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T Walle

Publications and source records attributed to T Walle.

At least 55 records · Page 3Linked to original sources
PubMed1995

Dexamethasone induction of taxol metabolism in the rat.

This study examined the effect of dexamethasone (DEX), 100 mg/kg/day for 3 days, on [3H]taxol metabolism in liver microsomes and hepatocytes of male and female Sprague-Dawley rats. In control rats, two isomeric monohydroxylated metabolites, M1 and M2, were formed. The formation of both metabolites was 2-3 times greater in the male than in the female animals. After DEX treatment, M1 increased 2.6-fold in the male animals and 6.5-fold in the female animals. This was accompanied by similar increases in hepatic cytochrome P4503A protein and testosterone 6 beta-hydroxylation. Three additional metabolites (U1, U2, and U3) were formed in the DEX-treated animals only. Isolation of these metabolites from rat hepatocyte incubates by reversed-phase HPLC permitted structure identification of U2 and U3, using tandem MS. The mass spectrum of U3 was consistent with deacetylation of taxol, whereas the mass spectrum of U2 was consistent with deacetylation of the monohydroxylated taxol metabolite M2. A comparison of HPLC and MS data for U3 with those of standard 10-deacetyltaxol suggested that the site of deacetylation might be the 4-position of the taxane ring. Preliminary observations indicate that the deacetylation is caused by a DEX-inducible cytochrome P450.

Animals↗
PubMed1994

Human phenol sulfotransferases: chiral substrates and expression in Hep G2 cells.

Enzymatic sulfation of chiral phenolic ethanolamine drugs, e.g. beta-agonists, has been shown to be stereoselective in humans. The reaction appears to be specific for the monoamine (M) form of the phenol sulfotransferases (PSTs). In further studies of the stereochemistry of this reaction, we have found the hepatoblastoma-derived cell line Hep G2 to be an excellent human model. These cells contain the M form PST in quantities exceeding those of human liver by about 4-fold. Thus, sulfate conjugates of the beta-agonist drugs can easily be synthesized for subsequent structural and enzyme kinetic studies. Although less abundant, the phenol (P) form PST as well as dehydroepiandrosterone sulfotransferase are also expressed in the Hep G2 cells.

Adrenergic beta-Agonists↗
PubMed1994

Propranolol metabolism in normal subjects: association with sex steroid hormones.

The objective of this study was to determine the potential role of circulating testosterone and estradiol in regulation of the activity of the sex-dependent pathways of propranolol metabolism (i.e., alpha-naphthoxylactic acid and propranolol glucuronide). The pharmacokinetics of a single 80 mg oral dose of propranolol and the plasma levels of the sex steroid hormones were therefore determined in normal volunteers. In 33 young men there was a positive correlation between the testosterone levels and the propranolol clearances through both alpha-naphthoxylactic acid (p < 0.001) and propranolol glucuronide (p < 0.002), as well as the total clearance (p < 0.05), but not through aromatic ring hydroxylation. Testosterone cypionate administration led to an increased clearance of propranolol through alpha-naphthoxylactic acid in nine of the 11 men studied, further supporting a stimulatory effect of testosterone on propranolol metabolism. In 23 young women there was no significant association between the circulating levels of either estradiol or testosterone and any of the clearances of propranolol. These observations may be clinically relevant for propranolol therapy and may provide improved insight into the influence of gender and circulating gonadal hormones on drug metabolism in humans.

Adult↗
PubMed1994

The influence of gender and sex steroid hormones on the plasma binding of propranolol enantiomers.

1. Plasma binding of tritium-labelled racemic propranolol (P) was measured by equilibrium dialysis. The unbound enantiomers were separated by h.p.l.c. after chiral derivatization. The binding of (-)-P was higher than that of (+)-P. 2. Contrary to previous suggestions, a sex difference in the plasma binding of the P enantiomers (9 young women, 12 young men) was not observed. The unbound percentage of (-)-P was 9.2 +/- 1.8 (mean +/- s.d.) in women vs 9.1 +/- 1.7 in men; for (+)-P it was 10.8 +/- 1.8 vs 10.8 +/- 2.1. 3. In the nine women, the binding did not change with fluctuating plasma oestradiol concentrations during the menstrual cycle. Testosterone cypionate doubled the circulating concentrations of testosterone in eight men but had no effect on P binding. 4. Ethinyl oestradiol (50 micrograms day-1) alone or together with norethindrone (OCD) in eight of the women produced an increase in the unbound percentage of both (-)-P (11.4 +/- 2.6 vs 9.5 +/- 1.6 for control; P < 0.001) and (+)-P (13.2 +/- 2.5 vs 11.2 +/- 1.5 for control; P < 0.001). This was due to a decrease in the plasma concentrations of alpha 1-acid glycoprotein from 0.54 +/- 0.11 mg ml-1 in control to 0.37 +/- 0.08 mg ml-1 (P < 0.001) during ethinyl oestradiol treatment. 5. Enantioselectivity in the unbound fraction of P increased with increasing total binding from a (-)/(+)-ratio of 0.93 at 84% binding to a (-)/(+)-ratio of 0.78 at 94% binding (P < 0.001).

Adult↗
PubMed1994

Cytochrome P450 3A-mediated human liver microsomal taxol 6 alpha-hydroxylation.

The antitumor drug taxol was metabolized to one major (6 alpha-hydroxytaxol) and two minor metabolites by human liver microsomes. A 10-fold interindividual variability with a Vmax of 1.16 +/- 0.85 nmol/hr/mg of microsomal protein and a Km of 18.0 +/- 12.2 microM was observed for taxol 6 alpha-hydroxylation (mean +/- S.D.; n = 6). The NADPH-dependency and the inhibitory effect of carbon monoxide and piperonyl butoxide on taxol metabolism indicated the involvement of cytochrome P450 (CYP) monooxygenases. Chemical inhibition studies pointed to the CYP 3A subfamily as being responsible for taxol 6 alpha-hydroxylation. However, although some CYP 3A substrates were inhibitory (midazolam, 17 alpha-ethinyl estradiol, quercetin, verapamil and testosterone), others were not (troleandomycin, erythromycin and cyclosporin A). The inhibition was found to be competitive with low Ki values for midazolam (10.5 microM) and 17 alpha-ethinyl estradiol (4.5 microM). Taxol 6 alpha-hydroxylation correlated well with the metabolism of 17 alpha-ethinyl estradiol (r = 0.874; P < .05) and midazolam (r = 0.954; P < .01) in the same livers. Rabbit anti-rat CYP 3A1 antibodies, which cross-react with human CYP 3A isoforms, were inhibitory of taxol 6 alpha-hydroxylation. Although the evidence from these experiments supported the CYP 3A mediation of taxol 6 alpha-hydroxylation, the lack of effect of some inhibitors combined with the inability of a human CYP 3A4 transfected cell line to metabolize taxol point to a CYP 3A isoform other than 3A4. The findings in this study could prove clinically useful for the prediction of potential drug interactions, both inhibitory and inductive of taxol metabolism.

Cytochrome P-450 CYP2E1↗
PubMed1993

Taxol metabolism in rat hepatocytes.

Metabolism of the anticancer drug taxol was investigated in freshly isolated rat hepatocytes. Two main metabolites were separated by reversed-phase HPLC and shown by tandem mass spectrometry to be monohydroxylated metabolites. Kinetic studies revealed apparent Km values of 68 and 61 microM with identical Vmax values for the two metabolites. Verapamil and midazolam, but not phenacetin, showed concentration-dependent inhibition of taxol metabolism with both metabolites being affected equally. The IC50 was about 100 microM for verapamil and 25 microM for midazolam. These observations demonstrate for the first time in vitro metabolism of taxol and suggest that the metabolism may be subject to potentially important interactions with numerous other drugs.

Animals↗
PubMed1993

Immunoaffinity isolation of the sulfate conjugate of 4'-hydroxypropranolol from plasma.

Selective extraction of sulfate conjugates of basic drugs from biological matrices has been difficult because of their highly polar nature. Immunoaffinity isolation may be the best solution to this analytical problem. This was tested for a model compound, the metabolite 4'-hydroxypropranolol sulfate (HOPS), which was effectively extracted from plasma by a column containing antibodies to the parent drug propranolol. The specificity was very high, giving little interference from the biological material in subsequent high-performance liquid chromatographic analysis with fluorometric detection. The method for HOPS was highly reproducible and provided a sensitivity of 1 ng/ml plasma. The technique was applied to measurements of HOPS in plasma after therapeutic doses of propranolol as well as to the individual enantiomers after chiral derivatization.

Adult↗
PubMed1993

Stereoselective sulfate conjugation of isoproterenol in humans: comparison of hepatic, intestinal, and platelet activity.

The stereochemistry of sulfate conjugation of isoproterenol (ISO) was examined with human liver, intestine, and platelets as the phenolsulfotransferase (PST) enzyme source and PAP35S as the cosubstrate. With the hepatic cytosol, two distinct sulfation reactions were identified, a high affinity reaction (Km 5 to 50 microM) and a low affinity reaction (Km 360 to 2,900 microM). The efficiency of sulfation (Vmax/Km) for both reactions was 5-fold higher for (+)- than for (-)-ISO. When the hepatic PSTs were resolved by ion-exchange chromatography, it could be shown that the high affinity reaction was catalyzed by the monoamine (M) form and the low affinity reaction by the phenol (P) form of PST. Only the high affinity (M form) sulfation was detected in the jejunal cytosol with a Vmax/Km value 6.1-fold higher for (+)- than for (-)-ISO. Finally the platelet, as a potentially useful model tissue, also demonstrated only the high affinity M form reaction with a Vmax/Km value 5.7-fold higher for (+)- than for (-)-ISO. In summary, this study has shown that sulfation of ISO by PSTs in various human tissues is stereoselective and favors the inactive (+)-enantiomer over the active (-)-enantiomer by about 5-fold, a finding which should be considered in the therapeutic use of chiral drugs cleared by sulfate conjugation.

Adolescent↗
PubMed1993

Stereoselective sulphate conjugation of salbutamol in humans: comparison of hepatic, intestinal and platelet activity.

1. The oral bioavailability of the beta 2-adrenoceptor agonist salbutamol has been proposed to be stereoselective, presumably due to presystemic sulphate conjugation. In the present study we examined the stereochemistry of the sulphation reaction in vitro using human tissue preparations. 2. Sulphation of salbutamol was studied with partially purified hepatic M and P form phenol sulphotransferases (PSTs), 100,000 g cytosol of jejunal mucosa and platelet homogenate. The cosubstrate PAP35S was used as the sulphate donor. The acceptor substrate was either (+)-, (-)-or (+/)-salbutamol. 3. Sulphation was catalyzed by the M form PST of the liver but not the P form. The sulphation efficiency (Vmax/Km) was 11.9-fold greater for the (-)- than for the (+)- enantiomer, due entirely to a lower apparent Km for (-)-salbutamol, 103 microM, than for (+)-salbutamol, 1394 microM. 4. Sulphation by the jejunal mucosa (n = 3) was very similar to that of the M form PST with the efficiency being 9.8-fold greater for the (-)-enantiomer and apparent Km values 95 microM and 889 microM for (-)- and (+)-salbutamol, respectively. 5. Sulphation by the platelet (n = 3) was also very similar to that of the M form PST with the efficiency being 9.9-fold greater for the (-)-enantiomer and apparent Km values 141 microM and 1190 microM for (-)- and (+)-salbutamol, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Albuterol↗
PubMed1993

Obstetric shoulder injury. Associated risk factors, prediction and prognosis.

A prospective case-control study was made to estimate the incidence of clavicular fracture and brachial plexus palsy, to find out possible risk factors during pregnancy and labor associated with these injuries, and to ascertain the prognosis of injured infants. Clavicular fracture occurred in 165 (3.2%) and brachial plexus palsy in 10 (0.2%) of the 5082 infants born during the study period. From maternal characteristics the Body Mass Index (kg/m2) and pregnancy weight gain were significantly greater in the cases than in the controls. Symphysis-to-fundus height was, on average, higher in the case group. There were no more instrumental vaginal deliveries in cases than in controls. The shoulder injured infants were in every way bigger, but only in 30% of the cases macrosomic (birthweight over the 90th percentile), and they were more often males than in the controls. The prediction of shoulder injury by ultrasound was not successful in our study. The prognosis of injured infants was good, all children except one recovered soon after birth.

Adult↗
PubMed1993

Binding of taxol to human plasma, albumin and alpha 1-acid glycoprotein.

The binding of taxol to human plasma and to individual plasma proteins was studied by equilibrium dialysis. Taxol was found to bind extensively (about 95%) without a significant difference between healthy volunteers and cancer patients. At clinically relevant concentrations (0.1-6 microM), the binding was found to be concentration independent, indicating nonspecific hydrophobic binding. Human serum albumin and alpha 1-acid glycoprotein were found to contribute about equally to the binding, with a minor contribution from lipoproteins. None of the drugs commonly coadministered with taxol (dexamethasone, diphenhydramine, ranitidine, doxorubicin, 5-fluorouracil and cisplatin) altered the binding of taxol significantly. The protein binding of taxol was found to dramatically decrease the red blood cell uptake of taxol.

Albumins↗
PubMed1992

Induction of propranolol metabolism in the Hep G2 human hepatoma cell line.

Metabolism of propranolol by the human hepatoma cell line Hep G2 was studied. Although metabolism qualitatively was similar to that in-vivo, the P450-mediated N-desisopropylation clearly predominated. Pretreatment of cells with 3-methylcholanthrene increased the activity of this pathway 14-fold, whereas phenobarbitone had no effect. This is similar to the pathway-selective inductive response observed for cigarette smoking in-vivo. As in-vivo, secondary metabolism of N-desisopropylpropranolol was extensive. This could, however, be completely blocked by 0.1 microM clorgyline, a potent MAO type A inhibitor. As in human liver microsomes, the stereochemistry of propranolol metabolism demonstrated a preference for the R(+)-enantiomer. These observations emphasize the usefulness of the Hep G2 cell line as a model of man.

Carcinoma, Hepatocellular↗
PubMed1992

Hep G2 cell line as a human model for sulphate conjugation of drugs.

1. The objective of this study was to examine the usefulness of the hepatoma cell line Hep G2 as a model for human sulphoconjugation of drugs, in particular stereoselective conjugation. 2. Using the substrates p-nitrophenol and dopamine, we found sulphation activities consistent with the presence of both the phenol (P) and the monoamine (M) form of the human phenolsulphotransferases in these cells. 3. The Kmapp was 3.0 microM for the sulphation of p-nitrophenol. This activity was inhibited selectively by 2,6-dichloro-4-nitrophenol, IC50 6 microM. The Kmapp was 39 microM for the sulphation of dopamine. This activity was selectively inhibited by elevated temperature. 4. The chiral adrenergic drugs (+/-)-terbutaline and (+/-)-4-hydroxypropranolol were both sulphated stereoselectively with Kmapp and Vmaxapp values for each enantiomer virtually identical to previous observations with human liver cytosol. 5. In a direct comparison, the estimated activity of the P form of phenolsulphotransferase in the Hep G2 cell line was 30% of that in human liver, whereas, surprisingly, the activity of the M form of phenolsulphotransferase was 4.5 times higher in the Hep G2 cells than in the liver.

Arylsulfotransferase↗
PubMed1992

Potent inhibition of MAO mediated propranolol metabolism by dimethyl sulfoxide in Hep G2 cells.

This communication reports on the effects of dimethyl sulfoxide (DMSO) on the oxidative metabolism of propranolol by the homogenate of the human hepatoma cell line Hep G2. The formation of multiple metabolites was monitored in the absence and presence of 0.1 to 2.0% of DMSO. For the primary cytochrome P450-mediated pathways, ring oxidation was significantly reduced by 1.3 to 2.0% DMSO, whereas side-chain oxidation was not affected. A gradual inhibition of the oxidative deamination of N-desisopropylpropranolol to its glycol metabolite was seen with 20% inhibition at a DMSO concentration of 0.1% and a maximum 75% inhibition at a DMSO concentration of 0.7%. This highly selective inhibitory effect of DMSO on oxidative drug metabolism appears to be directed towards MAO type A.

Animals↗
PubMed1992

Enantiomeric interaction in the sulfate conjugation of the beta 2-agonist drug albuterol by the human liver.

Sulfate conjugation of the pure enantiomers of albuterol was compared to that of racemic drug using the 100,000g cytosol of four human livers as the catalyst. Whereas a trend towards a lower Kmapp was seen for sulfation of (-)-compared to (+)-albuterol, there was no enantiomeric difference in Vmaxapp. However, when the sulfation of racemic albuterol was compared to that of its pure enantiomers, there was a marked suppression of the reaction. This was due to a reduction in Vmaxapp by about 75% (p less than 0.01) with no difference in Kmapp. Thus, an enantiomeric interaction occurs in the sulfoconjugation of albuterol, which must be taken into account when evaluating the pharmacokinetics of the individual enantiomers following racemic drug administration.

Albuterol↗
PubMed1991

Stereoselective accumulation of the beta-receptor blocking drug atenolol by human platelets.

We have studied the stereochemistry of accumulation of the hydrophilic beta-adrenoceptor antagonist rac-atenolol by human platelets in vitro. The accumulation was slow, not reaching equilibrium until 90 min at 37 degrees C. The uptake was temperature dependent with the accumulation at 37 degrees C being 3-4 times greater than at 4 degrees C. The accumulation was also stereoselective at 37 degrees C, favoring the active (-)-enantiomer over the (+)-enantiomer by 2.3-fold. Reserpine, but not desipramine, inhibited the platelet accumulation of rac-atenolol enantiospecifically. This uptake profile is different from the platelet uptake of lipophilic beta-blockers, which is characterized by nonspecific membrane binding, but similar to the carrier-mediated accumulation of the neurotransmitter norepinephrine by storage granules within the platelet.

Adult↗
PubMed1990

Rabbit hepatocytes in primary culture: preparation, viability and use in studies of propranolol metabolism.

Isolated hepatocyte cultures have become a frequently used model system for investigating drug metabolism. Although rat and hamster hepatocytes are frequently used for this purpose, metabolism in these species differs in many respects from human metabolism. A species with a metabolism more closely resembling that of humans might be more useful. Our in vivo experiments demonstrated that the metabolism of propranolol in the rabbit is more similar to that in humans than in rats or hamsters. We therefore examined the usefulness of rabbit parenchymal liver cells for studies of propranolol metabolism. A detailed method is presented for their preparation and culture, along with data on their viability, structure and protein synthesizing capability. One- or two-day-old hepatocyte cultures were exposed to 10 mumol/L 3H-propranolol from 30 min to 2 hr; metabolites were identified by gas chromatography-mass spectrometry and quantitated by high-performance liquid chromatography. Propranolol metabolism was linear over 1 hr, with 15% of the substrate metabolized during this time. The cytochrome P-450 pathways, which result in ring oxidation and side-chain oxidation, were expressed in a reproducible fashion similar to that found in vivo in humans. The arylhydrocarbon hydroxylase inhibitor alpha-naphthoflavone (100 mumol/L) inhibited side-chain oxidation of propranolol by 90% without affecting ring oxidation. In contrast, chlorpromazine (100 mumol/L) was shown to inhibit ring oxidation of propranolol by 85% without affecting side-chain oxidation. Cimetidine (250 mumol/L) inhibited both pathways by about 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗