Rifampicin enhances anti-cancer drug accumulation and activity in multidrug-resistant cells

Conversion of Olmesartan to Olmesartan Medoxomil, A Prodrug that Improves Intestinal Absorption, Confers Substrate Recognition by OATP2B1

PURPOSE

Olmesartan medoxomil (olmesartan-MX), an ester-type prodrug of the angiotensin II receptor blocker (ARB) olmesartan, is predominantly anionic at intestinal pH. Human organic anion transporting polypeptide 2B1 (OATP2B1) is expressed in the small intestine and is involved in the absorption of various acidic drugs. This study was designed to test the hypothesis that OATP2B1-mediated uptake contributes to the enhanced intestinal absorption of olmesartan-MX, even though olmesartan itself is not a substrate of OATP2B1.

METHODS

Tetracycline-inducible human OATP2B1- and rat Oatp2b1-overexpressing HEK 293 cell lines (hOATP2B1/T-REx-293 and rOatp2b1/T-REx-293, respectively) were established to characterize OATP2B1-mediated uptake. Rat jejunal permeability was measured using Ussing chambers. ARBs were quantified by liquid chromatography-tandem mass spectrometry.

RESULTS

Significant olmesartan-MX uptake was observed in hOATP2B1/T-REx-293 and rOatp2b1/T-REx-293 cells, whereas olmesartan uptake was undetectable or much lower than olmesartan-MX uptake, respectively. Furthermore, olmesartan-MX exhibited several-fold higher uptake in Caco-2 cells and greater permeability in rat jejunum compared to olmesartan. Olmesartan-MX uptake in hOATP2B1/T-REx-293 cells and in Caco-2 cells was significantly decreased by OATP2B1 substrates/inhibitors such as 1 mM estrone-3-sulfate, 100 µM rifamycin SV, and 100 µM fluvastatin. Rat Oatp2b1-mediated uptake and rat jejunal permeability of olmesartan-MX were significantly decreased by 50 µM naringin, an OATP2B1 inhibitor. Oral administration of olmesartan-MX with 50 µM naringin to rats significantly reduced the area under the plasma concentration-time curve of olmesartan to 76.9%.

CONCLUSION

Olmesartan-MX is a substrate for OATP2B1, and the naringin-sensitive transport system contributes to the improved intestinal absorption of olmesartan-MX compared with its parent drug, olmesartan.

Fluorescence-based methods for studying activity and drug-drug interactions of hepatic solute carrier and ATP binding cassette proteins involved in ADME-Tox

In humans, approximately 70% of drugs are eliminated through the liver. This process is governed by the concerted action of membrane transporters and metabolic enzymes. Transporters mediating hepatocellular uptake of drugs belong to the SLC (Solute carrier) superfamily of transporters. Drug efflux either toward the portal vein or into the bile is mainly mediated by active transporters of the ABC (ATP Binding Cassette) family. Alteration in the function and/or expression of liver transporters due to mutations, disease conditions, or co-administration of drugs or food components can result in altered pharmacokinetics. On the other hand, drugs or food components interacting with liver transporters may also interfere with liver function (e.g., bile acid homeostasis) and may even cause liver toxicity. Accordingly, certain transporters of the liver should be investigated already at an early stage of drug development. Most frequently radioactive probes are applied in these drug-transporter interaction tests. However, fluorescent probes are cost-effective and sensitive alternatives to radioligands, and are gaining wider application in drug-transporter interaction tests. In our review, we summarize our current understanding about hepatocyte ABC and SLC transporters affected by drug interactions. We provide an update of the available fluorescent and fluorogenic/activable probes applicable in in vitro or in vivo testing of these ABC and SLC transporters, including near-infrared transporter probes especially suitable for in vivo imaging. Furthermore, our review gives a comprehensive overview of the available fluorescence-based methods, not directly relying on the transport of the probe, suitable for the investigation of hepatic ABC or SLC-type drug transporters.

Assessment of Clinical Drug-Drug Interactions of Elagolix, a Gonadotropin-Releasing Hormone Receptor Antagonist

Elagolix is an oral gonadotropin-releasing hormone receptor antagonist indicated for the management of endometriosis-associated pain and in combination with estradiol/norethindrone acetate indicated for the management of heavy menstrual bleeding associated with uterine leiomyomas (fibroids) in premenopausal women. Elagolix coadministered with estradiol/norethindrone acetate is in late-stage development for the management of heavy menstrual bleeding associated with uterine fibroids. Based on the in vitro profile of elagolix metabolism and disposition, 9 drug-drug interaction (DDI) studies evaluating the victim and perpetrator characteristics of elagolix were conducted in 144 healthy volunteers. As a victim of cytochrome P450 (CYPs) and transporter-mediated DDIs, elagolix area under the curve (AUC) increased by ∼2-fold following coadministration with ketoconazole and by ∼5- and ∼2-fold with single and multiple doses of rifampin, respectively. As a perpetrator, elagolix decreased midazolam AUC (90% confidence interval) by 54% (50%-59%) and increased digoxin AUC by 32% (23%-41%). Elagolix decreased rosuvastatin AUC by 40% (29%-50%). No clinically significant changes in exposure on coadministration with sertraline or fluconazole occurred. A elagolix 150-mg once-daily regimen should be limited to 6 months with strong CYP3A inhibitors and rifampin because of the potential increase in bone mineral density loss, as described in the drug label. A 200-mg twice-daily regimen is recommended for no more than 1 month with strong CYP3A inhibitors and not recommended with rifampin. Elagolix is contraindicated with strong organic anion transporter polypeptide B1 inhibitors (eg, cyclosporine and gemfibrozil). Consider increasing the doses of midazolam and rosuvastatin when coadministered with elagolix, and individualize therapy based on patient response. Clinical monitoring is recommended for P-glycoprotein substrates with a narrow therapeutic window (eg, digoxin). Dose adjustments are not required for sertraline, fluconazole, bupropion (or any CYP2B6 substrate), or elagolix when coadministered.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Identification of rifampin-regulated functional modules and related microRNAs in human hepatocytes based on the protein interaction network

BACKGROUND

In combination with gene expression profiles, the protein interaction network (PIN) constructs a dynamic network that includes multiple functional modules. Previous studies have demonstrated that rifampin can influence drug metabolism by regulating drug-metabolizing enzymes, transporters, and microRNAs (miRNAs). Rifampin induces gene expression, at least in part, by activating the pregnane X receptor (PXR), which induces gene expression; however, the impact of rifampin on global gene regulation has not been examined under the molecular network frameworks.

METHODS

In this study, we extracted rifampin-induced significant differentially expressed genes (SDG) based on the gene expression profile. By integrating the SDG and human protein interaction network (HPIN), we constructed the rifampin-regulated protein interaction network (RrPIN). Based on gene expression measurements, we extracted a subnetwork that showed enriched changes in molecular activity. Using the Kyoto Encyclopedia of Genes and Genomes (KEGG), we identified the crucial rifampin-regulated biological pathways and associated genes. In addition, genes targeted by miRNAs that were significantly differentially expressed in the miRNA expression profile were extracted based on the miRNA-gene prediction tools. The miRNA-regulated PIN was further constructed using associated genes and miRNAs. For each miRNA, we further evaluated the potential impact by the gene interaction network using pathway analysis. RESULTS AND DISCCUSSION: We extracted the functional modules, which included 84 genes and 89 interactions, from the RrPIN, and identified 19 key rifampin-response genes that are associated with seven function pathways that include drug response and metabolism, and cancer pathways; many of the pathways were supported by previous studies. In addition, we identified that a set of 6 genes (CAV1, CREBBP, SMAD3, TRAF2, KBKG, and THBS1) functioning as gene hubs in the subnetworks that are regulated by rifampin. It is also suggested that 12 differentially expressed miRNAs were associated with 6 biological pathways.

CONCLUSIONS

Our results suggest that rifampin contributes to changes in the expression of genes by regulating key molecules in the protein interaction networks. This study offers valuable insights into rifampin-induced biological mechanisms at the level of miRNAs, genes and proteins.

Infectious complications in children with acute lymphoblastic leukemia treated in low-middle-income countries

Infections are the most important cause of morbidity and mortality in children treated for acute lymphoblastic leukemia (ALL). The rates of infection-associated mortality are up to 10-times higher in low- and middle-income countries (LMIC) than in high-income countries. The prevention, early recognition and management of infectious complications is especially challenging in LMIC because of disease and poverty-related factors, as well as the shortage of trained personnel, supplies, diagnostic tools and adequate organizational infrastructure. Children in LMIC with ALL, who are frequently underweight, are at increased risk of community-acquired pathogens, nosocomial multidrug-resistant pathogens and opportunistic microorganisms. This review summarizes the challenges of managing the major categories of infections in children receiving treatment for ALL and provides updated practical recommendations for preventing and managing these infections in LMIC.

Three- and four-class classification models for P-glycoprotein inhibitors using counter-propagation neural networks

P-glycoprotein (P-gp) is an ATP binding cassette (ABC) transporter that helps to protect several certain human organs from xenobiotic exposure. This efflux pump is also responsible for multi-drug resistance (MDR), an issue of the chemotherapy approach in the fight against cancer. Therefore, the discovery of P-gp inhibitors is considered one of the most popular strategies to reverse MDR in tumour cells and to improve therapeutic efficacy of commonly used cytotoxic drugs. Until now, several generations of P-gp inhibitors have been developed but they have largely failed in preclinical and clinical studies due to lack of selectivity, poor solubility and severe pharmacokinetic interactions. In this study, three models (SION, SIO, SIN) to classify specific 'true' P-gp inhibitors as well as three other models (CPBN, CPB1, CPN) to distinguish between P-gp inhibitors, CYP 3A inhibitors and co-inhibitors of these proteins with rather high accuracy values for the test set and the external set were generated based on counter-propagation neural networks (CPG-NN). Such three and four-class classification models helped provide more information about the bioactivities of compounds not only on one target (P-gp), but also on a combination of multiple targets (P-gp, CYP 3A).

Atovaquone and quinine anti-malarials inhibit ATP binding cassette transporter activity

Background

Therapeutic blood plasma concentrations of anti-malarial drugs are essential for successful treatment. Pharmacokinetics of pharmaceutical compounds are dependent of adsorption, distribution, metabolism, and excretion. ATP binding cassette (ABC) transport proteins are particularly involved in drug deposition, as they are located at membranes of many uptake and excretory organs and at protective barriers, where they export endogenous and xenobiotic compounds, including pharmaceuticals. In this study, a panel of well-established anti-malarial drugs which may affect drug plasma concentrations was tested for interactions with human ABC transport proteins.

Methods

The interaction of chloroquine, quinine, artemisinin, mefloquine, lumefantrine, atovaquone, dihydroartemisinin and proguanil, with transport activity of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), bile salt export pump (BSEP) and multidrug resistance-associated proteins (MRP) 1–4 were analysed. The effect of the anti-malarials on the ATP-dependent uptake of radio-labelled substrates was measured in membrane vesicles isolated from HEK293 cells overexpressing the ABC transport proteins.

Results

A strong and previously undescribed inhibition of BCRP-mediated transport by atovaquone with a 50% inhibitory concentration (IC₅₀) of 0.23 μM (95% CI 0.17-0.29 μM) and inhibition of P-gp-mediated transport by quinine with an IC₅₀ of 6.8 μM (95% CI 5.9-7.8 μM) was observed. Furthermore, chloroquine and mefloquine were found to significantly inhibit P-gp-mediated transport. BCRP transport activity was significantly inhibited by all anti-malarials tested, whereas BSEP-mediated transport was not inhibited by any of the compounds. Both MRP1- and MRP3-mediated transport were significantly inhibited by mefloquine.

Conclusions

Atovaquone and quinine significantly inhibit BCRP- and P-gp- mediated transport at concentrations within the clinically relevant prophylactic and therapeutic range. Co-administration of these established anti-malarials with drugs that are BCRP or P-gp substrates may potentially lead to drug-drug interactions.

Integrated approach of in vivo and in vitro evaluation of the involvement of hepatic uptake organic anion transporters in the drug disposition in rats using rifampicin as an inhibitor

Cumulative studies describe the importance of drug transporters as one of the key determinants of pharmacokinetics that necessitate investigation and assessment of the involvement of drug transporters in drug discovery and development. The present study investigated an integrated in vivo and in vitro approach to determine the involvement of organic anion transporting polypeptides (Oatps) in the disposition of drugs in rats using rifampicin as an inhibitor. When bromosulfophthalein (BSP) and HMG-CoA reductase inhibitors (statins), which were used as model substrates for Oatps, were administered intravenously (3 and 1 mg/kg, respectively) to rats pretreated with rifampicin orally (30 mg/kg), the total plasma clearance of BSP and statins was attenuated compared with that in control rats, suggesting the involvement of Oatps in the disposition of these drugs in vivo. On the other hand, the pharmacokinetics of midazolam, used as a model substrate of cytochrome P450 3a (Cyp3a), was unchanged between control rats and rifampicin-pretreated rats. The involvement of Oatps in the disposition of statins observed in vivo was further clarified by employing an in vitro hepatic uptake study and media-loss assay in the presence or absence of 100 μM rifampicin. Hepatic intrinsic clearance was reduced in the presence of rifampicin in both the media-loss assay and hepatocyte uptake study. The present study suggests in vivo investigations in rats using rifampicin together with in vitro investigations with a media-loss assay and/or uptake assay using rat hepatocytes can help determine whether a clinical drug-drug interaction study is necessary in drug development.

Hepatobiliary disposition of 17-OHPC and taurocholate in fetal human hepatocytes: a comparison with adult human hepatocytes

Little information is available in the literature regarding the expression and activity of transporters in fetal human liver or cultured cells. A synthetic progesterone structural analog, 17α-hydroxyprogesterone caproate (17-OHPC), is used in the prevention of spontaneous abortion in women with a history of recurrent miscarriage (habitual abortion). 17-OHPC has been reported to traverse the placental barrier and gain access to fetal circulation. In this study, the role of transporters in the disposition of 17-OHPC in fetal and adult human hepatocytes was examined. Progesterone metabolites have been reported to induce trans-inhibition of bile acid transporter, ABCB11. Thus, we investigated the effect of 17-OHPC or its metabolites on [(3)H]taurocholic acid transport in sandwich-cultured human fetal and adult hepatocytes. 17-OHPC was taken up rapidly into the cells and transported out partially by an active efflux process that was significantly inhibited by cold temperature, cyclosporine, verapamil, and rifampin. The active efflux mechanism was observed in both adult and fetal hepatocyte cultures. 17-OHPC produced a concentration-dependent inhibition of taurocholate efflux into canaliculi in sandwich-cultured adult and fetal human hepatocytes. However, given the high concentrations required to cause inhibition of these transport processes, no adverse effects would be anticipated from therapeutic levels of 17-OHPC. We also evaluated the expression of various hepatic transporters (ABCB1, ABCB4, SLCO1B1, SLCO1B3, SLCO2B1, ABCB11, SLC10A1, ABCC2, ABCC3, ABCC4, and ABCG2) in fetal and adult hepatocytes. With the exception of ABCB4, all transporters examined were expressed, albeit at lower mRNA levels in fetal hepatocytes compared with adults.

Effects of rifampicin, dexamethasone, St. John's Wort and Thyroxine on maternal and foetal expression of Abcb1 and organ distribution of talinolol in pregnant rats

It is well accepted that ABCB1 plays a critical role in absorption, distribution and elimination of many xenobiotics and drugs. Only little is known about the regulation and function of ABCB1 during pregnancy. Thus, the aim of this study is to investigate maternal, placental and foetal Abcb1 expression and function in pregnant rats after induction with rifampicin, dexamethasone, St. John's wort (SJW) or thyroxine. Wistar rats were orally treated with rifampicin (250 mg/kg), SJW (1.0 g/kg), thyroxine (9 μg/kg), dexamethasone (1 mg/kg) or 0.5% methylcellulose suspension (control) for 9 days during late pregnancy (each N = 5). Afterwards, organ mRNA expression and protein content of Abcb1a were determined. Tissue concentrations of the ABCB1 probe drug talinolol were measured after repeated administration of the drug (100 mg/kg, 9 days) and after induction with oral rifampicin (250 mg/kg, 9 days, N = 5). Abcb1 expression was substantially lower in foetal than in maternal organs. Abcb1 was significantly induced by SJW in the maternal jejunum and placenta, by dexamethasone in foetal brain and liver and by thyroxine in the placenta and maternal and foetal brain. Rifampicin induced Abcb1 in all maternal and foetal organs. However, organ distribution of talinolol was not influenced by comedication of rifampicin. In conclusion, maternal and foetal Abcb1 organ expression in pregnant rats is inducible by nuclear receptor agonists. Although rifampicin regulates maternal and foetal Abcb1 expression, organ distribution of talinolol remains unchanged most likely caused by the known inhibitory effect of rifampicin on Abcb1 function.

Transporter-mediated drug-drug interactions

Drug-drug interactions are a serious clinical issue. An important mechanism underlying drug-drug interactions is induction or inhibition of drug transporters that mediate the cellular uptake and efflux of xenobiotics. Especially drug transporters of the small intestine, liver and kidney are major determinants of the pharmacokinetic profile of drugs. Transporter-mediated drug-drug interactions in these three organs can considerably influence the pharmacokinetics and clinical effects of drugs. In this article, we focus on probe drugs lacking significant metabolism to highlight mechanisms of interactions of selected intestinal, hepatic and renal drug transporters (e.g., organic anion transporting polypeptide [OATP] 1A2, OATP2B1, OATP1B1, OATP1B3, P-gp, organic anion transporter [OAT] 1, OAT3, breast cancer resistance protein [BCRP], organic cation transporter [OCT] 2 and multidrug and toxin extrusion protein [MATE] 1). Genotype-dependent drug-drug interactions are also discussed.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.

Rifampin's acute inhibitory and chronic inductive drug interactions: experimental and model-based approaches to drug-drug interaction trial design

We studied the time course for the reversal of rifampin's effect on the pharmacokinetics of oral midazolam (a cytochrome P450 (CYP) 3A4 substrate) and digoxin (a P-glycoprotein (P-gp) substrate). Rifampin increased midazolam metabolism, greatly reducing the area under the concentration-time curve (AUC(0-∞)). The midazolam AUC(0-∞) returned to baseline with a half-life of ~8 days. Rifampin's effect on the AUC(0-3 h) of digoxin was biphasic: the AUC(0-3 h) increased with concomitant dosing of the two drugs but decreased when digoxin was administered after rifampin. Digoxin was found to be a weak substrate of organic anion-transporting polypeptide (OATP) 1B3 in transfected cells. Although the drug was transported into isolated hepatocytes, it is not likely that this transport was through OATP1B3 because the transport was not inhibited by rifampin. However, rifampin did inhibit the P-gp-mediated transport of digoxin with a half-maximal inhibitory concentration (IC(50)) below anticipated gut lumen concentrations, suggesting that rifampin inhibits digoxin efflux from the enterocyte to the intestinal lumen. Pharmacokinetic modeling suggested that the effects on digoxin are consistent with a combination of inhibitory and inductive effects on gut P-gp. These results suggest modifications to drug-drug interaction (DDI) trial designs.

Drug interactions in childhood cancer

Children with cancer are increasingly benefiting from new treatment strategies and advances in supportive care, as shown by improvements in both survival and quality-of-life. However, the continuous emergence of new cancer drugs and supportive-care drugs has increased the possibility of harmful drug interactions; health-care providers need to be very cautious when combining drugs. We discuss the most common interactions between chemotherapeutic drugs and supportive-care drugs-such as anticonvulsants, antiemetics, uric-acid-lowering compounds, acid suppressants, antimicrobials, and pain-management medications in paediatric patients. We also review the interactions between chemotherapy drugs and food and herbal supplements, and provide recommendations to avoid unwanted and potentially fatal interactions in children with cancer. Because of the constant release of new drugs, health-care providers need to check the most recent references before making recommendations about drug interactions.

Effects of p-glycoprotein on steroid-induced osteonecrosis of the femoral head

P-glycoprotein (P-gp) activity may play an important role in steroid-induced osteonecrosis of the femoral head (ONF); however, the precise mechanism of its pathogenesis remains unknown. Therefore, we investigated the effects of increased P-gp activity on steroid-induced ONF using a rat model. Rats (n = 60) were treated with either a pharmacological stimulant of P-gp, rifampicin (group A); a suppressant, verapamil (group B); or normal saline (group C) administered in conjunction with methylprednisolone, an inducer of ONF. P-gp activity in bone marrow cells and expression in the femoral head significantly increased in group A (P < 0.05) but decreased in group B (P < 0.05). Likewise, the serum osteocalcin level, trabecular thickness and number, osteoclast and osteoblast numbers, and mean percentage of the epiphyseal ossification center were significantly increased in group A (P < 0.01) but decreased in group B (P < 0.01). In contrast, however, adipocytic variables, trabecular separation, and apoptotic cells decreased in group A (P < 0.01) but increased in group B (P < 0.01). The ONF incidence in group A (50%) and group B (100%) was significantly different from that in the control group C (80%, P < 0.05). Taken together, our findings suggested that enhanced P-gp activity was able to decrease the risk of steroid-induced ONF, possibly by inhibiting adipogenesis and apoptosis in the femoral head.

Control of Mycosphaerella graminicola on wheat seedlings by medical drugs known to modulate the activity of ATP-binding cassette transporters

Medical drugs known to modulate the activity of human ATP-binding cassette (ABC) transporter proteins (modulators) were tested for the ability to potentiate the activity of the azole fungicide cyproconazole against in vitro growth of Mycosphaerella graminicola and to control disease development due to this pathogen on wheat seedlings. In vitro modulation of cyproconazole activity could be demonstrated in paper disk bioassays. Some of the active modulators (amitriptyline, flavanone, and phenothiazines) increased the accumulation of cyproconazole in M. graminicola, suggesting that they reversed cyproconazole efflux. However, synergism between cyproconazole and modulators against M. graminicola on wheat seedlings could not be shown. Despite their low in vitro toxicity to M. graminicola, some modulators (amitriptyline, loperamide, and promazine) did show significant intrinsic disease control activity in preventive and curative foliar spray tests with wheat seedlings. The results suggest that these compounds have indirect disease control activity based on modulation of fungal ABC transporters essential for virulence and constitute a new class of disease control agents.

Multiple transporters affect the disposition of atorvastatin and its two active hydroxy metabolites: application of in vitro and ex situ systems

Atorvastatin (ATV) is primarily metabolized by CYP3A in the liver to form two active hydroxy metabolites. Therefore, the sequential transport system governed by hepatic uptake and efflux transporters is important for the drug disposition and metabolism. Here, we assessed the interaction of ATV with hepatic uptake transporter organic anion transporting polypeptide (Oatp) and efflux transporter multidrug resistance associated protein 2 (MRP2/Mrp2) in vitro and ex situ using the isolated perfused rat liver (IPRL). Rifampicin (RIF) was chosen as an inhibitor for Oatp in both uptake and IPRL studies. Its inhibitory effects on MRP2 and metabolism were also tested using MRP2-overexpressing cells and rat microsomes, respectively. Our results indicate that RIF effectively inhibits the Oatp-mediated uptake of ATV and its metabolites. Inhibition on MRP2-mediated efflux of ATV was also observed at a high RIF concentration. Compared with ATV alone in the IPRL, the area under the curve(s) (AUC) of ATV was significantly increased by RIF, whereas the AUC of both metabolites were also increased in a concentration-dependent manner. However, the extent of metabolism was significantly reduced, as reflected by the reduced amounts of metabolites detected in RIF-treated livers. In conclusion, inhibition of Oatp-mediated uptake seems to be the major determinant for interaction between ATV and RIF. Metabolites of ATV were subject to Oatp-mediated uptake as well, suggesting that they undergo a similar disposition pathway as the parent drug. These data emphasize the relevance of uptake transporter as being one of the major players in hepatic drug elimination, even for substrates that undergo metabolism.

Increased expression of MDR1 mRNAs and P-glycoprotein in placentas from HIV-1 infected women

P-glycoprotein transports several compounds including protease inhibitors, actually used in the clinical management of HIV-1 infection. Since P-glycoprotein is expressed in placental trophoblasts, its efflux activity could interfere with placental transfer of antiretrovirals. The purpose of this study was to investigate the expression of P-gp-encoding MDR1 gene and P-gp itself in full-term placentas from uninfected (n=35) and HIV-1 infected women (n=24). MDR1 transcripts were quantified by real-time PCR using relative (MDR1 normalized upon 28S levels) and absolute (copy number) determinations. P-glycoprotein localization and expression were evaluated by immunohistochemistry and western blot analysis, respectively. Relative or absolute PCR quantification showed a significant 3.3-fold (p<0.0009) or 3.7-fold (p<0.0002) mean increase in MDR1 placental transcription in HIV-infected compared to non-infected women, respectively. Ratios of individual HIV-positive values to HIV-negative mean ranged from 0.1 to 21.8. Moreover a significant 2.5-fold increased expression of immunoreactive P-glycoprotein was evidenced in placentas from HIV-infected women (p<0.0001). This MDR1 overexpression was observed in a similar extent in placentas from pregnant women treated with Zidovudine alone or in combination with Nelfinavir and/or Lamivudine. Our findings suggest that P-glycoprotein in placentas from HIV-infected women would contribute to modulate the materno-fetal transport of antiretrovirals across the placental barrier and consequently diminish fetal exposure to these compounds.

Recent advances in the pharmacotherapy for hyperbilirubinaemia in the neonate

Jaundice is a common cause for diagnostic works-up and therapeutic intervention in neonates. This is motivated by the risk for severe neurological sequelae (kernicterus). The mainstays of treatment for the past decades have been exchange transfusion and phototherapy. Exchange transfusion is now becoming rare due to immune prophylaxis in Rhesus-negative women, and treatment of sensitised infants with intravenous immunoglobulin. Several different pharmacological approaches have been studied as far as the treatment of neonatal jaundice. Of these, the focus of attention in recent years has been on the haem oxygenase inhibitors (metal meso- and protoporphyrins). These are effective inhibitors of bilirubin production and have been shown to significantly reduce peak serum bilirubin levels in several clinical trials, both when used prophylactically and therapeutically. However, questions remain regarding long-term safety, as well as the advisability of whole-scale inhibition of bilirubin production. Nevertheless, in selected infants with a high risk of severe jaundice, the use of haem oxygenase inhibitors may be acceptable. Pharmacotherapy in jaundiced infants is fraught with risks, as many drugs may increase the entry of bilirubin into the brain and presumably, the risk for neurotoxicity. Both the displacement of bilirubin from its albumin binding and interference with the function of phosphoglycoprotein in the blood-brain barrier are documented mechanisms in this respect.

Transport characteristics of fexofenadine in the Caco-2 cell model

PURPOSE

To investigate the membrane transport mechanisms of fexofenadine in the Caco-2 model.

METHODS

Transport studies were performed in Caco-2 cell monolayers 21-25 days after seeding. The apparent permeability (Papp) of fexofenadine was determined in the concentration range 10-1000 microM in the basolateral-to-apical (b-a) and 50-1000 microM in the apical-to-basolateral (a-b) direction. The concentration-dependent effects of various inhibitors of P-glycoprotein (P-gp) (GF120918, ketoconazole, verapamil, erythromycin), multidrug resistant associated protein (MRP) (indomethacin, probenecid), and organic anion transporting polypeptide (OATP) (rifamycin SV) on the bidirectional transport of 150 microM fexofenadine were also examined.

RESULTS

Fexofenadine displayed polarized transport, with the Pappb-a being 28- to 85-fold higher than the Papp(a-b). The Papp(a-b) was independent of the concentration applied, whereas Pappb-a decreased with increasing concentration (Vmax = 5.21 nmol cm(-2)s(-1) and K(M) = 150 microM), suggesting saturation of an apical efflux transporter. All four P-gp inhibitors had a strong, concentration-dependent effect on the Papp of fexofenadine in both directions, with GF 120918 being the most specific among them. The IC50 of verapamil was 8.44 microM on the P-gp-mediated secretion of fexofenadine. The inhibitors of OATP or MRP appeared not to affect the Papp(a-b) of fexofenadine in the Caco-2 model.

CONCLUSIONS

This study clearly indicates that P-gp was the main transport protein of fexofenadine in the Caco-2 model. Even though P-gp was completely inhibited, fexofenadine was predicted to have a low fraction dose absorbed in humans due to poor intestinal permeability, and low passive diffusion seems to be the major absorption mechanism.

ASSESSMENT OF DRUG INTERACTIONS IN HEPATOBILIARY TRANSPORT USING RHODAMINE 123 IN SANDWICH-CULTURED RAT HEPATOCYTES

The purpose of the present study was to explore the utility of sandwich-cultured rat hepatocytes as an in vitro tool to examine drug interactions at the hepatic transport level. Rhodamine 123 was used as a model substrate for P-glycoprotein-mediated biliary excretion. Effects of various types of P-glycoprotein modulation on the biliary excretion index (BEI; a relative measure of the extent of biliary excretion) and the in vitro biliary clearance (CL(bile)) were determined. Significant reductions in rhodamine 123 BEI and CL(bile) were noted in the presence of the P-glycoprotein inhibitors verapamil (30-100 microM) and progesterone (100 microM). The P-glycoprotein activator quercetin (10-100 microM) enhanced rhodamine 123 CL(bile) by approximately 4-fold, with only a minor effect on BEI, suggesting that quercetin had a more pronounced effect on uptake at the basolateral membrane rather than excretion across the canalicular membrane. Treatment of hepatocytes for 48 h with dexamethasone (10 microM) resulted in significant enhancement of CL(bile), whereas rifampin (5-50 microM) increased both BEI and CL(bile), indicating that the inducing effects of dexamethasone and rifampin were occurring at the basolateral and canalicular membranes, respectively. Total rhodamine 123 uptake in sandwich-cultured rat hepatocytes was partly saturable and was affected by the presence of typical Oatp1a4 substrates (digoxin, quinine, d-verapamil, 17beta-estradiol-d-17beta-glucuronide). In summary, sandwich-cultured rat hepatocytes are a useful tool to study mechanisms of hepatobiliary drug disposition and to predict the potential for drug interactions in hepatic transport.

Evidence of Efflux-Mediated and Saturable Absorption of Rifampicin in Rat Intestine Using the Ligated Loop and Everted Gut Sac Techniques

This study was carried out to explore whether efflux-mediated and saturable mechanisms play any role toward poor and variable intestinal absorption of rifampicin. In situ segmental permeability of rifampicin at various residence times was determined in rat gastrointestinal tract using the ligated loop technique. The involvement of efflux-mediated and saturable absorption of rifampicin was studied in rat intestine using the everted sac method. The samples were analyzed by a validated HPLC method. Rifampicin showed decreased permeability in jejunum and ileum with an increase in residence time. The permeation of rifampicin from the serosal to the mucosal side (secretion) was significantly higher than permeation from the mucosal to the serosal side (absorption) of jejunum and ileum. This indicated the involvement of efflux-mediated transport. Addition of verapamil, an inhibitor for P-glycoprotein (Pgp), multidrug resistance associated protein-2 (MRP-2), and other related transporters, increased absorption of rifampicin in jejunum and ileum by 2-3-fold and decreased secretion by almost 4-fold. The permeation rate (flux) of rifampicin through duodenum increased with concentration up to 300 microg/mL, becoming constant thereafter, indicating the existence of saturable absorption. There was no saturable permeation in jejunum and ileum. Thus the present study indicates the involvement of efflux-mediated and saturable absorption mechanisms of rifampicin in rat intestine, which act as barriers to the absorption of the drug. This explains the drug's poor absolute bioavailability. As Pgp varies from person to person to an extent of 2-8-fold, it can be one direct reason for the interindividual variable bioavailability shown by rifampicin.

Pharmacokinetic interactions with rifampicin : clinical relevance

The antituberculosis drug rifampicin (rifampin) induces a number of drug-metabolising enzymes, having the greatest effects on the expression of cytochrome P450 (CYP) 3A4 in the liver and in the small intestine. In addition, rifampicin induces some drug transporter proteins, such as intestinal and hepatic P-glycoprotein. Full induction of drug-metabolising enzymes is reached in about 1 week after starting rifampicin treatment and the induction dissipates in roughly 2 weeks after discontinuing rifampicin. Rifampicin has its greatest effects on the pharmacokinetics of orally administered drugs that are metabolised by CYP3A4 and/or are transported by P-glycoprotein. Thus, for example, oral midazolam, triazolam, simvastatin, verapamil and most dihydropyridine calcium channel antagonists are ineffective during rifampicin treatment. The plasma concentrations of several anti-infectives, such as the antimycotics itraconazole and ketoconazole and the HIV protease inhibitors indinavir, nelfinavir and saquinavir, are also greatly reduced by rifampicin. The use of rifampicin with these HIV protease inhibitors is contraindicated to avoid treatment failures. Rifampicin can cause acute transplant rejection in patients treated with immunosuppressive drugs, such as cyclosporin. In addition, rifampicin reduces the plasma concentrations of methadone, leading to symptoms of opioid withdrawal in most patients. Rifampicin also induces CYP2C-mediated metabolism and thus reduces the plasma concentrations of, for example, the CYP2C9 substrate (S)-warfarin and the sulfonylurea antidiabetic drugs. In addition, rifampicin can reduce the plasma concentrations of drugs that are not metabolised (e.g. digoxin) by inducing drug transporters such as P-glycoprotein. Thus, the effects of rifampicin on drug metabolism and transport are broad and of established clinical significance. Potential drug interactions should be considered whenever beginning or discontinuing rifampicin treatment. It is particularly important to remember that the concentrations of many of the other drugs used by the patient will increase when rifampicin is discontinued as the induction starts to wear off.

Ex Situ Inhibition of Hepatic Uptake and Efflux Significantly Changes Metabolism: Hepatic Enzyme-Transporter Interplay

The disposition of digoxin and the influence of the organic anion transporting polypeptide (Oatp)2 inhibitor rifampicin and the P-glycoprotein (P-gp) inhibitor quinidine on its hepatic disposition were examined in the isolated perfused rat liver. Livers from groups of rats were perfused in a recirculatory manner after a bolus dose of digoxin (10 microg), a dual substrate for Oatp2 and P-gp as well as CYP3A. Perfusions of digoxin were also examined in groups of rats in the presence of the inhibitors: rifampicin (100 microM) or quinidine (10 microM). In all experiments, perfusate samples were collected for 60 min. Digoxin and its primary metabolite were determined in perfusate and liver by liquid chromatography/mass spectrometry. The area under the curve (AUC) from 0 to 60 min was determined. The AUC +/- S.D. of digoxin was increased from control (3880 +/- 210 nM x min) by rifampicin (5200 +/- 240 nM x min; p < 0.01) and decreased by quinidine (3220 +/- 340 nM x min; P < 0.05). It is concluded that rifampicin limits the hepatic entrance of digoxin and reduced the hepatic exposure of digoxin to CYP3A by inhibiting the basolateral Oatp2 uptake transport, whereas quinidine increased the hepatic exposure of digoxin to CYP3A by inhibiting the canalicular P-gp transport. These data emphasize the importance of uptake and efflux transporters on hepatic drug metabolism.

Administration of drugs known to inhibit P-glycoprotein increases brain bilirubin and alters the regional distribution of bilirubin in rat brain

P-glycoprotein (P-gp) is an ATP-dependent integral plasma membrane efflux pump, expressed in abundance in brain capillary endothelial cells and astrocytes. P-gp contributes to the blood-brain barrier in limiting the influx and retention of a variety of lipophilic compounds, including unconjugated bilirubin. Several drugs block P-gp function and thereby enhance intracellular accumulation of P-gp substrates. In this study we proposed that pretreatment with drugs known to inhibit P-gp function in clinically relevant doses would alter the uptake of bilirubin in the brain of 32- to 36-d-old rats. In the first arm of the study, the animals received pretreatment with an i.v. infusion of either propanolol, erythromycin, verapamil, ceftriaxone, rifampin, or saline, 10 min before an i.v. bolus of 50 mg/kg bilirubin was given. Except for the erythromycin-treated rats, all treatment groups had significantly higher brain-to-serum bilirubin ratios than control animals (p < 0.05, Welch's t test). In the second arm of the study, treatment with either ceftriaxone or rifampin or saline i.v. preceded a 50 mg/kg i.v. bolus of radioactive bilirubin. Analysis of seven different brain regions by scintillation counting showed that the distribution patterns differed significantly between the study groups (p < 0.001, ANOVA), however, not in accordance with a kernicteric staining pattern. Because of limited knowledge of expression and function of P-gp and other membrane transport proteins in the newborn, the implications of this study remain to be seen. We speculate that drugs known to inhibit P-gp function may increase the risk of bilirubin encephalopathy in the hyperbilirubinemic infant.

Modulation of P-glycoprotein transport activity in the mouse blood-brain barrier by rifampin

The objective of the present study was to examine the time course and concentration dependence of modulation of P-glycoprotein (P-gp) activity in the blood-brain barrier (BBB) with consequent influence on substrate uptake into brain tissue. Potential P-gp inducers (rifampin and morphine) were administered subchorionically to P-gp-competent [mdr1a(+/+)] mice to induce P-gp expression in brain; the impact of rifampin pretreatment on brain penetration of verapamil also was evaluated with an in situ brain perfusion technique. In addition, the effect of single-dose rifampin on P-gp BBB transport activity was assessed with brain perfusion using verapamil and quinidine as model P-gp substrates. Chronic exposure to rifampin or morphine induced P-gp expression in mouse brain to a modest extent. However, single-dose rifampin treatment increased the brain uptake of verapamil and quinidine in mdr1a(+/+) mice in a dose- and concentration-dependent manner, consistent with P-gp inhibition. Maximum inhibition of P-gp-mediated efflux of verapamil by rifampin pretreatment in vivo (150 mg/kg) was approximately 55%, whereas there was only approximately 12% inhibition of P-gp-mediated efflux of quinidine at that rifampin dose. Coperfusion of rifampin at a concentration of 500 microM abolished P-gp-mediated efflux of verapamil at the BBB. However, only approximately 40% inhibition of P-gp-mediated efflux of quinidine was observed with coperfusion of rifampin, even at a 2-fold higher rifampin concentration (1000 microM). The present studies demonstrate that P-gp function at the BBB can be modulated by rifampin in a dose- and concentration-dependent manner. The degree to which rifampin inhibits P-gp-mediated transport is dependent on the substrate molecule.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Lymphocyte P-glycoprotein expression and activity before and after rifampicin in man

It has recently been shown that P-glycoprotein (P-gp) is inducible by rifampicin in the human gut as shown in intestinal biopsies. The present study was performed in order to test the hypothesis that human peripheral lymphocytes can be used to assess such an inducibility. We also assessed inter- and intra-individual variability of P-gp expression and activity in peripheral lymphocytes. Blood samples from 13 healthy volunteers were collected 1.7, 14 and 19 days after inclusion. Rifampicin treatment (600 mg/day) was administered from day 15 to day 18. Lymphocyte P-gp expression was measured at the messenger RNA level by semi-quantitative RT-PCR and at the protein level by immunostaining flow cytometry. P-gp activity was determined by flow cytometry with rhodamine 123 efflux. Cytochrome P4503A4 (CYP3A4) inducibility was measured by comparing the urinary metabolic ratio of 6beta-hydroxycortisol/cortisol on day 14 and 19, Lymphocyte P-gp expression and activity was not induced by rifampicin, while it increased CYP3A4 activity from 5.0 +/- 4.0 to 22.9 +/- 16.6 (P < 0.001). There was a 3 - 4-fold inter-individual variability and a 3 - 44 % intra-individual variability of lymphocyte P-gp expression and activity. Peripheral lymphocytes are not an appropriate material to assess P-gp inducibility in humans. P-gp shows significant inter- and intra-individual variability in human lymphocytes.

Antibiotic efflux pumps

Active efflux from procaryotic as well as eucaryotic cells strongly modulates the activity of a large number of antibiotics. Effective antibiotic transport has now been observed for many classes of drug efflux pumps. Thus, within the group of primary active transporters, predominant in eucaryotes, six families belonging to the ATP-binding cassette superfamily, and including the P-glycoprotein in the MDR (Multi Drug Resistance) group and the MRP (Multidrug Resistance Protein), have been recognized as being responsible for antibiotic efflux. Within the class of secondary active transporters (antiports, symports, and uniports), ten families of antibiotic efflux pumps have been described, distributed in five superfamilies [SMR (Small Multidrug Resistance), MET (Multidrug Endosomal Transporter), MAR (Multi Antimicrobial Resistance), RND (Resistance Nodulation Division), and MFS (Major Facilitator Superfamily)]. Nowadays antibiotic efflux pumps are believed to contribute significantly to acquired bacterial resistance because of the very broad variety of substrates they recognize, their expression in important pathogens, and their cooperation with other mechanisms of resistance. Their presence also explains high-level intrinsic resistances found in specific organisms. Stable mutations in regulatory genes can produce phenotypes of irreversible multidrug resistance. In eucaryotes, antibiotic efflux pumps modulate the accumulation of antimicrobials in phagocytic cells and play major roles in their transepithelial transport. The existence of antibiotic efflux pumps, and their impact on therapy, must now be taken fully into account for the selection of novel antimicrobials. The design of specific, potent inhibitors appears to be an important goal for the improved control of infectious diseases in the near future.

Inhibition of multidrug resistance-associated protein (MRP) activity by rifampicin in human multidrug-resistant lung tumor cells

The multidrug resistance-associated protein (MRP) is a drug efflux membrane pump conferring multidrug resistance on tumor cells. In order to look for compounds that can lead to reversal of such a resistance, the antituberculosis compound rifampicin, belonging to the chemical class of rifamycins, was examined for its effect on MRP activity in human multidrug resistant lung cancer GLC4/ADR cells. Rifampicin was shown to increase accumulation of the MRP substrate calcein in GLC4/ADR cells in a dose-dependent manner by inhibiting its MRP-mediated efflux from the cells; it also enhanced intracellular retention of another substrate of MRP such as the anticancer drug vincristine in the resistant cells. By contrast, the antituberculosis drug did not alter cellular levels of accumulation of either calcein or vincristine in parental drug-sensitive GLC4 cells. Other rifamycins such as rifamycin B and rifamycin SV were also demonstrated to increase intracellular accumulation of calcein in GLC4/ADR cells. These results therefore indicate that rifamycins, including rifampicin, probably constitute a new chemical class of modulators down-regulating MRP-mediated drug transport.

Evidence for a multidrug resistance-associated protein 1 (MRP1)-related transport system in cultured rat liver biliary epithelial cells

Cellular accumulation and efflux of the anionic fluorescent dye carboxy-2',7'-dichlorofluorescein (CF) were studied in rat liver SDVI cells thought to derive from primitive bile ductules, in order to characterize carrier-related membrane transport of organic anions in epithelial cells. Probenecid, a common blocker of anion transport, was found to strongly enhance CF levels in SDVI cells in a dose-dependent manner through inhibition of dye efflux. Such an outwardly-directed transport was demonstrated to be temperature-dependent and down-regulated by various metabolic inhibitors, therefore outlining its requirement for energy; it was shown to be Na+- and membrane potential-independent and inhibited by anionic drugs such as indomethacin, indoprofen and rifamycin B. These functional features are closed to those described for multidrug resistance-associated protein 1 (MRP1) that was furthermore demonstrated, in contrast to P-glycoprotein, to be expressed in SDVI cells and to lower CF accumulation in MRP1-overexpressing drug-resistant tumor cells. These data therefore suggest that active membrane transport of organic anions such as CF occurs in epithelial cells like cultured liver biliary SDVI cells through a MRP1-related efflux system.

Reduced intracellular activity of antibiotics against Listeria monocytogenes in multidrug resistant cells

Multidrug resistance is expressed not only by bacteria, but also by tumor cells and by some normal cells of the body. It enables eukaryotic cells to exclude not only cytostatic drugs but also non-cytostatic antibiotics. This was demonstrated in genetically engineered multidrug resistant (MDR) cells infected with the facultative intracellular bacterium Listeria monocytogenes for all macrolide antibiotics tested (azithromycin, clarithromycin, erythromycin, josamycin, roxithromycin and spiramycin). In these cells and in conventionally selected MDR cells higher concentrations of the macrolides were necessary to inhibit the growth of L. monocytogenes than in the respective parental cells. This effect was due to a reduced intracellular accumulation, which was shown with a biological assay for all macrolides tested. For azithromycin, the results of this test were confirmed by measurement of the intracellular concentrations with high-performance liquid chromatography (HPLC). Besides the macrolides, MDR cells excluded also antibiotics of other chemical groups which was shown for ciprofloxacin, clindamycin, rifampicin and the streptogramin derivative RP 59500. In addition, in conventionally selected cells higher concentrations of chloramphenicol, doxycyclin, ofloxacin and trimethoprim than in the respective parental cells were necessary to inhibit the growth of L. monocytogenes. In contrast, when using genetically engineered cells, no significant differences were found for these antibiotics. These differences might be due to a higher expression of multidrug resistance in the conventionally selected cells because these cells were also more effective in excluding rhodamine 123 in a flow cytometric assay. In conclusion, expression of multidrug resistance by eukaryotic cells leads to a reduced concentration of macrolides and other antibiotics in these cells and to an impairment of activity against intracellular bacteria.

Contributions of hepatic and intestinal metabolism and P-glycoprotein to cyclosporine and tacrolimus oral drug delivery

The objective of this section is to evaluate the contributions of hepatic metabolism, intestinal metabolism and intestinal p-glycoprotein to the pharmacokinetics of orally administered cyclosporine and tacrolimus. Cyclosporine and tacrolimus are metabolized primarily by cytochrome P450 3A4 (CYP3A4) in the liver and small intestine. There is also evidence that cyclosporine is metabolized to a lesser extent by cytochrome P450 3A5 (CYP3A5). Cyclosporine and tacrolimus are also substrates for p-glycoprotein, which acts as a counter-transport pump, actively transporting cyclosporine and tacrolimus back into the intestinal lumen. Traditional teaching of clinical drug metabolism has been that hepatic metabolism is of primary importance, and other sites of metabolism play a relatively minor role. It appears as though intestinal metabolism plays a much greater role in the pharmacokinetics of orally administered drugs than previously thought. Intestinal metabolism may account for as much as 50% of oral cyclosporine metabolism. There are at least two components of intestinal metabolism for cyclosporine and tacrolimus, intestinal CYP3A4/CYP3A5 and intestinal p-glycoprotein activities. The quantity of intestinal enzymes, although highly variable, do not appear to be the key to explaining the variability of oral cyclosporine pharmacokinetics in kidney transplant patients. However, the quantity of intestinal p-glycoprotein accounts for approximately 17% of the variability in oral cyclosporine pharmacokinetics. It may be that p-glycoprotein maximizes drug exposure to intestinal enzymes, thus decreasing the importance of enzyme quantity. Since cyclosporine's FDA approval in 1983, there have been many reports of clinically significant drug interactions of other agents when given concomitantly with cyclosporine. With the FDA approval of tacrolimus in 1994, a similar pattern of clinically significant drug interactions appears to be emerging. It seems that compounds that alter (either induce or inhibit) CYP3A4 and/or p-glycoprotein will alter the oral pharmacokinetics of cyclosporine and tacrolimus. It should be expected that, until further data are available, the drugs which interact with cyclosporine will also interact with tacrolimus.

The P-glycoprotein multidrug transporter

1. P-glycoprotein (P-gp) is a transmembrane protein involved in ATP-dependent efflux of various structurally unrelated anticancer drugs. Its overexpression in cancer cells decreases intracellular drug concentrations and, thus, confers a multidrug resistance phenotype. 2. P-gp is encoded by MDR genes, which constitute a small gene family comprising two genes in humans and three genes in rodents. Only the MDR1 gene in humans and mdr1 and mdr3 genes in rodents have been demonstrated to be involved in drug resistance. 3. P-gp encoded by the human MDR1 gene is a phosphorylated and glycosylated protein 1289 amino acids long, and consists of 2 halves that share a high degree of similarity. 4. A wide variety of cancers have been shown to express P-gp, including solid tumors and hematological malignancies. This P-gp positivity can be evidenced at the time of diagnosis prior to chemotherapy or at relapse after treatment, and has been correlated with treatment failure and poor prognosis in several types of cancer. In addition, P-gp is also expressed by some normal tissues, such as liver and kidney. 5. P-gp expression is regulated by various factors, including xenobiotics and hormones. 6. P-gp-mediated multidrug resistance can be reversed by various unrelated compounds called chemosensitizers or reversing agents. These drugs act through inhibition of P-gp function and have entered clinical trials.