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Biomedical subjects

M Eichelbaum

Publications and source records attributed to M Eichelbaum.

At least 127 records · Page 7Linked to original sources
PubMed1991

Effects of verapamil enantiomers and major metabolites on the cytotoxicity of vincristine and daunomycin in human lymphoma cell lines.

Verapamil, a calcium channel blocker, is used as the racemate. Recently, racemic verapamil has been shown to increase the cytotoxicity of vinca alkaloid and anthracycline derivatives in several resistant tumour cell lines. With respect to its cardiovascular activity S-verapamil is an order of magnitude more potent than R-verapamil. Since it was not known whether the effect on multi-drug resistance was also enantioselective a comparison has been made of the potency of the R and S enantiomers and racemic verapamil in their ability to increase the cytotoxicity of vincristine and daunomycin in sensitive (MOLT 4B) and drug resistant human T-lymphoma cell lines (MOLT/VCR-5 x 9, MOLT/DAU-8 and VCR 1000, a highly resistant subline of CCRF-CEM). Two major metabolites, norverapamil and D617 were tested in the same system. (+)-R, (-)-S-, racemic verapamil, norverapamil and D617 alone had no effect on cell growth in sensitive or resistant cell lines in concentrations up to 20 microM. In combination with vincristine, verapamil and norverapamil but not D617 produced a concentration dependent increase in the sensitivity of the resistant lines. Racemic verapamil, its individual enantiomers and norverapamil were equipotent. The concentration of the modifiers required to elicit 50% of the maximum effect (EC50) was of the order of 0.5 microM. No significant difference in the slopes of the concentration-effect curves were observed. The effect of verapamil and norverapamil was additive. In the sensitive MOLT 4B cell line both enantiomers and norverapamil increased sensitivity towards vincristine. However, the EC50 values were at least an order of magnitude higher (2.5-8 microM) than in the resistant cell lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Daunorubicin↗
PubMed1991

Debrisoquine/sparteine hydroxylation genotype and phenotype: analysis of common mutations and alleles of CYP2D6 in a European population.

Four different mutations of the cytochrome P450 CYP2D6 gene associated with the poor metabolizer phenotype (PM) of the debrisoquine/sparteine polymorphism were analyzed by Xba I restriction fragment length polymorphism (RFLP) analysis and a polymerase chain reaction (PCR)-based DNA amplification method in DNA of 394 healthy European subjects; 341 of these were phenotyped by sparteine or debrisoquine administration and urinary metabolic ratios (MR). Our study demonstrates the efficiency of the PCR-test for phenotype prediction; 96.4% of individuals were correctly predicted, i.e., 100% of the extensive metabolizers (EMs) and 86.0% of the poor metabolizers (PMs). In contrast, Xba I RFLP analysis was far less informative, predicting the phenotype in only 26.8% of PMs. By combining both DNA tests, the prediction rate of the PM phenotype increased to 90.6%. A point mutation at a splice-site consensus sequence termed D6-B represented the most common mutant CYP2D6 gene and accounted for more than 75% of mutant alleles. In addition, other known mutations such as D6-D (14%), D6-A (5%), and the rare D6-C mutation bring the identified mutant alleles to greater than 95% of all mutant PM-alleles. Most of Xba I 44-kb alleles were confirmed as mutant alleles carrying the D6-B mutation. However, 9.7% did not have this mutation and may express a functional CYP2D6 gene. Moreover, all Xba I 16 + 9-kb alleles contained the D6-B mutation. Heterozygous EM individuals had a significantly higher MR when compared to homozygous EMs. Genotyping provides an important advantage for investigations of the influence of CYP2D6 activity on drug therapy and its association with certain diseases.

Alleles↗
PubMed1991

Pharmacokinetics, bioavailability, metabolism and acute and chronic antihypertensive effects of nitrendipine in patients with chronic renal failure and moderate to severe hypertension.

1. The pharmacokinetics, bioavailability, metabolism and antihypertensive effects of nitrendipine have been studied in 12 patients with impaired renal function and moderate to severe hypertension. The drug was administered simultaneously by the i.v. [13C4] and oral (commercial tablet 20 mg) routes. 2. No differences in the pharmacokinetic parameters were observed between the two routes of administration. The systemic clearance after i.v. administration in patients with renal impairment (18.2 +/- 6.1 ml min-1 kg-1) was similar to that observed in healthy volunteers. Despite complete absorption of drug from the tablet the bioavailability of the parent compound was 21.2 +/- 12.5%. Cumulative urinary excretion of nitrendipine metabolites was correlated with the creatinine clearance (r = 0.946). 3. Significant reductions in mean arterial blood pressure (mean: 23.6%) at the end of the nitrendipine infusion and after oral administration of 20 mg (mean: 17.5%) were observed. The blood pressure lowering effect of nitrendipine could be correlated within individuals with serum nitrendipine concentrations using a log linear model. 4. Following 4 weeks of therapy an average dose of 77 mg nitrendipine day-1 was required to achieve a systolic blood pressure below 160 mm Hg or a diastolic blood pressure below 90 mm Hg. The reduction in blood pressure during multiple dosing was related to the nitrendipine steady-state concentration. There was a significant relationship between the nitrendipine bioavailability and the dose required for sufficient blood pressure control. 5. No accumulation of nitrendipine caused by impaired renal function was observed during multiple dosing. Thus, no reduction of the nitrendipine dose in patients with renal impairment is necessary.

Adult↗
PubMed1991

Metabolism of verapamil in a family pedigree with deficient N-oxidation of trimethylamine.

The oxidative N-dealkylation of verapamil has been studied in a family of five members with two propositi with an inherited deficiency of trimethylamine N-oxidation (fish-odour syndrome). The results were assessed for possible co-segregation of the trimethylamine N-oxidation phenotype and any observed deficiency in oxidative N-dealkylation. The general pattern of metabolism of verapamil in the five subjects studied was similar to that reported in earlier investigations. Moreover, there were no differences between the two affected subjects and other family members with respect to the metabolic pattern. It is concluded that there is no functional segregation with respect to the mechanisms controlling trimethylamine N-oxidation and verapamil N-dealkylation.

Adult↗
PubMed1991

Regioselectivity and stereoselectivity of the metabolism of the chiral quinolizidine alkaloids sparteine and pachycarpine in the rat.

1. The metabolism of (-)-sparteine and (+)-sparteine (pachycarpine) was investigated in male Sprague-Dawley rats by g.l.c.-mass spectrometry, and 13C- and 2H-n.m.r. spectroscopy. The structure of the major metabolite of (-)-sparteine was confirmed to be 2,3-didehydrosparteine by g.l.c.-mass spectrometry after alkaline sample work-up. 2H-n.m.r. spectroscopy showed that this metabolite exhibits the structure of the carbinolamine (2S)-hydroxysparteine in aqueous solution of neutral pH. No other metabolites with an enamine structure were observed by g.l.c.-mass spectrometry and 13C-n.m.r. spectroscopy. 2. Pachycarpine is metabolized in vivo and in vitro stereoselectively to the aliphatic alcohol (4S)-hydroxypachycarpine as the main metabolite. 3. The formation of the 2,3-didehydrosparteine proceeds via stereospecific abstraction of the axial 2 beta hydrogen atom. Inhibition in vitro studied with purified rat liver microsomes demonstrated that both sparteine enantiomers are metabolized by the same cytochrome P450 isozyme. Therefore this enzyme exhibits marked substrate and product stereoselectivity for the metabolism of the two enantiomeric quinolizidine alkaloids.

Administration, Oral↗
PubMed1991

Thebaine O-demethylation to oripavine: genetic differences between two rat strains.

1. Codeine O-demethylation to morphine is mediated by cytochrome P450 IID1 (rat), or P450 IID6 (man), and exhibits genetic polymorphism. Thebaine is a precursor in the formation of endogenous morphine and codeine in man, being O-demethylated to oripavine. 2. The objective of the present study was to ascertain whether the O-demethylation of thebaine to oripavine was mediated by cytochrome P450 IID1 in rat liver microsomes. 3. Thebaine O-demethylation showed strain differences in female Sprague-Dawley (SD) and female Dark-Agouti (DA) rats, which serve as a model for the human debrisoquine/sparteine metabolism phenotypes. 4. The total intrinsic clearance of thebaine to oripavine was high (19.7 ml/h per mg protein) in SD rats, indicating that oripavine is a major metabolite of thebaine. A 3-fold lower intrinsic clearance was observed in DA rats (6.7 ml/h per mg protein). 5. Thebaine O-demethylation was inhibited by quinine and known substrates of cytochrome P450 IID1/P450 IID6, supporting the major involvement of cytochrome P450 IID1 in oripavine formation in rats.

Animals↗
PubMed1991

Deletion of the entire cytochrome P450 CYP2D6 gene as a cause of impaired drug metabolism in poor metabolizers of the debrisoquine/sparteine polymorphism.

The debrisoquine/sparteine polymorphism is associated with a clinically important genetic deficiency of oxidative drug metabolism. From 5% to 10% of Caucasians designated as poor metabolizers (PMs) of the debrisoquine/sparteine polymorphism have a severely impaired capacity to metabolize more than 25 therapeutically used drugs. The impaired drug metabolism in PMs is due to the absence of cytochrome P450IID6 protein. The gene controlling the P450IID6 protein, CYP2D6, is located on the long arm of chromosome 22. A pseudogene CYP2D8P and a related gene CYP2D7 are located upstream from CYP2D6. This gene locus is highly polymorphic. After digestion of genomic DNA with XbaI endonuclease, restriction fragments of 11.5 kb and 44 kb represent mutant alleles of the cytochrome CYP2D6 gene locus associated with the PM phenotype. In order to elucidate the molecular mechanism of the mutant allele reflected by the XbaI 11.5-kb fragment, a genomic library was constructed from leukocyte DNA of one individual homozygous for this fragment and screened with the human IID6 cDNA. The CYP2D genes were isolated and characterized by restriction mapping and partial sequencing. We demonstrate that the mutant 11.5-kb allele results from a deletion involving the entire functional CYP2D6 gene. This result provides an explanation for the total absence of P450IID6 protein in the liver of these PMs.

Base Sequence↗
PubMed1991

Enantiomer/enantiomer interaction of (S)- and (R)-propafenone for cytochrome P450IID6-catalyzed 5-hydroxylation: in vitro evaluation of the mechanism.

Many drugs are used as racemates, and the enantiomers may differ in terms of pharmacological properties and disposition. Stereoselective disposition of the enantiomers can arise from metabolism of the enantiomers via different routes catalyzed by different enzymes. In contrast, the enantiomers may be metabolized by the same enzyme at different rates. In the latter case, the enantiomers can compete for this metabolic step, giving rise to the possibility of an enantiomer/enantiomer interaction. We have chosen the antiarrhythmic propafenone, for which in vivo data indicated an interaction between (S)- and (R)-propafenone, as a model substance to study the mechanism underlying that interaction in human liver microsomes. We used the cytochrome P450IID6-mediated 5-hydroxylation of propafenone as a model pathway, because this metabolic step constitutes the major route of biotransformation of propafenone. The Michaelis-Menten kinetics for 5-hydroxylation were determined after incubation of (R)- and (S)-propafenone and a pseudoracemate consisting of (S)-[2H4]propafenone and (R)-propafenone. Inhibition experiments were performed using (S)-[2H4]propafenone as an inhibitor of the 5-hydroxylation of (R)-propafenone, and vice versa. The kinetic model of mixed alternative substrates was used to simulate inhibition experiments. Experimental data were compared with those predicted by this model. We observed a substantial stereoselectivity after incubation of the individual enantiomers [(S)-propafenone: Vmax, 10.2 pmol/micrograms/hr, and Km, 5.3 microM; (R)-propafenone: Vmax, 5.5 pmol/micrograms/hr, and Km, 3.0 microM]. In contrast, no substrate stereoselectivity was observed after incubation of the pseudoracemate [3.1 pmol/micrograms/hr for (S)-[2H4]propafenone and 3.3 pmol/micrograms/hr for (R)-propafenone]. Application of the model revealed Ki values of 2.9 and 5.2 microM for the inhibition of 5-hydroxylation of (S)-[2H4]-propafenone by (R)-propafenone and for inhibition of 5-hydroxylation of (R)-propafenone by (S)-[2H4]-propafenone, respectively. The predicted and the experimental data were in good agreement, and both indicated the mode of inhibition to be competitive. In conclusion, the enantiomers of propafenone interact with respect to 5-hydroxylation, with (R)-propafenone being a more potent inhibitor than the S-enantiomer with respect to cytochrome P450IID6-mediated 5-hydroxylation. Because beta-blocking properties of propafenone reside in the S-enantiomer, inhibition of metabolism of this enantiomer by (R)-propafenone may have therapeutic consequences.

Adolescent↗
PubMed1991

Influence of debrisoquine phenotype and of quinidine on mexiletine disposition in man.

Mexiletine is a low clearance drug which undergoes extensive metabolism in man. In vitro studies with human liver microsomes have suggested that major oxidation pathways of mexiletine are predominantly catalyzed by the genetically determined debrisoquine 4-hydroxylase (cytochrome P450IID6) activity. In this study, we investigated the role of debrisoquine polymorphism and the effects of low dose quinidine, a selective inhibitor of cytochrome P450IID6, on the disposition of mexiletine. Fourteen healthy volunteers, 10 with the extensive metabolizer (EM) and 4 with the poor metabolizer (PM) phenotype, received a single 200-mg dose of mexiletine hydrochloride orally on two occasions (1 week apart), once alone and once under steady-state conditions for quinidine (50 mg QID). During the phase mexiletine alone, total clearance, nonrenal clearance and partial metabolic clearance of mexiletine to hydroxymethylmexiletine, to m-hydroxymexiletine and to p-hydroxymexiletine were decreased in PM compared to EM (all P less than .05). In EM, quinidine decreased mexiletine total clearance from 621 +/- 298 to 471 +/- 214 ml/min (mean +/- S.D.; P less than .05) and mexiletine nonrenal clearance from 583 +/- 292 to 404 +/- 188 ml/min (P less than .05). Moreover, quinidine increased mexiletine elimination half-life in EM from 9 +/- 1 to 11 +/- 2 h (P less than .05). In these subjects, partial metabolic clearance to hydroxymethylmexiletine, m-hydroxymexiletine and p-hydroxymexiletine were decreased by quinidine coadministration 5-, 4- and 7-fold, respectively, whereas partial metabolic clearance to N-hydroxymexiletine was unaffected. Changes induced by quinidine in EM were correlated to their debrisoquine metabolic ratio. Thus, genetically determined or pharmacologically induced modulation of cytochrome P450IID6 activity represents a major determinant of mexiletine disposition.

Administration, Oral↗
PubMed1990

Identification of the primary gene defect at the cytochrome P450 CYP2D locus.

The mammalian cytochrome P450-dependent monooxygenase system is involved in the metabolism of drugs and chemical carcinogens. The role of these enzymes in toxicological response is exemplified by an autosomal recessive polymorphism at the cytochrome P450 CYP2D6 debrisoquine hydroxylase locus which results in the severely compromised metabolism of at least 25 drugs, and which in some cases can lead to life-threatening side-effects. In addition, this polymorphism, which affects 8-10% of the caucasian population, has been associated with altered susceptibility to lung and bladder cancer. Here we report the identification of the primary mutation responsible for this metabolic defect and the development of a simple DNA-based genetic assay to allow both the identification of most individuals at risk of drug side-effects and clarification of the conflicting reports on the association of this polymorphism with cancer susceptibility.

Base Sequence↗
PubMed1990

Patients with type II autoimmune hepatitis express functionally intact cytochrome P-450 db1 that is inhibited by LKM-1 autoantibodies in vitro but not in vivo.

Liver-kidney microsomal-1 autoantibodies characterize a subgroup of autoimmune chronic active hepatitis. The liver antigen of liver-kidney microsomal-1 antibodies has been identified as cytochrome P450 db1, a microsomal enzyme catalyzing the oxidative metabolism of more than 20 drugs, including debrisoquine, sparteine and bufuralol. A genetic polymorphism (debrisoquin-sparteine polymorphism) is responsible for the lack of P450 db1 protein in the livers of 5% to 10% of Caucasians, leading to impaired drug metabolism and a distinct poor metabolizer phenotype. We investigated whether liver-kidney microsomal-1 positive autoimmune chronic active hepatitis patients express functionally intact P450 db1 in their livers. In four patients with liver-kidney microsomal-1 positive chronic active hepatitis, but not in five patients with various liver-kidney microsomal-1 negative liver diseases, the presence of circulating liver-kidney microsomal-1 antibodies was confirmed by immunofluorescence, radioimmunoassay and immunoblotting analysis using recombinant P450 db1. Moreover, only sera from liver-kidney microsomal-1 positive autoimmune chronic active hepatitis patients strongly inhibited the enzymatic activity of P450 db1 in human liver microsomes in vitro. Immunoblotting detected 50-kd P450 db1 protein in liver biopsy specimens from all patients. The in vivo function of P450 db1 was investigated by determining the metabolic ratio for sparteine and its 2-dehydro and 5-dehydro metabolites in 12-hr urine samples after oral administration of sparteine sulfate. In vivo P450 db1-mediated drug metabolism was of the extensive metabolizer phenotype and did not differ significantly between liver-kidney microsomal-1 positive (metabolic ratio = 1.15 +/- 0.32) and liver-kidney microsomal-1 negative (metabolic ratio = 1.18 +/- 0.48) patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Effects of verapamil on anthracycline-induced cardiomyopathy: preliminary results of a prospective multicenter trial.

Previous investigations in animals and one retrospective study in man suggest that verapamil can prevent anthracycline-induced cardiomyopathy. In the following study, patients with acute myeloid leukemia (AML) treated with double induction and consolidation chemotherapy (AML COOP study 1986, [3]) were randomized in a group with and without accompanying low-dose oral verapamil treatment. Since July 1986, 64 patients have been included. Thirty patients have been evaluated for pre- and posttreatment cardiological investigations. So far, no significant difference in cardiotoxicity has been observed either between the verapamil and nonverapamil group or between the two induction chemotherapy regimens (TAD/TAD - TAD/HAM).

Adolescent↗
PubMed1990

The teaching and organisation of clinical pharmacology in European medical schools (W.H.O. Working Group on Clinical Pharmacology).

A World Health Organisation (European Regional Office) working party has been established to review the progress of clinical pharmacology in European countries. As part of this review a questionnaire on the teaching of clinical pharmacology was sent to the Deans of all 350 medical schools in the region. Very few replies were received from U.S.S.R., Greece and Portugal and these countries' returns were not analysed further. The overall compliance rate (excluding these countries) was 82% with a figure of 84% from Western Europe and 74% from Eastern Europe. An average time of 96 h (range 0-320) was devoted to pharmacology teaching in the medical curriculum in Western Europe with 124 (0-240) h in Eastern Europe. In contrast 28 h (0-210) was devoted to clinical pharmacology teaching in Western Europe and 27 h (0-90) in Eastern Europe. On average in Western Europe each medical school had 2 individuals trained in clinical pharmacology with 1.3 posts in the subject and the figures for Eastern Europe were 2.3 and 1.1 respectively. However these figures hide a wide variance in the teaching of clinical pharmacology. Particularly in Western Europe there are a number of medical schools in Italy, Spain and the Federal Republic of Germany (FRG) where clinical pharmacology is not taught and there is a dearth of individuals trained in the subject. Every effort to encourage clinical pharmacology and its teaching should be made, particularly in these countries.

Curriculum↗
PubMed1990

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects.

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

Animals↗
PubMed1990

Acetylation pharmacogenetics. The slow acetylator phenotype is caused by decreased or absent arylamine N-acetyltransferase in human liver.

The biochemical basis underlying the genetic polymorphism of drug N-acetylation was investigated using a combination of in vivo and in vitro assays for arylamine N-acetyltransferase (NAT) activity and content in human liver. The acetylator phenotype of 26 surgical patients was determined using caffeine as an innocuous probe drug by measurement of the 5-acetyl-amino-6-formylamino-3-methyluracil to 1-methylxanthine molar ratio in urine. Liver wedge biopsies from these patients and livers from 24 organ donors were then used for measurement of N-acetyltransferase activity with the substrate sulfamethazine and for quantitation of immunoreactive N-acetyl-transferase protein. In vivo (caffeine metabolites in urine) and in vitro (sulfamethazine acetylation) measures of N-acetyl-transferase activity correlated very highly (r = 0.98). Moreover, in all subjects tested, slow acetylation both in vivo and in vitro was associated with a decrease in the quantity of immunodetectable N-acetyltransferase protein in liver cytosol relative to that seen in cytosols from rapid acetylator livers. Two kinetically distinct enzyme activities, designated NAT-1 and NAT-2, were partially purified from low- and high-activity livers and their relationship to acetylator status was determined. Low acetylation capacity was related to decreases in the liver content of both of these immunologically related proteins. The results demonstrate that genetically defective arylamine N-acetylation is due to a parallel decrease in the quantity of two structurally and functionally similar acetylating enzymes.

Acetylation↗
PubMed1990

Interaction of verapamil and cimetidine: stereochemical aspects of drug metabolism, drug disposition and drug action.

The pharmacokinetics, metabolism and pharmacodynamics of verapamil (160 mg p.o. of a pseudoracemic mixture) were evaluated in six healthy volunteers before and after coadministration of cimetidine (400 mg b.i.d.). Enantiomers of verapamil and enantiomers of three major urinary metabolites (norverapamil, D-617 and D-620) were determined in plasma and urine by gas chromatography-mass spectrometry. Coadministration of cimetidine led to a significant increase in the area under the plasma concentration vs. time curve of S-verapamil (29.2 +/- 31.8 min x nmol x ml-1 vs. 41.2 +/- 33.7 min x nmol x ml-1; P less than .003) and R-verapamil (124.7 +/- 112.2 min x nmol x ml-1 vs. 156.8 +/- 105.0 min x nmol x ml-1; P less than .01). The increase was significantly greater for the pharmacologically more potent S-enantiomer compared to R-verapamil (150.3 +/- 37 vs. 117.8 +/- 15%; P less than .05). As a consequence, coadministration of cimetidine increased the negative dromotropic effect of verapamil on atrioventricular conduction in five of six subjects. In addition, fractional metabolic clearance to D-620 and D-617 decreased for both enantiomers. Tubular secretion of S-D-617 was inhibited by cimetidine (342 +/- 104 vs. 238 +/- 52 ml x min-1; P less than .05) whereas secretion of the R-enantiomer remained unchanged (276 +/- 91 vs. 222 +/- 43 ml x min-1). Thus, cimetidine interacts with both hepatic and renal verapamil elimination in a stereoselective manner. The increase in total plasma concentration of verapamil combined with an increase in eutomer/distomer ratio produces a more pronounced pharmacological effect of verapamil when cimetidine is coadministered.

Administration, Oral↗