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Biomedical subjects

W Kalow

Publications and source records attributed to W Kalow.

At least 199 records · Page 11Linked to original sources
PubMedDate not supplied

Pharmacogenetics and pharmacogenomics: origin, status, and the hope for personalized medicine.

Pharmacogenetics arose with studies of single genes, which had major effects on the action of particular drugs. It turned into pharmacogenomics through realization that the controls of most drug responses are multifactorial. Then, variable gene expression posed new problems, for example what do drugs do to genes, or how useful is any genetic pretesting of a person? A common disease may be caused by different groups of genes in different people, who therefore require different drugs for treatment. Personalized medicine is currently represented by a physician's attention to a patients age, sex, or ethnic background, that is groups showing smaller genetic variation than is typical for general humanity. Occasionally, there is also the use of single-gene pretesting of a patient before drug administration. Over time, improvements in multigenic testing promise to increase the role of personalized medicine. However, the many pharmacogenomic complexities, and particularly time-dependent changes of gene expression, will never allow personalized medicine to become an error-free entity.

Biomarkers↗
PubMedDate not supplied

Ethnic differences in drug metabolism.

Interethnic differences in drug-metabolising capacity may be substantial, and they are sufficiently frequent to warrant attention. Such differences may consist of different mean values of quantitative traits in separate populations, or of different frequency distributions as produced by the occurrence of genetic enzyme variants. The collection of population data requires the investigation of substantial numbers of subjects. This may be no problem if drug-metabolising enzymes occur in blood or are sufficiently stable in their tissues to allow investigation in vitro. However, if investigations require the use of probe drugs, new efforts are needed to adapt pharmacokinetic methods to make them suitable for population studies. This development of methods is further called for because genetic variants seem to be more easily detected through the assessment of particular metabolites than through the determination of pharmacokinetic parameters of the parent drug. Many studies with probe drugs comparing different populations have given results that are equivocal in terms of the nature-nurture interplay. However, a set of data with antipyrine has pointed to environmental factors as the principal determinant of differences in metabolising capacity, while data with debrisoquine have indicated monogenically controlled variation of one facet of the cytochrome P-450 system. In several instances, statistically significant differences between population means have been established by testing small numbers of subjects, numbers insufficient to establish distribution patterns that would allow the recognition of genetic polymorphism. The populations studied range from Greenlanders to South African Blacks, but most comparisons pertain to Caucasians and Orientals.

Acetylation↗
PubMedDate not supplied

N-hydroxyamobarbital: the second major metabolite of amobarbital in man.

After oral administration of 14C-labeled amobarbital to healthy subjects, most of the radioactivity was recovered in urine and only 4-5% in feces over a period of 6 days. No unchanged amobarbital was excreted. Two major metabolites were found and isolated. One was 3'-hydroxyamobarbital, which has been previously identified by Maynert. The second could be identified as N-hydroxyamobarbital on the basis of its spectral and chemical properties.

Amobarbital↗
PubMedDate not supplied

Catalytic and immunologic similarities between monkey and human liver cytochrome P-450db1 (human cytochrome P-450 2D6).

In vivo pharmacogenetic studies have suggested that the monkey may be an animal model for the human polymorphism of cytochrome P-450 2D6 (also called cytochrome P-450db1). In the present study, the catalytic, immunologic, and electrophoretic properties of cytochrome P-450db1 in liver microsomes from African green monkeys (Cercopithecus aethiops) were examined and compared with P-450db1 in human liver microsomes. Using sparteine as the substrate, the activity of microsomal P-450db1 from the two sources was indistinguishable in terms of the pattern of sparteine metabolites produced, the apparent Ki values of 8 competitive inhibitors (r = 0.94, p less than 0.001), and the extent of immunoinhibition by anti-rat P-450db1 antibody. Kinetic analyses demonstrated that the apparent KM values of the high affinity component of sparteine oxidation in monkey liver microsomes fell within the range observed in human livers; the Vmax of this component was as much as six times greater than the highest value reported for human liver. Western immunoblots showed a protein band in monkey liver microsomes that co-migrated with P-450db1 in human liver. The high degree of similarity observed here between P-450db1 of monkey and human liver microsomes suggests that the monkey will be a good animal model for P-450db1 enzyme studies, and possibly for studies of the role of this enzyme in drug abuse and dependence.

Animals↗
PubMedDate not supplied

N-Hydroxylation of pentobarbital in man.

After oral administration of 14C-labeled pentobarbital to healthy subjects, most ot the radioactivity was recovered in urine over a period of 6 days. Only a minute amount (approximately 1%) of unchanged pentobarbital was found in the urine. Four major metabolites were found and isolated. One was 3'-hydroxypentobarbital, which has been previously identified by Maynert. The second could be identified as N-hydroxypentobarbital on the basis of its spectral and chemical properties. The other two metabolites were not identified.

Chromatography, Thin Layer↗
PubMedDate not supplied

Evidence in humans for variant allozymes of the nondeficient sparteine/debrisoquine monooxygenase (P45OIID 1) in vitro.

Sparteine oxidation is part of a genetic polymorphism that affects the metabolism of many drugs and is under monogenic control. By examining the sparteine oxidation kinetics and the ratio of the dehydrogenated metabolites and through the use of the potent inhibitor, quinidine, two sites of metabolism were found for all 10 of the livers studied. The mean Km (N = 10) for the quinidine-sensitive enzyme is 73 +/- 46 (SD) microM and the mean Vmax is 4.51 +/- 4.16 nmol/mg microsomal protein/30 min, indicating a large interindividual variation. Because the polymorphic defect is due to at least three variants of a mRNA splicing error with consequent lack of enzyme formation [Gonzalez et al.: Nature 331, 442 (1988)], the variation that we observed in Km is most likely due to variation of allozymes from extensive metabolizer alleles. The low affinity enzyme also demonstrates a large interindividual variation, is not competitively inhibited by quinidine, and produces a higher ratio of 5-dehydrosparteine to 2-dehydrosparteine than the high affinity enzyme. This low affinity enzyme must be part of a separate enzyme system from that controlling the sparteine/debrisoquine polymorphism because of its different characteristics and the 100% frequency with which it is found in the livers. The two dehydrosparteine metabolites are thought to be formed by the spontaneous breakdown of a primary metabolite. The different ratio of these two dehydrosparteines, which was found at low and high substrate concentrations, suggests that the reactions producing the primary metabolite are different between the quinidine-sensitive and -insensitive enzymes.

Cytochrome P-450 CYP2D6↗
PubMedDate not supplied

In vitro studies of human liver alcohol dehydrogenase variants using a variety of substrates.

Alcohol dehydrogenase (ADH) is genetically polymorphic, and large differences in allele frequencies exist between the major human races. Genetic variants at the ADH2 gene locus include the beta 2-ADH ("atypical" ADH) present in 85% of Orientals and the beta 1-ADH ("normal" ADH) present in 85 to 95% of whites. Although the presence of one or the other of these ADH variants does not significantly affect the rate of ethanol oxidation in the living subject, it may affect that of other substrates. The overall objective of this work was to screen in vitro for ADH substrates which might be differentially metabolized by these ADH2variants in living subjects. In an in vitro screening method using autopsy livers at pH 8.5, the formation or disappearance of NADH at 340 nm was measured before and after exposure to 4-methylpyrazole, an ADH-specific competitive inhibitor. The screening test revealed three new substrates and suggested that alcohol substrates fall into two groups. The majority of substrates belonged to a group which was oxidized at a significantly lower rate by the beta 2-ADH as compared to the beta 1-ADH, but this was not the case for a small group which included ethanol. Subsequent kinetic studies of selected alcohols tended to indicate a uniqueness of ethanol kinetics in that both KM and Vmax favored its oxidation by beta 2-ADH rather than by the beta 1 variant. None of the other seven tested alcohols showed a similar differential. Also, reduction of aldehydes and ketones tended to be moderately slower by beta 2- than by beta 1-ADH.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMedDate not supplied

Mephenytoin metabolism in vitro by human liver.

Human liver was used in investigations of mephenytoin p-hydroxylase, the enzyme presumably responsible for the genetic polymorphism in mephenytoin metabolism. A gas chromatographic assay method was developed to measure p-hydroxylation and N-demethylation which is the other major metabolic pathway. Both reactions were localized in the microsomal fraction and required NADPH. Inhibition of p-hydroxylation by CO, SKF 525-A, and metyrapone was demonstrated. It was concluded that a form of cytochrome P-450 catalyzes the reaction. The velocity of N-demethylation in human liver did not show saturation even at 500 microM substrate concentration. The p-hydroxylation, however, followed Michaelis-Menten kinetics. The Km, determined in five different livers, ranged from 59 to 143 microM. The linearity in Eadie-Hofstee plots was consistent with the involvement of a single catalytic site.

Humans↗
PubMedDate not supplied

In vitro inhibition studies of two isozymes of human liver cytochrome P-450. Mephenytoin p-hydroxylase and sparteine monooxygenase.

Human liver preparations were used to screen various drugs for their capability of binding to mephenytoin p-hydroxylase and sparteine monooxygenase, two cytochrome P-450-catalyzed activities that are independently heritable. For this screening, any indication of competitive inhibition by the drug was interpreted as an indication of binding. Among 64 drugs and alkaloids tested, 24 compounds caused inhibition of mephenytoin p-hydroxylation but the inhibition was weak in most cases; by contrast, 40 of the 64 compounds inhibited sparteine oxidation, the inhibition being potent in many cases. The only fairly strong inhibitors of mephenytoin p-hydroxylation were the alkaloid papaverine and the monoamine oxidase inhibitors tranylcypromine and nialamide. The results of these inhibition studies confirm the independence of the two monogenic defects observed in different populations. Metabolism is possibly altered in poor metabolizers of mephenytoin with fewer drugs than in poor metabolizers of sparteine.

Alkaloids↗
PubMedDate not supplied

Biotransformation of caffeine, paraxanthine, theophylline, and theobromine by polycyclic aromatic hydrocarbon-inducible cytochrome(s) P-450 in human liver microsomes.

The microsomal metabolism of caffeine and its primary dimethylxanthine metabolites, paraxanthine, theophylline, and theobromine, was investigated in 15 different human livers, including those from two known nonsmokers and one known smoker. At least two distinct enzymes with differing substrate affinities have the potential to catalyze most methylxanthine N-demethylations and C8-hydroxylations in vitro; however, at the low methylxanthine concentrations routinely encountered in vivo, participation by the high affinity site is expected to predominate. It appears that the high affinity enzyme is a polycyclic aromatic hydrocarbon-inducible isozyme of cytochrome P-450, based on competitive inhibition by 7-ethoxyresorufin and benzo[a]pyrene, and based on a significant (p less than 0.001) correlation between 7-ethoxyresorufin-O-deethylation and methylxanthine demethylation rates. alpha-Naphthoflavone inhibited all methylxanthine demethylations in excess of 80% in two high activity livers, whereas 8-hydroxylations were generally inhibited less. Kinetic analysis of paraxanthine 7-demethylation in four different liver preparations resulted in similar Km values of 1.2 +/- 0.5 mM (mean +/- SD), whereas Vmax values varied 8-fold, compatible with participation by the same high affinity isozyme. Notable was the high degree of inter-liver variation in metabolic rates, with the known smoker showing the second highest activity among a 20-fold range in paraxanthine demethylation rates, consistent with polycyclic aromatic hydrocarbon-related enzyme induction. Maximal inhibition of paraxanthine 8-hydroxylation by alpha-naphthoflavone left similar residual activities in the 15 liver preparations, indicating the presence of an enzyme activity that was not inducible. Furthermore, in low activity livers, more than 80% of paraxanthine 8-hydroxylation was mediated by an isozyme of cytochrome P-450 insensitive to inhibition by alpha-naphthoflavone. Our in vitro data show that the proportion of demethylation relative to hydroxylation products of paraxanthine correlate with 7-ethoxyresorufin O-deethylation rates. Taken together, the data provide a rationale for a potential in vivo marker of polycyclic aromatic hydrocarbon-inducible cytochrome P-450 activity based on a urinary metabolite ratio of paraxanthine 7-demethylation to 8-hydroxylation products after caffeine intake.

Adult↗
PubMedDate not supplied

Metabolism of diazepam in vitro by human liver. Independent variability of N-demethylation and C3-hydroxylation.

The interindividual variability of diazepam metabolism was studied using human livers. The formation of N-desmethyldiazepam (NDZ) and 3-hydroxydiazepam (temazepam, TMZ), was monitored by gas chromatography. In 10 livers, Km values for NDZ and TMZ formation varied independently from each other, each by a factor of 4, from 100 to 400 microM. This variability is consistent with the presence of at least two different cytochrome P-450 species controlling formation of these metabolites. In the same livers, Vmax for NDZ and TMZ production varied 6-fold and 15-fold, respectively.

Anti-Anxiety Agents↗
PubMedDate not supplied

Metabolic fate of phenobarbital in man. N-Glucoside formation.

1-(beta-D-Glucopyranosyl)phenobarbital was identified as the major metabolite of phenobarbital in man. Proof of structure was based on the comparison of the UV, NMR, and mass spectrometry and TLC data for the acetylated metabolite with an authentic compound. The previous erroneous structure assignment of this metabolite as N-hydroxyphenobarbital was based on insufficient data. After oral administration of 14C-labeled phenobarbital to two healthy male subjects, most of the radioactivity (87 and 78% of the dose) was recovered in urine over a period of 16 days. The N-glucopyranoside, p-hydroxyphenobarbital, and unchanged phenobarbital accounted for 30 and 24%, 18 and 19%, and 33 and 25% of the dose, respectively.

Acetylation↗