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Kristian Linnet

Publications and source records attributed to Kristian Linnet.

13 recordsLinked to original sources
PubMed2006

[Forensic chemistry: analytical developments and pharmacological aspects].

Analytical development within the field of forensic chemistry has been dominated by the increasing application of mass spectrometry techniques combined with gas or liquid chromatography, all of which improve analytical specificity and sensitivity. The impact of accreditation of standardized method validation is another important aspect. Pharmacokinetic interactions and genetic variation of the cytochrome P450 system are considered in relation to the risk of intoxication with psychoactive drugs. The role of P-glycoprotein in the distribution of drugs over the blood-brain barrier is highlighted. In conclusion, epidemiological trends in drug addiction are touched upon, and newer approaches, such as testing of hair, are considered.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2006

The exponentially weighted moving average (EWMA) rule compared with traditionally used quality control rules.

BACKGROUND: The exponentially moving average (EWMA) rule for internal quality control is a well-known type of control rule in industry. Here, the power of the EWMA rule is evaluated to outline the potential of this type of control rule in clinical chemistry. METHODS: Using simulations, the power of the EWMA rule was explicitly compared with that of commonly used rules in clinical chemistry. The type I error levels were standardized to common values to achieve unbiased comparisons. RESULTS: For small to moderately large errors (systematic errors up to 2-3 standard deviations), the EWMA rule outperforms simple rules (N=1) and multi-rules (N=2-6). For example, for a systematic error of 2s, the EWMA rule equivalent to the 1(3s) rule has a power of 0.30, whereas the 1(3s) rule only displays a power of approximately 0.15. For N=4, comparison was carried out with the 1(3s)/2(2s)/R(4s)/4(1s) rule. Here the common type I error level is 0.017. At all error levels, the EWMA rule is superior to the multi-rule. For example, given a 1s systematic error, the EWMA rule has a power (0.4) of twice the value of the multi-rule (0.2). CONCLUSION: The EWMA rule is an efficient control rule with regard to systematic errors that should be considered for general application in the field of clinical chemistry.

Chemistry, Clinical↗
PubMed2006

The influence of P-glycoprotein on cerebral and hepatic concentrations of nortriptyline and its metabolites.

The impact of P-glycoprotein (P-gp) on the distribution of nortriptyline (NT) and its metabolites between brain, liver and serum was studied experimentally. The interaction of NT with P-gp in vitro was confirmed by measurement of P-gp stimulated ATPase activity (Km = 257.6 microM, Vmax = 51.0 nmol phosphate released/mg protein-min). Administration of NT (5 mg/kg, s.c.) to mdrla knockout mice resulted in enhanced brain-serum (1.6-fold, p = 0.012) and liver-serum (1.4-fold, p = 0.019) ratios, as compared to the wild-type mice. For a series of NT doses (2.5, 5, 10, 25, 30 mg/kg, i.p.) inhibition of P-gp with cyclosporine A (CsA, 200 mg/kg, i.p.) in rats led to NT brain- and liver-serum ratios that were on average 1.3- (p = 0.005) and 2.1- (p = 0.001) fold higher than those of the controls, respectively. Verapamil (50 mg/kg) (NT, 5 mg/kg) increased the ratios by a factor of 1.6 (p <0.001) and 10.3 (p <0.001) for brain and liver, respectively. Finally, co-administration of methadone (1 mg/kg) did not alter the brain-serum ratio of NT, but in the liver a slight increase (1.5-fold, p = 0.035) was observed. In conclusion, verapamil yielded complete inhibition of P-gp at the blood-brain barrier and CsA had an effect corresponding to about 50% inhibition. The results show that P-gp influences the penetration of NT into the brain, and that drug-drug interactions may take place.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2006

In vitro studies on quetiapine metabolism using the substrate depletion approach with focus on drug-drug interactions.

The metabolism of the atypical antipsychotic quetiapine was investigated by in vitro methods. Pharmacokinetic parameters were determined in human liver microsomes and recombinant cytochrome P450 measuring substrate depletion and product formation. The cytochrome P450 isozymes CYP3A4 and CYP2D6 displayed activity towards quetiapine. The isozyme CYP2D6 played a minor role in the metabolism of quetiapine as CYP3A4 contributed 89% to the overall metabolism. A Km value of 18 microM was determined by substrate depletion, suggesting linear kinetics under therapeutic conditions. Drugs known to inhibit CYP3A4, such as ketoconazole and nefazodone, displayed almost complete inhibition at low concentrations, whereas inhibitors of CYP2D6 do not seem to have a clinically relevant effect.

Antipsychotic Agents↗
PubMed2005

Influence of P-glycoprotein inhibition on the distribution of the tricyclic antidepressant nortriptyline over the blood-brain barrier.

The distribution of the antidepressant drug nortriptyline (NT) and its main metabolite E-10-hydroxy-nortriptyline (E-10-OH-NT) across the blood-brain barrier was considered in relation to inhibition of the multidrug transporter P-glycoprotein (P-gp). Rats received NT in doses of 25 mg/kg orally, 10 mg/kg i.p. or 25 mg/kg i.p. Half the rats were treated with the P-glycoprotein inhibitor cyclosporine A (CsA) (200 mg/kg) 2 h prior to NT administration, and the other half served as a control group. NT and the metabolite were extracted from brain and serum by liquid-liquid extraction and analysed by HPLC with UV-detection. The brain to serum ratio of NT was increased in the CsA treated groups (22.3-26.8) compared with the control groups (16.5-22.7), the difference being statistically significant in two of the three experiments (p<0.05). Increased brain-serum ratios were also found for E-10-OH-NT, but the differences were not statistically significant. These results suggest that inhibition of P-gp by CsA increases the accumulation of NT in the brain. Administration of the antipsychotic drug risperidone (0.5 mg/kg s.c.), which is a P-gp substrate, instead of CsA did not exert any measurable influence on the blood-brain ratio of NT concentrations. In conclusion, the results show that drug-drug interaction at P-gp may influence the intracerebral NT concentration, but apparently, a major inhibition of P-gp is necessary to attain a measurable effect.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2005

P-glycoprotein interaction with risperidone and 9-OH-risperidone studied in vitro, in knock-out mice and in drug-drug interaction experiments.

The drug transporter P-glycoprotein (P-gp) influences drug distribution across the blood-brain barrier (BBB) by actively extruding drugs into the neural capillaries. Several psychotropic drugs, including nortriptyline (NT) and risperidone (Risp), are substrates of P-gp. Here we compared the in vitro P-gp interactions of Risp and its major metabolite, 9-OH-Risperidone (OH-Risp), with their distribution over the BBB in P-gp knock-out mice and in rats where P-gp was inhibited. K(m) and V(max) were determined by an in vitro ATPase assay, and V(max)/K(m) ratios of 2.7 and 0.5 were recorded for Risp and OH-Risp, respectively, suggesting that Risp is a better substrate for P-gp than OH-Risp. In Mdr1a (-/-) knock-out mice, the brain-serum ratios of both Risp and OH-Risp were more than ten times those of control mice (14 and 11, respectively). When P-gp was inhibited with cyclosporine A (CsA) in Wistar rats, the effect was an order of magnitude less than that observed for the knock-out mice experiments (1-1.5 times the controls), and co-administration of NT had no effect. In conclusion, both Risp and OH-Risp interact with P-gp in vitro, and P-gp has a profound effect on Risp and OH-Risp distribution over the BBB, as is evident from the knock-out mice experiments. Drug-drug interaction effects in relation to P-gp, however, appear to be more limited.

ATP Binding Cassette Transporter, Subfamily B↗
PubMed2005

Estimation of the limit of detection with a bootstrap-derived standard error by a partly non-parametric approach. Application to HPLC drug assays.

A recently proposed procedure for estimating the limit of detection (LoD) of an assay using a partly non-parametric approach is applied to five HPLC drug assays. The non-parametrically determined 95th percentile of the blank measurements (LoB) is obtained as the value of the N (95/100) + 0.5 ordered observations. The LoD is the lowest level of analyte that is likely to yield a measured result exceeding the LoB. The LoD equals LoB + c(beta) x SD(S), where SD S is the analytical SD at low concentrations, and c(beta) = z(1-beta)/(1-1/(4 x f )) (f = degrees of freedom). c(beta) approximately 1.65 for a type II error of 5%. The blank distributions deviated significantly from normality for four of the five assays because of skewness to the right. The estimated LoB ranged from 3.3 to 10.2 nmol/L. The LoDs were in the interval from 7.8 to 17.2 nmol/L. A new procedure for estimation of the standard error (SE) of the estimated LoD that is partly based on the bootstrap principle showed good performance in simulation studies. In conclusion, the partly non-parametric procedure for estimation of the LoD of typical HPLC drug assays was found to be a suitable approach.

Antidepressive Agents↗
PubMed2004

Partly nonparametric approach for determining the limit of detection.

BACKGROUND: According to recent International Organization for Standardization (ISO) standards, the limit of detection (LoD) of an assay should be estimated taking both type I (alpha) and II (beta) errors into account. The suggested procedure, however, supposes gaussian distributions of both blank and sample measurements and a linear calibration curve. In clinical chemistry, asymmetric, nongaussian blank distributions are common, and the calibration curve may be nonlinear. We present a partly nonparametric procedure that takes these aspects into account. METHODS: Using theoretical distribution models and simulation studies, we developed a LoD estimation procedure suitable for the field of clinical chemistry that is partly based on nonparametric statistics. RESULTS: For sample size n, the nonparametrically determined 95th percentile of the blank measurements obtained as the value of the [n(95/100) + 0.5]th ordered observation defines the limit for results significantly exceeding zero [limit of blank (LoB)]. The LoD is the lowest value that is likely to yield a result exceeding the LoB. LoD is estimated as: LoB + cbeta x SDS, where SDS is the analytical SD of a sample with a low concentration; cbeta = z(1 - beta)/[1 - 1/(4 x f)]; z(1 - beta) is the standard normal deviate; and f is the number of degrees of freedom for estimation of SD(S). c(beta) is approximately equal to 1.65 for a type II error of 5%. Approaches and needed tabular values for calculation of confidence limits are presented as well as sample size. Worked examples are given to illustrate estimation and verification of the limit of detection. Simulation results are used to document performance. CONCLUSION: The proposed procedure appears useful for application in the field of clinical chemistry and promotes a standardized approach for estimating LoDs of clinical chemistry assays.

Calibration↗
PubMed2004

In vitro microsomal metabolism of imipramine under conditions mimicking the in vivo steady-state situation.

A steady-state model is presented of the metabolism of imipramine. In this model, the N-demethylation and 2-hydroxylation rates of imipramine were measured with and without the presence of metabolites by using a tritium isotope of imipramine. At an imipramine concentration of 5 micromol/l, desipramine, in the concentration ratio 1:2, decreased the total metabolic rate by 70%. The 2-hydroxylation pathway was mainly inhibited, thereby increasing the N-demethylation pathway from 25% to 62% in the presence of desipramine. The additional presence of 2-hydroxy-imipramine did not change this situation. A study on the relative influence of CYP1A2 and 3A4 only revealed minor changes in the presence of desipramine. In conclusion, the presence of metabolites in metabolism studies undertaken in vitro may reflect the changes from the single- to the multiple-dose situation observed clinically and therefore constitute a better model for the clinical application of a drug.

Cytochrome P-450 Enzyme System↗
PubMed2004

Quetiapine serum concentrations in psychiatric patients: the influence of comedication.

Steady-state serum concentrations of quetiapine were recorded in 62 psychiatric patients under routine conditions. Doses were administered twice daily, and serum quetiapine levels were measured in the morning about 12 hours after the last dose. Eight patients were in monotherapy, whereas the rest received various additional psychotropic drugs. For the whole group, the concentration-to-dose ratio (C/D) varied 238-fold, with a median value of 0.41 nmol/L/(mg/24 h). For the administered dose range (37.5-1200 mg/24 h), the serum concentrations ranged from below the detection limit (10 nmol/L) to 999 nmol/L. With the exception of 2 subjects receiving carbamazepine and patients receiving quetiapine outside the recommended dose interval, 80% of the rest had serum levels within the range 50 to 650 nmol/L, which may serve as an orienting interval for serum concentrations observed under routine treatment conditions. Patients (n = 38) comedicated with drugs competing for metabolism by CYP3A4 displayed a median C/D value of 0.48 nmol/L/(mg/24 h), which was 70% higher than the C/D value of the monotherapy group [0.28 nmol/L/(mg/24 h)], in contrast to patients receiving drugs metabolized by CYP2D6 with a median C/D of 0.23 nmol/L/(mg/24 h). None of the comedicated groups were significantly different from the monotherapy group. Two subjects comedicated with carbamazepine had very low C/D values [0.02-0.04 nmol/L/(mg/24 h)].

Adult↗
PubMed2002

Glucuronidation of olanzapine by cDNA-expressed human UDP-glucuronosyltransferases and human liver microsomes.

Olanzapine is a widely used, newer antipsychotic agent, which is metabolized by various pathways: hydroxylation and N-demethylation by cytochrome P450, N-oxidation by flavin monooxygenase and direct glucuronidation. In vivo studies have pointed towards the latter pathway as being of major importance. Accordingly, the glucuronidation reaction was studied in vitro using cDNA-expressed human UDP-glucuronosyltransferase (UGT) enzymes and a pooled human liver microsomal preparation (HLM). Glucuronidated olanzapine was determined by HPLC after acid or enzymatic hydrolysis. The following UGT-isoenzymes were screened for their ability to glucuronidate olanzapine: 1A1, 1A3, 1A4, 1A6, 1A9, 2B7 and 2B15. Only UGT1A4 was able to glucuronidate olanzapine obeying saturation kinetics. The K(m) value was 227 micromol/l (SE 43), i.e. of the same order of magnitude as for other psychotropic drugs, and the V(max) value was 2370 pmol/(min mg) (SE 170). Glucuronidation was also mediated by the HLM preparation, but a saturation level was not reached. The olanzapine glucuronidation reaction was inhibited by several drugs known as substrates for UGT1A4, e.g. amitriptyline, trifluoperazine and lamotrigine. Thus, competition for glucuronidation by UGT1A4 represents a possibility for drug-drug interactions in subjects receiving several of these psychotropic drugs at the same time. Whether such possible interactions are of any clinical importance may await further studies in patients.

Benzodiazepines↗
PubMed2002

Free and glucuronidated olanzapine serum concentrations in psychiatric patients: influence of carbamazepine comedication.

The influence of carbamazepine on the glucuronidation of the antipsychotic olanzapine was studied in a group of psychiatric patients. Steady-state serum concentrations of free and glucuronidated olanzapine were measured in 31 psychiatric patients in monotherapy (dose range, 2.5-30 mg/d; median, 15 mg/d) and in 16 patients being comedicated with carbamazepine (dose range, 5-50 mg/d; median, 20 mg/d). The concentrations were determined by HPLC with and without acid hydrolysis of glucuronidated olanzapine. For the monotherapy group, the concentrations of free and glucuronidated olanzapine ranged from 0 nmol/L to 292 nmol/L (median, 94 nmol/L) and from 0 nmol/L to 180 nmol/L (median, 27 nmol/L), respectively. The serum concentrations of the carbamazepine-treated group ranged from 21 nmol/L to 310 nmol/L (median, 81 nmol/L) and from 0 to 376 nmol/L (median, 57 nmol/L) for free and glucuronidated olanzapine, respectively. Two patients with outlying values were excluded from further analysis. The median concentration-to-daily dose ratios (C/D) of free and glucuronidated olanzapine in the monotherapy group were 5.8 nmol/L/mg and 2.2 nmol/L/mg, respectively (n =30). The corresponding values for the group comedicated with carbamazepine were 3.6 and 3.1 nmol/L/mg (n =15). Thus, the median C/D of free olanzapine in the carbamazepine group was 38% lower than that of the monotherapy group (P <0.01), confirming that carbamazepine accelerates the metabolism of olanzapine. Further, for the carbamazepine group the median glucuronidated olanzapine fraction constituted 79% of the free fraction compared with 43% for the monotherapy group (P <0.01), which suggests that an increased rate of olanzapine glucuronidation contributes to the increased rate of metabolism of olanzapine induced by carbamazepine.

Adult↗