggesting that intestinal metabolism may be a great contributor to the first-pass effect. Therefore, evaluation of intestinal and hepatic glucuronidation for new chemical entities is important to improve their pharmacokinetic profiles.

First-pass metabolism via UDP-glucuronosyltransferase: a barrier to oral bioavailability of phenolics

Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endobiotics and xenobiotics, including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug-induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions, including the important properties such as binding pocket size and phosphorylation requirements.

The pharmacokinetics of raloxifene and its interaction with apigenin in rat

Purpose: Raloxifene is a selective estrogen receptor modulator which is structurally similar to tamoxifen. As flavonoids can interact with raloxifene in vitro, we evaluated the in vivo pharmacokinetics of raloxifene in rats when co-administered with apigenin. Methods: The pharmacokinetics of raloxifene in the absence or presence of apigenin was investigated in rats after different dosage regimens. The plasma concentrations before and after enzymatic hydrolysis were analyzed by HPLC, and the pharmacokinetic profiles of raloxifene administered alone and in combination with apigenin were compared. Results: Co-administration of apigenin with raloxifene in a 1:2 ratio by weight resulted in a 55% and 37% increase in the C_max and AUC of intact raloxifene, respectively. When equal proportions of raloxifene and apigenin (1:1) were administered, the C_max and AUC of intact raloxifene were increased by 173% and 97% respectively. This increase in intact raloxifene was not associated with an increase in total raloxifene (intact plus conjugated raloxifene) because AUC and C_max of total raloxifene when administered alone or in combination with apigenin were found to be similar. The results indicated that apigenin inhibited the glucuronidation and sulfation of raloxifene in the intestine bringing about an increased bioavailability of the drug. Conclusions: The results showed that apigenin decreased the first-pass metabolism of raloxifene but did not increase its absorption from the gastrointestinal tract.

Identification of the position of mono-O-glucuronide of flavones and flavonols by analyzing shift in online UV spectrum (lambdamax) generated from an online diode array detector

The beneficial pharmacological effects of flavonoids such as chemoprevention against cancer, aging, and heart diseases are severely limited due to their extensive in vivo glucuronidation by UDP-glucuronosyltransferases (UGTs). UGTs showed regiospecificity (i.e., position preference) in the glucuronidation of the flavonoids based on the substrate's chemical structure. In this paper, glucuronide(s) of 36 flavones and flavonols were generated using an in vitro glucuronidation reaction. UPLC/MS/MS was used to confirm the degree (mono- or di-) of glucuronidation in flavonoids with up to four hydroxyl groups. UV spectra of flavonoids and their respective mono-O-glucuronides were generated using UPLC with an online diode array detector. Analysis of the extent of shift in spectra of glucuronides in band I (300-385 nm) and band II (240-280 nm) regions as reflected by changes in lambdamax value was used to identify the position of glucuronidation. The data showed that glucuronidation of the 3- and 4'-hydroxyls resulted in band I lambdamax hypsochromic shifts (or blue shift) of 13-30 and 5-10 nm, respectively. Glucuronidation of the 5-hydroxyl group caused a band II lambdamax hypsochromic shift of 5-10 nm. In contrast, glucuronidation of the 7-hydroxyl group did not cause any lambdamax change in band I or II lambdamax, whereas glucuronidation of the 6-hydroxyl group did not cause predictable changes in lambdamax values. The paper demonstrated for the first time that a rapid and robust analysis method using lambdamax changes in online UV spectra can be used to pinpoint region-specific glucuronidation of flavones and flavonols with hydroxyl groups at the 4'-, 3-, 5-, and/or 7-position(s).

Breast cancer resistance protein (BCRP) and sulfotransferases contribute significantly to the disposition of genistein in mouse intestine

The low bioavailability of genistein has impeded its development into a therapeutic agent. Our earlier studies indicate that glucuronidation is one of the major barriers to genistein oral bioavailability. This study will determine how sulfotransferases and efflux transporters affect its intestinal disposition. A rodent intestinal perfusion model and S9 fractions were used. Sulfate excretion rates were comparable to glucuronide excretion in mouse small intestine but significantly higher than glucuronide excretion in mouse colon, which is different from rat intestinal disposition but similar to disposition in Caco-2 cells. To define efflux transporter(s) involved in sulfate excretion, two organic anion inhibitors (estrone sulfate and dihydroepiandrosterone sulfate) or a multidrug resistance protein inhibitor (MK-571) were used but neither was able to decrease the excretion of genistein sulfates. In contrast, the excretion of genistein sulfate decreased substantially (>90%) in small intestine of breast cancer resistance protein (BCRP) knockout mice and became undetectable in colon of the knockout mice. The excretion rates of genistein glucuronide in the small intestine of BCRP knockout mice were also significant decreased (78%). This study shows clearly that BCRP facilitates the cellular genistein sulfate excretion by removing sulfates to prevent their backward hydrolysis and to limit substrate inhibition, indicating that BCRP plays a dominant role in genistein sulfate excretion and a significant role in genistein glucuronide excretion in the mouse intestine.

Site-specific accumulation of the cancer preventive dietary polyphenol ellagic acid in epithelial cells of the aerodigestive tract

Ellagic acid (EA), a polyphenol present in berries, has been demonstrated to prevent oesophageal and colon cancer in animals. To better understand the site-specificity of these effects, we studied the accumulation and transport of [14C]EA in rat aerodigestive epithelial cells in-vivo and in cultured human cells. When [14C]EA was administered to rats by gavage, a high content of EA was found in the oesophagus and small intestine at 0.5h after oral administration and in the colon at 12h, with very low amounts in plasma and peripheral tissues. Studies in human intestinal Caco-2 and human oesophageal HET-1A cells found very limited transcellular transport (Caco-2) of EA but high accumulation (Caco-2 and HET-1A) in the cells. In more detailed studies in the Caco-2 cells, accumulation of EA displayed ATP- and Na+-dependency. Multiple interventions permitted the exclusion of a number of transporters as mediators of this uptake. A dramatically reduced transport of EA at low pH (5.5) compared with high pH (7.4) suggested an important role for the negative charge of EA. This was supported by the organic anion transport inhibitors 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid and bromosulfophthalein. The latter produced as much as 78% inhibition at the 100 μm concentration. Finally, Caco-2 cells were shown to express organic anion transporter 4 (OAT4) mRNA, as was the human large intestine. EA appears to be accumulated along the aerodigestive tract using OATlike transporters, one of which might be OAT4.

Involvement of P-glycoprotein in regulating cellular levels of Ginkgo flavonols: quercetin, kaempferol, and isorhamnetin

Quercetin, kaempferol, and isorhamnetin were the most important flavonoid constituents in extracts from Ginkgo biloba leaves. Transport studies of Ginkgo flavonols were performed in Caco-2 cell mono-layers. Their apparent permeability in absorptive and secretion directions was determined, and quercetin, kaempferol and isorhamnetin displayed polarized transport, with the Papp,B-A being higher than the Papp,A-B (P &lt; 0.01 for quercetin, P &lt; 0.001 for kaempferol and isorhamnetin, Student's t-test). Bcap37/MDR1 cells, which were transfected with a P-glycoprotein (P-gp) gene construct, were treated with quercetin, kaempferol or isorhamnetin. The concentrations of Ginkgo flavonol in Bcap37/MDR1 cells were lower than those in parent cells (P &lt; 0.05 for quercetin, P &lt; 0.01 for isorhamnetin, Mann-Whitney U test). The concentrations of the flavonol in transfected cells increased when incubated with the P-gp inhibitor verapamil (P &lt; 0.05 for kaempferol, Mann-Whitney U test). A colorometric assay for ATPase activity was applied to the detection of interaction of flavonol with P-gp. Quercetin and kaempferol inhibited the ATPase activity, and isorhamnetin stimulated the ATPase activity (P &lt; 0.05 for isorhamnetin, Mann Whitney U test). The results indicated that Ginkgo flavonols quercetin, kaempferol and isorhamnetin were substrates of P-gp. The P-gp type efflux pump might limit the bioavailability of Ginkgo flavonols.

MRP isoforms and BCRP mediate sulfate conjugate efflux out of BeWo cells

The breast cancer resistance protein (BCRP) and the multidrug resistance-associated proteins (MRPs) have the ability to eliminate sulfate conjugates but it is not known if this constitutes one of their roles in the placenta. To determine this, the BeWo cell line was used as a model of placental trophoblast cells and the mechanisms of elimination of sulfate metabolites of two common sulfotransferase substrates, 4-nitrophenol and acetaminophen were examined. At 0.5-200 microM, neither 4-nitrophenyl sulfate nor acetaminophen sulfate affected the accumulation of the BCRP substrates BODIPY FL prazosin or mitoxantrone in BeWo monolayers, indicating a lack of interaction of BCRP with the sulfates. Examination of the effect of BCRP/MRP inhibitors on the efflux of intracellularly generated 4-nitrophenyl sulfate and acetaminophen sulfate, indicated that one or more of the MRP isoforms play a major role in the elimination of 4-nitrophenyl sulfate and acetaminophen sulfate across the basolateral (fetal-facing) and apical (maternal-facing) membranes respectively. BCRP played a minor role in the elimination of these two sulfate conjugates across the apical membrane. This study demonstrates that a yet undetermined role of trophoblast efflux transporters is the elimination of sulfate conjugates.

Caco-2 cell monolayers as a tool to study simultaneous phase II metabolism and metabolite efflux of indomethacin, paracetamol and 1-naphthol

The human intestinal cell line, Caco-2, was used to study compounds - indomethacin, paracetamol and 1-naphthol - that undergo intestinal phase II metabolism followed by apical and/or basolateral efflux of the metabolites and/or parent compounds. The interplay was studied during permeability experiments across fully differentiated Caco-2 cell monolayers. The parent compounds and their glucuronide and/or sulfate metabolites were detected by LC-MS/MS. Conjugation of the model compounds and effluxes of their metabolites were observed. The efflux of indomethacin glucuronide was apical, but complementary basolateral efflux was observed at the highest indomethacin concentration (500 microM), probably due to apical saturation. Paracetamol glucuronide was not formed in these experiments, but apical and basolateral effluxes of paracetamol sulfate were observed. A typical bell-shaped inhibition curve was observed for the formation of 1-naphthol glucuronides, indicating substrate or product inhibition of the UGT enzyme(s) at higher 1-naphthol concentrations (200 microM and 500 microM). Based on these results, the fully differentiated Caco-2 cell monolayers can be applied as a platform for qualitative in vitro studies, where phase II metabolism and efflux activities are ongoing simultaneously.

Metabolic inhibition and kinetics of raloxifene by pharmaceutical excipients in human liver microsomes

This study was originally undertaken to establish the in vitro metabolic conditions and then evaluate the effect of pharmaceutical excipients (PEs) on drug metabolism in uridine diphosphoglucuronic acid-supplemented human liver microsomes. Poorly bioavailable raloxifene was chosen as a model drug. Intact drug and its two glucuronide metabolites were successfully isolated using gradient HPLC analysis and LC/MS analysis. Formation of raloxifene metabolites was affected by buffer compositions, incubation time and initial raloxifene concentrations. Under optimized metabolic conditions, 41.0% of raloxifene was converted to its metabolites after 2h incubation. This metabolic inhibition of raloxifene by the PEs occurred in a dose-dependent manner and accordingly formed two glucuronide metabolites. In the metabolic kinetics using Lineweaver-Burk analyses, Cremophor EL competitively inhibited formation of metabolites while sodium lauryl sulfate (SLS), polyvinylpyrrolidone K30 (PVP) and Tween 80 significantly inhibited in a mixed competition. Although some PEs showed inhibition on glucuronidation of raloxifene in vitro, their effects on in vivo bioavailability of raloxifene need to be confirmed directly due to the dilution factors and other complicated situations influencing the bioavailability.

Disposition of flavonoids via enteric recycling: enzyme stability affects characterization of prunetin glucuronidation across species, organs, and UGT isoforms

We characterized the in vitro glucuronidation of prunetin, a prodrug of genistein that is a highly active cancer prevention agent. Metabolism studies were conducted using expressed human UGT isoforms and microsomes/S9 fractions prepared from intestine and liver of rodents and humans. The results indicated that human intestinal microsomes were more efficient than liver microsomes in glucuronidating prunetin, but rates of metabolism were dependent on time of incubation at 37 degrees C. Human liver and intestinal microsomes mainly produced metabolite 1 (prunetin-5- O-glucuronide) and metabolite 2 (prunetin-4'- O-glucuronide), respectively. Using 12 human UGT isoforms, we showed that UGT1A7, UGT1A8, and UGT1A9 were mainly responsible for the formation of metabolite 1, whereas UGT1A1, UGT1A8, and UGT1A10 were mainly responsible for the formation of metabolite 2. This isoform-specific metabolism was consistent with earlier results obtained using human liver and intestinal microsomes, as the former (liver) is UGT1A9-rich whereas the latter is UGT1A10-rich. Surprisingly, we found that the thermostability of the microsomes was isoform- and organ-dependent. For example, human liver microsomal UGT activities were much more heat-stable (37 degrees C) than intestinal microsomal UGT activities, consistent with the finding that human UGT1A9 is much more thermostable than human UGT1A10 and UGT1A8. The organ-specific thermostability profiles were also evident in rat microsomes and mouse S9 fractions, even though human intestinal glucuronidation of prunetin differs significantly from rodent intestinal glucuronidation. In conclusion, prunetin glucuronidation is species-, organ-, and UGT-isoform-dependent, all of which may be impacted by the thermostability of specific UGT isoforms involved in the metabolism.

Interactions of the human cytosolic sulfotransferases and steroid sulfatase in the metabolism of tibolone and raloxifene

Sulfation is important in the metabolism and inactivation of steroidal compounds and hormone replacement therapeutic (HRT) agents in human tissues. Although generally inactive, many steroid sulfates are hydrolyzed to their active forms by sulfatase activity. Therefore, the specific sulfotransferase (SULT) isoforms and the levels of steroid sulfatase (STS) activity in tissues are important in regulating the activity of steroidal and HRT compounds. Tibolone (Tib) is metabolized to three active metabolites and all four compounds are readily sulfated. Tib and the Delta4-isomer are sulfated at the 17beta-OH group by SULT2A1 and the 17-sulfates are resistant to hydrolysis by human placental STS. 3alpha-OH and 3beta-OH Tib can form both 3- and 17-monosulfates as well as disulfates. Only the 3beta-sulfates are susceptible to STS hydrolysis. Raloxifene monosulfation was catalyzed by at least seven SULT isoforms and SULT1E1 also synthesizes raloxifene disulfate. SULT1E1 forms both monosulfates in a ratio of approximately 8:1 with the more abundant monosulfate migrating on HPLC identical to the SULT2A1 synthesized monosulfate. The raloxifene monosulfate formed by both SULT isoforms is sensitive to STS hydrolysis whereas the low abundance monosulfate formed by SULT1E1 is resistant. The benzothiophene sulfates of raloxifene and arzoxifene were hydrolyzed by STS whereas the raloxifene 4'-phenolic sulfate was resistant. These results indicate that tissue specific expression of SULT isoforms and STS could be important in the inactivation and regeneration of the active forms of HRT agents.

Mechanistic study on the intestinal absorption and disposition of baicalein

The present study aims to investigate the mechanisms of intestinal absorption and disposition of flavonoid baicalein (B) in Caco-2 cell monolayer model, transporter overexpressing membrane, and cellular models. The bidirectional transport studies of B and its metabolite baicalein-7-glucuronide (BG) were conducted at various concentrations and in the absence or presence of the selected transporter inhibitors. To identify specific interactions of BG with ABC transporters, ABC transporter-ATPase assays were carried out on membrane vesicles prepared from Sf9 cells overexpressing human MDR1, MRP1, MRP2, MRP3 and MXR. To further confirm the interactions between BG and specific ABC transporters, inhibition of BG on the transport of substrates of specific transporters were evaluated using membrane vesicles overexpressing MRP1-3 and MXR, or K562MDR cells with overexpressing MDR1. The results showed that B could readily pass through Caco-2 cell monolayer, but with significant glucuronidation and sulfation. The extent of phase II metabolism of B during its transport was in dose-dependent manner. The intracellularly formed glucuronide and sulfate of B were efficiently extruded to both apical and basolateral sides of the Caco-2 monolayer, which were reduced in the presence of MRP inhibitors. Although BG was not permeable from apical to basolateral side, it exhibited significant efflux transport that was inhibited in the presence of MRPs inhibitors. Moreover, BG seemed to activate the ATPase activity of both MRP3 and MXR at a pharmacologically relevant concentration range.

Chemoprevention of breast cancer with selective oestrogen-receptor modulators

Twenty years ago, a new therapeutic dimension was conceived that not only had the potential to treat and prevent osteoporosis, but to prevent breast and endometrial cancer at the same time. As osteoporosis was known to be caused by oestrogen withdrawal after menopause, whereas breast and endometrial cancer are caused by unopposed oestrogen action, the new tissue-selective oestrogens and anti-oestrogens, or selective oestrogen-receptor modulators (SERMs), had to recruit new networks to activate or suppress target tissues selectively. New medicines now promise to provide chemoprevention strategies for women at risk for the development of many diseases.

The Role of P-glycoprotein in the Bioactivation of Raloxifene

Drug transporters have been shown to alter drug metabolism. Similarly, bioactivation of drugs may also be altered by drug transporters. The aim of this work was to examine the role of P-glycoprotein (Pgp) in the bioactivation of a Pgp substrate, raloxifene, and a non-Pgp substrate, naphthalene. To evaluate the extent of bioactivation, covalent binding was measured. In both freshly isolated and cryopreserved hepatocytes, the extent of raloxifene covalent binding increased significantly (p < 0.05) in the presence of verapamil, whereas no change was observed with the covalent binding of naphthalene. To ascertain that the change was a Pgp effect, covalent binding was examined in microsomes in which raloxifene and naphthalene covalent binding was not altered in the presence of verapamil. In addition, the measure of raloxifene-glutathione adducts in the cryopreserved hepatocytes showed that the formation of the adducts increased in the presence of verapamil, which supports the idea that blocking Pgp in the liver increases metabolism and, therefore, the bioactivation of raloxifene. Because raloxifene and naphthalene are known to undergo bioactivation mediated by CYP3A4, covalent binding in the presence of ketoconazole was examined. In both hepatocytes and microsomes, raloxifene covalent binding decreased significantly (p < 0.01). It is interesting that naphthalene covalent binding was not affected. In the presence of the CYP2E inhibitor 4-methylpyrazole, a decrease in naphthalene covalent binding was observed, suggesting that the formation of the 1,2-epoxide may be the main culprit contributing to naphthalene covalent binding. In conclusion, these data suggest that in addition to other "protective" mechanisms, Pgp may attenuate bioactivation of drugs.

Disposition of flavonoids via enteric recycling: structural effects and lack of correlations between in vitro and in situ metabolic properties

The purpose of this study is to determine the importance of coupling of efflux transporters and metabolic enzymes in the intestinal disposition of six isoflavones (genistein, daidzein, formononetin, glycitein, biochanin A, and prunetin), and to determine how isoflavone structural differences affect the intestinal disposition. A rat intestinal perfusion model was used, together with rat intestinal and liver microsomes. In the intestinal perfusion model, significant absorption and excretion differences were found between isoflavones and their respective glucuronides (p <0.05), with prunetin being the most rapidly absorbed and formononetin glucuronides being the most excreted in the small intestine. In contrast, glucuronides were excreted very little in the colon. In an attempt to account for the differences, we measured the glucuronidation rates of six isoflavones in microsomes prepared from rat intestine and liver. Using multiple regression analysis, intrinsic clearance (CL(int)) and other enzyme kinetic parameters (V(max) and K(m)) were determined using appropriate kinetic models based on Akaike's information criterion. The kinetic parameters were dependent on the isoflavone used and the types of microsomes. To determine how metabolite excretion rates are controlled, we plotted excretion rates versus calculated microsomal rates (at 10 microM), CL(int) values, K(m) values, or V(max) values, and the results indicated that excretion rates were not controlled by any of the kinetic parameters. In conclusion, coupling of intestinal metabolic enzymes and efflux transporters affects the intestinal disposition of isoflavones, and structural differences of isoflavones, such as having methoxyl groups, significantly influenced their intestinal disposition.