Role of P-glycoprotein as a secretory mechanism in quinidine absorption from rat small intestine

Cardiovascular Ion Channel Inhibitor Drug-Drug Interactions with P-glycoprotein

P-glycoprotein (Pgp) is an ATP-binding cassette (ABC) transporter that plays a major role in cardiovascular drug disposition by effluxing a chemically and structurally diverse range of cardiovascular therapeutics. Unfortunately, drug-drug interactions (DDIs) with the transporter have become a major roadblock to effective cardiovascular drug administration because they can cause adverse drug reactions (ADRs) or reduce the efficacy of drugs. Cardiovascular ion channel inhibitors are particularly susceptible to DDIs and ADRs with Pgp because they often have low therapeutic indexes and are commonly coadministered with other drugs that are also Pgp substrates. DDIs from cardiovascular ion channel inhibitors with the transporter occur because of inhibition or induction of the transporter and the transporter's tissue and cellular localization. Inhibiting Pgp can increase absorption and reduce excretion of drugs, leading to elevated drug plasma concentrations and drug toxicity. In contrast, inducing Pgp can have the opposite effect by reducing the drug plasma concentration and its efficacy. A number of in vitro and in vivo studies have already demonstrated DDIs from several cardiovascular ion channel inhibitors with human Pgp and its animal analogs, including verapamil, digoxin, and amiodarone. In this review, Pgp-mediated DDIs and their effects on pharmacokinetics for different categories of cardiovascular ion channel inhibitors are discussed. This information is essential for improving pharmacokinetic predictions of cardiovascular therapeutics, for safer cardiovascular drug administration and for mitigating ADRs emanating from Pgp.

ABC transporters in multidrug resistance and pharmacokinetics, and strategies for drug development

Multidrug resistance (MDR) is a serious problem that hampers the success of cancer pharmacotherapy. A common mechanism is the overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2) that limit the exposure to anticancer drugs. One way to overcome MDR is to develop ABC efflux transporter inhibitors to sensitize cancer cells to chemotherapeutic drugs. The complete clinical trials thus far have showen that those tested chemosensitizers only add limited or no benefits to cancer patients. Some MDR modulators are merely toxic, and others induce unwanted drug-drug interactions. Actually, many ABC transporters are also expressed abundantly in the gastrointestinal tract, liver, kidney, brain and other normal tissues, and they largely determine drug absorption, distribution and excretion, and affect the overall pharmacokinetic properties of drugs in humans. In addition, ABC transporters such as P-gp, MRP1 and BCRP co-expressed in tumors show a broad and overlapped specificity for substrates and MDR modulators. Thus reliable preclinical assays and models are required for the assessment of transporter-mediated flux and potential effects on pharmacokinetics in drug development. In this review, we provide an overview of the role of ABC efflux transporters in MDR and pharmacokinetics. Preclinical assays for the assessment of drug transport and development of MDR modulators are also discussed.

In vivo induction of P-glycoprotein expression at the mouse blood-brain barrier: an intracerebral microdialysis study

Intracerebral microdialysis was utilized to investigate the effect of P-glycoprotein (a drug efflux transporter) induction at the mouse blood-brain barrier (BBB) on brain extracellular fluid concentrations of quinidine, an established substrate of P-glycoprotein. Induction was achieved by treating male CD-1 mice for 3 days with 5 mg/kg/day dexamethasone (DEX), a ligand of the nuclear receptor, pregnane X receptor, and a P-glycoprotein inducer. Tandem liquid chromatography mass spectrometric method was used to quantify analytes in dialysate, blood and plasma. P-glycoprotein, pregnane X receptor and Cyp3a11 (metabolizing enzyme for quinidine) protein expression in capillaries and brain homogenates was measured by immunoblot analysis. Following quinidine i.v. administration, the average ratio of unbound quinidine concentrations in brain extracellular fluid (determined from dialysate samples) to plasma at steady state (375-495 min) or Kp, uu, ECF /Plasma in the DEX-treated animals was 2.5-fold lower compared with vehicle-treated animals. In DEX-treated animals, P-glycoprotein expression in brain capillaries was 1.5-fold higher compared with vehicle-treated animals while Cyp3a11 expression in brain capillaries was not significantly different between the two groups. These data demonstrate that P-gp induction mediated by DEX at the BBB can significantly reduce quinidine brain extracellular fluid concentrations by decreasing its brain permeability and further suggest that drug-drug interactions as a result of P-gp induction at the BBB are possible. Applying microdialysis, distribution of quinidine, a P-gp substrate, in mouse brain extracellular fluid (ECF) was investigated following ligand-mediated P-glycoprotein (P-gp) induction at the blood-brain barrier (BBB). We demonstrated that a PXR agonist (dexamethasone) significantly up-regulated P-gp in brain capillaries and reduced quinidine brain ECF concentrations. Our data suggest that drug-drug interactions as a result of P-gp induction at the BBB are possible.

Coexisted components of Salvia miltiorrhiza enhance intestinal absorption of cryptotanshinone via inhibition of the intestinal P-gp

Cryptotanshinone, derived from the roots of Salvia miltiorrhiza Bge and Salvia przewalskii Maxim, is the major active component and possesses significant antibacterial, antidermatophytic, antioxidant, anti-inflammatory and anticancer activities. The objective of this study was to investigate the intestinal absorptive characteristics of cryptotanshinone as well as the absorptive behavior influenced by co-administration of the diterpenoid tanshinones and danxingfang using an in vitro everted rat gut sac model. The results showed a good linear correlation between cryptotanshinone of absorption and the incubation time from 10 to 70min. The concentration dependence showed that a non-linear correlation existed between the cryptotanshinone absorption and the concentration at 100 μg/ml. Coexisting diterpenoid tanshinones and danxingfang could significantly enhance the absorption of cryptotanshinone. Coexisting diterpenoid tanshinones and danxingfang, which influenced cryptotanshinone's absorption, manifested as similar to that of the P-glycoprotein inhibitor. The underlying mechanism of the improvement of oral bioavailability was proposed that coexisting diterpenoid tanshinones and danxingfang could decrease the efflux transport of cryptotanshinone by P-glycoprotein.

The effects of notoginsenoside R₁ on the intestinal absorption of geniposide by the everted rat gut sac model

ETHNOPHARMACOLOGICAL RELEVANCE

Geniposide is derived from Gardenia jasminoides Ellis (Rubiaceae). Its anti-inflammatory, antithrombotic effects as well as its preventive effect against ischemic stroke have been reported. Radix notoginseng (Chinese name tienchi or sanqi) is the dried root of Panax notoginseng (Burk.) F.H. Chen, an herb noted for its promotion of blood circulation, blood stasis removal and pain alleviation, and has been widely utilized for the prevention and treatment of microcirculatory disturbances in China and other Asian countries for many years. Notoginsenoside R₁ is an effective and structurally representative bioactive constituent of R. notoginseng. In our preliminary study, notoginsenoside R₁ was able significantly to improve the bioavailability of geniposide in beagle dogs, but the underlying mechanisms remain unknown.

MATERIALS AND METHODS

The present study aimed to investigate the intestinal kinetic absorptive characteristics of geniposide as well as the absorptive behavior influenced by the co-administration of notoginsenoside R₁ using an in vitro everted rat gut sac model.

RESULTS

The results showed good linear correlation between the geniposide absorption in sac contents and the incubation time from 0 to 120 min. The concentration dependence showed a non-linear correlation between the geniposide absorption and the concentrations 0.356-1.424 mg/mL, the absorption was saturated about 1.424 mg/mL. Notoginsenoside R₁ at 0.1 and 0.2mg/mL concentrations was able significantly to enhance the absorption of geniposide (1.424 mg/mL) by 1.7- and 1.4-fold. Moreover, verapamil, a well-known P-glycoprotein inhibitor, was able significantly to elevate the absorption of geniposide 2.4-fold. Notoginsenoside R₁ influenced geniposide's absorption in a way similar to that of a P-glycoprotein inhibitor.

CONCLUSIONS

In conclusion, notoginsenoside R₁ significantly enhances the intestinal absorption of geniposide. As for the mechanism underlying the improvement of geniposide's bioavailability, it is proposed that notoginsenoside R₁ was able to decrease the efflux transport of geniposide by P-glycoproteins.

Evaluation of in vivo P-glycoprotein phenotyping probes: a need for validation

Drug transporters are involved in clinically relevant drug-drug interactions. P-glycoprotein (P-gp) is an efflux transporter that displays genetic polymorphism. Phenotyping permits evaluation of real-time, in vivo P-gp activity and P-gp-mediated drug-drug interactions. Digoxin, fexofenadine, talinolol and quinidine are commonly used probe drugs for P-gp phenotyping. Although current regulatory guidance documents highlight methodologies for evaluating transporter-based drug-drug interactions, whether current probe drugs are suitable for phenotyping has not been established, and validation criteria are lacking. This review proposes validation criteria and evaluates P-gp probes to determine probe suitability. Based on these criteria, digoxin, fexofenadine, talinolol and quinidine have limitations to their use and are not recommended for P-gp phenotyping.

Modulating effect of polyethylene glycol on the intestinal transport and absorption of prednisolone, methylprednisolone and quinidine in rats by in-vitro and in-situ absorption studies

The effects of polyethylene glycol 20000 (PEG 20000) on the intestinal absorption of prednisolone, methylprednisolone and quinidine, three P-glycoprotein (P-gp) substrates, across the isolated rat intestinal membranes were examined by an in-vitro diffusion chamber system. The serosal-to-mucosal (secretory) transport of these P-gp substrates was greater than their mucosal-to-serosal (absorptive) transport, indicating that their net movement across the intestinal membranes was preferentially in the secretory direction. The polarized secretory transport of these drugs was remarkably diminished and their efflux ratios decreased in the presence of PEG 20000. In addition, PEG 20000 did not affect the transport of Lucifer yellow, a non-P-gp substrate. The intestinal membrane toxicity of PEG 20000 was evaluated by measuring the release of alkaline phosphatase (ALP) and protein from the intestinal membranes. The release of ALP and protein was enhanced in the presence of 20 mM sodium deoxycholate (NaDC), a positive control, while these biological parameters did not change in the presence of 0.1–5% (w/v) PEG 20000. These findings indicated that the intestinal membrane damage caused by PEG 20000 was not a main reason for the enhanced absorptive transport of these P-gp substrates in the presence of PEG 20000. Furthermore, the transepithelial electrical resistance (TEER) of rat jejunal membranes in the presence or absence of PEG 20000 was measured by a diffusion chamber method. PEG 20000 (0.1–5.0 % w/v) did not change the TEER values of the rat jejunal membranes, indicating that the increase in the absorptive transport of these P-gp substrates might not be due to the increased transport of these P-gp substrates via a paracellular pathway caused by PEG 20000. Finally, the effect of PEG 20000 on the intestinal absorption of quinidine was examined by an in-situ closed-loop method. The intestinal absorption of quinidine was significantly enhanced in the presence of 0.1-1.0% (w/v) PEG 20000. These findings suggest that PEG 20000 might be a useful excipient to improve the intestinal absorption of quinidine, which is mainly secreted by a P-gp-mediated efflux system in the intestine.

Influence of quinidine, fluvoxamine, and ketoconazole on the enantioselective pharmacokinetics of citalopram in rats

Citalopram (CITA) is available as a racemic mixture or as (+)-(S)-CITA. In humans, CITA is metabolized to demethylcitalopram (DCITA) by CYP2C19, CYP2D6, and CYP3A and to didemethylcitalopram by CYP2D6. There are no data regarding the enzymes involved in CITA and DCITA metabolism in rats. The present study investigated the influence of CYP inhibitors on the enantioselective metabolism of CITA in rats. Male Wistar rats (n = 6) received a single dose of 20 mg·kg–1 CITA after pretreatment with 80 mg·kg–1 quinidine, 10 mg·kg–1 fluvoxamine, 50 mg·kg–1 ketoconazole, or vehicle (control). Blood samples were collected up to 20 h after CITA administration. The CITA and DCITA enantiomers were analyzed by LC-MS/MS using a Chiralcel OD-R column. The kinetic disposition of CITA was enantioselective in rats (AUCS/R ratio = 0.4). Coadministration with quinidine resulted in non-enantioselective inhibition of the metabolism of CITA. Coadministration with fluvoxamine or ketoconazole, however, inhibited only the metabolism of (+)-(S)-CITA, but not of (–)-(R)-CITA when the racemic drug was administered to rats.

Novel oral absorption system containing polyamines and bile salts enhances drug transport via both transcellular and paracellular pathways across Caco-2 cell monolayers

The combinatorial use of spermine (SPM), a typical polyamine, and sodium taurocholate (STC), a typical bile salt, was found to be a promising safe preparation for improving the oral absorption of poorly water-soluble and/or poorly absorbable drug in our previous studies utilizing rats and dogs. To clarify the mechanisms behind the synergistic enhancement effect of the polyamine and bile salt, the transport of rebamipide, which is classified into Biopharmaceutics Classification System Class IV, was investigated in Caco-2 cell monolayers. The synergistic enhancement of rebamipide transport by SPM and STC was certainly observed in Caco-2 cells as well, while the separate use of either SPM or STC did not significantly improve the transport of rebamipide. The combinatorial use of SPM and STC significantly decreased the transepithelial electrical resistance (TEER) in Caco-2 cell monolayers, suggesting that the opening of paracellular pathway. On the other hand, it was also confirmed that the decrease in TEER was transient and reversible after removal of SPM and STC and that cell viability was maintained. Voltage-clamp study clearly showed that their combinatorial use improved rebamipide transport via both paracellular and transcellular pathways, and that the contribution of transcellular route could be larger than paracellular route.

Marked impact of P-glycoprotein on the absorption of TAK-427 in rats

The role of P-glycoprotein (P-gp, ABCB1) on the absorption process was investigated by drug-drug interaction studies of TAK-427 with P-gp inhibitors (erythromycin, ketoconazole or quinidine) in rats and by transport studies using rat multidrug resistance (MDR1) stably expressing cells and rat small intestine mounted in a Ussing-type chamber. TAK-427 showed high efflux activity with low permeability in rat MDR1a and MDR1b stably expressing cells and was revealed to be a typical substrate for P-gps. Although TAK-427 was mainly absorbed from the small intestine in rats, a large part of the dosed compound remained in the gastrointestinal tract. Orally co-administered P-gp inhibitors (50 mg/kg) increased the AUC of TAK-427 after a 5 mg/kg oral dose 5.4- to 18.3-fold, whereas orally administered P-gp inhibitors had a minor effect on the increase in the AUC of TAK-427 (1.3- to 2.2-fold) after a 0.5 mg/kg intravenous dose. Thus, the bioavailability of TAK-427 after oral administration in rats (7.3%) markedly increased when co-administered with P-gp inhibitors (28.6-57.6%). Moreover, the transport of TAK-427 was predominantly secretory throughout the rat small intestine and was inhibited by P-gp inhibitors. In conclusion, P-gp can markedly reduce the absorption of a typical P-gp substrate by its efflux activity throughout the absorption site.

The effects of sinomenine on intestinal absorption of paeoniflorin by the everted rat gut sac model

Paeoniflorin and sinomenine, derived from the root of Paeonia lactiflora Pall. (family Ranunculaceae) and the stem of Sinomenium acutum Rehder & Wilson (family Menispermaceae), respectively, have been, and are currently, widely used for treatment of rheumatic and arthritic diseases in China and Japan. Our previous studies demonstrated that sinomenine could significantly improve the bioavailability of paeoniflorin in rats, but the underlying mechanisms remain unknown. The present study aims to investigate the intestinal kinetic absorptive characteristics of paeoniflorin as well as the absorptive behavior influenced by co-administration of sinomenine using an in vitro everted rat gut sac model. The results showed a good linear correlation between the paeoniflorin absorption in sac contents and the incubation time from 0 to 90 min. However, the concentration dependence showed that a non-linear correlation exists between the paeoniflorin absorption and its concentrations from 10 to 160 microM, and the absorption was saturated at about 80 microM of the drug. Sinomenine at 16 and 136 microM concentrations could significantly enhance the absorption of paeoniflorin (20 microM) by 1.5- and 2.5-fold, respectively. Moreover, two well-known P-glycoprotein inhibitors, verapamil and quinidine, could significantly elevate the absorption of paeoniflorin by 2.1- and 1.5-fold, respectively. Furthermore, sinomenine in a pattern, which influenced paeoniflorin's absorption, manifested as similar to that of P-glycoprotein inhibitors. In conclusion, sinomenine significantly enhance the intestinal absorption of paeoniflorin, subsequently improve the bioavailability of paeoniflorin. The mechanism underlying the improvement of paeoniflorin's bioavailability was proposed that sinomenine could decrease the efflux transport of paeoniflorin by P-glycoprotein.

pH-dependent functional activity of P-glycoprotein in limiting intestinal absorption of protic drugs: Kinetic analysis of quinidine efflux in situ

The purpose of this investigation is to evaluate the quantitative contribution of pH-dependent passive permeability on the functional activity of P-glycoprotein (P-gp) in limiting intestinal absorption of weakly basic drugs, in order to include this effect in prediction models. pH-dependent octanol/buffer partition coefficient, artificial membrane permeability and in situ rat intestinal permeability of quinidine were determined in the physiological pH range of gastrointestinal tract. In situ permeability, as a function of luminal pH, was also determined in the presence of P-gp inhibitor, verapamil (500 microM). Octanol/buffer partition coefficient, transport across artificial membrane, and rat in situ permeability showed high pH-dependency. Absorption quotient (AQ), calculated from in situ permeability to express the functional activity of P-gp, declined with increase in luminal pH or increase in luminal quinidine concentration because of the increased passive permeability or saturation of P-gp. AQ was 0.57 +/- 0.02 and 0.41 +/- 0.05, while passive permeability was 0.32 +/- 0.01 x 10(-4) cm/sec and 0.43 +/- 0.02 x 10(-4) cm/sec, in jejunum and ileum, respectively, at pH 7.4. Further, apparent Michaelis-Menten constants (K(M), J(P-gp,max)) for the quinidine efflux in jejunum indicated that efflux activity was more at luminal pH 4.5 over pH 7.4. K(M) values for jejunum quinidine efflux at pH 4.5 and pH 7.4 were determined to be 77.63 +/- 10.90 and 22.86 +/- 5.22 microM, with J(P-gp,max) values of 1.47 +/- 0.08 and 0.62 +/- 0.04 nM/cm2/sec, respectively. AQ vs passive permeability showed significant relationship indicating dependency of P-gp-mediated efflux on pH-dependent passive permeability, which is dictated by ionization status for a protic or ampholytic drug. In conclusion, an orally administered drug is absorbed from various segments of intestine, which inherit difference in luminal pH, transcellular permeability and P-gp expression. In situ data suggests that pH-dependency and regional variability in passive permeability of protic substrates significantly influence their P-gp-mediated efflux and may have implications on predictions of the in vivo drug absorption.

Comparison of Human Tracheal/Bronchial Epithelial Cell Culture and Bovine Nasal Respiratory Explants for Nasal Drug Transport Studies

Ten drug compounds with varying physicochemical properties and transporter substrate specificities were investigated to compare their in vitro permeabilities across bovine nasal respiratory explants and the EpiAirway system, both established models for the assessment of nasal drug absorption. Permeability across the bovine explants and EpiAirway correlated well with the partitioning behavior of compounds whose clogDC values were greater than 0. The permeabilities of all ten compounds were well-correlated between the two tissue models, with the permeability values through the EpiAirway tissues being approximately 10-fold higher than through the bovine explants due to the thickness differences between the models. For more lipophilic compounds, the in vitro permeabilities measured with both tissue systems were also predictive of the reported in vivo nasal bioavailabilities. Deviations from these correlations were observed for compounds reported to be substrates of p-glycoprotein or OCT transporters, and differences were also seen between the permeabilities measured in the tissue models for these compounds. Both models can be used to estimate the systemic bioavailability of moderately lipophilic compounds administered intranasally, while each may have particular advantages or disadvantages in estimating the bioavailability of drug compounds that are subject to local mucosal metabolism or to carrier-mediated uptake or efflux.

Functional assessment of multiple P-glycoprotein (P-gp) probe substrates: influence of cell line and modulator concentration on P-gp activity

Compounds known to modulate P-glycoprotein (P-gp) activity were evaluated in cell monolayers expressing P-gp for their effects on the secretory transport of P-gp substrates paclitaxel, vinblastine, and digoxin. Paclitaxel has been proposed to selectively interact with a binding site on P-gp that is distinct from the vinblastine and digoxin-binding site. Using Madin-Darby canine kidney (MDCK)-multidrug resistance-1 (MDR1), MDCK-wild-type (WT), and Caco-2 cell monolayers, the basal-to-apical (BL-AP) apparent permeability (Papp) of [3H]paclitaxel, [3H]vinblastine, and [3H]digoxin in the presence of various concentrations of a series of structurally diverse P-gp substrates and modulators of P-gp function were determined. MDCK-WT cell monolayers demonstrated active secretory transport of all P-gp substrate probes, although the sensitivity to inhibition by verapamil was lower than that demonstrated in MDCK-MDR1 cell monolayers. When evaluated as competitive inhibitors, several known P-gp substrates had no effect or only a slight modulatory effect on the BL-AP Papp of all probe substrates in MDCK-MDR1 cells. The secretory transport of P-gp substrates in MDCK-WT cells was more sensitive to inhibition by known P-gp modulators compared with MDCK-MDR1 cells. Low concentrations of ketoconazole (1-3 microM) activated the BL-AP Papp of [3H]vinblastine and [3H]digoxin in MDCK-MDR1 cells but not in MDCK-WT or Caco-2 cells. Determination of secretory transport in P-gp expressing cell monolayers, such as MDCK-MDR1 and Caco-2, may be complicated by substrate cooperativity and allosteric binding, which may result in the activation of P-gp. In addition, expression of other efflux transporters in these cell lines introduces additional complexity in distinguishing which transporter is responsible for substrate recognition and transport.

Localization and Differential Activity of P-glycoprotein in the Bovine Olfactory and Nasal Respiratory Mucosae

PURPOSE

The purpose of this study was to demonstrate that P-glycoprotein (P-gp) is localized in the olfactory mucosa and is capable of limiting the nose-to-brain transport of substrates. Bovine olfactory and nasal respiratory mucosae were compared to both localize P-gp and to measure its activity within the epithelia.

METHODS

Immunolocalization was performed on the bovine olfactory and nasal respiratory mucosa using the C219 monoclonal antibody. Flux of etoposide, a substrate reported to be primarily effluxed by P-gp, across bovine olfactory and nasal respiratory mucosae was measured using Sweetana-Grass (Navicyte) vertical diffusion cells. Experiments were performed to evaluate the effect of directionality, donor concentration, and the presence of inhibitors.

RESULTS

Dense staining was observed on the apical surface of the ciliated epithelial cells and within the submucosal lymphatics/vasculature and mucosal glands of the bovine olfactory and nasal respiratory mucosae. Staining in the nasal respiratory epithelium was weak and patchy when compared to that observed in the olfactory mucosa. The secretory transport (Js-m) kinetics of etoposide in the olfactory (Km = 260.5 microM, Vmax = 0.179 microM/cm(2) min) and nasal respiratory (Km = 46.9 microM, Vmax = 0.034 microM/cm(2) min) mucosae were observed to be saturable and concentration-dependent. The flux of etoposide in the submucosal-mucosal (Js-m) direction was significantly greater than the flux in the mucosal-submucosal (Jm-s) direction in both the olfactory and nasal respiratory mucosa. The efflux ratios (Js-m/Jm-s) of etoposide across the olfactory and the nasal respiratory mucosae were 2.02 and 2.10, respectively. In the presence of inhibitors such as 2,4-dinitrophenol (1 mM) and quinidine (1 mM), etoposide showed an increase in Jm-s and a decrease in Js-m. The etoposide efflux was unaffected in the presence of a specific multiresistance associated protein 1 (MRP1) inhibitor (MK571) and methotrexate, a substrate for BCRP and MRP1-4.

CONCLUSIONS

P-gp was localized in the epithelial cells, nasal glands, and the vascular endothelium of both the bovine olfactory and nasal respiratory mucosae, and the expressed P-gp was capable of effluxing a substrate such as etoposide. The Km and Vmax of etoposide efflux were higher in the olfactory mucosa compared to the nasal respiratory mucosa, and the expression of P-gp seems to be greater in the olfactory epithelium compared to the nasal respiratory epithelium based on the staining density observed using immunohistochemistry.

Carrier mediated transport of chlorpheniramine and chlorcyclizine across bovine olfactory mucosa: Implications on nose‐to‐brain transport

Delivery to the CNS via the nasal cavity has been pursued as a means to circumvent the blood-brain barrier (BBB), yet the mechanism of drug transport across this novel route is not well understood. Hydroxyzine and triprolidine have been reported to readily reach the CNS following nasal administration, whereas no measurable amounts of chlorcyclizine or chlorpheniramine, structurally similar antihistamines, were observed in the CSF. The permeation of chlorpheniramine and chlorcyclizine in vitro across the bovine olfactory mucosa was studied to investigate the biological and physicochemical characteristics that contribute to the limited CNS disposition of these compounds following nasal administration. The submucosal to mucosal fluxes (J(s-m)) of chlorcyclizine and chlorpheniramine across the olfactory mucosa were significantly greater than the mucosal to submucosal fluxes (J(m-s)). Moreover, the submucosal-mucosal permeability of both compounds was temperature dependent and saturable. In the presence of metabolic inhibitors (ouabain and 2,4-dinitrophenol) and P-glycoprotein (P-gp)/multidrug resistance protein 1 (MRP1) inhibitors (quinidine and verapamil), the J(m-s) increased and J(s-m) decreased significantly. These results indicate that chlorpheniramine and chlorcyclizine are effluxed from the olfactory mucosa by efflux transporters such as P-gp and MRP1. Transport studies across inert polymeric membranes demonstrated that the permeability of chlorpheniramine and chlorcyclizine decreased at donor concentrations higher than 3 mM suggesting that physicochemical properties such as self-aggregation also play a role in the reduced olfactory mucosal permeability of these compounds at higher concentrations.

Regional Difference in P-glycoprotein Punction in Rat Intestine

It has been reported that inhibition of the P-glycoprotein (P-gp) results in the improved absorption of P-gp substrate in the intestinal tract. In fact, the increased permeability of P-gp substrate across the intestinal epithelium was observed following inhibition of P-gp in in vitro experiments. To develop the formulation containing P-gp inhibitor and P-gp substrate for practical use, it is necessary to know whether the results obtained in the in vitro experiments are reproducible at whole body level. It is also important to find out the regional difference of the P-gp activity in the intestinal tract. In this study, we examined whether verapamil, a specific inhibitor of P-gp, improves the absorption of rhodamine123 (Rho123), a substrate of P-gp, from the jejunum, ileum, and colon of rats using the in situ loop method. The water content in the loop decreased during the experiment, resulting in a significant change of the Rho123 concentration in the loop. Thus, to accurately determine the absorption rate of Rho123, it was necessary to measure the water movement. It was found that there was a regional difference in the water movement, i.e., greatest in colon, followed by ileum. Verapamil did not change the water movement in any intestinal regions. When the concentration of Rho123 in the loop was corrected by water movement, the Rho123 clearance was in the order of ileum (1.15 microL/min/cm), colon (0.83 microL/min/cm) and jejunum (0.47 microL/min/cm). In the presence of verapamil, the Rho123 clearance was significantly increased at jejunum and ileum but not in colon (ileum: 2.08 microL/min/cm, colon: 1.14 microL/min/cm, jejunum: 1.28 microL/min/cm). These results suggest that P-gp inhibits the drug absorption in jejunum and ileum. From these results, it is possible to evaluate the role of P-gp and its regional difference in the in situ experiments. In particular, the inhibition of P-gp results in an increase in absorption of the P-gp substrate limited to jejunum and ileum.

Exposure-response relationships and drug interactions of sirolimus

Sirolimus (rapamycin, RAPAMUNE, RAPA) is an immunosuppressive agent used for the prophylaxis of renal allograft rejection and exhibits an immunosuppressive mechanism that is distinct from that for cyclosporine and tacrolimus. The purpose of this manuscript is to discuss the exposure-response relationships and drug interactions of sirolimus. The various factors affecting sirolimus whole blood exposure included first-pass extraction, formulation, food, demographics, liver disease, assay method, and interacting drugs. Clinically significant effects caused by food, pediatric age, hepatic impairment, and interacting drugs require recommendations for the safe and efficacious use of sirolimus in renal allograft patients. An exposure-response model based on multivariate logistic regression was developed using the interstudy data from 1832 renal allograft patients. The analysis revealed an increased probability of acute rejection for sirolimus troughs <5 ng/mL, cyclosporine troughs <150 ng/mL, human leukocyte antigen (HLA) mismatches > or =4, and females. The outcomes suggested that individualization of sirolimus doses immediately after transplantation, based on HLA mismatch and sex, would likely decrease the probability of acute rejections in renal allograft recipients who receive concomitant sirolimus, cyclosporine (full-dose), and corticosteroid therapy. Sirolimus is a substrate for both Cytochrome P450 3A (CYP3A) and P-glycoprotein (P-gp) and undergoes extensive first-pass extraction. Drugs that are known to inhibit or induce these proteins may potentially affect sirolimus whole blood exposure. In healthy volunteers, cyclosporine, diltiazem, erythromycin, ketoconazole, and verapamil significantly increased sirolimus whole blood exposure, and rifampin significantly decreased sirolimus exposure. However, sirolimus whole blood exposure was not affected by acyclovir, atorvastatin, digoxin, ethinyl estradiol/norgestrel, glyburide, nifedipine, or tacrolimus. Among the 15 drugs studied, sirolimus significantly increased the exposures of only erythromycin and S-(-)verapamil.

Up-regulation of P-glycoprotein expression in small intestine under chronic serotonin-depleted conditions in rats

To investigate the role of serotonin (5-HT), an important neurotransmitter and hormone/paracrine agent in the small intestine, in the transport activity of P-glycoprotein (P-gp), the intestinal transport of quinidine, a P-gp substrate, was examined in 5-HT-depleted rats prepared by intraperitoneal administration of p-chlorophenylalanine, a specific inhibitor of tryptophan hydroxylase in 5-HT biosynthesis. In the in vitro transport study, quinidine transport across rat jejunum was significantly enhanced in both the secretory and absorptive directions under 5-HT-depleted conditions, although the secretory transport was still predominant. The electrophysiological study suggested that the quinidine transport via passive diffusion was enhanced presumably through a paracellular route. This might be due to looser tight junctions under 5-HT-depleted conditions. The voltage-clamp technique clearly indicated that the secretory transport of quinidine through the transcellular pathway was also enhanced by the depletion of 5-HT. Furthermore, 5-HT depletion increased verapamil-sensitive secretory transport of quinidine in rat jejunum. These results indicate that the secretory transport of quinidine via P-gp was significantly enhanced under 5-HT-depleted conditions. The level of ATP, an energy source for functioning P-gp, wet weight of jejunum, and total protein level in rat jejunal mucosa were not changed by 5-HT depletion, but the expression of P-gp in the brush-border membrane of rat jejunum was significantly induced, which is partly responsible for the enhancement of P-gp activity under the 5-HT-depleted condition.

The influence of buffer composition on tissue integrity during permeability experiments "in vitro"

A well-balanced incubation saline is necessary for permeability experiments with the rat jejunal tissue in the diffusion chambers. At the same time the investigated substance must be chemically stable and sufficiently soluble in this incubation saline. To investigate whether the absence of some ions in incubation salines influences the tissue viability and integrity or the diffusional characteristics of the epithelial membrane the electrical parameters were monitored and the permeability of fluorescein and acyclovir was evaluated during the experiments in side-by-side diffusion chambers. Our results show that the tissue integrity and viability are seriously impaired when Ca(2+) and Mg(2+)-free conditions are applied on both sides of the diffusion chambers, but not when only mucosal or only serosal side is Ca(2+) and Mg(2+)-free. Bicarbonate-free incubation salines can also alter the measured apparent permeability coefficients even though the tissue viability and integrity do not change. This change in the apparent permeability is most likely due to a change in the pH of the mucosal surface and can be prevented if the buffer capacity of the incubation saline is increased.

Modulation of Intestinal P-Glycoprotein Function by Cremophor EL and Other Surfactants by an In Vitro Diffusion Chamber Method Using the Isolated Rat Intestinal Membranes

Effects of various surfactants on the transport of rhodamine123, a P-glycoprotein (P-gp) substrate, across the isolated rat intestinal membranes were examined by an in vitro diffusion chamber system. The jejunal serosal-to-mucosal transport (Jsm) of rhodamine123 was more than threefold greater than its mucosal-to-serosal transport (Jms), suggesting that the net movement of rhodamine123 across the rat jejunum was preferentially secretory direction. There exists a regional difference in the intestinal transport of rhodamine123 and the secretory directed transport was remarkably observed in the jejunum. The Jsm/Jms ratio of rhodamine123 decreased in the presence of 0.3 mM verapamil and 10 mM sodium azide (NaN3) + 1 mM sodium fluoride (NaF), confirming that rhodamine123 might be secreted from the intestinal tissue into the lumen by a P-gp-mediated efflux system. Nonionic surfactants [0.1% Cremophor EL, Tween 80 and n-dodecyl-beta-D-maltopyranoside (LM)] reduced the Jsm/Jms ratio of rhodamine123, whereas its ratio was not influenced in the presence of 0.1% cationic surfactant (hexadecyltrimethylammonium bromide, C16TAB) and anionic surfactant (sodium dodecyl sulfate, SDS). Therefore, these findings suggested that charge of surfactants was possibly related to the action of these surfactants on the intestinal absorption of P-gp substrates. On the other hand, the transfer of rhodamine123 was not affected by the addition of Cremophor EL to the serosal side. Because the c.m.c. of Cremophor EL is 0.0095 w/v%, interactions between rhodamine123 and the micellar form of Cremophor EL may decrease the P-gp-mediated efflux of rhodamine123 at higher concentrations. In the kinetic analysis, the Vmax value (nmol/min/g wet tissue) of rhodamine123 decreased, although the Km value (mM) was constant in the presence of Cremophor EL. Therefore, Cremophor EL inhibited the efflux transport of rhodamine123 in a noncompetitive manner. Cremophor EL did not affect the transport of [14C]Gly-Sar and [3H]3-O-methyl-D-glucose, suggesting that the action of Cremophor EL might be P-gp specific. These findings indicated that nonionic surfactants including Cremophor EL and Tween 80 may be useful pharmaceutical excipients for inhibiting the function of P-gp, thereby increasing the intestinal absorption of various drugs, which are secreted by a P-gp-mediated efflux system in the intestine.

Regulation of Drug Absorption from Small Intestine by Enteric Nervous System I: a Poorly Absorbable Drug Via Passive Diffusion

To investigate the regulation of drug absorption from the small intestine by the enteric nervous system (ENS), the vascular-luminal perfusion study and the in-vitro transport study were performed by employing phenol red as a poorly absorbable model compound. The effect of ENS on the intestinal absorption of phenol red was examined by adding epinephrine, an adrenergic agonist, or bethanechol, a cholinergic agonist into the vascular perfusate in the vascular-luminal perfused rat small-intestine preparation. The viability of the perfused intestine was checked by the recovery of the vascular perfusate, net water flux and absorbability of antipyrine, a well absorbable drug, and it was confirmed that the function of the perfused small-intestine preparation was maintained for at least 1 hr. The effect of epinephrine or bethanechol on the function of the small intestine was recognized as the increase in net water absorption, or the promotion of the water secretion, respectively. These phenomena are ones that are typically observed when adrenergic or cholinergic neuron is stimulated. Then, we investigated the small-intestinal absorption of phenol red in the vascular-luminal perfused preparation. Absorption clearance (CL(abs)) of phenol red was gradually increasing during the perfusion for 1 hr, but the 20-min vascular perfusion with the perfusate containing epinephrine made CL(abs) of phenol red constant and significantly lower than those for control study. Furthermore, after the perfusate was changed with the one without any agonist, again, CL(abs) of phenol red started to increase. These results clearly indicate that the stimulation of adrenergic neuron by epinephrine leads to the decrease in the small-intestinal absorption of phenol red. On the other hand, the vascular perfusion of bethanechol resulted in the increase in CL(abs) of phenol red comparing to the control study. Removing bethanechol from the vascular perfusate decreased CL(abs) of phenol red, again. The in-vitro transport study using the isolated jejunum sheet also showed that epinephrine in the serosal solution significantly decreased the transport of phenol red, which can be ascribed to the paracellular pathway tightened by the action of epinephrine because of the increase in transmucosal electrical resistance (TER). On the other hand, although the effect of bethanechol on both the transport of phenol red and TER was not statistically significant, the transport of phenol red tended to increase and the values of TER are smaller than those of control study.

In vivo effect of 5- and 8-methoxypsoralens and cimetidine on R,S-warfarin metabolism in rat

Several forms of cytochrome P-450 (CYP) metabolize R,S-warfarin in a regio- and enantioselective manner, therefore R,S-warfarin can be recognized as a metabolic probe for a number of CYP isoforms. We have applied a warfarin model in vivo in order to estimate the inhibitory properties of 5- and 8-methoxypsoralens on the activity of rat CYP isoforms. The area under the serum concentration versus time curve (AUC) values from time zero to 5 h for R- and S-warfarin and their metabolites were calculated. R,S-Warfarin kinetics measurements were made three times on each rat: a week before the 7-days inhibitor treatment, 3 h after the last dose of inhibitor and 3-7 days after the inhibitor was withdrawn. The inhibitory effect of cimetidine on CYP 2C11 and CYP 2C6 activities was confirmed in this approach and can be recognized as a positive control in validation of the in vivo experiment. Both 5- and 8-methoxypsoralen inhibited CYP 2C6 activity as the respective AUC for metabolite/warfarin enantiomer ratio decreased significantly. The activity of CYP 2C6 in 5- and 8-methoxypsoralen-treated rats increased over control values after the inhibitor was withdrawn. It was also observed that cimetidine additionally inhibits the absorption of R,S-warfarin and a decrease in the sum of AUC for R- and S-enantiomers became evident in spite of inhibition of the activity of both CYPs. 5-Methoxypsoralen modified the serum R-warfarin/S-warfarin ratio and a selective increase in AUC(S-warfarin) was observed, the most pronounced being after the inhibitor was withdrawn. This effect is not likely to be mediated by P-glycoprotein (P-gp) because quinidine--, a P-gp inhibitor at a dose of 15 mg kg(-1) body wt.--did not influence the AUC for either enantiomer.

The impact of P-glycoprotein efflux on enterocyte residence time and enterocyte-based metabolism of verapamil

P-glycoprotein (P-gp) can limit the intestinal permeability of a number of compounds and may therefore influence their exposure to metabolizing enzymes within the enterocyte (e.g. cytochrome P450 3A, CYP 3A). In this study, the intestinal metabolic profile of verapamil, the influence of P-gp anti-transport on the cellular residence time of verapamil, and the impact of this change in residence time on the extent of enterocyte-based metabolism have been investigated in-vitro, utilizing segments of rat jejunum and side-by-side diffusion chambers. Verapamil exhibited concentration-dependent P-gp efflux and CYP 3A metabolism. The P-gp efflux of verapamil (1 microM) increased the cellular residence time across the intestinal membrane (approximately 3-fold) in the mucosal to serosal (m to s) direction relative to serosal to mucosal (s to m), yielding significantly greater metabolism (approximately 2-fold), presumably as a result of the prolonged exposure to CYP 3A. Intestinal metabolism of verapamil generated not only norverapamil, but resulted also in the formation of an N-dealkylated product (D-617). Norverapamil and D-617 accumulated significantly in mucosal chambers, relative to serosal chambers, over the time course of the experiment. Based on these in-vitro data, it was apparent that P-gp efflux prolonged the cellular residence time of verapamil (m to s) and therefore increased the extent of intestinal metabolism, and also played a role in metabolite secretion from within the enterocyte.

Permeability of the peptidic GH secretagogues hexarelin and EP 51389, across rat jejunum

The intestinal permeability of hexarelin and EP 51389, two growth hormone releasing hexa- and tri- peptide analogues, was assessed in vitro with side-by-side diffusion chambers in the apical-to-basolateral (AP-to-BL) and in the basolateral-to-apical (BL-to-AP) direction using excised rat jejunal segments. The effect of EP 51389 on P-glycoprotein (P-gp) was evaluated by rhodamine 123 accumulation on monolayers of CH(R)C5 cells with increasing concentrations of EP 51389. Hexarelin and EP 51389 permeability were found to be < 1%. Permeability coefficients (P(app)) were 18.87 +/- 2.86 (x10(-7) cm/s) and 5.87 +/- 0.45 (x10(-7) cm/s) for hexarelin and EP 51389, respectively. Bidirectional studies revealed that hexarelin transport was similar in both directions. EDTA did not influence hexarelin permeability. Permeability was predominantly secretory for EP 51389 as P(app) in the BL-to-AP direction [32.56 +/- 6.11 (x10(-7) cm/s)] was greater than AP-to-BL. Confirming involvement of a secretory transport system, chlorpromazine inhibited EP 51389 transport across the jejunum. EP 51389 inhibited P-gp in a dose dependent manner resulting in the intracellular accumulation of rhodamine in CH(R)C5 cells. These results suggest that: 1) the intestinal permeability of hexarelin and EP 51389 is poor; 2) the passage of hexarelin is mainly via a transcellular passive pathway since the contribution of paracellular permeability to the overall permeability is rather low; 3) P-gp may act as a potential barrier for the intestinal absorption of EP 51389.

Transport functions of riboflavin carriers in the rat small intestine and colon: site difference and effects of tricyclic-type drugs

The present study was aimed at kinetically characterizing the newly found carrier-mediated riboflavin transport system in the rat colon, comparing it with that in the small intestine, and also probing the potential roles of these transport systems in intestinal drug absorption. Riboflavin transport, evaluated by measuring the initial uptake into everted intestinal tissue sacs, was saturable with a Michaelis constant (Km) of 0.13 microM and a maximum transport rate (Jmax) of 0.74 pmol/min/100 mg wet tissue weight (wtw) in the colon. Both the Km and the Jmax were smaller than those (0.57 microM and 4.26 pmol/min/100 mg wtw, respectively) in the small intestine, suggesting that the transport system in the colon has a higher affinity to substrates and a smaller transport capacity than its counterpart in the small intestine. The carrier-mediated riboflavin transport in the colon, similarly to that in the small intestine, was Na+-dependent and inhibited by lumiflavin, a riboflavin analogue with an isoalloxazine ring, but not by D-ribose, which forms the side-chain attached to the isoalloxazine ring in riboflavin. To further clarify the substrate specificities of the transport systems, we examined the effects of several drugs with a tricyclic structure similar to isoalloxazine ring on riboflavin transport. Chlorpromazine, a phenothiazine derivative, was found to inhibit riboflavin transport in both the small intestine and the colon. Methylene blue also was found to be a potent inhibitor in both sites. These results suggest that some tricyclic-type drugs could interfere with intestinal riboflavin absorption by specific carrier-mediated transport systems. These transport systems may play roles in the absorption of tricyclic-type drugs.

Effect of P-glycoprotein modulators on the human extraneuronal monoamine transporter

The aim of this work was to investigate the effect of P-glycoprotein modulators on human extraneuronal monoamine transporter (EMT)-mediated transport. The experiments were performed using a cell line from human embryonic kidney (HEK293 cells) stably transfected with pcDNA3hEMT (293(hEMT)), or with pcDNA3 alone (293(control)). Of the P-glycoprotein modulators tested, rhodamine123, verapamil and daunomycin concentration-dependently inhibited EMT-mediated uptake of [3H]1-methyl-4-phenylpyridinium ([3H]MPP(+)). The corresponding IC(50)'s were found to be 3.6, 37 and 130 microM, respectively. By contrast, vinblastine, digitoxin and cyclosporine A were devoid of effect. The endogenous organic cation tyramine, but not choline, inhibited EMT-mediated transport (IC(50) of 468 microM). Moreover, L-arginine and L-histidine (up to 1 mM) did not affect [3H]MPP(+) uptake. Finally, MPP(+) and tyramine trans-stimulated [3H]MPP(+) uptake, but rhodamine123 had no effect, and verapamil and daunomycin trans-inhibited [3H]MPP(+) uptake. In conclusion, this study shows that several cationic modulators of P-glycoprotein inhibit EMT-mediated transport. As a consequence, the interaction of P-glycoprotein modulators with EMT must be taken into account, and the consequences of this interaction must not be forgotten when using such drugs in vivo.

Improved intestinal absorption of sulpiride in rats with synchronized oral delivery systems

The goals of this study were to examine whether formulations, capable of releasing sulpiride (SP) in synchrony with the p-Glycoprotein (P-gp) inhibitors, verapamil (Ver) or quinidine (Qn) can increase SP relative bioavailability and to suggest a rationale approach for oral administration of SP. Jejunum of anesthetized rats was perfused with 200 or 400 microg/ml of SP either alone or together with 98 microg/ml of Ver. It was observed that while an increasing SP concentration did not cause an increase in SP blood levels, the addition of Ver or Qn to the perfusion solution caused a profound increase in SP absorption. Erodible matrix tablets, exhibiting a range of erosion rates, were prepared by manipulating the ratios of hydroxypropylmethylcellulose (HPMC) in the matrices. The tablets were designed to release the low water soluble SP and the highly water soluble Qn concomitantly over 1, 2 or 4 h. In all cases, the synchronous release increased SP bioavailability after intra-intestinal administration. The increase varied from 2.6- to 3.9-fold for the fast and the slow release formulations, respectively (compared with a control administration of a powdered mixture of SP and Qn). It is speculated that the poor oral bioavailability of SP was caused by brush border P-gp efflux. Synchronous release delivery systems of SP containing also the P-gp inhibitor Qn were able to increase SP bioavailability after intestinal administration in the rat. It is concluded that oral bioavailability of poorly absorbed drugs, in which absorption is restricted by gut wall secretory transport, may be improved by formulating them with functional adjuvants in synchronous release drug carriers.

Region-dependent disappearance of vinblastine in rat small intestine and characterization of its P-glycoprotein-mediated efflux system

This study was aimed to characterize the absorption behavior of vinblastine (VLB), a well-known substrate of P-glycoprotein (P-gp), from rat small intestine, especially focusing on the regional-dependence of its efflux mediated by P-gp. VLB disappeared from duodenal and ileal loops of male Wistar rats fairly rapidly (30-60% in 30 min). In contrast, its disappearance from the jejunal loop was almost negligible and in some rats >100% of the jejunal dose was recovered. The radioactivity derived from [3H]VLB, which was absorbed from duodenum and ileum, was detected in the jejunal region. The jejunal appearance of radioactivity was increased when unlabeled VLB was present in the region in advance. The basolateral-to-apical transport of [3H]VLB across Caco-2 cell monolayers was greater when unlabeled VLB was added to the apical medium than when VLB-free buffer was applied to the apical side. When verapamil or cyclosporin A, potent modulators of P-gp, was added to the apical medium together with unlabeled VLB, enhanced basolateral-to-apical transport of [3H]VLB was disappeared. It is suggested that VLB absorption is strongly restricted by P-gp, especially in the jejunal region of the rat small intestine, and that the secretory transport via intestinal P-gp may be subject to trans-stimulation. Moreover, intravenously administered methylprednisolone and intramuscularly administered progesterone significantly enhanced the absorption of VLB, suggesting that parenterally administered P-gp modulators could influence the intestinal absorption of P-gp substrates.

Molecular identification and functional characterization of Mdr1a in rat cholangiocytes

BACKGROUND & AIMS

The multidrug resistance P-glycoprotein 170 gene products (mdr1a and 1b) are glycosylated plasma membrane proteins that function as adenosine triphosphate-dependent transmembrane export pumps for lipophilic xenobiotics of widely different structure. We assessed whether these P-glycoproteins are functionally expressed in cholangiocytes.

METHODS

A reverse-transcription polymerase chain reaction was performed on RNA from a normal rat cholangiocyte cell line using mdr1-specific primers. Northern and Western blot analyses were performed on cholangiocytes immunoisolated from 2-week bile duct-ligated rats and cholangiocytes and isolated cholangiocyte membrane subfractions, respectively. Functional assays were performed in isolated bile duct units from bile duct-ligated rats and incubated with rhodamine 123, a P-glycoprotein substrate, with or without the P-glycoprotein inhibitors verapamil or GF120918.

RESULTS

A 400-base pair fragment with 99% homology to the cytosolic domain of rat intestinal mdr1a (5' 1953-2350 3') was identified that hybridized to a 5.2-kilobase RNA transcript in a normal rat cholangiocyte cell line, isolated rat cholangiocytes, and ileum. Western analysis localized mdr1 to the apical membrane of cholangiocytes. Confocal microscopy showed active secretion of rhodamine 123 into the lumen of isolated bile duct units that was abolished by vanadate and P-glycoprotein competitive antagonists, verapamil and GF120918, in a dose-dependent manner.

CONCLUSIONS

These findings provide the first molecular and functional evidence for the expression of mdr1a on the luminal membrane of cholangiocytes, where it may have a protective role.

Pharmacokinetic interaction between diltiazem and tolbutamide

The effect of co-administered tolbutamide and diltiazem on each drug's pharmacokinetics was studied in eight healthy male volunteers aged 21-25 years, with a 3 x 3 randomised crossover design. Each subject received orally 60 mg of diltiazem hydrochloride or 500 mg of tolbutamide, or both drugs. The washout period between each treatment was 7 days. Serum levels of diltiazem and tolbutamide were determined by HPLC. Serum profiles were analysed using a non-compartmental model. There was no change in the pharmacokinetics of diltiazem in the presence of tolbutamide. There was approximately 10% increase in AUC0-24 and Cmax for tolbutamide in the presence of diltiazem.

Rifampicin treatment greatly increases the apparent oral clearance of quinidine

We investigated the effect of cytochrome P450 induction by rifampicin on the in vivo oxidative metabolism of quinidine. The pharmacokinetics of a 200 mg oral single dose quinidine were studied before and after one week of daily treatment with 600 mg rifampicin in six healthy young male volunteers. Biomarker reactions of cytochrome P450 isozyme activities in the form of caffeine, sparteine, mephenytoin, tolbutamide and cortisol metabolism were applied. The median total apparent oral clearance and partial clearance by 3-hydroxylation of quinidine increased 9 times. The partial clearance by N-oxidation increased 6 times. The Cmax and the elimination half life were reduced 3 times. No statistically significant changes were found for quinidine tmax and renal clearance. The cortisol metabolic ratio increased 5 times, while no statistically significant effects were seen for other CYP marker reactions. The results indicate that the inductive effect of rifampicin is likely to be of clinical relevance particularly when used concomitantly with drugs metabolized by CYP3A4.

Effect of diclofenac, disulfiram, itraconazole, grapefruit juice and erythromycin on the pharmacokinetics of quinidine

AIMS

In vitro studies suggest that the oxidation of quinidine to 3-hydroxyquinidine is a specific marker reaction for CYP3A4 activity. To assess the possible use of this reaction as an in vivo marker of CYP3A4 activity, we studied the involvement of cytochromes CYP2C9, CYP2E1 and CYP3A4 in the in vivo oxidative metabolism of quinidine.

METHODS

An open study of 30 healthy young male volunteers was performed. The pharmacokinetics of a 200 mg single oral dose of quinidine was studied before and during daily administration of 100 mg diclofenac, a CYP2C9 substrate (n=6); 200 mg disulfiram, an inhibitor of CYP2E1 (n=6); 100 mg itraconazole, an inhibitor of CYP3A4 (n=6); 250 ml single strength grapefruit juice twice daily, an inhibitor of CYP3A4 (n=6); 250 mg of erythromycin 4 times daily, an inhibitor of CYP3A4 (n=6). Probes of other enzyme activities, caffeine (CYP1A2), sparteine (CYP2D6), mephenytoin (CYP2C19), tolbutamide (CYP2C9) and cortisol (CYP3A4) were also studied.

RESULTS

Concomitant administration of diclofenac reduced the partial clearance of quinidine by N-oxidation by 27%, while no effect was found for other pharmacokinetic parameters of quinidine. Concomitant administration of disulfiram did not alter any of the pharmacokinetic parameters of quinidine. Concomitant administration of itraconazole reduced quinidine total clearance, partial clearance by 3-hydroxylation and partial clearance by N-oxidation by 61, 84 and 73%, respectively. The renal clearance was reduced by 60% and the elimination half-life increased by 35%. Concomitant administration of grapefruit juice reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 15, 19 and 27%, respectively. The elimination half-life of quinidine was increased by 19%. The caffeine metabolic index was reduced by 25%. Concomitant administration of erythromycin reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 34, 50 and 33%, respectively. Cmax was increased by 39%.

CONCLUSIONS

The results confirm an important role for CYP3A4 in the oxidation of quinidine in vivo, and this applies particularly to the formation of 3-hydroxyquinidine. While a minor contribution of CYP2C9 to the N-oxidation of quinidine is possible, a major involvement of the CYP2C9 or CYP2E1 enzymes in the oxidation of quinidine in vivo is unlikely.

P-glycoprotein, secretory transport, and other barriers to the oral delivery of anti-HIV drugs

Orally administered anti-HIV drugs must be adequately and consistently absorbed for therapy to be successful. This review discusses the barriers to achieving oral bioavailability for the currently available anti-HIV drugs. Most reverse transcriptase inhibitors have good oral bioavailabilities. Didanosine bioavailability could be reduced by acid instability, first-pass hepatic metabolism, and possibly poor intestinal permeation. Bioavailability of zidovudine is also reduced by first-pass metabolism. The non-nucleoside reverse transcriptase inhibitors have oral bioavailabilities most probably limited by poor aqueous solubility. For each of the currently marketed HIV protease inhibitors, solubility, intestinal permeability, and first-pass metabolism could contribute to reducing oral bioavailability. The intestinal permeabilities of these agents is influenced by secretory transport. In vitro, secretory transport, which appears to be P-glycoprotein-mediated, is much greater than permeation in the absorptive direction for indinavir, nelfinavir, ritonavir, and saquinavir. The mechanisms of secretory intestinal transport are reviewed, and the factors that may influence the impact of secretory transport in vivo are considered.

The roles of P-glycoprotein and intracellular metabolism in the intestinal absorption of methadone: in vitro studies using the rat everted intestinal sac

Methadone is used as a treatment for opiate detoxification in methadone maintenance programs. Intra- and inter-patient variations in methadone bioavailability have been observed after oral methadone treatment and this makes it difficult to predict a dosing regimen. Intestinal absorption and metabolism could explain these variations. The in vitro gut sac model was used to study the intestinal absorption of methadone, and it confirmed that methadone is a substrate for P-glycoprotein. The transport of methadone was increased in presence of P-gp inhibitors verapamil and quinidine. The appearance of a major metabolite of methadone, 2-ethylidene-1, 5-dimethyl-3, 3-diphenyl pyrrolidine (EDDP) in the gut sac contents also demonstrated the existence of intestinal metabolism of methadone.

Active intestinal elimination of ciprofloxacin in rats: modulation by different substrates

1. Two in vivo models, in the rat, were used to investigate, in the presence of different substrates, the overall and net intestinal elimination of ciprofloxacin: an open-intestinal perfusion model and an intestinal loop model respectively. 2. In the presence of quinidine, verapamil and cyclosporin (substrates of the P-glycoprotein (P-gp)), plasma AUCs of ciprofloxacin were 1.5 - 2 fold increased, while biliary clearance (1.5 - 2 fold), intestinal overall and net clearances (2 - 4 fold and 1.5 - 8 fold respectively) decreased. The weak effect obtained with cyclosporin as compared to verapamil and especially quinidine, suggests, for ciprofloxacin, the existence of transport systems distinct from the P-gp, as the OCT1 transporter which could be inhibited by quinidine. 3. With cephalexin and azlocillin, two beta-lactam antibiotics, plasma AUCs of ciprofloxacin increased and biliary and intestinal overall clearances decreased in a similar fashion (1.3 - 2 fold), suggesting the involvement of organic anion and/or cation transporters. 4. In the presence of structural analogues, the effect was dependent on the compound administered: Sparfloxacin had no effect on intestinal clearance of ciprofloxacin. In contrast, with pefloxacin, overall intestinal clearance of ciprofloxacin was decreased and net intestinal clearance increased. 5. The specificity of ciprofloxacin intestinal transport appears to be different from P-gp as outlined by the lack of competition with sparfloxacin, a P-gp substrate. Ciprofloxacin intestinal elimination seems to be mediated by organic anion and/or cation transporters and a mechanism sensitive to quinidine and verapamil.

Sister of P-glycoprotein expression in different tissues

Sister of P-glycoprotein (spgp) is a gene that is closely related to the P-glycoprotein family (Pgps). This class of proteins belongs to the superfamily of ATP-binding cassette transporters and is known for its involvement in pharmacological drug interactions. Therefore, this study investigated the distribution of spgp expression in different tissues known for their high levels of Pgps expression such as brain, liver, kidney, small- and large-gut mucosa. Analysis was done by using the reverse transcription-polymerase chain reaction. In addition to a high expression in the liver, we were able to demonstrate a significant spgp expression in brain grey cortex, small- and large-gut mucosa. Although Pgps are expressed in the kidney and brain capillary endothelial cells, no expression of spgp was detected in these tissues, which might indicate that spgp has no function in the blood-brain barrier and is not involved in the renal excretion of drugs.

Structure-transport relationship for the intestinal small-peptide carrier: is the carbonyl group of the peptide bond relevant for transport?

PURPOSE

The objective of this study was to determine the influence of the peptide bond with emphasis on the carbonyl group on the interaction with and transport by the intestinal small-peptide carrier. Therefore enalapril, a known substrate for the small-peptide carrier, has been modified to an analogue with a reduced peptide bond, enamipril. The transport characteristics of both compounds have been determined.

METHODS

The in vitro transport studies were performed using rat ileum in Ussing chambers. The transport of enalapril and enamipril were measured in a concentration range from 0.5-8 mM in both directions across the ileum. in the presence and absence of inhibitors. The interaction with the small-peptide carrier was studied by evaluating the ability of enalapril and its analogue enamipril to inhibit the transport rate of amoxycillin.

RESULTS

Enalapril shows, besides passive diffusion (P(m)3.06+/-0.14 . 10(-6)cm/s), saturable transport kinetics (Jmax = 16+/-5 nmol/h.cm2, Km = 1.86+/-0.64 mM) which can be inhibited with 10 mM cephalexin. The analogue with a reduced peptide bond does not show saturable transport from the mucosal to the serosal side, and cephalexin does not inhibit the flux of enamipril. However, the transport of enamipril from the serosal to mucosal side of the intestinal membrane is saturable and can be inhibited by 100 microM verapamil. Although enamipril is not a substrate for the small-peptide carrier in contrast to enalapril, both enalapril and enamipril are able to inhibit the active transport of amoxycillin with a K(i) of 0.41+/-0.24 mM and 0.24+/-0.12 mM respectively.

CONCLUSIONS

The reduction of the peptide bond of enalapril results in a compound, enamipril, which does not show polarized and saturable transport from the mucosal to the serosal side of the intestinal tissue. Also because the transport of enamipril cannot be inhibited by cephalexin, the analogue with the reduced peptide bond is no longer a substrate for the intestinal small-peptide carrier. Therefore, it can be concluded that the carbonyl group is an essential structural requirement for transport by the small-peptide carrier. In contrast, the interaction with the small-peptide carrier is still present, shown by the inhibition of the fluxes of amoxycillin. Reduction of the peptide bond of enalapril resulted in a new substrate for the P-glycoprotein efflux pump.