Encequidar is a multispecies gut-restricted P-glycoprotein inhibitor that delineates between intestinal secretion and biliary elimination in animals and predicts human disposition pathways.

Digoxin is a drug with a narrow therapeutic index, which is a substrate of the ATP-dependent efflux pump P-glycoprotein. Increased or decreased digoxin plasma concentrations occur in humans due to the inhibition or induction of this drug transporter in organs with excretory function such as small intestine, liver and kidney. It is well known that serum concentrations of digoxin increase considerably in humans if propafenone is given simultaneously. However, it has not been investigated in detail whether propafenone and its metabolites are substrates and/or inhibitors of human P-glycoprotein. The aim of this study, therefore, was to investigate the P-glycoprotein-mediated transport and inhibition properties of propafenone and its major metabolites 5-hydroxypropafenone and N-desalkylpropafenone in Caco-2 cell monolayers. Inhibition of P-glycoprotein-mediated transport by propafenone and its metabolites was determined using digoxin as a P-glycoprotein substrate. No polarised transport was observed for propafenone and N-desalkylpropafenone in Caco-2 cell monolayers. However, 5-hydroxypropafenone translocation was significantly greater from basal-to-apical compared with apical-to-basal (P(app) basal-apical vs. P(app) apical-basal, 10.21+/-2.63 x 10(-6) vs. 4.34+/-1.84 x 10(-6) cm/s; P<0.01). Moreover, propafenone, 5-hydroxypropafenone and N-desalkylpropafenone inhibited P-glycoprotein-mediated digoxin transport with IC(50) values of 6.8, 19.9, and 21.3 microM, respectively. In summary, whereas propafenone and N-desalkylpropafenone are not substrates of P-glycoprotein, 5-hydroxypropafenone is translocated by human P-glycoprotein across cell monolayers. In addition, propafenone and its two major metabolites 5-hydroxypropafenone and N-desalkylpropafenone are inhibitors of human P-glycoprotein and therefore contribute to the digoxin-propafenone interaction observed in humans.

Albendazole is a clinically important anthelminthic agent known to have variable and low oral bioavailability. The aim of this work was to determine whether albendazole, a CYP3A4 substrate, is also a substrate for the multidrug efflux transporter P-glycoprotein. Both in vitro and in vivo methods were used to assess the role of P-glycoprotein-mediated albendazole transport. In cultured LLC-PK1, L-MDR1, and Caco-2 cells, albendazole was found not to be a P-glycoprotein substrate; the transport across LLC-PK1 and L-MDR1 cells revealed basal to apical versus apical to basal transport to a similar extent. In addition, there was no inhibitory effect of albendazole on digoxin transport in Caco-2 cells, and P-glycoprotein inhibitors (verapamil and quinidine) did not affect transport across Caco-2 cells. The in vivo relevance of P-glycoprotein to albendazole disposition was assessed using mdr1a/1b(-/-) mice after intravenous administration of albendazole (15 mg/kg). A similar pattern of tissue distribution in both P-glycoprotein-deficient and wild-type mice was observed. In conclusion, albendazole is neither a substrate nor an inhibitor of P-glycoprotein. Therefore, interactions between albendazole and P-glycoprotein substrates or inhibitors are unlikely to be clinically important.

To report a case of colchicine intoxication occurring with institution of clarithromycin. A 76-year-old man with familial Mediterranean fever (FMF) had received colchicine 1.5 mg daily for 6 years. The patient underwent 7 days of clarithromycin, amoxicillin, and omeprazole treatment for Helicobacter pylori-associated gastritis. Fever, abdominal pain, and diarrhea occurred 3 days after treatment initiation. On day 8, dehydration, pancytopenia, metabolic acidosis, and increased lipase level necessitated hospitalization. Alopecia was observed 2 weeks later. The patient recovered fully after the colchicine dosage was reduced to 0.5 mg/day and rehydration was performed. The previous dosage was then reinstituted without adverse reaction. An objective causality assessment revealed that the adverse event was probable. Continuous colchicine administration is used in treatment of microcrystalline arthritis, Behcet's disease, and FMF. Colchicine is primarily eliminated through biliary excretion. Renal elimination and cytochrome P450 metabolism play a less significant role. Colchicine is also a substrate of P-glycoprotein, a transporter involved in cellular efflux and elimination of numerous drugs. Three cases of intoxication have been reported when colchicine was combined with erythromycin, josamycin, or clarithromycin. Macrolides are inhibitors of P-glycoprotein and cytochrome P450-dependent enzymes and may decrease colchicine's biliary excretion through P-glycoprotein inhibition. Coadministration of colchicine and macrolides may impair colchicine elimination, resulting in excess drug exposure and toxicity. To this end, colchicine should be used with extreme caution in patients receiving P-glycoprotein inhibitors, particularly if they are elderly and/or renally compromised.

The absorption, metabolism, and excretion of (2R,3R,4R)-4-hydroxy-2-(hydroxymethyl)pyrrolidin-3-yl 4-O-(6-deoxy-β-d-glucopyranosyl)-α-d-glucopyranoside (CS-1036), a novel and potent pancreatic and salivary α-amylase inhibitor, were evaluated in F344/DuCrlCrlj rats and cynomolgus monkeys. The total body clearance and volume of distribution of CS-1036 were low (2.67-3.44 ml/min/kg and 0.218-0.237 l/kg for rats and 2.25-2.84 ml/min/kg and 0.217-0.271 l/kg for monkeys). After intravenous administration of [(14)C]CS-1036 to rats and monkeys, radioactivity was mainly excreted into urine (77.2% for rats and 81.1% for monkeys). After oral administration, most of the radioactivity was recovered from feces (80.28% for rats and 88.13% for monkeys) with a low oral bioavailability (1.73-2.44% for rats and 0.983-1.20% for monkeys). In rats, intestinal secretion is suggested to be involved in the fecal excretion as a minor component because fecal excretion after intravenous administration was observed (15.66%) and biliary excretion was almost negligible. Although intestinal flora was involved in CS-1036 metabolism, CS-1036 was the main component in feces (70.3% for rats and 48.7% for monkeys) and in the intestinal contents (33-68% for rats up to 2 hours after the dose) after oral administration. In Zucker diabetic fatty rats, CS-1036 showed a suppressive effect on plasma glucose elevation after starch loading with a 50% effective dose at 0.015 mg/kg. In summary, CS-1036 showed optimal pharmacokinetic profiles: low oral absorption and favorable stability in gastrointestinal lumen, resulting in suppression of postprandial hyperglycemia by α-amylase inhibition.

1. P-Glycoprotein is a 170-kDa transmembrane glycoprotein active efflux system that confers multidrug resistance in tumors, as well as normal tissues including brain. 2. The classical model of multidrug resistance in brain places the expression of P-glycoprotein at the luminal membrane of the brain microvascular endothelial cell. However, recent studies have been performed with human brain microvessels and double-labeling confocal microscopy using (a) the MRK16 antibody to human P-glycoprotein, (b) an antiserum to glial fibrillary acidic protein (GFAP), an astrocyte foot process marker, or (c) an antiserum to the GLUT1 glucose transporter, a brain endothelial plasma membrane marker. These results provide evidence for a revised model of P-glycoprotein function at the brain microvasculature. In human brain capillaries, there is colocalization of immunoreactive P-glycoprotein with astrocytic GFAP but not with endothelial GLUT1 glucose transporter. 3. In the revised model of multidrug resistance in brain, P-glycoprotein is hypothesized to function at the plasma membrane of astrocyte foot processes. These astrocyte foot processes invest the brain microvascular endothelium but are located behind the blood-brain barrier in vivo, which is formed by the brain capillary endothelial plasma membrane. 4. In the classical model, an inhibition of endothelial P-glycoprotein would result in both an increase in the blood-brain barrier permeability to a given drug substrate of P-glycoprotein and an increase in the brain volume of distribution (VD) of the drug. However, in the revised model of P-glycoprotein function in brain, which positions this protein transporter at the astrocyte foot process, an inhibition of P-glycoprotein would result in no increase in blood-brain barrier permeability, per se, but only an increase in the VD in brain of P-glycoprotein substrates.

Voclosporin is a novel calcineurin inhibitor intended for prevention of organ graft rejection and treatment of lupus nephritis. Pharmacokinetic drug interactions between voclosporin and a CYP3A inhibitor, inducer and substrate and a P-glycoprotein inhibitor and substrate were evaluated. Voclosporin 0.4 mg kg(-1) was administered to 24 subjects in each of five studies, as follows: every 12 h (Q12H) alone and concomitantly with ketoconazole 400 mg once daily (QD); single dose before and single dose after rifampin 600 mg QD; Q12H where midazolam 7.5 mg was administered as a single dose alone before voclosporin and with last the dose of voclosporin; Q12H alone and concomitantly with verapamil 80 mg every 8 h; and Q12H with digoxin 0.25 mg QD. The noncompartmental pharmacokinetic parameters maximal concentration (Cmax ) and area under the concentration-time curve (AUC) were obtained, and geometric least squares mean ratios and 90% confidence intervals were evaluated. Ketoconazole increased voclosporin Cmax (6.4-fold) and AUC (18-fold); rifampin reduced voclosporin AUC (0.9-fold); voclosporin did not change exposure of midazolam or α-hydroxy-midazolam; verapamil increased voclosporin Cmax (2.1-fold) and AUC (2.7-fold); and voclosporin increased digoxin Cmax (0.5-fold), AUC (0.25-fold) and urinary excretion (0.2-fold). Administration of voclosporin concomitantly with strong inhibitors and inducers of CYP3A resulted in increased and decreased exposures, respectively, and should be considered contraindicated. Drug-drug interactions involving voclosporin and CYP3A substrates are not expected. Administration of voclosporin concomitantly with inhibitors and substrates of P-glycoprotein resulted in increased voclosporin and substrate exposures, respectively. Appropriate concentration and safety monitoring is recommended with co-administration of voclosporin and P-glycoprotein substrates and inhibitors.

Methadone is an opioid, which has a high oral bioavailability (>70%) and a long elimination half-life (>20 h) in human beings. The purpose of this study was to evaluate the effects of ketoconazole [a CYP3A and p-glycoprotein (p-gp) inhibitor] and omeprazole (an H+,K(+)-ATPase proton-pump inhibitor) on oral methadone bioavailability in dogs. Six healthy dogs were used in a crossover design. Methadone was administered i.v. (1 mg/kg), orally (2 mg/kg), again orally following oral ketoconazole (10 mg/kg q12 h for two doses), and following omeprazole (1 mg/kg p.o. q12 h for five doses). Plasma concentrations of methadone were analyzed by high-pressure liquid chromatography or fluorescence polarization immunoassay. The mean +/- SD for the elimination half-life, volume of distribution, and clearance were 1.75 +/- 0.25 h, 3.46 +/- 1.09 L/kg, and 25.14 +/- 9.79 mL/min.kg, respectively following i.v. administration. Methadone was not detected in any sample following oral administration alone or following oral administration with omeprazole. Following administration with ketoconazole, detectable concentrations of methadone were present in one dog with a 29% bioavailability. MDR-1 genotyping, encoding p-gp, was normal in all dogs. In contrast to its pharmacokinetics humans, methadone has a short elimination half-life, rapid clearance, and low oral bioavailability in dogs and the extent of absorption is not affected by inhibition of CYP3A, p-gp, and gastric acid secretion.

The pharmacokinetics of cefoperazone in normal subjects, and in patients with hepatic and renal dysfunction are reviewed. After intravenous administration of 2 g of cefoperazone, levels in serum ranged from 202 to 375 microgram/ml depending on the period of drug administration. After intramuscular injection of 2 g of cefoperazone, the mean peak serum level was 111 microgram/ml at 1.5 hours. At 12 hours after dosing, mean serum levels were still 2 to 4 microgram/ml. Cefoperazone was 90% bound to serum proteins. The apparent volume of distribution was 10 to 13L. The half-life of the drug varied from 1.6 to 2.4 hours; serum clearance was between 75 and 96 ml/min. Urinary excretion was rapid, but only 15 to 36% of the cefoperazone dose was recovered in the urine. Renal clearance ranged from 14 to 25 ml/min. Urine levels of cefoperazone in excess of 32 microgram/ml were maintained for at least 12 hours. Biliary levels of cefoperazone were many-fold higher than serum levels; peak bile concentrations from 675 to 6000 microgram/ml were obtained. Severe hepatic dysfunction was associated with a 2- to 4-fold increase in the half-life of cefoperazone. In patients with relatively complete biliary obstruction, over 90% of the dose was recovered in the urine. In contrast, the serum kinetics of cefoperazone were not significantly altered in patients with renal impairment. The human pharmacology of cefoperazone is similar to cephazolin in terms of serum concentrations, half-life, protein binding, and apparent volume of distribution, but markedly different in terms of biliary and renal excretion. Since biliary excretion is normally the primary route of cefoperazone elimination, dosage modification should only be required in the presence of severe biliary obstruction or concomitant renal and hepatic dysfunction.

Preliminary studies in our laboratory demonstrated low oral bioavailability of Drug X in male Sprague Dawley rats. However, the factors responsible for the observed poor bioavailability were not well understood. The objective of this study was to investigate the contribution of cytochrome P450(s) metabolism to the observed poor oral bioavailability of Drug X in male Sprague-Dawley rats in the presence of 1-aminobenzotriazole, a non-specific irreversible inhibitor of cytochrome P450s. Male Sprague-Dawley rats were pre-treated with or without oral 1-aminobenzotriazole (50 mg/kg) two hours prior to receiving a single intravenous or oral dose of Drug X (3 mg/kg). Blood samples were collected from animals at different time points over six hours following Drug X dosing. Plasma concentrations of Drug X were determined using LC/MS/MS. Pharmacokinetic data obtained from an intravenous dose study in rats suggested that Drug X exhibited a high clearance (55 mL/min/kg) and moderate volume of distribution (1.3 L/kg) with short half-life in rats (0.7 hr). Oral dosing of Drug X to rats resulted in low oral bioavailability (19%). 1-aminobenzotriazole pre-treatment of male Sprague Dawley rats followed by an intravenous dose of Drug X resulted in a decrease in plasma clearance by 71% and an increase in half-life by 100%, without affecting the volume of distribution. Furthermore, the oral bioavailability of Drug X increased markedly with 1-aminobenzotriazole pre-treatment. However, the fraction absorbed of Drug X did not significantly change with 1-aminobenzotriazole pre-treatment. The results of this study indicated that CYP-mediated metabolism played a major role in limiting the oral bioavailability of Drug X in rats. The data suggests that 1-aminobenzotriazole can be used as an effective tool in assessing the factors contributing to the poor oral bioavailability of drugs.

Interspecies differences in biliary excretion and the differences in bile flow rates make scaling across species difficult for drugs that are excreted in the bile. The objective of this study is to predict clearance (CL) and volume of distribution (V) for humans from animals for drugs that are excreted in the bile. Clearance values of 10 drugs known to be excreted in the bile were selected from the literature. Scaling of CL was performed using at least three animal species. Using simple allometry and the rule of exponents (ROE), clearances of studied drugs were predicted in humans. Besides using the ROE, a 'correction factor' was applied adjusting bile flow rate based on the species body weight (bile flow mL/day/kg body weight) or liver weight (bile flow mL/day/kg liver weight). Using the ROE and combining it with the 'correction factor', the clearances of biliary excreted drugs were predicted for humans. V for 15 drugs (without any correction factor) that are excreted in the bile was also predicted for humans. The results of the study indicated that the ROE in association with the correction factors developed for the biliary excreted drugs substantially improved the prediction of human clearance for drugs that are excreted in the bile. In this study, there was no indication (unlike clearance) that the prediction of volume of drug distribution was affected (systematically under- or over-prediction) because of biliary excretion. The clearance of drugs that are excreted in the bile can be predicted with reasonable accuracy using ROE and a correction factor.

Interactions of HIV protease inhibitors with ATP-dependent drug export proteins.

Colchicine in acute gout: the need for a reappraisal

Proton pump inhibitors are a class of drugs which are widely prescribed for acid-related diseases. They are primarily metabolized by CYP2C19 and CYP3A4. It is unknown so far whether proton pump inhibitors are also substrates of the ATP-dependent efflux transporter P-glycoprotein. Moreover, it is not established whether proton pump inhibitors are also inhibitors of P-glycoprotein function. The aim of our study was therefore to characterize omeprazole, lansoprazole and pantoprazole as P-glycoprotein substrates and inhibitors. Polarized transport of these compounds was assessed in P-glycoprotein-expressing Caco-2 and L-MDR1 cells. Inhibition of P-glycoprotein-mediated transport was determined using the cyclosporine analogue PSC-833 (valspodar) as P-glycoprotein inhibitor. Inhibition of efflux transport by omeprazole, lansoprazole and pantoprazole was assessed using digoxin as P-glycoprotein substrate. At concentrations of 5 microM, basal-to-apical transport of omeprazole, lansoprazole and pantoprazole was greater than apical-to-basal transport in Caco-2 and L-MDRI cells. Addition of PSC-833 (1 microM) showed a clear effect only for lansoprazole, suggesting that other transporters contribute to omeprazole and pantoprazole cellular translocation. Furthermore, all of the tested compounds inhibited digoxin transport with IC50 values of 17.7, 17.9 and 62.8 microM for omeprazole, pantoprazole and lansoprazole, respectively. In summary, our data provide evidence that proton pump inhibitors are substrates and inhibitors of P-glycoprotein. These findings might explain some of the drug interactions with proton pump inhibitors observed in vivo.

The purpose of this study was to clarify the contribution of P-glycoprotein to the bioavailability and intestinal secretion of grepafloxacin and levofloxacin in vivo. Plasma concentrations of grepafloxacin and levofloxacin after intravenous and intraintestinal administration were increased by cyclosporin A, a P-glycoprotein inhibitor, in rats. The total body clearance and volume of distribution at steady state of grepafloxacin were significantly decreased to 60 and 63% of the corresponding control values by cyclosporin A. The apparent oral clearance of grepafloxacin was decreased to 33% of the control, and the bioavailability of grepafloxacin was increased to 95% by cyclosporin A from 53% in the controls. Intestinal clearance of grepafloxacin and levofloxacin were decreased to one-half and one-third of the control, respectively, and biliary clearance of grepafloxacin was also decreased to one-third with cyclosporin A in rats. Intestinal secretion of grepafloxacin in mdr1a/1b (-/-) mice, which lack mdr1-type P-glycoproteins, was significantly decreased compared with wild-type mice, although the biliary secretion was similar. Intestinal secretion of grepafloxacin in wild-type mice treated with cyclosporin A was comparable to those in mdr1a/1b (-/-) mice with or without cyclosporin A, indicating that cyclosporin A completely inhibited P-glycoprotein-mediated intestinal transport of grepafloxacin. In conclusion, our results indicated that P-glycoprotein mediated the intestinal secretion of grepafloxacin and limited the bioavailability of this drug in vivo.

BMS-310705, a novel semisynthetic derivative of epothilone B, is a tubulin-polymerization agent currently in phase I clinical trials for anticancer therapy. The in vitro and in vivo pharmacokinetics and oral bioavailability of BMS-310705 were investigated in mice, rats, and dogs. In addition, comparison of the pharmacokinetics of BMS-310705 using various formulations was conducted in rats. The permeability of BMS-310705 was evaluated in Caco-2 cells, an in vitro model of the human intestinal epithelium. Human liver microsomes were used to determine the cytochrome P450 enzymes involved in the metabolism of BMS-310705. Plasma protein binding of BMS-310705 was determined in mouse, rat, dog, and humans. BMS-310705 was administered to female nude mice as single doses of 5 mg/kg intravenously or 15 mg/kg orally. Male Sprague-Dawley rats were treated with single doses of BMS-310705 either intraarterially (2 mg/kg) or orally (8 mg/kg). The effect of Cremophor on the pharmacokinetics of BMS-310705 was evaluated in rats using various formulations with and without Cremophor. Male dogs were treated with 0.5 mg/kg intravenously or 1 mg/kg orally in a crossover study design. Systemic clearance of BMS-310705 was high in mice (152 ml/min/kg), rats (39 ml/min/kg), and dogs (25.7 ml/min/kg). The volume of distribution (Vss) in mice, rats, and dogs was 38, 54, and 4.7 l/kg, respectively, and greater than total body water. BMS-310705 showed moderate binding to plasma proteins in all four species tested. The clearance in humans may be intermediate to high based on both allometric scaling using parameters obtained from three species, and in vitro human liver microsomal stability data. In rats, the presence of Cremophor in the formulation resulted in a significant increase in exposure compared to buffered vehicles not containing Cremophor. Inhibition of p-glycoprotein and/or CYP3A4 by Cremophor may be responsible for this phenomenon, and studies in Caco-2 cells and human liver microsomes suggested that BMS-310705 may be a substrate for both p-glycoprotein and CYP3A4. The oral bioavailability of BMS-310705 in pH buffered formulations was 21% in mice, 34% in rats and 40% in dogs. In summary, BMS-310705 is cleared rapidly and distributes extensively in mice, rats, and dogs. The presence of Cremophor in the formulation could significantly increase exposure in rats, possibly due to interactions with p-glycoprotein and/or CYP3A4. Oral bioavailability using formulations not containing Cremophor were found to be adequate, suggesting potential for development of BMS-310705 as an oral anticancer drug.