Fate of quinidine, a P-glycoprotein substrate, in the gastrointestinal tract after oral administration in rats

P-Glycoprotein (P-gp), an ATP-dependent efflux transporter, is expressed in brush-border membranes in the intestines of humans and rodents. In this study, the fate of quinidine, a P-gp substrate, in the gastrointestinal tract after oral administration was examined in conscious rats. The animals received quinidine (3 mg/ml/kg) or FITC-dextran of molecular weight of 10,000 (FD-10S, 5 mg/ml/kg, a poorly absorbable compound) orally, and the remaining amount of the compound in the upper gastrointestinal tract was measured at designated time intervals. As a control, FD-10S was distributed almost evenly throughout the gastrointestinal tract at 30 min, and most of FD-10S was accumulated in the distal small intestine at 60 min after administration. In contrast, most of the orally administered quinidine disappeared at the proximal intestine, and only small amounts reached the distal region. Also, the gastrointestinal transit of FD-10S was markedly slowed by stopping or inhibiting the bile flow, indicating that bile flow significantly affects the transit of drugs in the gastrointestinal tract. In conclusion, this study has demonstrated that compounds with high solubility and high permeability, such as quinidine, can be absorbed rapidly at the proximal intestine,escaping the barrier function of P-gp, because P-gp is mostly expressed in the distal intestine.     

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. Copyright (c) 2008 John Wiley & Sons, Ltd.     

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.     

Characterization of gastrointestinal absorption of digoxin involving influx and efflux transporter in rats: application of mdr1a knockout (−/−) rats into absorption study of multiple transporter substrate

Abstract

1.?This study was aimed to characterize gastrointestinal absorption of digoxin using wild-type (WT) and multidrug resistance protein 1a [mdr1a; P-glycoprotein (P-gp)] knockout (?/?) rats.

2.?In WT rats, the area under the plasma concentration–time curve (AUC) of oral digoxin increased after oral pretreatment with quinidine at 30?mg/kg compared with non-treatment, but the increasing ratio tended to decrease at a high dose of 100?mg/kg. In mdr1a (?/?) rats, however, quinidine pretreatment caused a dose-dependent decrease in the AUC.

3.?Quinidine pretreatment did not alter the hepatic availability of digoxin, indicating that the changes in the digoxin AUC were attributable to inhibition of the absorption process by quinidine; i.e. inhibition of influx by quinidine in mdr1a (?/?) rats and inhibition of efflux and influx by quinidine in WT rats.

4.?An in situ rat intestinal closed loop study using naringin implied that organic anion transporting peptide (Oatp) 1a5 may be a responsible transporter in the absorption of digoxin.

5.?These findings imply that the rat absorption behavior of digoxin is possibly governed by Oatp1a5-mediated influx and P-gp-mediated efflux. The mdr1a (?/?) rat is therefore a useful in vivo tool to investigate drug absorption associated with multiple transporters including P-gp.     

Induction of P-glycoprotein, glutathione-S-transferase and cytochrome P450 in rat liver by atrazine

We studied the effects of intraperitoneally administered atrazine on two hepatic neoplastic markers, P-glycoprotein (P-gp), and glutathione-S-transferase (GST), and several phase I drug-metabolizing enzyme cytochrome P450 (CYP) subfamilies in hepatic microsomes and cytosol of Fischer rats. The P-gp content was increased after 24 h of atrazine administration at 50 mg/kg, and maximum P-gp induction was observed at 300 mg/kg for 3 days. GST-P was induced at a lower dose than P-gp, from 10 mg/kg, but no other form of GST, such as GST1A1, was induced by the same dose. Among the CYP families, CYP1A2 was highly and CYP2B was slightly induced by atrazine while the CYP3A content remained unchanged. The liver plasma membrane marker alkaline phosphatase (AP) was not induced by the same doses. The inductions of P-gp, GST-P and CYP1A2 observed may explain some of the reported tumor-promoting properties and toxicity of atrazine in vivo.     

Concentration-dependent exsorption of quinidine in the rat intestine.

During intravenous infusion, the luminal concentration of quinidine was higher than the plasma concentration. The intestinal clearance (CLi) of the drug was measured by dividing the rate of appearance of the drug in the intestinal luminal perfusate by the plasma concentration. The CLi of quinidine was therefore much higher than the rate of luminal perfusion. Over the infusion dose range of 0.1-2 mg h-1, the CLi of quinidine decreased with increasing plasma concentration of quinidine. Adding quinidine into the luminal perfusate had little effect on the CLi of quinidine. Co-administration of quinidine with other agents intravenously did not alter the CLi of salicylic acid and urea, while the same treatment decreased the CLi of theophylline and S-disopyramide. In-vitro experiments on brush-border membrane vesicles showed that quinidine decreased the rate of Na+ uptake and H+ efflux. The inhibition was significant at quinidine concentrations above 20 microM. Quinidine was a more potent inhibitor than amiloride. At quinidine infusion rates less than 2 mg h-1, quinidine concentration in plasma or in the luminal perfusate was at the lower limit of the inhibitory concentration. Microclimate pH at the intestinal surface was also measured. At mid-jejunum, the microclimate pH increased 0.3 pH units by infusing 2 mg h-1 of quinidine, while the microclimate pH at most other measuring sites was not significantly altered by quinidine infusion. It was concluded that quinidine is exsorbed from blood into the intestinal lumen by a carrier-mediated pathway in addition to the passive diffusion. At high plasma concentration, quinidine exsorption becomes saturated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of curcumin on the pharmacokinetics of tamoxifen and its active metabolite, 4-hydroxytamoxifen, in rats: possible role of CYP3A4 and P-glycoprotein inhibition by curcumin

The effects of curcumin, a natural anti-cancer compound, on the bioavailability and pharmacokinetics of tamoxifen and its metabolite, 4-hydroxytamoxifen, were investigated in rats. Tamoxifen and curcumin interact with cytochrom P450 (CYP) enzymes and P-glycoprotein, and the increase in the use of health supplements may result in curcumin being taken concomitantly with tamoxifen as a combination therapy to treat or prevent cancer. A single dose of tamoxifen was administered orally (9 mg x kg(-1)) with or without curcumin (0.5, 2.5 and 10 mg x kg(-1)) and intravenously (2mg x kg(-1)) with or without curcumin (2.5 and 10 mg x kg(-1)) to rats. The effects of curcumin on P-glycoprotein (P-gp) and CYP3A4 activity were also evaluated. Curcumin inhibited CYP3A4 activity with 50% inhibition concentration (IC50) values of 2.7 microM. In addition, curcumin significantly (P < 0.01 at 10 microM) enhanced the cellular accumulation of rhodamine-123 in MCF-7/ADR cells overexpressing P-gp in a concentration-dependent manner. This result suggested that curcumin significantly inhibited P-gp activity. Compared to the oral control group (given tamoxifen alone), the area under the plasma concentration-time curve (AUC(0-infinity)) and the peak plasma concentration (C(max)) of tamoxifen were significantly (P < 0.05 for 2.5 mg x kg(-1); P < 0.01 for 10 mg x kg(-1)) increased by 33.1-64.0% and 38.9-70.6%, respectively, by curcumin. Consequently, the absolute bioavailability of tamoxifen in the presence of curcumin (2.5 and 10 mg x kg(-1)) was 27.2-33.5%, which was significantly enhanced (P < 0.05 for 2.5 mg x kg(-1); P < 0.01 for 10 mg x kg(-1)) compared to that in the oral control group (20.4%). Moreover, the relative bioavailability of tamoxifen was 1.12- to 1.64-fold greater than that in the control group. Furthermore, concurrent use of curcumin significantly decreased (P < 0.05 for 10 mg x kg(-1)) the metabolite-parent AUC ratio (MR), implying that curcumin may inhibit the CYP-mediated metabolism of tamoxifen to its active metabolite, 4-hydroxytamoxifen. The enhanced bioavailability of tamoxifen by curcumin may be mainly due to inhibition of the CYP3A4-mediated metabolism of tamoxifen in the small intestine and/or in the liver and to inhibition of the P-gp efflux transporter in the small intestine rather than to reduction of renal elimination of tamoxifen, suggesting that curcumin may reduce the first-pass metabolism of tamoxifen in the small intestine and/or in the liver by inhibition of P-gp or CYP3A4 subfamily.     

Distribution of a lipidic 2.5 nm diameter dendrimer carrier after oral administration.

The biodistribution of a lipidic peptide dendrimer has been studied after oral administration to female Sprague-Dawley rats (180 g, 9 weeks old). Uptake by gut epithelial tissue of the radiolabelled dendrimer molecule (mol. wt. 6300; diameter 2.5 nm; log P=1.24) was studied in rats after a single oral dose by gavage (14 mg/kg). The maximum levels of dendrimer observed were 3% (blood), 1.5% (liver), 0.1% (spleen), 0.5% (kidneys), 15% (small intestine) and 5% (large intestine). Approximately 6% of a single administered dose (28 mg/kg) was recovered from the entire gastrointestinal tract while 1% was absorbed via the small intestine lymphoid tissue after 3 h; after 12 h, 0.1% was detected. The maximum uptake by the non-lymphoid small intestine was 4% of the dose after 3 h. After 12 h, 0.3 and 4% dendrimer was measured in the lymphoid large intestine and the non-lymphoid large intestine, respectively. The total percentage of the administered dose absorbed through the lymphoid tissue was comparatively greater than through the non-lymphoid tissue of the small intestine with respect to organ weight after 3 and 24 h.     

Modulation of mdr1a and CYP3A gene expression in the intestine and liver as possible cause of changes in the cyclosporin A disposition kinetics by dexamethasone

We investigated the effect of dexamethasone (DEX) on the disposition kinetics of cyclosporin A (CyA) and the mechanism of this drug interaction. Rats were treated with DEX (1 or 75mg/kg per day, i.p.) once a day for 1-7 days, and the blood concentration of CyA was measured after an i.v. or p.o. dose of CyA (10mg/kg) at 1.5hr after the last DEX treatment. In rats treated with a low dose of DEX (1mg/kg), the blood concentration of CyA after i.v. administration was unchanged compared with that of untreated rats, whereas the blood concentration after oral administration was significantly decreased, and this decrease was dependent on the duration of DEX administration. The total clearance (CL(tot)) of CyA was unchanged, but the bioavailability was significantly decreased to about one-third of that in DEX-untreated rats after 7 days of DEX treatment. At this time, the expression of mdr1a mRNA and P-gp in the liver and intestine was increased, whereas CYP3A2 was unaffected at both the mRNA and protein levels. In rats treated with a high dose of DEX (75mg/kg), the blood concentration of CyA was significantly decreased after both i.v. and p.o. administrations compared with those of untreated rats. The bioavailability of CyA was decreased, and the CL(tot) was significantly increased. The P-gp and CYP3A2 in the liver and intestine were increased at both the mRNA and protein levels. Our results indicate that the drug interaction between CyA and DEX is a consequence of modulation of P-gp and CYP3A2 gene expression by DEX, with differential dose-dependence.     

Lack of carcinogenicity of chlorpyrifos insecticide in a high-dose, 2-year dietary toxicity study in Fischer 344 rats.

Chlorpyrifos (CPF) was administered daily in the feed to evaluate toxicity and oncogenicity potential in male and female Fischer 344 rats, according to U.S. EPA guidelines. Doses for the 2-year study were based on findings in a 13-week feeding study in which lower body weights, urinary perineal staining, adrenal cortical vacuolization, and inhibition (slightly more than 60%) of brain cholinesterase (ChE) occurred at 15 mg/kg/day. The high dose in the subsequent 2-year study was 10 mg/kg/day, with lower doses of 0, 0.05, 0.1, or 1.0 mg/kg/day chosen to define dose-response patterns. Rats given 10 mg/kg/day for 2 years were healthy and there was no evidence of premature deaths. Mild toxicity occurred only in rats given 10 mg/kg/day and consisted of perineal urine soiling in females and a 6-8% body-weight decrease in males. Males given 10 mg/kg/day also had increased adrenal weights and vacuolation of the adrenal zona fasciculata. ChE was considered a measure of exposure. Plasma, RBC, and brain ChE activities were inhibited in rats given 10 mg/kg/day, and the plasma and RBC ChE activities were inhibited in rats given 1.0 mg/kg/day. Chronic exposure to 0.1 mg/kg/day was considered a threshold exposure level for inhibition of plasma ChE. Rats given 10 mg/kg/day, considered a maximum-tolerated dose, had approximately 60% chronic inhibition of brain ChE. This group had similar numbers and types of neoplasms as control rats. Consequently, CPF was not carcinogenic at dose levels up to 10 mg/kg/day.     

Effects of morin on the pharmacokinetics of etoposide in rats

This study investigated the effects of orally administered morin, an inhibitor of CYP isozyme and P-glycoprotein (P-gp), on the pharmacokinetics of intravenous and orally administered etoposide in rats. It was reported that etoposide is a substrate for P-gp and metabolized mainly via CYP3A4 and to a lesser degree via CYP1A2 and 2E1. Etoposide was administered through intravenous (2 mg/kg) or oral (6 mg/kg) routes to rats with or without orally administered morin (5 or 15 mg/kg), which was administered 30 min before etoposide. The pharmacokinetic parameters of etoposide intravenously administered were not significantly different from other groups, suggesting that CYP 3A-mediated metabolism and the P-gp mediated efflux of etoposide in the liver and kidney seemed not to be markedly inhibited by orally administered morin. However, orally administered morin (15 mg/kg) significantly increased the AUC (45.8%), C(max) (32.0%) and the absolute bioavailability (35.9%) of orally administered etoposide compared with the control, which could be mainly due to inhibition of CYP isoenzyme and P-gp in the intestine by morin. The dosage regimen of etoposide should be taken into consideration for toxic reactions when combined with morin or dietary supplements containing morin in patients.     

Pharmacokinetics of baicalin in rats and its interactions with cyclosporin A, quinidine and SKF-525A: a microdialysis study

Baicalin, a flavone glucuronide derived mainly from the root of Scutellaria baicalensis, has been used in traditional Chinese medicine as an anti-inflammatory and anti-viral agent. To explore whether the disposition of baicalin is related to multidrug resistance P-glycoprotein (P-gp), baicalin (3, 10 and 30 mg kg(-1); i. v.) was injected to rats for a pharmacokinetic study using microdialysis coupled with HPLC. The results indicate that baicalin goes through hepatobiliary excretion against a concentration gradient based on the blood-to-bile distribution ratio (AUCbile/AUCblood), but that AUCblood or AUCbile did not show any dose-related increase in the range from 3 to 30 mg kg(-1). Coadministration of cyclosporin A (CsA) or quinidine (both are P-gp inhibitors) was used to delineate the role of P-gp on baicalin disposition, while SKF-525A (a cytochrome P450 inhibitor) could specifically inhibit the cytochrome P450 catalysis of baicalin without crossing with P-gp function. Both CsA and quinidine promoted the active transport of baicalin into bile and reduced its level in blood, and this result was the same as that obtained by treating with SKF-525A. Hence, the association of the involvement of P-gp in active baicalin efflux into bile seems to be excluded since CsA and quinidine are also cytochrome P450 inhibitors. In addition, baicalin was not detected in the brain striatum after treating with baicalin alone in the present study. Also, neither CsA nor quinidine co-administered with baicalin is able to induce measurable levels of baicalin in rat brain, which suggests that baicalin might not be able to pass through the blood-brain barrier (BBB).     

Commonly used surfactant,Tween 80, improves absorption of P-glycoprotein substrate,digoxin, in rats

Tween 80 (Polysorbate 80) is a hydrophilic nonionic surfactant commonly used as an ingredient in dosing vehicles for pre-clinical in vivo studies (e.g., pharmacokinetic studies, etc.). Tween 80 increased apical to basolateral permeability of digoxin in Caco-2 cells suggesting that Tween 80 is an in vitro inhibitor of P-gp. The overall objective of the present study was to investigate whether an inhibition of P-gp by Tween 80 can potentially influence in vivo absorption of P-gp substrates by evaluating the effect of Tween 80 on the disposition of digoxin (a model P-gp substrate with minimum metabolism) after oral administration in rats. Rats were dosed orally with digoxin (0.2 mg/kg) formulated in ethanol (40%, v/v) and saline mixture with and without Tween 80 (1 or 10%, v/v). Digoxin oral AUC increased 30 and 61% when dosed in 1% and 10% Tween 80, respectively, compared to control (P < 0.05). To further examine whether the increase in digoxin AUC after oral administration of Tween 80 is due, in part, to a systemic inhibition of digoxin excretion in addition to an inhibition of P-gp in the GI tract, a separate group of rats received digoxin intravenously (0.2 mg/kg) and Tween 80 (10% v/v) orally. No significant changes in digoxin IV AUC was noted when Tween 80 was administered orally. In conclusion, Tween 80 significantly increased digoxin AUC and Cmax after oral administration, and the increased AUC is likely to be due to an inhibition of P-gp in the gut (i.e., improved absorption). Therefore, Tween 80 is likely to improve systemic exposure of P-gp substrates after oral administration. Comparing AUC after oral administration with and without Tween 80 may be a viable strategy in evaluating whether oral absorption of P-gp substrates is potentially limited by P-gp in the gut.     

Effect of oral ketoconazole on intestinal first-pass effect of midazolam and fexofenadine in cynomolgus monkeys.

Because the expression of drug-metabolizing enzymes and drug efflux transporters has been shown in the intestine, the contribution of this tissue to the first-pass effect has become of significant interest. Consequently, a comprehensive understanding of the absorption barriers in key preclinical species would be useful for the precise characterization of drug candidates. In the present investigation, we evaluated the intestinal first-pass effect of midazolam (MDZ) and fexofenadine (FEX), typical substrates for CYP3A and P-glycoprotein (P-gp), respectively, with ketoconazole (KTZ) as a potent dual CYP3A/P-gp inhibitor in cynomolgus monkeys. When MDZ or FEX was administered i.v. at doses of 0.3 or 1 mg/kg, respectively, the plasma concentration-time profiles were not influenced by p.o. coadministration of KTZ (20 mg/kg). On the other hand, when MDZ or FEX was administered p.o. at doses of 1 or 5 mg/kg, respectively, concomitant with a dose p.o. of KTZ (20 mg/kg), significant increases were observed in the area under the plasma concentration-time curves of MDZ or FEX (22-fold in MDZ and 3-fold in FEX). These findings indicate that both CYP3A and P-gp play a key role in the intestinal barrier and that inhibition of intestinal CYP3A/P-gp activities contributes exclusively toward the drug-drug interactions (DDI) with KTZ. Additionally, the K(i) values of the antifungal agents, KTZ, itraconazole, and fluconazole, for MDZ 1'-hydroxylation in monkey intestinal and liver microsomes were comparable with those in the respective human samples. These results suggest that monkeys may be an appropriate animal species for evaluating the intestinal first-pass effect of p.o. administered drugs and predicting intestinal DDI related to CYP3A4 and P-gp in humans.     

Effects of myricetin on the bioavailability of doxorubicin for oral drug delivery in Rats: Possible role of CYP3A4 and P-glycoprotein inhibition by myricetin

The purpose of this study was to investigate the effect of oral myricetin on the bioavailability and pharmacokinetics of orally and intravenously administered doxorubicin (DOX) in rats for oral delivery. The effect of myricetin on the P-glycoprotein (P-gp) and CYP3A4 activity was also evaluated. Myricetin inhibited CYP3A4 enzyme activity with 50% inhibition concentration of 7.8 μM. In addition, myricetin significantly enhanced the cellular accumulation of rhodamine 123 in MCF-7/ADR cells overexpressing P-gp. The pharmacokinetic parameters of DOX were determined in rats after oral (40 mg/kg) or intravenous (10 mg/kg) administration of DOX to rats in the presence and absence of myricetin (0.4, 2 or 10 mg/kg). Compared to the control group, myricetin significantly (p < 0.05, 2 mg/kg; p < 0.01, 10 mg/kg) increased the area under the plasma concentration-time curve (AUC, 51–117% greater) of oral DOX. Myricetin also significantly (p < 0.05, 2 mg/kg; p < 0.01, 10 mg/kg) increased the peak plasma concentration of DOX. Consequently, the absolute bioavailability of DOX was increased by myricetin compared to that in the control group, and the relative bioavailability of oral DOX was increased by 1.51- to 2.17-fold. The intravenous pharmacokinetics of DOX were not affected by the concurrent use of myricetin in contrast to the oral administration of DOX. Accordingly, the enhanced oral bioavailability in the presence of myricetin, while there was no significant change in the intravenous pharmacokinetics of DOX, could be mainly due to the increased intestinal absorption via P-gp inhibition by myricetin rather than to the reduced elimination of DOX. These results suggest that the increase in the oral bioavailability of DOX might be mainly attributed to enhanced absorption in the gastrointestinal tract via the inhibition of P-gp and to reduced first-pass metabolism of DOX due to inhibition of CYP3A in the small intestine and/or in the liver by myricetin.     

Multiple pathways are involved in the oral absorption of BMS-262084, a tryptase inhibitor, in rats: role of paracellular transport, binding to trypsin, and P-glycoprotein efflux

BMS-262084 is a potent and selective beta-lactam tryptase inhibitor with therapeutic potential for treating asthma. The oral bioavailability of BMS-262084 was low in rats (4% at a dose of 0.5 mg/kg) due to poor absorption. BMS-262084 was excreted mainly unchanged in the urine, suggesting minimal metabolism in rats. The objective of this study was to investigate the mechanisms of oral absorption of BMS-262084 in rats. Modulation of intestinal tight junctions, binding to trypsin, and involvement of the intestinal dipeptide transport system and P-glycoprotein (P-gp) in the absorption of BMS-262084 were examined. Coadministration of BMS-262084 with SQ-29852, a substrate of the intestinal dipeptide transport system, did not change the oral absorption of BMS-262084. An increase in the dose of BMS-262084 from 0.5 to 50 mg/kg resulted in a 3.7-fold increase in its oral absorption. Inulin absorption was enhanced upon coadministration with BMS-262084, suggesting the opening of tight junctions in the intestinal epithelium. Coadministration of aprotinin, a trypsin inhibitor, increased the oral absorption of BMS-262084 several fold. In vitro, using Caco-2 cells, BMS-262084 appeared to be a P-gp substrate, with an efflux ratio of 14. These results suggest that absorption of BMS-262084 is mediated by several concurrent mechanisms. At higher doses of BMS-262084, increased absorption may be primarily due to opening of tight junctions in the intestinal epithelium and consequent absorption via the paracellular pathway, while at lower doses, binding to trypsin may contribute to limiting its absorption. P-gp efflux may also play a role in influencing the absorption of BMS-262084. The intestinal dipeptide transporter system does not appear to be involved in the absorption of BMS-262084.     

Enhanced bioavailability of paclitaxel after oral coadministration with flavone in rats

The purpose of this study was to investigate the effect of flavone on the bioavailability of paclitaxel orally coadministered in rats. Paclitaxel (40 mg/kg) and flavone (2, 10, 20 mg/kg) were orally administered to rats orally. The plasma concentration of paclitaxel with flavone increased significantly (P < 0.01) compared to that of paclitaxel control. Area under the plasma concentration-time curve (AUC) of paclitaxel with the dose of 2-20 mg/kg flavone was significantly (P < 0.05 at 10 mg/kg, P < 0.01 at 20 mg/kg) higher than that of control. AUCs of paclitaxel were increased dose-dependently in the dose range of flavone. The absorption rate constant (Ka) of paclitaxel with the dose of 10-20 mg/kg flavone was significantly increased (P < 0.05 at 10 mg/kg, P < 0.01 at 20 mg/kg) compared to that of control. Peak concentration (Cmax) of paclitaxel with the dose of 10-20 mg/kg flavone were significantly increased (P < 0.05 at 10 mg/kg, P < 0.01 at 20 mg/kg) compared to that of control. Half-life (t(1/2)) of paclitaxel with the dose of 10-20 mg/kg flavone was significantly prolonged (P < 0.05 at 10 mg/kg, P < 0.01 at 20 mg/kg) compared to that of control. The relative bioavailability increased significantly by about 2.38- or 3.10-fold (P < 0.05 at 10 mg/kg, P < 0.01 at 20 mg/kg) compared to that of paclitaxel control. Based on these results, It might be considered that the bioavailability of paclitaxel coadministered with flavone was significantly enhanced by the both inhibition of cytochrome P-450 and the P-gp efflux pump in the intestinal mucosa. It could be possible to administer paclitaxel orally besides the established i.v. route.     

The pharmacokinetics of aspirin in rats and the effect of buffer

Aspirin (acetylsalicyclic acid) was administered to rats intravenously, orally, and intraintestinally at different doses or in different dosage forms. The distribution and elimination kinetics of aspirin in rats following intravenous administration were best described by a two-compartmental open system and were dose independent up to 15 mg/kg. The terminal elimination half-life following intravenous dosing (10 mg/kg) was 3.36 +/- 0.85 min (n = 15) with the clearance being 8.40 +/- 1.24 L/(kg.hr). Intravenous distribution and elimination kinetics of aspirin in rats were not influenced by an orally administered buffered solution with a buffer capacity of 0.933 mEq ANC (acid neutralizing capacity) per kg of body weight. However, this orally buffered solution did change the gastrointestinal absorption kinetics of aspirin in rats. The absolute bioavailable dose of aspirin was 56.6 +/- 10.4% (n = 6) following its administration in an unbuffered solution while it was only 31.8 +/- 8.0% (n = 6) following administration in the buffered solution. The corresponding values of the absolute bioavailable doses were 43.4 +/- 3.7% and 25.5 +/- 1.8% following intraintestinal administration. The lower systemic availability of aspirin in the presence of buffer is attributed to a greater fraction of the administered dose becoming available for absorption from the intestine where the extraction efficiency is higher than that in the stomach.     

Studies on the metabolism of piperine: absorption, tissue distribution and excretion of urinary conjugates in rats

Upon administration of piperine, the major bite factor of black pepper, to male albino rats at a dose of 30 mg (170 mg/kg) by gavage or 15 mg (85 mg/kg) intraperitoneally, about 97% was absorbed irrespective of the mode of dosing. Three per cent of the administered dose was excreted as piperine in the feces. Piperine was not detectable in urine. When everted sacs of rat intestines were incubated with 200-1000 micrograms of piperine, about 47-64% of the added piperine disappeared from the mucosal side. Only piperine was present in the serosal fluid and also the intestinal tissue, indicating that piperine did not undergo any metabolic change during absorption. Examination of the passage of piperine through the gut indicated that the highest concentration in the stomach and small intestine was attained at about 6 h. Only traces (less than 0.15%) of piperine were detected in serum, kidney and spleen from 30 min to 24 h. About 1-2.5% of the intraperitoneally administered piperine was detected in the liver during 0.5-6 h after administration as contrasted with 0.1-0.25% of the orally administered dose. The increased excretion of conjugated uronic acids, conjugated sulphates and phenols indicated that scission of the methylenedioxy group of piperine, glucuronidation and sulphation appear to be the major steps in the disposition of piperine in the rat.     

Enhanced oral paclitaxel absorption with vitamin E-TPGS: Effect on solubility and permeability in vitro, in situ and in vivo

Solubility and permeability being important determinants of oral drug absorption, this study was aimed to investigate the effect of d--tocopheryl polyethylene glycol 1000 succinate (TPGS) on the solubility and intestinal permeability of paclitaxel in vitro, in situ and in vivo, in order to estimate the absorption enhancement ability of TPGS. Aqueous solubility of paclitaxel is significantly enhanced by TPGS, where a linear increase was demonstrated above a TPGS concentration of 0.1 mg/ml. Paclitaxel demonstrated asymmetric transport across rat ileum with significantly greater (26-fold) basolateral-to-apical (B–A) permeability than that in apical-to-basolateral (A–B) direction. Presence of P-glycoprotein (P-gp) inhibitor, verapamil (200 μM), diminished asymmetric transport of paclitaxel suggesting the role of P-gp-mediated efflux. TPGS showed a concentration-dependent increase in A–B permeability and decreased B–A permeability. The maximum efflux inhibition activity was found at a minimum TPGS concentration of 0.1 mg/ml, however, further increase in TPGS concentration resulted in decreased A–B permeability with no change in B–A permeability. Thus, the maximum paclitaxel permeability attained with 0.1 mg/ml TPGS was attributed to the interplay between TPGS concentration dependent P-gp inhibition activity and miceller formation. In situ permeability studies in rats also demonstrated the role of efflux in limiting permeability of paclitaxel and inhibitory efficiency of TPGS. The plasma concentration of [¹⁴C]paclitaxel following oral administration (25 mg/kg) was significantly increased by coadministration of TPGS at a dose of 50 mg/kg in rats. Bioavailability is enhanced about 4.2- and 6.3-fold when [¹⁴C]paclitaxel was administrated with verapamil (25 mg/kg) and TPGS, respectively, as compared to [¹⁴C]paclitaxel administered alone. The effect of verapamil on oral bioavailability of [¹⁴C]paclitaxel was limited relative to the TPGS, consistent with the in vitro solubility and permeability enhancement ability of TPGS. In conclusion, the current data suggests that the coadministration of TPGS may improve the bioavailability of BCS class II–IV drugs with low solubility and/or less permeable as a result of significant P-gp-mediated efflux.