Extrahepatic sulfation and glucuronidation in the rat in vivo. Determination of the hepatic extraction ratio of harmol and the extrahepatic contribution to harmol conjugation

After 50 Years of Hepatic Clearance Models, Where Should We Go from Here? Improvements and Implications for Physiologically Based Pharmacokinetic Modeling

There is overwhelming preference for application of the unphysiologic, well-stirred model (WSM) over the parallel tube model (PTM) and dispersion model (DM) to predict hepatic drug clearance, CLH , despite that liver blood flow is dispersive and closer to the DM in nature. The reasoning is the ease in computation relating the hepatic intrinsic clearance ( CLint ), hepatic blood flow ( QH ), unbound fraction in blood ( fub ) and the transmembrane clearances ( CLin and CLef ) to CLH for the WSM. However, the WSM, being the least efficient liver model, predicts a lower EH that is associated with the in vitro CLint ( Vmax / Km ), therefore requiring scale-up to predict CLH in vivo. By contrast, the miniPTM, a three-subcompartment tank-in-series model of uniform enzymes, closely mimics the DM and yielded similar patterns for CLint versus EH , substrate concentration [S] , and KL / B , the tissue to outflow blood concentration ratio. We placed these liver models nested within physiologically based pharmacokinetic models to describe the kinetics of the flow-limited, phenolic substrate, harmol, using the WSM (single compartment) and the miniPTM and zonal liver models (ZLMs) of evenly and unevenly distributed glucuronidation and sulfation activities, respectively, to predict CLH For the same, given CLint ( Vmax and Km ), the WSM again furnished the lowest extraction ratio ( EH,WSM = 0.5) compared with the miniPTM and ZLM (>0.68). Values of EH,WSM were elevated to those for EH, PTM and EH, ZLM when the Vmax s for sulfation and glucuronidation were raised 5.7- to 1.15-fold. The miniPTM is easily manageable mathematically and should be the new normal for liver/physiologic modeling. SIGNIFICANCE STATEMENT: Selection of the proper liver clearance model impacts strongly on CLH predictions. The authors recommend use of the tank-in-series miniPTM (3 compartments mini-parallel tube model), which displays similar properties as the dispersion model (DM) in relating CLint and [ S ] to CLH as a stand-in for the DM, which best describes the liver microcirculation. The miniPTM is readily modified to accommodate enzyme and transporter zonation.

Pulmonary metabolism of resveratrol: in vitro and in vivo evidence

The role of pulmonary metabolism in trans-resveratrol (RES) pharmacokinetics was studied in a mouse model. Plasma concentrations of RES and its major metabolites trans-resveratrol-3-sulfate (R3S) and trans-resveratrol-3-glucuronide (R3G) were compared after administration of RES by intravenous (IV) and intra-arterial (IA) routes. Total area under the curve (AUC) of RES decreased by approximately 50% when RES was administered by the IV route compared with the IA route. The AUC of R3G was also significantly higher in mice administered RES by the IV route compared with the IA route. In vitro studies performed with mouse and human lung fractions confirmed pulmonary metabolism of RES. Interestingly, mouse-lung fractions gave rise to both R3S and R3G, whereas human lung fractions yielded R3S. This indicates marked interspecies variation in RES conjugation, especially in the context of extrapolating rodent data to humans. Taken together, the results presented here underline, for the first time, the impact of pulmonary metabolism on resveratrol pharmacokinetics and interspecies differences in RES pulmonary metabolism.

Sulfate conjugating and transport functions of MDCK distal tubular cells

BACKGROUND

Transfected Madin-Darby canine kidney (MDCK) cells (of distal tubular origin) have been used to study transport of organic anions. These cells have not been shown to possess sulfate-conjugating activity. Neither has transport activity been demonstrated in nontransfected MDCK cells.

METHODS

Polarized and monolayers of nontransfected MDCK type II cells were incubated with prototype substrates of phenolsulfotransferase (PST) and sodium sulfate in the absence or presence of known inhibitors of multidrug resistance protein (MRP): (3-3-(2-(7-chloro-2-quinionlinyl) ethenyl)phenyl)(3-dimethylamino-3-oxopropyl)thio)methyl)thio) propanoic acid (MK571), cyclosporin A (CsA), and probenecid. Effects of glutathione (GSH) and buthionine sulfoximine (BSO), potential modulators of the organic anion transporting protein/polypeptide (OATP) isoform, OATP1 were also examined. Sulfated conjugates were identified by high-performance liquid chromatography (HPLC)-radiometry or HPLC-fluorimetry.

RESULTS

Uptake, sulfate conjugation, and efflux of the sulfated conjugates of harmol, p-nitrophenol, N-acetyldopamine and acetaminophen were demonstrated. Activities in MDCK type II cells were higher than those in HepG2, human fetal liver, and Chang liver cells. A significant decrease in extracellular with a reciprocal increase in intracellular harmol sulfate was observed with MK571, CsA, and probenecid and with preloading of glutathione. Depletion of intracellular glutathione by BSO had the opposite effects.

CONCLUSIONS

Normal (nontransfected) MDCK type II cells provide a suitable system for the study of the physiologic processes of uptake, sulfate conjugation, and transport of sulfated conjugates in kidney cells. Based on the action of specific inhibitors and modulators of MRP2 and OATP1, it was concluded that MRP2-like and OATP1-like transporters are possibly responsible for the transport of sulfated conjugates.

Propofol metabolism in man during the anhepatic and reperfusion phases of liver transplantation

1. An i.v. dose of 14C-propofol (0.53 mg/kg) was administered to three male and three female patients during the anhepatic phase of liver transplantation, which lasted 30-56 min after dosing. Arterial and venous blood samples, bile (T-tube drainage) and urine were collected at various times afterwards and submitted to h.p.l.c. and radioassay or specific fluorescence detection for the unchanged drug. 2. Extrahepatic metabolism was apparent during the anhepatic phase, since at 30 min post-dose, unchanged propofol comprised only 42-89% of the blood radioactivity. 3. Examination of the plasma radioactivity during the anhepatic phase in two subjects showed evidence of propofol glucuronide and 4-quinol sulphate, confirming extrahepatic metabolism of the drug. Quinol glucuronides were only detected in the liver reperfusion phase. 4. There was no evidence that the lungs contribute to the extrahepatic metabolism of propofol, since drug concentrations in the arterial blood were not less than in central venous samples. 5. During the first 24 h period, urine collected from five patients contained 7-74% dose, whilst the bile contained 0.1-0.9%. In three patients with normal renal function recovery in urine was 66-74% dose. Examination of urinary radioactivity in one subject showed the main component to be propofol glucuronide during the anhepatic phase.

Species differences in blood profiles, metabolism and excretion of 14C-propofol after intravenous dosing to rat, dog and rabbit

1. Bolus i.v. doses of 14C-propofol (7-10 mg/kg) to rat, dog and rabbit, or an infusion dose (0.47 mg/kg per min for 6 h) to dog were eliminated primarily in urine (60-95% dose); faecal elimination (13-31%) occurred for rat and dog, but was minimal (less than 2%) for rabbit. 2. After bolus administration, blood 14C concentrations were maximal (8-30 micrograms equiv./ml) at 2-15 min; these declined rapidly during the 0-2 h period and thereafter more slowly. Propofol concentrations were maximal (4-16 micrograms/ml) at 2 min and the profiles were best fitted by a tri-exponential (rat and dog) or bi-exponential (rabbit) equation. Duration of sleep ranged from 5 to 8 min. 3. Infusion of 14C-propofol in dog gave a blood 14C concentration of 117 micrograms equiv./ml at the end of the 6 h infusion period; this declined at a similar rate to that after the bolus dose. Propofol concentration on termination of infusion was 13 micrograms/ml; thereafter, propofol concentrations declined less rapidly than after the bolus dose. Waking occurred about 44 min post-infusion. 4. Propofol was cleared by conjugation of the parent molecule or its quinol metabolite; hydroxylation of an isopropyl group also occurred in rat and rabbit. Biliary excretion leading to enterohepatic recirculation, and in turn increased sulphate conjugation, occurred in rat and dog, but not rabbit, resulting in a marked interspecies variation in drug clearance and metabolite profiles.

UDP-glucuronyltransferase activity toward harmol in human liver and human fetal liver cells in culture

This paper presents a fast HPLC assay for measuring UDP-glucuronyltransferase (UDPGT) activity in extracts of adult human liver and human fetal liver cells in culture. Harmol glucuronide formed was quantitated directly without prior hydrolysis after a simple step of selective extraction of harmol. The method is applicable to crude liver homogenates as well as to partially fractionated preparations. It is several fold more sensitive than the conventional detection of harmol glucuronide by TLC, making it possible to distinguish the low and high affinity forms of UDPGT of adult human liver and to detect the low affinity form in fetal cells. The possible participation of both forms of GT in adults under different conditions and the apparent lack of the high affinity form in the fetal liver is discussed.

High capacity for pulmonary first-pass elimination of propranolol in rats

Plasma propranolol concentrations after i.v. and i.a. 1, 2.5, 5 and 10 mg kg-1 doses of the drug given intravenously or intra-arterially have been compared in 7-week-old male Wistar rats. The areas under the curves after i.a. dosing were almost twice those after i.v. dosing at any dose, despite the elimination half-lives being the same. The difference in total body clearance after i.a. dosing from that after i.v. dosing indicated a significant contribution by pulmonary clearance, which ranged from about 20 to 30 mL min-1 kg-1, to the overall first-pass elimination after the i.v. administration. The pulmonary extraction ratio was approximately 0.4 to 0.5 at the i.v. doses used. Mean pulmonary transit time was estimated to be about 1 to 2 min. There was no dose-dependence in the pulmonary first-pass elimination kinetics of propranolol.

Drug disposition in obese humans. An update

Drug disposition for many drugs has now been studied in obese individuals and some general conclusions can be drawn. Absorption of drugs evaluated to date is unchanged due to obesity. Apparent volume of distribution is greatly increased for some drugs including most benzodiazepines, thiopentone, phenytoin, verapamil and lignocaine (lidocaine). Modest increases in volume of distribution have been noted for methylxanthines, aminoglycosides, vancomycin, ibuprofen, prednisolone and heparin. Distribution of digoxin, cimetidine and procainamide is unchanged in obesity. The mechanism for the increased distribution of some drugs and unchanged distribution of others in obesity is unclear at present. It may be in part due to the lipophilic character of the drug molecule; however, other complex and as yet poorly understood factors contribute to the variability in drug distribution in obese patients. Protein binding of drugs bound to albumin is not dramatically changed in obesity. In contrast, some studies report that drugs bound to alpha 1-acid glycoprotein (AAG) may have increased binding that is related to increased serum AAG concentration; however, this is not a consistent finding. Oxidative drug biotransformation is minimally changed in obesity with the exceptions of ibuprofen and prednisolone, for which clearance increases as a highly correlated function of total bodyweight. Drug conjugation uniformly increases as a function of bodyweight in obesity, with paracetamol (acetaminophen), lorazepam and oxazepam having been studied. Drug acetylation may be unchanged in obesity, with only procainamide evaluated at this time. High clearance drugs, including lignocaine, verapamil and midazolam, have no change in clearance in obese individuals compared to normal bodyweight controls. Renal clearance of drugs is little changed for some drugs evaluated (digoxin, cimetidine), and increased for others (aminoglycosides, unmetabolised procainamide). Characterisation of appropriate animal models of obesity is underway to clarify the mechanisms for these in vivo pharmacokinetic observations in obese man. Two models, the Zucker obese and the obese cafeteria-fed male Sprague-Dawley rat, have provided preliminary physiological pharmacokinetic data with evaluations of theophylline, phenobarbitone and verapamil.(ABSTRACT TRUNCATED AT 400 WORDS)

Secretion of the organic anion harmol sulfate from liver into blood. Evidence for a carrier-mediated mechanism

In the liver, drugs with phenolic groups can be converted to sulfate or glucuronide conjugates and are then transported into bile or back into the bloodstream. The mechanism for transport of drugs and drug conjugates from the hepatocytes into the blood at the sinusoidal side of the cell are not well defined. In the case of carrier-mediated transport of these strongly polar conjugates, saturability of transport and mutual competition between structurally related compounds would be anticipated. This competitive aspect was investigated by using two organic anions, dibromosulfophthalein (DBSP) and harmol sulfate. The latter compound was generated by the hepatocytes from harmol, which was continuously infused into the rat in vivo and in isolated perfused rat livers. In addition we loaded the perfused rat livers with preformed harmol sulfate and studied its efflux rate to the perfusate as influenced by DBSP. In steady state, about 80% of harmol was sulfated and 20% was glucuronidated. Harmol sulfate was mainly excreted in the urine, the glucuronide was equally excreted in urine and bile. DBSP lowered the urinary excretion of harmol sulfate by about 30% which was due to a decrease in plasma concentration. However, renal clearance of harmol sulfate (3.2 +/- 0.2 ml/min) was unchanged. At the same time DBSP doubled the biliary clearance of harmol sulfate (1.0 +/- 0.1 and 2.2 +/- 0.2 ml/min in controls and DBSP studies respectively). DBSP decreased glucuronide excretion both in urine and bile. The increase in biliary output and decrease in urinary excretion of harmol sulfate is explained by competitive interaction between the organic anion DBSP and harmol sulfate at the sinusoidal level. Efflux experiments in single pass perfused isolated livers showed a clear decrease of harmol sulfate transport from liver into plasma by DBSP and provided evidence for such an inhibitory phenomenon (t 1/2 of efflux was 3.58 +/- 0.21 compared with 2.46 +/- 0.07 min for controls). These results indicate that organic anion transport from the hepatocyte into the blood stream is very likely carrier-mediated. A decrease in renal output of drug conjugates therefore may not only be due to a decrease in the conjugation process but also to a lower liver to blood transport rate which at the same time may produce a higher biliary output.

Glucuronidation and sulfation in the rat in vivo. The role of the liver and the intestine in the in vivo clearance of 4-methylumbelliferone

The role of the liver in the conjugation of 4-methylumbelliferone (4MU), mainly glucuronidation, was investigated in the rat in vivo. The liver extracted 4MU almost completely (97%) during steady-state infusion, as measured by the difference between 4MU concentration in portal and hepatic venous blood. Previously, it was shown that the intestinal region extracts 40% of the 4MU of the incoming arterial blood. The liver and the gastrointestinal region are so efficient that their conjugation activity can account for total body clearance of 4MU (50-60 ml/min per kg). These results and other evidence on extrahepatic conjugation of phenolic substrates suggest that glucuronidation may be limited to the liver, (the kidney) and the gastrointestinal region, while sulfation may occur more widespread throughout the body. Protein binding studies showed the sulfate conjugate to be even more protein-bound than unconjugated 4MU, while 4MU glucuronide was much less bound to rat plasma proteins.