Contributions of five human cytochrome P450 isoforms to the N-demethylation of clozapine in vitro at low and high concentrations

The authors assessed the in vitro contribution of cytochrome P450 (CYP) isoforms 1A2, 3A4, 2C9, 2C19, and 2D6 to the N-demethylation of clozapine mediated by human liver microsomal preparations (HLM). In contrast to previous studies, the authors focused on a relatively low hepatic concentration level, 5 microM, to assess the conditions at a therapeutically relevant hepatic concentration level of clozapine. The optimal concentrations of specific inhibitors were initially established using cDNA-expressed CYP isoforms. The mean contributions of CYPs 1A2, 2C19, 3A4, 2C9, and 2D6 amounted to 30%, 24%, 22%, 12%, and 6%, respectively, with regard to the total HLM-mediated N-demethylation. Thus, the present in vitro study on clozapine N-demethylation suggests that CYP1A2 is the most important form at low concentrations, which is in agreement with clinical findings. CYP2C19 is also of considerable importance, while the roles of CYP2C9 and 2D6 are more modest. CYP3A4 attained a dominating role with an average contribution of 37% at a high clozapine concentration (50 microM). The rate of other metabolic routes mediated by CYP2D6 only corresponded to about one fifth of the CYP2D6 catalyzed N-demethylation rate.

Estimation of the linear relationship between the measurements of two methods with proportional errors

The linear relationship between the measurements of two methods is estimated on the basis of a weighted errors-in-variables regression model that takes into account a proportional relationship between standard deviations of error distributions and true variable levels. Weights are estimated by an interative procedure. As shown by simulations, the regression procedure yields practically unbiased slope estimates in realistic situations. Standard errors of slope and location difference estimations are derived by the jackknife principle. For illustration, the linear relationship is estimated between the measurements of two albumin methods with proportional errors.

Studies on the stereoselective metabolism of citalopram by human liver microsomes and cDNA-expressed cytochrome P450 enzymes

The involvement of CYP enzymes in the metabolism of citalopram was studied, inclusive the conversion of demethylcitalopram to didemethylcitalopram and the formation of citalopram N-oxide, which both have not been considered previously. Using human mixed liver microsomes and cDNA-expressed CYP enzymes, we confirmed that CYP3A4, 2C19 and 2D6 are involved in the first demethylation step of citalopram, all favouring conversion of the biologically active S-enantiomer. Inhibitor studies indicated that at therapeutic citalopram concentrations CYP3A4 was responsible for 40-50% of demethylcitalopram formation, while the contribution of CYP2C19 increased and that of CYP2D6 tended to decrease with increasing drug concentration. CYP2D6 exclusively mediated the second demethylation step, and citalopram N-oxide was also exclusively formed by CYP2D6. None of the studied CYP enzymes mediated deamination to the propionic acid derivative.

Olanzapine serum concentrations in psychiatric patients given standard doses: the influence of comedication

We recorded steady state serum concentrations of olanzapine in 56 psychiatric patients under routine conditions. Twenty-two patients (39%) underwent monotherapy; the rest received additional psychotropic drugs. Doses were given once daily in the evening, and serum olanzapine levels were measured 12 hours later. For the whole group, the concentration-to-dose ratio (C/D) varied 26-fold, with a median value of 4.8 (nmol/L)/(mg/24 hours), but 80% of the patients had C/D values within the range 2 to 10 (nmol/L)/(mg/24 hours). All but three patients received standard doses (5-20 mg/24 hours), of whom 80% had serum concentrations of olanzapine within the range 22 to 146 nmol/L. Patients comedicated with potential inhibitors of CYP2D6 and other drugs displayed a median C/D approximately 40% higher than the group undergoing monotherapy. Patients comedicated with carbamazepine had a median C/D 36% lower than that of the monotherapy group. Therefore, the serum concentration range (12-hour values) of 25 to 150 nmol/L can be expected for patients receiving a standard daily dose.

Serum concentrations and side effects in psychiatric patients during risperidone therapy

Steady state serum concentrations of risperidone and 9-hydroxyrisperidone (9-OH-risperidone), the active moiety, were measured in 42 patients. The concentration-to-dose ratios (C/D) varied by a factor of 20, from 1.8 to 36.8 (nmol/l)/(mg/24 hours), and 90% of the active moiety was constituted of 9-OH-risperidone. No correlation between the serum concentration of the active moiety and the side effects evaluated by the UKU Side Effect Scale was found. The absence of CYP2D6 (poor metabolizers) or the coadministration of drugs other than benzodiazepines increased the ratio between parent compound and metabolite but did not significantly influence the C/D of the total active moiety. A therapeutic range for serum risperidone has not been established, but 6 mg/day is considered the optimum dose for most patients. The authors found that in 90% of 22 patients administered 6 mg/day risperidone, the serum levels were within 50 to 150 nmol/l.

Two-stage transformation systems for normalization of reference distributions evaluated

In two-stage transformation systems for normalization of reference distributions, the asymmetry is first corrected, and any deviation of kurtosis is then adjusted. The simulation studies reported here show that these systems have previously been assessed too optimistically because the sample variation of the transformation parameters was neglected. Applying a goodness-of-fit test to transformed values shows that one should accept gaussianity only for p-values greater than 0.15 instead of those greater than 0.05. Further, the calculated 90% confidence intervals of reference limits should be expanded by 25%. When the correct level of significance is used, only real reference distributions that deviate moderately from the gaussian form are normalized. Calculation of confidence intervals demonstrated that 50 to 450 subjects are needed for a precise parametric estimation of the 95% reference interval. For the nonparametric approach, 125 to 700 reference subjects are necessary. The larger sample sizes are needed when distributions show pronounced skewness.

Determination of olanzapine in serum by high-performance liquid chromatography using ultraviolet detection considering the easy oxidability of the compound and the presence of other psychotropic drugs

A method for determination of the atypical neuroleptic drug olanzapine in serum was developed. After a single-step liquid-liquid extraction, the compound was separated from other constituents on a normal-phase silica gel column using a buffer-methanol mobile phase and measured by UV absorption at 270 nm. Addition of 0.25% ascorbic acid to serum protects olanzapine against oxidation during extraction and stabilizes the easily oxidised compound during storage. Inter-day variation was <8% at serum levels found in olanzapine treated patients. Analytical interference from coadministered psychoactive drugs and their metabolites were studied. Only risperidone, also a relatively newly developed antipsychotic drug, interfered, but the most commonly used antidepressants and traditional antipsychotics and their metabolites did not interfere.

Metabolism of the tricyclic antidepressant amitriptyline by cDNA-expressed human cytochrome P450 enzymes

The metabolism of amitriptyline was studied in vitro using cDNA-expressed human cytochrome P450 (CYP) enzymes 1A2, 3A4, 2C9, 2C19, 2D6 and 2E1. CYP 2C19 was the most important enzyme with regard to the demethylation of amitriptyline, the quantitatively most important metabolic pathway. CYP 1A2, 3A4, 2C9 and CYP 2D6 also participated in the demethylation of amitriptyline. CYP 2D6 was the sole enzyme mediating the hydroxylation of amitriptyline, and (E)-10-OH-amitriptyline was exclusively produced. CYP 2E1 did not metabolize amitriptyline. Concerning the quantitative relations, CYP 2C19 and 2D6 exhibited high affinities with Km values in the range of 5-13 mumol/l, whereas the affinities of 1A2, 3A4 and 2C9 were somewhat lower with Km values ranging from 74 to 92 mumol/l. CYP 2C19 displayed the highest reaction capacity per mole with Vmax equal to 475 mol h-1 (mol CYP)-1. The other enzymes had Vmax values in the range of 90-145 mol h-1 (mol CYP)-1. Allowing for the typical relative distribution of amounts of CYP enzymes in the liver, a simulation study suggested that, at therapeutic doses, on average about 60% of the metabolism depended on CYP 2C19. At toxic doses, CYP 2C19 is expected to be saturated, and CYP 3A4 may now play a dominant role in the metabolism.

Simplified high-performance liquid chromatographic method for determination of risperidone and 9-hydroxyrisperidone in serum from patients comedicated with other psychotropic drugs

A HPLC method was developed for determination of risperidone and its therapeutically active main metabolite 9-hydroxyrisperidone in serum. After a single-step liquid-liquid extraction the analytes were separated on a C18 column and measured by UV detection at 280 nm. Inter-day coefficient of variation was <7% for both compounds at serum levels occurring in patients treated with ordinary doses. Studies of analytical interference showed that the most commonly coadministered antidepressants and benzodiazepines did not interfere. Some conventional low dose neuroleptics and clozapine did interfere, but this is of minor importance, because risperidone is intended as an alternative to these drugs.

Comparison of quantitative diagnostic tests: type I error, power, and sample size

For a quantitative laboratory test the 0.975 fractile of the distribution of reference values is commonly used as a discrimination limit, and the sensitivity of the test is the proportion of diseased subjects with values exceeding this limit. A comparison of the estimates of sensitivity between two tests without taking into account the sampling variation of the discrimination limits can increase the type I error to about seven times the nominal value of 0.05. Correct statistical procedures are considered, and the power and required sample size are studied for Gaussian and log-Gaussian distributions of diagnostic test values. The results may be useful for the planning phase of studies to evaluate quantitative diagnostic tests.

A review on the methodology for assessing diagnostic tests

Evaluation of diagnostic tests by the following principles are reviewed: error rates, scores based on posterior probabilities, and the excess loss considered in a decision theoretic context. Error rates or the complementary non-error rates, specificity and sensitivity, are simple measures which provide a rough indication of the discriminative value. In clinical practice, where a test serves as a decision support together with other information, conversion of test results to posterior probabilities is recommended. An aggregate score of these probabilities expresses the value of the test. Finally, in simple, well-defined cases--for example, screening situations, where the prevalence of disease and the relative consequences of false-positive and -negative classifications can be estimated--a Bayesian decision analysis is appropriate. The optimal discrimination limit is selected, and the total loss is minimized. The likelihood ratio LR(x) plays a central role in probability calculations and in the decision analysis. An example illustrates application of the procedures.

Fluvoxamine-Clozapine drug interaction: inhibition in vitro of five cytochrome P450 isoforms involved in clozapine metabolism

Administration of fluvoxamine to patients receiving clozapine therapy may increase the steady-state serum concentrations of clozapine by a factor of 5 to 10. The authors undertook in vitro studies to disclose the mechanism behind this clinically important interaction. In a human liver microsome preparation, fluvoxamine showed a concentration-dependent inhibition of clozapine N-demethylation. Fluvoxamine was much less effective as an inhibitor of clozapine N-oxidation. Fluvoxamine also inhibited in a concentration-dependent manner the activity of all five cytochrome P450 (CYP) isoforms previously determined to be capable of catalyzing the demethylation of clozapine. Fluvoxamine inhibited CYP1A2 and 2C19 with the highest affinities (Ki values of 0.041 and 0.087 microM, respectively). The Ki values for CYP2C9 and 2D6 were 2.2 and 4.9 microM, respectively, whereas the Ki for CY3A4 was 24 microM. Assuming a hepatic tissue concentration of fluvoxamine in the range of 4 to 7 microM under therapeutic conditions, a clinically significant inhibition of all but CYP3A4 is expected in relation to clozapine N-demethylation. No significant effect of fluvoxamine on clozapine N-oxidation is to be expected under therapeutic conditions. Because of the large interindividual variability of the quantity of the various CYP isoforms in liver tissue, it is not possible to predict the fluvoxamine-induced increase in the plasma concentration of clozapine of an individual patient.

Steady-state serum concentrations of the neuroleptic perphenazine in relation to CYP2D6 genetic polymorphism

Steady-state serum concentration to dose ratios of the neuroleptic agent perphenazine were related to CYP2D6 metabolizer status for 96 psychiatric inpatients: 88 extensive metabolizers and eight poor metabolizers. The median concentration per dose of the poor metabolizer group (0.195 nmol/L per milligram) was about twice the median (0.098 nmol/L per milligram) of the 56 extensive metabolizers without interacting medicine (p < 0.01). The rest of the extensive metabolizers (n = 32), who were comedicated with drugs that compete with perphenazine for metabolism by CYP2D6, had an intermediate median value of 0.140 nmol/L per milligram. The range of concentration/dose values for the total extensive metabolizer group extended from 0.025 to 0.688 nmol/L per milligram, that is, an almost thirtyfold variation. The concentration/dose range of the eight poor metabolizer subjects was 0.096 to 0.750 nmol/L per milligram. Serum levels not corrected for dose overlapped to a large degree among the groups, with a total range from 0.5 to 12 nmol/L. This study points toward a limited information value of CYP2D6 genotyping in the context of therapeutic drug monitoring of perphenazine.

Citalopram and desmethylcitalopram concentrations in breast milk and in serum of mother and infant

A case is presented with measurements of the selective serotonin reuptake inhibitor (SSRI)-antidepressant citalopram in serum of mother and breast-fed infant and in breast milk. We found a milk-to-serum concentration ratio of approximately 3 for both citalopram and the main metabolite desmethylcitalopram. Peak milk concentrations of citalopram occurred 3-9 h after drug intake by the mother. The infant received approximately 5% of the mother's dose, adjusted for weight. Accordingly, the serum level in the infant of approximately 7 nM corresponded to approximately 1/15 of the trough serum concentration of the mother (104 nM). No signs of drug effects in the infant were observed.

Identification of the human cytochrome P450 isoforms mediating in vitro N-dealkylation of perphenazine

AIMS

To identify the human cytochrome P450 (CYP) isoforms mediating the N-dealkylation of the antipsychotic drug perphenazine in vitro and estimate the relative contributions of the CYP isoforms involved.

METHODS

cDNA-expressed CYP isoforms were used to identify the isoforms that are able to mediate the N-dealkylation of perphenazine, which is considered a major metabolic pathway for the drug. Using human liver microsomal preparations (HLM), inhibition studies were carried out to establish the relative contributions of the CYP isoforms involved in the N-dealkylation reaction.

RESULTS

CYP isoforms 1A2, 3A4, 2C8, 2C9, 2C18, 2C19 and 2D6 were able to mediate the N-dealkylation of perphenazine. Reaction velocities and their relative abundance in HLM suggested that CYP1A2, 3A4, 2C19 and 2D6 were the most important contributors to N-dealkylation. Apparent Km values of CYP1A2 and CYP2D6 were in the range 1-2 microM, and Km values of CYP2C19 and CYP3A4 were 14 microM and 7.9 microM, respectively. Ketoconazole inhibition of N-dealkylation mediated by a mixed HLM indicated that CYP3A4 accounted for about 40% of perphenazine N-dealkylation at therapeutically relevant concentrations. The contribution of the CYP isoforms 1A2, 2C19 and 2D6 amounted to 20-25% each as measured by the percentage inhibition obtained by addition of furafylline, fluvoxamine or quinidine, respectively. HLM-mediated N-dealkylation of perphenazine accounted for 57% of the total amount of substrate consumed during incubation.

CONCLUSIONS

The present in vitro study suggests that CYP isoforms 1A2, 3A4, 2C19 and 2CD6 are primarily involved in the N-dealkylation of perphenazine. The relatively modest role of CYP2D6 is at variance with in vivo studies, which indicate a greater contribution of this isoform. Alternative metabolic pathways, corresponding to 43% of the HLM-mediated metabolism of the drug, may depend more strongly on CYP2D6.

Simplified high-performance liquid chromatographic method for the determination of citalopram and desmethylcitalopram in serum without interference from commonly used psychotropic drugs and their metabolites

A simplified method for the determination of racemic citalopram and its main metabolite desmethylcitalopram in serum using HPLC was developed. The compounds were extracted with heptane-isoamyl alcohol (98:2) and subsequently transferred into phosphate buffer pH 2.5 for direct injection into the HPLC apparatus. The analytes were separated with an acetonitrile-phosphate buffer, pH 2.5-tetraethylamine mobile phase on a C18 column and measured by UV detection at 240 nm. Within the typical range of serum concentrations (30-100 ng/ml) the inter-day variation was < 6% for both compounds. Possible analytical interference from a number of commonly coadministered psychoactive drugs and their metabolites was studied by extracting sera from patients receiving these drugs. Interference was not a problem for the developed method.

Serotonin depletion decreases serotonin transporter mRNA levels in rat brain

In order to study the impact of serotonin depletion on gene expression of the serotonin transporter (5-HTt) we measured 5-HTt mRNA levels by Northern blot in rats treated with p-chlorophenylalanine methyl ester (PCPA) for 10 days. Six rats received PCPA i.p. only, and another 6 rats receiving 0.9% NaCl served as controls. An additional group of 6 rats received both PCPA i.p. and imipramine, 5 mg/kg/day by osmotic minipumps. 5-HTt mRNA levels decreased to 81.1% (P = 0.05) and 76.0% (P = 0.05) of the control level for PCPA treated animals without and with concomitant imipramine treatment, respectively. The average level of the PCPA treated groups was 78.6% (P = 0.03). The isolated effect of 21 days of imipramine treatment was a 5-HTt mRNA level of 89.4%, which was not significantly different from the control level. In conclusion, 5-HTt gene expression is suppressed in the serotonin depleted state. A decreased synaptic reuptake of 5-HT may be interpreted as a compensatory mechanism aiming at preserving adequate synaptic 5-HT levels in a generally deficient state.

Methaemoglobinaemia among neonates in a neonatal intensive care unit

After detection of a few clinical cases of methaemoglobinaemia (methb) in our NICU, a prospective clinical study was undertaken to determine the extent of the problem and to identify the causes. Consequently, during the following 8 months all haemoglobin tests included simultaneous measurements of methb on an OSM 3 hemoximeter (Radiometer): 8% (n = 33) of 415 neonates were found to be methb positive (defined as > or = 6% methb). Mean methb was 19% (range 6.5-45.5%). Maximum methb concentrations were found on day 4-31 postpartum (mean 12 days) and the number of days with a positive methb sample ranged from 1 to 18 days (mean 6 days). About 40% of the neonates born at 25-30 weeks of gestation and 60% with a birth weight < 1000 g were methb positive. Also, there was a negative correlation between the size of the methb positive concentration and gestational age (r = -0.38, p = 0.02). Measurements of C-reactive protein and leucocytes, NADH reductase, pH, Cl, nitrate and nitrite were carried out in methb positive patients. The tests were repeated 1 week after cessation of methb. The only significant difference was an increase in NADH reductase at the second measurement. Likewise, a wide range of clinical parameters were registered and they occurred with a higher frequency among the methb positive patients when compared with a methb negative control group matched with regard to gestational age and the closest possible birth weight. The mean birth weight of methb positive patients was 1170 g and that of negative controls 1380 g (p < 0.006).(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of nortriptyline in human serum by fully automated solid-phase extraction and on-line high-performance liquid chromatography in the presence of antipsychotic drugs

A fully automated on-line method for determination of nortriptyline in human serum was developed using an ASPEC XL (Gilson) solid-phase extraction apparatus in combination with high-performance liquid chromatography. Solid phase extraction was performed on cyanopropyl cartridges. HPLC was carried out using a C18 column with a mobile phase of acetonitrile-0.01 M triethylamine (34:66 v/v) buffer, pH 3.0. UV detection was at 242 nm. The Inter-day CV% was <5%. Comparison with liquid-liquid extraction of serum from patients treated with nortriptyline showed good agreement. Studies of analytical interference from coadministered psychoactive drugs revealed that only imipramine and a methotrimeprazine metabolite interfered.

Choosing quality-control systems to detect maximum clinically allowable analytical errors

Critical systematic and random analytical errors for 17 common clinical chemical components were estimated from published values for analytical imprecision, biological variation, and "medically important changes." Appropriate quality-control systems for these analytes are discussed on the basis of power considerations. The simple rule 1(3)s, with one control per run, is minimally sufficient for the analytes (about one quarter of those considered here) for which the magnitude of critical error is at least 3 analytical standard deviations. The more powerful rule 1(2)s, with one control per run, is the minimal requirement for analytes for which critical errors are about 2 analytical standard deviations; these are about half the remaining analytes. Greater power values are achieved by using multiple rules based on several controls per run. In general, this study does not support the view put forward by some authors that the quality-control rules in use today are too restrictive.

Assessing diagnostic tests once an optimal cutoff point has been selected

The specificity and sensitivity of a quantitative diagnostic test depends on the chosen cutoff point. The common practice of selecting a cutoff point that maximizes the specificity plus the sensitivity, as judged from the observed test results, is studied here by simulation. Test performance is on average assessed too optimistically by this procedure--a phenomenon of importance when sample sizes are small. For example, the average positive bias is up to 15% of the test performance for sample sizes of 25. Furthermore, binomial calculated standard errors of specificity and sensitivity estimates are incorrect. A Monte Carlo statistical method--the "bootstrap procedure"--is applied to correct for bias and to estimate standard errors, including the standard error of the optimal cutoff point. Independent and paired comparisons of two diagnostic tests are also considered when optimal cutoff points have been selected. For this purpose, binomial statistical tests behave satisfactorily. Examples of power functions are presented.

HPLC with enzymatic detection as a candidate reference method for serum creatinine

We present a candidate Reference Method for determining the concentration of serum creatinine. The method is based on HPLC combined with enzymatic determination. Creatinine plus 14[C]creatinine is extracted by cation-exchange chromatography, subjected to reversed-phase HPLC, and finally quantified enzymatically. Enzymatic measurement ensures no interference from co-eluting compounds, which has been a problem for some reported HPLC assays relying on ultraviolet detection. The average corrected recovery was 100.1% (SEM = 1.1%; n = 15). The accuracy was verified by assaying five sera with target values determined by isotope dilution mass spectrometry. The total interassay CV was less than or equal to 2.5%. We applied the method to study the specificity of HPLC-ultraviolet detection, using 72 plasma samples from hospitalized patients; no interference was noted. Thus, HPLC-ultraviolet detection appears to be specific, provided that sample cleanup is based on cation-exchange chromatography. Our diode-array detector studies of peak homogeneity supported this conclusion. Still, combined HPLC-enzymatic determination ensures even greater accuracy, ranking close to that by isotope dilution mass spectrometry.

Postmortem analysis of three methoxyacetylfentanyl-related deaths in Denmark and in vitro metabolite profiling in pooled human hepatocytes

Methoxyacetylfentanyl belongs to the group of fentanyl analogues and has been associated with several deaths in recent years. We present three case reports of deceased individuals that tested positive for methoxyacetylfentanyl consumption, as well as in vitro and in vivo metabolite profiles. Methoxyacetylfentanyl was quantified by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) in femoral blood, as well as in urine and brain tissue when these were available. Metabolite profiling was performed by incubating methoxyacetylfentanyl with pooled human hepatocytes (pHH) in Leibovitz's L-15 medium supplemented with fetal bovine serum. Metabolites were identified in vivo and in vitro using UHPLC-high resolution (HR)-MS/MS. The measured methoxyacetylfentanyl concentration was 0.022-0.056mg/kg (N=3) in femoral blood, 0.12mg/kg (N=1) in urine, and 0.074mg/kg (N=1) in brain tissue homogenate. A total of 10 metabolites were identified. The observed metabolic pathways were: hydroxylation(s), N-dealkylation, O-demethylation, deamination, glucuronidation, and combinations thereof. Major analytical targets in vitro and across measured biological samples in vivo were methoxyacetylfentanyl, the O-demethyl- metabolite, and the deamide-metabolite. Intoxication with methoxyacetylfentanyl was judged as the cause of death or a major contributing factor in all three presented cases.