The role of mdr1a P-glycoprotein in the biliary and intestinal secretion of doxorubicin and vinblastine in mice.

Drug-transporting P-glycoproteins are abundantly present in the liver and the intestinal wall. We have now investigated their role in the biliary and intestinal secretion of the anticancer drugs doxorubicin (unlabeled: 5 mg/kg) and vinblastine ((3)H-labeled: 1 mg/kg) i.v. administered to wild-type and mdr1a P-glycoprotein knockout [mdr1a(-/-)] mice. At 90 min after drug administration, levels of unchanged drug and metabolites in plasma, intestinal contents, and bile were determined by high-performance liquid chromatography and radioactivity by liquid scintillation counting. The bile of both wild-type and mdr1a(-/-) mice contained only minor amounts of unchanged vinblastine, whereas the total biliary secretion of unknown (3)H-labeled breakdown products was about 25 to 30% of the dose. The direct secretion of unchanged vinblastine through the gut wall was 6.7 and 3.3% of the dose in wild-type and mdr1a(-/-) mice, respectively. The biliary secretion of unchanged doxorubicin decreased from 13.3% of the dose to only 2.4% in the absence of mdr1a P-glycoprotein. Approximately 10% of the dose was secreted as unchanged doxorubicin into the intestinal contents of both types of mice. Thus, the absence of mdr1a P-glycoprotein affects the fate of vinblastine chiefly by diminishing secretion into the lumen of the small intestine, whereas it affects the fate of doxorubicin chiefly by diminishing secretion of parent drug into bile.     

Kinetics of biliary elimination of pentacaine in rats

Pentacaine, a local anaesthetic of the carbanilate type, administered intravenously as 3H-pentacaine, 2 mg kg-1 to rats, was eliminated in the bile mainly in the form of metabolites (20.5 +/- 1.5 per cent of the dose) and as parent drug (0.1 per cent of the dose) within 3 days. In control rats 32.2 +/- 2.5 and 37.8 +/- 2.5 per cent of 3H-dose, representing pentacaine metabolites, were excreted in the urine and faeces, respectively. In bile-duct-cannulated rats 35.7 +/- 6.8, 11.2 +/- 3.4, and 20.5 +/- 1.5 per cent of 3H-dose were excreted in the urine, faeces, and bile, respectively. The high 3H recovered in faeces in animals with diverted bile flow indicated passage of pentacaine metabolites through the intestinal wall. The excretion of pentacaine and its metabolites in bile was an active process, since the ratio bile/plasma concentration rapidly attained values approaching 10. In rats with ligated ureters the biliary elimination of pentacaine and its metabolites was enhanced, compensating for impaired urinary excretion. This was accompanied by increased plasma and brain levels of 3H compared with untreated controls.     

Breast cancer resistance protein (Bcrp1/Abcg2) limits net intestinal uptake of quercetin in rats by facilitating apical efflux of glucuronides

The intestinal absorption of the flavonoid quercetin in rats is limited by the secretion of glucuronidated metabolites back into the gut lumen. The objective of this study was to determine the role of the intestinal efflux transporters breast cancer resistance protein (Bcrp1)/Abcg2 and multidrug resistance-associated protein 2 (Mrp2)/Abcc2. To study the possible involvement of Mrp2, we compared intestinal uptake of quercetin-3-glucoside between control and Mrp2-deficient rats, using an in situ intestinal perfusion system. The contribution of Bcrp1 was determined using the specific inhibitor fumitremorgin C (FTC) in Mrp2-deficient rats. Furthermore, vectorial transport of quercetin was studied in Madin-Darby canine kidney (MDCK)II cells transfected with either human MRP2 or murine Bcrp1. In these MDCKII cells, we showed an efficient efflux-directed transport of quercetin by mouse Bcrp1, whereas in control and MRP2-transfected cells no vectorial transport of quercetin was observed. In Mrp2-deficient rats, intestinal uptake of quercetin from quercetin-3-glucoside, efflux of quercetin glucuronides to the gut lumen, and plasma concentration of quercetin were similar to that in control rats. When intestinal Bcrp1 was inhibited by FTC in Mrp2-deficient rats, total plasma concentrations of quercetin and its methylated metabolite isorhamnetin after 30 min of perfusion were more than twice that of controls (12.3 +/- 1.5 versus 5.6 +/- 1.3 muM; p < 0.01), whereas uptake of free quercetin from the intestinal lumen was not affected. Instead, inhibition of Bcrp1 lowered the efflux of quercetin glucuronides into the perfusion fluid by approximately 4-fold. In conclusion, Bcrp1 limits net intestinal absorption of quercetin by pumping quercetin glucuronides back into the lumen.     

Biliary secretion of moxifloxacin in obstructive cholangitis and the non-obstructed biliary tract

BACKGROUND: Biliary secretion of antibiotic agents into the bile is considerably compromised by biliary obstruction, a precondition of bacterial cholangitis. Moxifloxacin may be advantageous according to secretion and antimicrobial spectrum. AIM: To establish the secretion of moxifloxacin into obstructed and non-obstructed bile. METHODS: Biliary excretion of moxifloxacin was determined in plasma and bile of 10 patients with biliary obstruction and cholangitis and 10 patients without biliary obstruction 30 min after administration of 400 mg of moxifloxacin intravenously. RESULTS: The plasma concentration of moxifloxacin was similar in both groups (4.45 +/- 1.58 microg/mL; 4.33 +/- 1.23 microg/mL). The concentration of moxifloxacin in the bile was significantly lower in patients with biliary obstruction than without (4.63 +/- 3.94 microg/mL; range 0.71-14.40; vs. 16.90 +/- 13.77 microg/mL; range 1.79-42.50; P = 0.043). Although significantly different, the penetration index was extensively high in those without biliary obstruction (4.41 +/- 4.40; range 0.35-14.45) but still sufficient in those patients with obstructive cholangitis (1.02 +/- 0.74; range 0.29-2.83; P = 0.035). CONCLUSION: These findings are suggestive of an active secretion mechanism for moxifloxacin into the obstructed bile, producing a biliary concentration sufficiently above the minimal inhibitory concentrations for most of the expected bacteria.     

Effects of butylated hydroxytoluene (BHT) on biliary excretion of xenobiotics and bile flow in rats

A significant enhancement in the biliary excretion of iv injected sulfobromophthalein (BSP), phenol- 3,6 -dibromphthalein disulfonate (DBSP), procaine amide ethobromide (PAEB) and ouabain was observed in rats maintained on diets containing 0.25% BHT for periods of 10 days. The enhanced biliary excretion of these drugs in BHT treated rats appears to be correlated with the increase in bile flow produced by BHT. The increased bile flow was due to an increase in canalicular bile production rather than a change in net ductular secretion or reabsorption of fluid since bile to plasma concentration ratios of erythritol were unchanged and no permeability change in the biliary tree was observed when mannitol was administered by retrograde intrabiliary injection. The increase in bile flow was not due to an enhanced excretion of bile salts into bile, because both the biliary bile acid concentration and total biliary excretion of bile acids were lower in BHT-treated rats than in control rats. It appears that the increase in bile flow produced by BHT is due to the osmotic choleresis related to the secretion of BHT and its metabolites into bile.     

Prediction of human pharmacokinetics-biliary and intestinal clearance and enterohepatic circulation

The main objective was to evaluate and propose methods for predicting biliary clearance (CL(bile)) and enterohepatic circulation (EHC) of intact drugs in man. Another aim was to evaluate to role of intestinal drug secretion and propose a method for prediction of intestinal secretion CL (CL(i)). Animal data poorly predict the CL and CL(bile) of biliary excreted drugs, and the suggested molecular weight threshold for bile excretion as the dominant elimination route does not seem to hold. Active transport, low metabolic intrinsic CL (CL(int)) and, as an approximation, permeability (P(e)) less than that of metoprolol is required for substantial CL(bile) to occur. The typical EHC plasma concentration vs time profile (multiple peaks) is demonstrated for many low metabolic CL(int)-compounds with efflux and moderate to high intestinal P(e) and fraction absorbed. Physiologically-based in-vitro to in-vivo (PB-IVIV) methodology with in-vitro intrinsic CL(bile)-data obtained with sandwich-cultured human hepatocytes has generated 2- and 5-fold underpredictions for two compounds with intermediate to high CL(bile). This is despite not considering the unbound fraction. Possible explanations include low transporter activity and diffusion limitations in the in-vitro experiments. Intestinal reabsorption and EHC were also neglected in these predictions and in-vivo CL(bile) estimations. The sandwich model and these reference data are still very useful. Consideration of an empirical scaling factor and a newly developed approach that accounts for intestinal reabsorption and EHC could potentially lead to improved PB-IVIV predictions of CL(bile). Apparently, no attempts have been made to predict CL(i). Elimination via the intestinal route does not appear to be of great importance for the few compounds with available data, but could be equally as important as bile excretion. Net secretion in-vitro P(e) and newly estimated in-vivo intrinsic CL(i) data for digoxin and rosuvastatin could be useful for approximation of CL(i) of other compounds.     

Inhibition of biliary lipid and protein secretion by cyclosporine A in the rat.

We investigated the effect of cyclosporine A (CyA) administered as a single i.v. dose of 20 and 40 mg/kg body wt, on biliary secretion of cholesterol, phospholipid, bile acid, and lysosomal marker and canalicular plasma membrane marker enzymes in anaesthetized Wistar rats. CyA reduced the concentration and biliary secretion of cholesterol, phospholipid and bile acid to a considerable extent; the inhibitory effect of CyA on the biliary secretion of phospholipid and bile acid was greater than that on cholesterol. The biliary outputs of acid phosphatase (AcP) and gamma-glutamyltransferase (gamma-GT) were also diminished by the drug, all these effects being dose-dependent. Maximum decreases in bile acid secretion were observed 10 min after administration, whereas those of cholesterol and phospholipid were delayed. Bile acid concentrations and secretion returned to pretest values at 30-50 min after CyA injection whereas those of cholesterol and phospholipid remained significantly reduced at this time point. The greater inhibitory effect of CyA on the biliary outputs of phospholipid and bile acid relative to cholesterol secretion together with the asynchronous fall and recovery of bile acid, cholesterol and phospholipid concentrations and secretion alter the cholesterol/bile acid, phospholipid/bile acid and cholesterol/phospholipid molar ratios as well as the lithogenic index, thus suggesting that CyA would uncouple biliary lipid secretion from bile acid secretion. Since under physiological conditions biliary lipid and gamma-GT secretion is related to and dependent upon bile acid secretion, we propose that the CyA-induced inhibition on lipid and gamma-GT secretion is, at least partly, secondary to the fall in bile acid output caused by the drug. However, since CyA inhibits secretory processes independent of the hepatobiliary flux of bile acid, such as the exocytic discharge of AcP, and because it also uncouples biliary lipid from bile acid secretion, other mechanisms and factors involved in lipid and protein secretion (such as intracellular transport, canalicular membrane fluidity and/or intracanalicular events) might also be altered by this drug.     

Effect of diphenylhydantoin on the biotransformation and biliary excretion of imipramine in rats.

1. Rats with biliary fistula excrete 18% of an intraperitoneal dose of [14C]imipramine (80 mg/kg) in bile within 2-5 h. Diphenylhydantoin (46 mg/kg) simultaneously administered intravenously decreases the biliary excretion of imipramine plus metabolites to 7% dose. With or without diphenylhydantoin, the highest biliary concentration of imipramine plus metabolites occurs 30-60 min after dosage. 2. With or without administration of diphenylhydantoin, 83% of the bile radioactivity is present as the conjugated 2-hydroxylated metabolites of imipramine. With imipramine alone, more conjugated 2-hydroxydesmethyl-imipramine than conjugated 2-hydroxyimipramine is excreted in the bile. Diphenylhydantoin reverses this order. 3. Administration of diphenylhydantoin does not significantly alter the concentration of imipramine plus metabolites in plasma, liver, lung and brain measured at five consecutive 30 min periods after dosage.     

Plasma concentration, uptake by liver, and biliary excretion of tritiated cardiac glycosides in the isolated perfused guinea-pig liver

1. Investigations were carried out on isolated perfused guinea-pig livers. Different doses of tritiated ouabain, digoxin, and digitoxin were added to the perfusion medium and the subsequent plasma elimination, hepatic uptake, and biliary excretion quantitatively measured. After the perfusion, extracts of liver, bile and plasma were subjected to thin layer chromatography in order to detect the radioactively labelled glycosides and their metabolites.

2. The ouabain concentration in the plasma approached the equilibrium stage within 45 minutes. At this time 40% of the administered dose had been taken up by the liver, and no further elimination occurred. The elimination curve for ouabain followed a simple exponential function. After 1 h the tissue medium (T/M) ratio was approximately 3. In bile hardly any radioactivity could be detected. Ouabain was therefore not excreted by the liver.

3. Up to 80% of the digitoxin was eliminated from the plasma within 4 hours. The elimination of radioactive material for the dose range studied could be described by a hyperbolic function. The T/M ratio in the liver varied with time. At the beginning it was as high as 10 and after 4 h reduced to approximately 3. After 45-60 min the concentration of radioactive material in the bile was 500 times as high as that in the plasma. Almost 70% of the administered radioactivity was excreted with the bile within 4 hours. At the end of the perfusion almost all the identifiable substances in plasma and bile were polar metabolites, as shown by thin layer radiochromatography.

4. Digoxin behaved similarly to digitoxin.

5. The findings led to the following hypothesis: uptake of cardiac glycosides into the liver cells occurs by a passive diffusion process and is related to their lipid solubility. On the other hand excretion in the bile occurs in general if polar metabolites are formed in the liver cells.

     

Probenecid-impaired biliary excretion of acetaminophen glucuronide and sulfate in the rat.

Acetaminophen (APAP; 100 mg/kg iv) and probenecid (50 mg/kg bolus + 11.4 mg/hr/kg infusion) were administered to male Sprague-Dawley rats to examine the disposition of APAP, and its glucuronide (AG) and sulfate (AS) conjugates in plasma, bile, and urine. Probenecid significantly decreased the formation clearance of AG from 3.65 +/- 0.434 to 1.94 +/- 0.441 ml/min/kg and the renal clearance of AS from 9.32 +/- 2.26 to 3.15 +/- 1.21 ml/min/kg. The biliary excretion of AG was reduced approximately 3- to 4-fold by probenecid, from 6.54 to 1.87% of the APAP dose, and the AG biliary excretion rate was decreased 4- to 5-fold during probenecid treatment. The more extensive impairment of AG biliary excretion relative to AG formation suggests that probenecid may inhibit the hepatobiliary transport of AG. The significant reduction in the biliary excretion rate at early time points for AS suggests that probenecid may inhibit hepatic AS transport. The study results indicate that probenecid impairs AG and AS formation, AS renal secretion, and AG and AS biliary excretion.     

High-density lipoproteins decrease the biliary concentration of chlordecone in isolated perfused pig liver

Chlordecone (CD) is an organochlorine pesticide associated with albumin and high-density lipoproteins (HDL) in the plasma. It is found in higher concentrations in the liver than in other tissues and is excreted in the bile. The influence of plasma HDL on the biliary excretion of CD was studied using isolated pig liver perfused with a Krebs-Ringer bicarbonate solution containing albumin, dextrose, and pig red blood cells. Within 5 min after administration into the perfusion medium of [14C]CD bound to albumin or to HDL, only 13% of the [14C]CD dose remained in the perfusate, showing that CD is rapidly taken up by the liver. After 60 min the plasma concentration was constant at 0.008% dose/ml when [14C]CD was administered bound to albumin in the absence of HDL and at 0.004% dose/ml when administered bound to HDL. The mean concentration of CD in the bile was higher when CD was administered bound to albumin in the absence of HDL (0.039% dose/ml) than when it was administered bound to HDL (0.010% dose/ml). The elimination rate constant of CD from the liver into the bile was 0.007/min whether CD was administered bound to albumin or to HDL. The addition of HDL to the perfusion system after the administration of albumin-bound CD resulted in lower biliary CD concentrations. The results suggest that HDL affects the distribution of CD between the perfusate and liver and between liver and bile. In both cases, distribution toward the liver is favored.     

Pharmacokinetics and bioavailability of diclofenac in the rat.

Diclofenac sodium is a widely used drug with interesting absorption and disposition features when administered to laboratory animals. The present study was undertaken to assess the pharmacokinetics of the drug after iv and gastrointestinal dosing to rats. Renal excretion of unchanged drug was negligible, but biliary excretion of the drug (unchanged and conjugated) was detected in bile duct-cannulated rats; it accounted for 27.2 and 31.2% of the total dose following iv and intraduodenal administration, respectively. Most of the drug excreted in the bile was conjugated diclofenac; unchanged drug accounted for only 4.7 and 5.4% of total diclofenac excreted in the bile after iv and intraduodenal dosing, respectively. In normal animals, intestinal absorption of the drug excreted in the bile resulted in higher drug concentrations in plasma than those obtained in bile duct-cannulated rats, but only after 60 min of dosing. When administered directly into the duodenum, diclofenac absorption was extremely fast and the maximum plasma diclofenac concentration was reached within 2 min. After oral dosing, an early peak was also observed, but it was lower than that obtained after intraduodenal dosing: 71% diclofenac bioavailability was found in bile duct-cannulated rats intraduodenally dosed, whereas in normal animals dosed by mouth a bioavailability of 79% was obtained. In normal animals intraduodenally dosed, an apparent bioavailability of 106% was observed. All of these features, particularly the influence of enterohepatic circulation on drug bioavailability, are discussed.     

Phenylbutazone-hydroxycoumarol interactions. Effects on steady state disposition, hepatocellular distribution, and biliary excretion of (S)-acenocoumarol in rats

The effect of phenylbutazone on the disposition of (S)-acenocoumarol in the rat was studied at steady state conditions of distribution and elimination. (S)-Acenocoumarol was administered by constant rate infusions (1 microgram/min). The biliary excretion of 6- and 7-hydroxylated acenocoumarol was followed and the intrahepatic distribution was investigated. Phenylbutazone (50 mg/kg) increased the plasma unbound fraction about 4-fold. (S)-Acenocoumarol plasma clearance was enhanced (2.8 +/- 0.15 vs. 1.54 +/- 0.14 ml/min) but the unbound plasma clearance was reduced by 50% (67 +/- 9 vs. 140 +/- 27 ml/min). Phenylbutazone caused an intrahepatic redistribution of (S)-acenocoumarol, i.e. the drug shifted from the cytosol to the 10,000g pellet. The cytosolic unbound concentration, however, was increased. The (S)-acenocoumarol content in the microsomal fraction was not affected. The biliary excretion rate of total metabolite (free plus conjugated) comprised 50% of the (S)-acenocoumarol infusion rate in controls and was slightly stimulated (+20%) by phenylbutazone. The biliary excretion of free metabolites, however, was greatly increased (62 +/- 7 vs. 22 +/- 6 ng/min for 6-hydroxy-acenocoumarol; 337 +/- 38 vs. 141 +/- 32 ng/min for 7-hydroxy-acenocoumarol). This effect is probably due to stimulation of a hepatic biliary transport system; the rate constant for transport of 7-hydroxy-acenocoumarol was enhanced 5-fold (0.107 +/- 0.03 vs. 0.021 +/- 0.007 min-1).     

Increased Absorption of Digoxin from the Human Jejunum Due to Inhibition of Intestinal Transporter-Mediated Efflux

BACKGROUND AND OBJECTIVES: The contribution of transport in the small intestine by the apically located efflux pump P-glycoprotein to variable drug absorption in humans is still poorly understood. We therefore investigated whether inhibition of intestinal P-glycoprotein-mediated efflux by quinidine leads to increased absorption of the P-glycoprotein substrate digoxin. METHODS: Using a multilumen perfusion catheter, we investigated the impact of P-glycoprotein inhibition on absorption of two compounds: the P-glycoprotein substrate digoxin and the marker for passive transcellular absorption antipyrine. Two 20cm adjacent jejunal segments were isolated with the multilumen perfusion catheter in seven healthy subjects. Unlabelled and deuterated digoxin and antipyrine, respectively, were simultaneously infused into either of the intestinal segments. One of the segments was additionally perfused with the P-glycoprotein inhibitor quinidine. Intestinal perfusates were collected for 3 hours, and drug concentrations were determined in the intestinal perfusates, plasma and urine. RESULTS: Quinidine did not affect the disposition of antipyrine. In contrast, coadministration of quinidine into one jejunal segment caused a considerable increase in the amount of digoxin absorbed from this segment compared with the absorption from the other quinidine-free segment (22.3 +/- 8.9% vs 55.8 +/- 21.2% of the dose; p < 0.05). Accordingly, the area under the plasma concentration-time curve and the maximum plasma concentration of digoxin were considerably higher when luminal quinidine was coadministered (p < 0.05 and p < 0.001, respectively). Differences in digoxin absorption from the two intestinal segments were also reflected by pronounced differences in urinary digoxin elimination (5.5 +/- 3.3% vs 19.2 +/- 8.1% of the dose; p < 0.01). CONCLUSIONS: P-glycoprotein inhibition in enterocytes increases systemic exposure of orally administered drugs that are P-glycoprotein substrates. These data highlight the importance of the small intestine as an active barrier against xenobiotics.     

Disposition and tissue distribution of angiopeptin in the rat.

The disposition and tissue distribution of angiopeptin, a long-acting octapeptide analogue of somatostatin, were studied in rats following single iv and sc administration of the drug. Similar plasma levels and excretion values of angiopeptin were observed by using radioimmunoassay and radiolabeling techniques. Angiopeptin was absorbed fairly rapidly, with a mean peak plasma level of 25 +/- 4.1 ng/ml at 10-15 min after administration. The kinetics of angiopeptin following sc administration closely resembled those following iv administration due to rapid absorption. The pharmacokinetics of angiopeptin can be described by a two-compartment model. The plasma half-life of the drug ranged from 2.6-2.9 hr when administered sc and 1.98-2.5 hr when given iv. Distribution of angiopeptin was rapid, with the highest concentration appearing in the liver. Half-lives in the liver and bile were short. Most of the drug was excreted in the feces via the bile, while approximately 10% was excreted in the urine. Angiopeptin was also found to be secreted in the saliva. TLC and HPLC of blood, urine, feces, and bile samples did not reveal the presence of any metabolites. In conclusion, the in vivo fate of angiopeptin is characterized by little or no hepatic metabolism and rapid biliary excretion.     

Seromucosal transport of intravenously administered carbamazepine is not enhanced by oral doses of activated charcoal in rats

The fate of carbamazepine after intravenous injection in rats (n = 24) and the influence of activated charcoal on the kinetics was investigated. After randomization to four groups (n = 6, each), plasma concentration and the quantities of carbamazepine and metabolites excreted into bile, urine and intestine were determined using an in situ perfusion model of the small intestine (Ringer's solution) with or without orally administered activated charcoal (AC+; AC-) and with or without bile duct cannulation (BD+; BD-). The cumulative amount of carbamazepine and metabolites exsorbed into the small intestine within 3.5 hr after intravenous injection was about 15% in BD- animals and about 3% in BD+ animals. About 20% of the dose was detected in the externalized bile. Activated charcoal did not influence the amount exsorbed into the small intestine. Terminal half-life in plasma ranged from 159 min. to 194 min. within the four treatment groups without statistical significant difference (P = 0.751). Correspondingly, the area under the curve did not vary significantly and ranged between 1.13 and 1.41 g/min./l (P = 0.378). Excretion of carbamazepine and metabolites into urine varied between 3% and 6% of dose within all groups and showed close correlation with diuresis. In an identical experimental approach using a 2-fold intestinal perfusion rate (50 ml/hr; n = 8), no fundamental changes compared to the main experiment regarding pharmacokinetics of carbamazepine were observed. The lack of effect of activated charcoal on the elimination of carbamazepine and metabolites must be contributed to the small amount of the drug being exsorbed into the intestine and may be further influenced by reduced intestinal permeability of carbamazepine and metabolites or inadequate luminal stirring.     

Effects of ursodeoxycholate on maximal biliary secretion of bilirubin in the rat

The effect of sodium ursodeoxycholate (0.5 and 1.0 mumol/min/100 g) on the maximal biliary secretion (Tm) of bilirubin and on the concentration of bilirubin in liver and plasma at the end of a bilirubin load was studied in Wistar rats. Administration of ursodeoxycholate at 0.5 mumol/min/100 g caused a 0.8-fold increase in bile flow and a significant increase in the bilirubin Tm (+24%). This was associated with a significant reduction of liver and plasma bilirubin concentrations (-16% and -17%, respectively). Bilirubin UDP-glucuronosyltransferase activity was not significantly enhanced. There was a significant increase in the biliary excretion of bilirubin conjugates (+30%) and in the diconjugates/monoconjugates ratio in bile (+31%). When ursodeoxycholate was given at 1.0 mumol/min/100 g, it produced a 1.7-fold increase in bile flow, but the bilirubin Tm was significantly reduced (-21%). Liver bilirubin concentrations were decreased (-20%) and there was a significant enhancement in total pigment concentration in plasma (+19%). Both the excretion of unconjugated bilirubin and that of bilirubin conjugates were significantly reduced (-60% and -18%, respectively). There was a significant decrease in the bilirubin-UDP glucuronosyltransferase activity and the diconjugates/monoconjugates ratio in bile (-27% and -27%, respectively). These results indicate that ursodeoxycholate is able to increase maximal bilirubin secretion only when administered at low doses and that infusion at higher rates can significantly interfere with different steps in the hepatobiliary transport of the pigment.     

Human proximal tubular epithelium actively secretes but does not retain rosuvastatin

Rosuvastatin is a potent HMG-CoA reductase inhibitor that has proven to be effective in the treatment of dyslipidemia. Rosuvastatin is cleared from the body by both biliary and renal clearance, the latter believed to be due to active tubular secretion. Whereas the mechanisms of hepatic clearance of rosuvastatin are well documented, those of renal clearance are not. Because rosuvastatin (and other statins) may alter proximal tubular function, this study aimed to characterize the mechanisms of tubular rosuvastatin secretion to define the factors that could influence the presence/concentration of rosuvastatin in proximal tubular cells. Hereto, polarized monolayers of primary human tubular cells were used. We found rosuvastatin net secretion across proximal tubule cells, which was saturable (K50=20.4+/-4.1 microM). The basolateral uptake step was rate-limiting and mediated by OAT3. Rosuvastatin efflux at the apical membrane was mediated by MRP2/4 and ABCG2 together with a small contribution from MDR1 or P-glycoprotein. These data, obtained in an intact human tubule cell model, provide a detailed insight into rosuvastatin's renal handling and the possible factors influencing it.     

Determination of the biliary excretion of piperacillin in humans using a novel method

AIM: To evaluate the applicability of a novel method to determine the biliary excretion of piperacillin. METHODS: Healthy volunteers were administered piperacillin i.v. Duodenal aspirates were collected via a custom-made oroenteric catheter; blood and urine also were collected. Gallbladder ejection fraction (EF) was determined by gamma scintigraphy and pharmacokinetic parameters were calculated using noncompartmental analysis. RESULTS: The fraction of the piperacillin dose excreted unchanged into bile was 1.1 +/- 0.3% (biliary clearance corrected for EF was 0.032 +/- 0.008 ml min(-1) kg(-1)). CONCLUSIONS: This methodology can be used to determine reliably the biliary clearance of drugs that are excreted only marginally into bile. Normalization of biliary clearance for EF significantly reduces intersubject variability of this parameter.     

Regional intestinal absorption and biliary excretion of fluvastatin in the rat: possible involvement of mrp2

The first purpose of this study was to investigate the in vivo absorption, biliary secretion, and first-pass effect of fluvastatin following regional intestinal dosing in the rat. We also examined the membrane transport mechanisms and made in silico predictions of the relative importance of various intestinal regions to the human absorption of fluvastatin. Fluvastatin was administered intravenously (2, 10, and 20 micromol/kg) and into the duodenum (1.46, 2.92, 7.32, and 14.6 micromol/kg), jejunum (14.6 micromol/kg), ileum (1.46 and 14.6 mciromol/kg), and colon (1.46 and 14.6 micromol/kg) as a solution to conscious rats. In a separate group of rats, bile was collected after an i.v. dose of fluvastatin (2 micromol/kg). In the Caco-2 model the bidirectional transport of fluvastatin (16 microM) was investigated with and without various efflux inhibitors (verapamil, vinblastine, probenecid, and indomethacin, 160 microM). The human in vivo absorption of fluvastatin from an oral immediate release tablet and that from an oral extended release tablet (both 40 mg) were simulated in GastroPlus. Neither the dose nor the intestinal region influenced the bioavailability of fluvastatin significantly. The rate of absorption was, however, affected by both the dose and the site of administration; duodenum = jejunum > colon > ileum, and higher following the high dose. Increasing the i.v. dose from 2 to 20 micromol/kg decreased the clearance (26 +/- 3 to 12 +/- 1 mL/min/kg), the hepatic extraction (66 +/- 8 to 30 +/- 2%), and the volume of distribution (7.3 +/- 0.3 to 2.1 +/- 0.7 L/kg) for fluvastatin (p < 0.05). Neither bile cannulation nor bile sampling affected the pharmacokinetics. Fluvastatin was secreted into the bile, probably by active transport. The in vitro permeability for fluvastatin was high (>10 x 10(-6) cm/s). Indomethacin, but not the other inhibitors, affected the transport in both directions suggesting mrp2 to be involved. In silico, 93% of the dose was absorbed from the small intestine and 6% from the colon when given as an immediate release formulation. The corresponding values for an extended release formulation were 21% and 74%, respectively. In conclusion, fluvastatin exhibits dose-dependent pharmacokinetics in the rat. The rate of absorption (Cmax, Tmax, and Cmax/AUC(lqc)) from the intestinal tract is both region and dose-dependent in the rat. This may be due to the involvement of mrp2 in the intestine and/or in the liver. These absorption properties have to be considered in the development of an extended release formulation of fluvastatin.