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D K Meijer

Publications and source records attributed to D K Meijer.

At least 163 records · Page 9Linked to original sources
PubMed1989

Covalent and noncovalent protein binding of drugs: implications for hepatic clearance, storage, and cell-specific drug delivery.

This review deals with the mechanisms by which the liver disposes of drugs that are covalently or noncovalently associated with proteins. Many drugs bind to plasma proteins such as albumin (mainly anionic compounds) and alpha 1-acid glycoprotein (cationic compounds). Nevertheless, the liver is able to clear such drugs efficiently from the circulation because of intrahepatic dissociation of the drug-protein complex. This clearance may involve spontaneous dissociation because of progressive removal of the unbound drug during liver passage, a process that can be rate limiting in hepatic uptake. Alternatively, the porous endothelial lining of the hepatic sinusoids may allow extensive surface interactions of the drug-protein complexes with hepatocytes, leading to facilitation of drug dissociation. Binding to plasma proteins and intracellular proteins in the cytoplasm or cell organelles is an important factor determining the hepatic storage and elimination rate of drugs. Drugs noncovalently associated with glycosylated proteins, which can be endocytosed by various liver cells, are not coendocytosed with such proteins. However, covalently bound drugs can be internalized by receptor-mediated endocytosis, which permits specific targeting to hepatocytes, endothelial cells, Kupffer cells, and lipocytes by coupling to different glycoproteins that are recognized on the basis of their terminal sugar. The endocytosed drug-carrier complex is routed into endosomes and lysosomes, where the active drug is liberated by cleavage of acid-sensitive linkages or proteolytic degradation of peptide linkers. This concept has been applied to antineoplastic, antiparasitic, and antiviral drugs.

Animals↗
PubMed1989

The role of sulfation in the metabolic activation of N-hydroxy-4'-fluoro-4-acetylaminobiphenyl.

The role of sulfation in the metabolic activation of the liver carcinogen N-hydroxy-4'-fluoro-4-acetylaminobiphenyl (N-OH-FAABP) in male rat liver was investigated. N-OH-FAABP was a substrate for sulfotransferases in vitro and sulfation was inhibited by the sulfotransferase inhibitors pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DCNP). The main metabolite of N-OH-FAABP excreted in bile in vivo, and in the isolated perfused liver, was identified as the N-O-glucuronide conjugate. Inhibition of sulfation in vivo by PCP or DCNP, or in vitro by omission of inorganic sulfate, resulted in a large increase in the excretion of the N-O-glucuronide conjugate. It was estimated that at least 21% of the dose was sulfated in control animals. Inhibition of sulfation in vivo by PCP or DCNP prevented the covalent binding of N-OH-FAABP to liver (and kidney) macromolecules by 70% and 20% respectively. HPLC analysis of the fluorobiphenyl DNA and RNA adducts showed that the formation of both N-acetylated and deacetylated (deoxy)-guanosine adducts was prevented. Furthermore, omission of inorganic sulfate in the isolated perfused liver prevented the formation of all fluorobiphenyl DNA adducts by 70-80%. It is concluded that two sulfotransferase-dependent pathways exist for the metabolic activation of N-OH-FAABP in male rat liver: (i) direct sulfation of the hydroxamic acid, resulting, upon decomposition of the FAABP-N-sulfate ester, in the formation of N-acetylated DNA adducts and (ii) deacetylation followed by sulfation of the hydroxylamine to FABP-N-sulfate, leading to the formation of deacetylation DNA adducts.

Aminobiphenyl Compounds↗
PubMed1989

High-performance liquid chromatographic assay of flecainide and its enantiomers in serum.

Two analytical methods are described for the determination of flecainide in serum by high-performance liquid chromatography employing spectrofluorimetric detection. One method concerns total flecainide, whereas the other covers the assay of flecainide enantiomers separately. The stereo-specific assay is based on a precolumn derivatization of flecainide enantiomers to urea derivatives with the chiral compound R-(+)-1 phenyl-ethyl-isocyanate. By formation of the permanent diastereomers, the racemic mixture can be resolved with a traditional reversed-phase column. The run time of the total flecainide assay is 15 min, whereas that of its enantiomers is 20 min. The extraction recovery from serum in both methods is 85%. The intraassay precision of the nonstereospecific assay in serum ranged from 99 +/- 3% (coefficient of variation) to 101 +/- 5%. The accuracy of the estimation in serum ranged from 100 to 103%. The intraassay precision of the stereospecific assay in serum ranged from 98 +/- 5% (coefficient of variation) to 103 +/- 7%. The accuracy of the estimation in serum ranged from 101 to 104%. The limit of quantitation was 0.05 mg/L for both methods.

Chromatography, High Pressure Liquid↗
PubMed1989

Unequal disposition of enantiomers of the organic cation oxyphenonium in the rat isolated perfused liver.

This paper describes the results of pharmacokinetic experiments in the rat isolated perfused liver with enantiomers of oxyphenonium. The study was performed with the [14C]methyl labelled compounds. In this preparation both metabolism and biliary excretion were significantly different for the (+)- and the (-)-isomer. Hepatic uptake rate was similar, but total biliary excretion (including metabolites) of the (-)-isomer was only 55% compared with the excretion of the (+)-isomer. In line with these data, after 2 h only 30% of the dose of the (+)-isomer and over 50% of the dose of the (-)-isomer was still found in the liver, predominantly in the form of metabolites. The metabolic profile was investigated using ion pair TLC. At least two metabolites were detected in bile for both enantiomers. However, unchanged (-)-oxyphenonium persisted for longer in bile, indicating either a more rapid canalicular transport of the (+)-isomer and/or a more rapid metabolism of (+)-oxyphenonium to cholephilic metabolites.

Animals↗
PubMed1989

Selective activity of several cholic acid derivatives against human immunodeficiency virus replication in vitro.

Several cholic acid derivatives such as taurolithocholic acid, lithocholic acid 3-sulfate, taurolithocholic acid 3-sulfate, and glycolithocholic acid 3-sulfate were shown to inhibit selectively the replication of human immunodeficiency virus type 1 (HIV-1) in vitro. These compounds completely protected MT-4 cells against HIV-1-induced cytopathogenicity at a concentration of 100 micrograms/ml, whereas no toxicity for the host cells was observed at 200 micrograms/ml. They also inhibited HIV-1 antigen expression in HIV-1-infected CEM cells. The bile acids (cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid) did not show any inhibitory effect on HIV-1 replication at concentrations that were not toxic to the host (MT-4) cells. From a structure-function analysis of a number of cholic acid derivatives, the presence of either a sulfonate (as in the tauro conjugates) or a sulfate group as well as the "litho" configuration appeared to be necessary for the expression of anti-HIV-1 activity. The active cholic acid derivatives did not directly inactivate the virus particles at the concentrations that were not toxic to the host cells. Lithocholic acid 3-sulfate, taurolithocholic acid 3-sulfate, and glycolithocholic acid 3-sulfate, but not taurolithocholic acid, partially inhibited virus adsorption to MT-4 cells. These three compounds were also inhibitory to the reverse transcriptase activity associated with HIV-1.

Adsorption↗
PubMed1989

Zonal compartmentation of perfused rat liver: plasma reappearance of rhodamine B explained.

Rhodamine B (RB) fluorescence reappears in perfusion medium of a cyclically perfused rat liver after a rapid initial removal phase. At the same time the compound redistributes in the liver from acinar zone 1 toward zone 3. By analysis of the metabolic profile of RB, and by inhibition of glucuronidation (the main metabolic route) with salicylamide, we show in this paper that formation and secretion of RB-conjugates from liver into perfusate is not involved in the reappearance and redistribution phenomena. We therefore sought the explanation in a kinetic model, in which the acinar heterogeneity of the liver was simulated by several sequential liver compartments. Most kinetic parameters we used in the simulation were calculated from previous experiments with RB (Braakman et al., Hepatology 7: 849-855, 1987). This led to an accurate simulation of the measured RB curves in bile, medium and the acinar zones of the liver. In this study we show that a secondary rise in medium concentration of an injected compound is not necessarily caused by metabolism, but can be easily explained by considering the liver a sequence of compartments, instead of one well-stirred compartment. The conditions for the reappearance as well as for the intrahepatic redistribution are: a fast uptake into the liver, combined with a fast sinusoidal secretion and a slow biliary excretion of the injected substance.

Animals↗
PubMed1989

Zonal distribution of the cation lucigenin in rat liver: influence of taurocholate.

The yellow fluorescent cation lucigenin (LU) was used as a model compound to study acinar heterogeneity in transport of hydrophilic cations that enter the liver by adsorptive endocytosis. Hepatic uptake was fast and saturable. The extraction was about 50% in a cyclically perfused rat liver preparation in which endogenous bile salts were replaced by the infusion of taurocholate (TC). Fluorescence microscopy on 8-microns liver sections revealed a striking distribution pattern. LU appeared to be concentrated in micro- and macrovesicular structures in the cell. At the same time, LU skipped the first cells of the acinus, zone 1 in an antegrade and zone 3 in a retrograde perfusion. A downstream localization of the dye was the result. Single-pass perfusions with TC concentrations ranging from 0 to 180 microM showed that hepatic clearance of LU negatively correlated with the TC concentration (p less than 0.005). Clearance fell from 1.99 +/- 0.06 ml/min-g of liver(mean +/- SD) without TC to 1.61 +/- 0.21 with 45 microM TC and 1.65 +/- 0.12 with 180 microM TC. Moreover, in the absence of TC we observed a homogeneous distribution of LU. TC induced in the acinus a nonfluorescent upstream area that expanded with increasing TC concentration. We concluded that TC inhibited uptake of LU; a high medium concentration of TC in zone 1 (antegrade) was accompanied by a low uptake of LU in this zone, resulting in a downstream increasing acinar gradient. Hepatic uptake and acinar distribution of certain cationic drugs in vivo may, therefore, vary with the variable input of bile salts in the portal circulation and, hence, with nutritional status and the time of day.

Acridines↗
PubMed1989

Vesicular uptake system for the cation lucigenin in the rat hepatocyte.

The hepatic transport mechanism for the fluorescent bivalent hydrophilic organic cation lucigenin (LU) was characterized employing kinetic and morphological methods. The extraction of LU by the perfused rat liver was 50% and uptake was saturable. LU did not inhibit the carrier-mediated hepatic uptake of the model organic cationic compounds tributylmethyl ammonium (type 1) and vecuronium (type 2) in isolated hepatocytes, whereas the uptake of LU in the perfused liver was not affected by either type of cation or by the cardiac glycoside cymarin, a potent type 2 inhibitor. The cytoskeleton-disrupting agents cytochalasin B and nocodazole, however, significantly lowered hepatic uptake of LU. In the intact liver, LU did not stimulate fluid phase endocytosis, as indicated by a lack of effect on the internalization of horseradish peroxidase. These kinetic data point to adsorptive endocytosis as the most probable uptake mechanism. This was confirmed by the inhibitory effect of neomycin and the polycation poly(L-lysine) on LU uptake. Fluorescence microscopy revealed that LU accumulated in the hepatocytes in discrete vesicular structures. Partial co-localization of rhodamine-dextran and acid phosphatase with LU indicated that part of the LU fluorescence was present in lysosomes, although not all lysosomes contained LU. Taken together, we conclude that we identified a novel vesicular pathway for uptake of organic cations by hepatocytes.

Acridines↗
PubMed1988

Pharmacokinetics of biliary excretion in man. VI. Indocyanine green.

The pharmacokinetics of Indocyanine Green (ICG) has been studied in 15 patients given 0.5, 1.0 and 2.0 mg.kg-1. The plasma disappearance and biliary excretion rate were measured in patients with tightly fitting catheters under slight negative pressure in order to achieve complete collection of bile. Recovery of unchanged ICG in bile over 18 h after the i.v. injection was 80% of the dose in all three dose groups. Plasma disappearance in all 3 groups was biphasic, showing an initial phase with a t1/2 of 3-4 min and a secondary phase with a dose-dependent apparent t1/2 of 67.6, 72.5 and 88.7 min, respectively. After 0.5 and 1.0 mg.kg-1 the biliary excretion rate curves showed an ascending phase with a mean t1/2 of 5 min and a descending phase with a mean t1/2 of 72 min. It was inferred that the secondary component of the plasma-decay mainly reflected the biliary excretion rate. After 2.0 mg.kg-1 in some patients the biliary excretion curve showed features of saturation; the t1/2 of the descending phase ranged from 73 to 440 min, and the time of maximal excretion was increased from 1.3 to 2.7 h after injection, whilst the mean maximal excretion rate was in the same range as the excretion rate after the 1.0 mg.kg-1 dose. The non-linear pharmacokinetics was only moderately reflected in the measured plasma disappearance patterns. Two compartment analysis of the plasma levels indicated a clearance of 230-260 ml.min-1, whereas the clearance conventionally calculated from the initial t1/2 was 475 ml.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Uptake and excretion of vecuronium bromide and pancuronium bromide in the isolated perfused rat liver.

Using the isolated perfused rat liver preparation, the disappearance from the perfusate and the excretion in the bile of vecuronium bromide and pancuronium bromide and their metabolites were followed for 2 h after the addition of 1 mg of either drug to the perfusate. In addition, the rate of change of the hepatic content of these two compounds was calculated by serially subtracting the amount of the compound and the metabolites in the bile and in the perfusate from the dose of drug added to the perfusate. It was found that, whereas the concentration of pancuronium in the perfusate declined slowly and monoexponentially, vercuronium concentration in the perfusate declined rapidly in a biexponential manner. No metabolites of either drug were detected in the perfusate. Approximately 40% of the injected dose of vecuronium was excreted in the bile as unchanged vecuronium and another 30% as the 3-hydroxy metabolite. No other metabolites of vecuronium were found in the bile. In total only about 7% of pancuronium (unchanged) was collected in the bile by the end of the experiment. It is concluded that, in comparison to pancuronium, the rat liver takes up large amounts of vecuronium rapidly, half of which is eliminated as unchanged vecuronium and half as the 3-hydroxy derivative. A small amount of vecuronium or its 3-hydroxy metabolite is returned to the perfusate from the liver. Some possible mechanisms underlying these differences are discussed.

Animals↗
PubMed1988

Mechanisms for the hepatic uptake of organic cations. Studies with the muscle relaxant vecuronium in isolated rat hepatocytes.

In hepatobiliary transport of organic cations some remarkable differences have been reported between the monovalent compounds (prototype procainamidethobromide) and the potentially bivalent cations, containing a second quaternary ammonium group or a protonated tertiary amine function (prototype d-tubocurarine). In order to characterize the hepatic uptake mechanism for such bivalent cations in more detail, we studied the uptake of the steroidal muscle relaxant vecuronium in isolated rat hepatocytes. Uptake occurred by both a saturable (Vmax = 181 pmol/min x 10(6) cells, Km = 15 microM) and a nonsaturable process (rate constant = 1.10 pmol/min/10(6) cells/microM). The uptake of vecuronium was reduced by various metabolic inhibitors and by sulfhydryl-blocking agents. The transport system showed temperature dependency with an activation energy of 85 kJ/mol. Sodium replacement by lithium or choline in the extracellular medium had no effect on the uptake of vecuronium. Replacement of sodium chloride by sucrose led to a decrease of the uptake, whereas chloride replacement by bicarbonate or iodide stimulated the vecuronium uptake. These data point to a significant anion-dependency of the uptake system and indicate electroneutral uptake of vecuronium. The uptake of vecuronium was inhibited by a variety of hepatic transport model compounds, including bile acids, uncharged compounds and high molecular weight organic cations. Low molecular weight monovalent cations had no effect on the uptake of vecuronium.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

The influence of binding to albumin and alpha 1-acid glycoprotein on the clearance of drugs by the liver.

The liver is a major site for synthesis and catabolism of plasma proteins. Albumin has various binding sites for anionic drugs, alpha 1-acid glycoprotein possesses a single binding site for cationic drugs. In spite of extensive protein binding, the liver can efficiently remove drugs from the circulation. Intrahepatic dissociation of the drug-protein complex may involve dissociation-limited debinding under non-equilibrium conditions or surface interaction-facilitated dissociation phenomena. During liver or renal disease and acute-phase conditions plasma protein binding of drugs may be affected. Changes in the unbound drug fraction do not always result in proportional changes in clearance or distribution volume. Potential changes in the unbound concentration in steady-state as well as the fluctuations in total plasma levels depend on the extent of protein binding of a drug, the relative change in the unbound drug fraction type of clearance, the size of the distribution volume, route of administration as well as concomitant changes in intrinsic (cellular) clearance function. Optimization of dosage regimens for certain drugs and interpretation of liver function tests with diagnostic dyes may largely benefit from determination of the unbound rather than the total concentration of the drugs involved.

Humans↗
PubMed1987

Hepatobiliary excretion of organic anions in double-mutant rats with a combination of defective canalicular transport and uridine 5'-diphosphate-glucuronyltransferase deficiency.

Mutant rats with a selective defect for the hepatobiliary excretion of organic anions (GT+TR- rats) are valuable models to study hepatic transport processes. However, retained conjugates in the livers of these rats may secondarily affect hepatic uptake, metabolism, and excretion of other compounds and this may confound the interpretation of test results. We have developed double mutants (GT-TR-) rats with both a conjugation and an excretion defect by cross-breeding uridine 5'-diphosphate-glucuronyl-transferase-deficient GT-TR+ Gunn rats with transport-deficient GT+TR- rats. Phenotypically, GT-TR- rats and Gunn rats are alike in that both have unconjugated hyperbilirubinemia. Intravenous administration of tetrabromosulphthalein, bilirubin diglucuronide, and bilirubin monoglucuronide revealed a significant difference in that the clearance of these compounds was reduced to 10%, 10%, and 20%, respectively, in GT-TR- rats when compared with Gunn rats. The hepatic elimination of tetrabromosulphthalein in GT-TR- rats and in GT+TR- rats is impaired to the same extent. Thus, both have a similar hepatic excretion defect. However, bile flow and bile acid excretion in GT+TR- rats are more depressed than in GT-TR- rats: bile flow, 88 +/- 3 vs. 36 +/- 1 microliters/min.kg and bile acid excretion, 3.4 +/- 0.2 vs. 1.5 +/- 0.1 mumol/min.kg in GT-TR- and GT+TR- rats, respectively. This suggests that accumulated glucuronides in the liver inhibit bile flow and bile acid excretion. To test whether conjugated bilirubin and the photoisomers of unconjugated bilirubin are excreted via the same transport pathways, the effect of phototherapy was studied in GT-TR- rats and in Gunn rats. Photoexposure caused a 120% increase in biliary excretion of bilirubin isomers in Gunn rats and only 40% in GT-TR- rats. This shows that the biliary excretion of bilirubin photoisomers is indeed affected by the hepatic excretion defect of GT-TR- rats and suggests that hepatic excretion of bilirubin photoisomers proceeds via the same route as other organic anions such as conjugated bilirubin and tetrabromosulphthalein.

Animals↗
PubMed1987

Pharmacokinetics of xamoterol glucuronidation in the rat in vivo and in liver perfusion.

Xamoterol has a phenolic hydroxyl group at which both sulphation and glucuronidation may occur. In the rat in vivo it is almost exclusively glucuronidated. In bile the glucuronide conjugate is the only metabolite of xamoterol. In urine equal amounts of the glucuronide conjugate and unchanged xamoterol are present, together with a small amount of the sulphate conjugate (approx. 1% of the dose). The t1/2 of excretion of the glucuronide in bile was the same as the t1/2 for excretion of unchanged xamoterol in urine. The t1/2 for urinary excretion of the glucuronide conjugate was longer than that for its biliary excretion. In the isolated perfused rat liver glucuronidation was also the almost exclusive metabolic pathway. The extraction ratio of xamoterol by the liver was approx. 0.15. Since xamoterol is a very water-soluble compound, and is almost exclusively glucuronidated in the rat, it is an excellent model substrate for investigations on the pharmacokinetics of glucuronidation in the rat in vivo and in isolated perfused organs.

Animals↗
PubMed1987

Hepatic disposition of cationic drugs bound to asialoorosomucoid: lack of coendocytosis and evidence for intrahepatic dissociation.

A combination of protein binding, liver clearance, subcellular distribution and cell separation experiments was employed to investigate the influence of binding of cationic drugs to asialoorosomucoid (ASOR) on their hepatic uptake and intrahepatic distribution. Two quaternary ammonium drugs, d-tubocurarine and N-methyldeptropine, were selected because of their marked differences in hepatic processing and binding to ASOR. In spite of an increase in protein binding of 560% for d-tubocurarine and 380% for N-methyldeptropine, perfusate clearance of both drugs in isolated perfused rat livers was not influenced by addition of 75 mg of ASOR. Absence of coendocytosis was indicated by subcellular distribution studies revealing no extra enrichment of quaternary ammonium drugs in lysosomal fractions compared with control studies. Isolation of parenchymal and sinusoidal liver cells demonstrated d-tubocurarine to be present solely in hepatocytes; binding to ASOR did not affect the relative distribution in the various cell types. It is concluded that binding of cationic drugs to ASOR does not result in endocytosis of a drug-protein-receptor complex by the liver. This result rather suggests that dissociation of the organic cations from the asialoglycoprotein occurs within the liver before endocytosis of the glycoprotein.

Animals↗
PubMed1986

Studies on the mechanism of binding and uptake of immune complexes by various cell types of rat liver in vivo.

Soluble heterologous immune complexes (IC) were used to study the mechanism of IC binding to rat liver in vivo. Binding of IC to the various cell types of the liver, endothelial cells, hepatocytes, and Kupffer cells, was only inhibited by aggregated swine immunoglobulins. Binding was not inhibited by the absence of complement components. Intravenous injection of asialoglycoproteins, to block the galactose receptor, could not prevent IC binding. We conclude that Fc receptors play an important role in the binding of soluble heterologous IC to hepatocytes, endothelial cells, and Kupffer cells.

Animals↗