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Aims: To study the influence of the CYP2D6*10 allele on the disposition of debrisoquine and nortriptyline., Methods: The pharmacokinetics of debrisoquine and nortriptyline and their main metabolites were determined in ten Koreans with the CYP2D6*1/*1 (n = 5) and CYP2D6*1/*10 (n = 5) genotypes after single oral doses of 20 mg debrisoquine and 25 mg nortriptyline, respectively. The data were compared with previously published findings from 21 Caucasians with 0, one, two, three, four or 13 functional CYP2D6 genes., Results: The AUC0-8 of 4-hydroxydebrisoquine was significantly lower in Koreans with CYP2D6*1/*10 genotype compared with CYP2D6*1/*1[95% confidence interval (CI) for the ratio between means 1.17, 1.85]. No other genotype-related differences were found in the plasma kinetics of nortriptyline and debrisoquine, or their hydroxy metabolites. The AUCnortriptyline/AUC10-hydroxynortriptyline ratio did not differ between the *1/*1 and *1/*10 genotype groups (95% CI for the ratio of means 0.60, 1.26). Similarly, there was no difference between these genotypes with respect to the AUCdebrisoquine/AUC4-hydroxydebrisoquine ratio (95% CI for the ratio of mean values 0.38, 1.46). Both Korean genotype groups had similar AUCs and parent compound/metabolite AUC ratios of debrisoquine and nortriptyline to Caucasians with two functional CYP2D6 genes., Conclusions: Heterozygosity for CYP2D6*10 decreases the CYP2D6-dependent elimination of nortriptyline and debrisoquine to only a limited degree. Further studies in subjects homozygous for CYP2D6*10 are required to elucidate fully the pharmacokinetic consequences of this CYP2D6 genotype in Orientals.

Background: The ultrarapid metabolizer phenotype of the cytochrome P4502D6 (CYP2D6) enzyme has been considered a relevant cause of nonresponse to antidepressant drug therapy. Prescribing high doses of antidepressants to such patients leads to high concentrations of potentially toxic metabolites and an increased risk for adverse reactions. Normalization of the metabolic status of ultrarapid metabolizers by inhibition of CYP2D6 activity could offer a clinically acceptable method to successfully treat such patients with antidepressants., Methods: Five ultrarapid metabolizers with a CYP2D6 gene duplication or triplication were treated with 25 mg nortriptyline twice a day for 3 consecutive weeks, alone during the first week and concomitantly with the CYP2D6 inhibitor paroxetine 10 mg or 20 mg twice a day, respectively, during the second and third weeks. After the third week, nortriptyline was discontinued and the subjects were treated with paroxetine 20 mg twice a day during the fourth study week. At the end of each study week, the steady-state pharmacokinetic parameters of nortriptyline or paroxetine were determined within the dose interval. In addition, the CYP2D6 phenotype was determined by debrisoquin (INN, debrisoquine) test at baseline and at the end of each study phase. Treatment-related adverse events were recorded during drug administration and for 1 week thereafter., Results: All 5 subjects had very low (subtherapeutic) nortriptyline concentrations after 7 days' treatment with nortriptyline only. Addition of paroxetine 10 mg twice a day to the nortriptyline regimen resulted in a change in all individuals to the "normal" extensive debrisoquine metabolizer phenotype, and therapeutic plasma nortriptyline concentrations were achieved in 4 of 5 subjects after a 3 times mean increase in nortriptyline trough concentration (P =.0011). Doubling the paroxetine dose caused a 15 times mean increase in paroxetine trough concentration (P <.001), indicating strong inhibition by paroxetine of its own metabolism. The high paroxetine concentrations in 2 subjects caused them to have the poor debrisoquine metabolizer phenotype and resulted in a further increase in plasma nortriptyline trough concentration (P =.0099). A strong correlation (rank correlation coefficient [r(s)] = 0.89; P <.0001) was observed between paroxetine and nortriptyline trough concentrations. Paroxetine also significantly decreased the fluctuation of nortriptyline concentrations within the dose interval. One subject discontinued the study after the second study week because of adverse effects; otherwise, the study drugs were well tolerated., Conclusions: Paroxetine, with a daily dosage from 20 to 40 mg, is an effective tool in normalizing the metabolic status of CYP2D6 ultrarapid metabolizers.

Objectives: To study the impact of the CYP2D6*10 allele on the disposition of nortriptyline in Chinese subjects., Methods: A single dose of 25 mg nortriptyline was given orally to 15 healthy Chinese volunteers who were classified as extensive metabolizers after phenotyping with debrisoquin (INN, debrisoquine) and who were genotyped by allele-specific polymerase chain reaction. Five subjects were homozygous for CYP2D6*1, 5 subjects were homozygous for CYP2D6*10, and 5 subjects were heterozygous for these 2 alleles. Plasma concentrations of nortriptyline and its main metabolite 10-hydroxynortriptyline were measured by liquid chromatography-mass spectrometry, and the pharmacokinetics were studied during 168 hours after the dose., Results: Subjects who were homozygous for CYP2D6*10 had significantly higher total areas under the plasma concentration-time curve (AUC), lower apparent oral clearances, and longer mean plasma half-life of nortriptyline than subjects in the CYP2D6*1/*1 and the heterozygous groups. For 10-hydroxynortriptyline, the AUC was lower and the plasma half-life was longer in subjects who were homozygous for CYP2D6*10 than in subjects in the other 2 groups., Conclusion: The CYP2D6*10 allele in Chinese subjects was associated with significantly higher plasma levels of nortriptyline compared with the CYP2D6*1 allele because of an impaired metabolism of nortriptyline to 10-hydroxynortriptyline, particularly in the subjects with the CYP2D6*10/*10 genotype. The results suggest that genotyping of CYP2D6 may be a useful tool in predicting the pharmacokinetics of nortriptyline.

Objective: To investigate the disposition and effects of nortriptyline and its major metabolite 10-hydroxy-nortriptyline line in panels of white subjects with different CYP2D6 genotypes, including those with duplicated and multiduplicated CYP2D6*2 genes and to evaluate the contribution of the number of functional C gamma P2D6 alleles to the metabolism of nortriptyline, used here as a model drug for CYP2D6 substrates., Methods: Oral single doses of 25 to 50 mg nortriptyline were given to five poor metabolizers of debrisoquin (INN; debrisoquine) with no functional CYP2D6 gene, five extensive metabolizers with one functional CY2D6 gene, five extensive metabolizers with two functional CYP2D6 genes, five ultrarapid metabolizers with duplicated CYP2D6*2 genes, and one ultrarapid metabolizer with 13 copies of the CYP2D6*2 gene. Plasma kinetics of nortriptyline and 10-hydroxynortriptyline were analyzed. Anticholinergic effects (inhibition of salivation and accommodation disturbances), sedation, blood pressure, and effect on supine and erect pulse rate were measured., Results: There was a clear relation between the C gamma P2D6 genotype and the plasma kinetics of nortriptyline and 10-hydroxynortriptyline. The proportion between the apparent oral clearances of nortriptyline in the groups with 0, 1, 2, 3, and 13 functional genes was 1:1:4:5:17. The proportions between AUC(nortriptyline) to AUC(10-hydroxynortriptyline) ratios in the groups with 0, 1, 2, 3, and 13 functional genes were 36:25:10:4:1. Oral plasma clearance of nortriptyline and AUC(nortriptyline) to AUC(10-hydroxynortriptyline) ratio both correlated significantly with the debrisoquin metabolic ratio (rS = -0.89, p = 0.0001; rS = 0.92, p = 0.0001). Although ultrarapid metabolizer subjects were given double the nortriptyline dose (50 mg), inhibition of salivation was not more pronounced compared with the other genotype groups given 25 mg nortriptyline., Conclusion: The results of this study show the quantitative importance of the CYP2D6 genotype, especially the presence of multiple functional CYP2D6 genes for the pharmacokinetics of nortriptyline and 10-hydroxynortriptyline. Genotyping of subjects with multiple copies of functional genes may be of great value for differentiating ultrarapid metabolizers from patients who do not comply with the prescription and for assuring adequate drug choice and dosage for these patients.

The relationship between the CYP2D6 genotype and the steady state plasma levels of nortriptyline (NT), its main active metabolite 10-hydroxynortriptyline (10-OH-NT) and the NT/10-OH-NT ratio were studied in 21 Caucasian depressed patients treated with 100-150 mg NT daily. The patients had participated in a previously published study investigating the role of NT and 10-OH-NT for the therapeutic effect of NT, and the plasma level data were from that study. In the present follow-up study, the patients were genotyped with respect to the polymorphic CYP2D6 by allele-specific PCR amplification and EcoRI RFLP. One poor metabolizer (PM) was identified and she had the highest plasma concentration of NT. Among the 20 extensive metabolizers (EM), the genotype (homozygous versus heterozygous EM) alone was not found to explain the variance in dose-corrected NT concentrations, but contributed significantly when gender was also taken into account. Together, these factors accounted for 59% of the variability in NT levels. Female patients had higher plasma levels of NT than male patients. 10-OH-NT levels were influenced by genotype, and NT/10-OH-NT ratio by genotype and gender. The present follow-up study confirms a relationship between the CYP2D6 genotype and the plasma levels of NT and its active metabolite. Identification of PM by genotyping should be of value for the prediction of the plasma levels and, consequently, the lower than average dose of NT required for optimal therapy. Also among EM, the genotype contributes to the variability in NT and 10-OH-NT levels but alone is of limited practical value for the prediction of optimal dosage.

Hydroxymetabolites of the antidepressants nortriptyline and desipramine, like the parent drugs, inhibit neuronal uptake of noradrenaline (norepinephrine). In both plasma and cerebrospinal fluid (CSF), the concentrations of the 10-hydroxymetabolites of nortriptyline (10-OH-NT) are usually higher than those of the parent drugs, but there is a pronounced interindividual variation in the plasma concentrations. This shows that during treatment with nortriptyline, hydroxymetabolites exert, at least in some patients, major effects on brain noradrenaline neurons. Hydroxymetabolites of antidepressants are formed by the polymorphic cytochrome P450 enzyme CYP2D6. Nortriptyline is hydroxylated by this enzyme in a highly stereospecific way to the (-)-enantiomer of E-10-OH-NT. Among Caucasians, 7% are poor metabolisers of the CYP2D6 probe drug debrisoquine. These patients will form very little hydroxymetabolite. The affinity of E-10-OH-NT for muscarinic acetylcholine receptors in vitro was only one-eighteenth of the affinity of nortriptyline for these receptors. In healthy individuals, nortriptyline decreased saliva flow to a significantly greater extent than either E-10-OH-NT or placebo. In an ultrarapid hydroxylator of nortriptyline treated with very high doses of nortriptyline, the plasma concentration of unconjugated 10-OH-NT was very high without any sign of anticholinergic adverse effects. These results show that hydroxymetabolites of nortriptyline have much less anticholinergic effect than the parent drug. When racemic E-10-OH-NT per se was given to healthy individuals, the plasma concentration of the (-)-enantiomer was 5-fold higher than that of (+)-E-10-OH-NT. The 2 enantiomers were eliminated in parallel with an elimination half-life of 8 to 10 hours. A combined in vitro and in vivo investigation showed that a mean of 64% of (+)-E-10-OH-NT was glucuronidated in the liver and subsequently eliminated in urine. Of the administered (-)-enantiomer, a mean of 36% was eliminated as glucuronide formed in the intestine and 35% was actively secreted as unchanged form in urine. Plasma protein binding, determined by ultrafiltration, of the (+)- and (-)-enantiomers of E-10-OH-NT was 54 and 69%, respectively, which is less than that of nortriptyline (92%). The concentration of E-10-OH-NT in CSF was 50% of the concentration of unbound in plasma. There seems to be a stereoselective active transport of E-10-OH-NT from the CSF to blood. We administered racemic E-10-OH-NT to 5 patients during a major depressive episode.(ABSTRACT TRUNCATED AT 400 WORDS)

Therapeutic drug monitoring data for amitriptyline (AT) and nortriptyline (NT) collected during 10 years (total of 4,278 analyses in 2,937 patients) were evaluated to study how other drugs affect the kinetics at steady state. The distribution of the ratio concentration/daily dose (C/D) in patients treated with the antidepressant only was compared with that in patients on different concomitant drugs. Patients on phenothiazines or dextropropoxyphene had a significantly higher mean C/D of NT than controls, both when AT and when NT had been given. The highest values were seen with levomepromazine and thioridazine. On the contrary, the mean C/D of both AT and NT in patients on carbamazepine was about 50% lower than in those treated with the antidepressant only. Benzodiazepines did not affect the steady-state kinetics of AT or NT. Intraindividual comparisons of the ratio C/D in subjects with analyses performed when off and on concomitant drugs corroborate previous results showing that drugs metabolized by the debrisoquine hydroxylase (CYP2D6) inhibit the metabolism of NT and that carbamazepine induces the metabolism of both AT and NT. Modeling of the dose dependency of the NT interactions with levomepromazine, perphenazine, and thioridazine revealed that the ratio C/D was most affected at low doses of the antidepressant and at high doses of the phenothiazine. The distribution of the doses given was the same in patients on monotherapy as in patients with interacting drugs, which means that many patients treated with phenothiazines had concentrations above the therapeutic range and that most patients treated with carbamazepine had subtherapeutic levels. The present study shows that therapeutic drug monitoring may serve as a valuable tool to discover and quantify drug interactions.

The enantioselectivity of hydroxylation of nortriptyline (NT) to E-10-hydroxynortriptyline (E-10-OH-NT) was studied in human liver microsomes, intestinal homogenate, and patients treated with NT. The rate of formation of (-)-E-10-OH-NT was higher than that of (+)-E-10-OH-NT both in the liver microsomes and in the intestinal homogenate. Quinidine, a prototype competitive inhibitor of the cytochrome P450IID6 ("debrisoquin hydroxylase"), inhibited the formation of (-)-E-10-OH-NT in a concentration-dependent manner in liver microsomes, while the formation of (+)-E-10-OH-NT was hardly affected. This indicates that P450IID6 catalyzes the hydroxylation of NT in a highly enantioselective manner to (-)-E-10-OH-NT in the liver. Another P450 isozyme besides IID6 seems to be responsible for the formation of the (+)-enantiomer in the liver. In intestinal homogenate, the formation of both enantiomers of E-10-OH-NT was inhibited to about the same extent by quinidine, the maximum inhibition being much less than in the liver. In the urine of six patients treated with NT, the (-)-enantiomer accounted for 91 +/- 2% of the unconjugated E-10-OH-NT, and for 78 +/- 6% of the glucuronide conjugates. The study shows that NT is hydroxylated in a highly enantioselective way, probably catalyzed by the polymorphic P450IID6, to (-)-E-10-OH-NT both in vitro in human liver as well as in vivo in patients treated with the drug.

Studies in humans have given strong support of an active transport of (E)-10-hydroxynortriptyline [E)-10-OH-NT) in the kidney and from the cerebrospinal fluid. After 3 days of intraperitoneal injections of (E)-10-OH-NT (10 mg/kg t.i.d.) in a rat model, we investigated the effect of propranolol and quinidine (25 and 2.5 mg/kg injected 6 times during day 3) on the concentrations of (E)-10-OH-NT in urine, plasma (total and unbound) and brain. In a group of control rats the renal clearance of (E)-10-OH-NT varied 10-fold between rats, but was reproducible from Day 2 to Day 3 (r = 0.83; n = 8; p less than 0.05). Quinidine markedly decreased the renal clearance of both total and unbound (E)-10-OH-NT from plasma, while the effect of propranolol was less pronounced. There was no difference in the ratio of (E)-10-OH-NT concentrations in brain and plasma (unbound) between treated and control rats. We conclude that (E)-10-OH-NT is actively secreted in the rat proximal tubule and that the activity of this system varies between rats, but is constant in the individual rat. Quinidine, and to a less extent propranolol, inhibits this renal secretion. By the present methodology we could not demonstrate an active transport of (E)-10-OH-NT from brain to blood in the rat. Studies using other methods are needed to further elucidate this.

In 5 patients treated with nortriptyline or amitriptyline for at least 9 months, the cerebrospinal fluid (CSF)/plasma ratio for 10-hydroxynortriptyline (10-OH-NT) ranged from 0.085 to 0.172, which is similar to the ratio previously measured in patients treated for 3 weeks. In 4 other patients treated with racemic (E)-10-OH-NT, the mean concentration ratio between (-)- and (+)-(E)-10-OH-NT was 3.56 in plasma, 2.39 in plasma ultrafiltrate and 1.42 in CSF (one-way ANOVA; P less than 0.001). The mean free fraction in plasma determined by ultrafiltration for (-)-(E)-10-OH-NT was 28.9 +/- S.D.1.1% and for the (+)-enantiomer 43.7 +/- 0.8% (P less than 0.001) confirming the difference in protein binding shown previously in healthy subjects. There was a correlation between the concentration of 10-OH-NT (sum of enantiomers) in CSF and plasma ultrafiltrate (r = 0.96; n = 7; P less than 0.001). The concentration in CSF was, however, only about 50% of that in the plasma ultrafiltrate and this seems to be due to a stereoselective transport of (E)-10-OH-NT out from the CSF. The secretion from the CSF is more pronounced for the (-)-compared to the (+)-enantiomer, which is consistent with the stereoselectivity of the renal secretion of these compounds.

The major metabolite of nortriptyline, i.e. E-10-hydroxynortriptyline (E-10-OH-NT), was given as a racemate in increasing doses from 75 to 225 mg/day to five patients with major depressive episode. Plasma concentrations of both the (-)- and (+)-enantiomers were linearly related to the doses. The mean ratio between them was 3.6 +/- 0.53, indicating stereospecific kinetics during maintenance treatment. Lumbar punctures were performed in four of the patients before and after 3 weeks of E-10-OH-NT treatment. There was a 18% mean decrease (P less than 0.01) in the noradrenaline metabolite HMPG in cerebrospinal fluid (CSF), supporting previous in vitro data showing that E-10-OH-NT inhibits noradrenaline uptake in vivo. During treatment, the median depression score measured by the Montgomery-Asberg Depression Rating Scale declined from 32 to 14 (P less than 0.05). As the study was open, the clinical outcome is not conclusive but does not contradict the hypothesis that E-10-OH-NT has antidepressant properties. If present at all, side effects were mild and did not interfere with the treatment.

Eight healthy subjects [who were phenotyped with a debrisoquine (D) hydroxylation test] were selected to cover a wide range in the ratio between D and 4-hydroxydebrisoquine (4-OH-D) in the urine. After a single oral dose of nortriptyline (NT) the metabolic clearance by 10-hydroxylation in the E-position, but not in the Z-position, correlated closely to the metabolic ratio D/4-OH-D (rs = -0.88, p less than 0.01). This indicates that common enzymatic mechanisms are involved in the hydroxylation of D and the E- but not the Z-10-hydroxylation of NT. Slow hydroxylators of NT and D excreted less 10-hydroxynortriptyline in urine and had lower plasma clearance of NT than the rapid hydroxylators. The strong correlation (r = 0.96) between the total plasma clearance of NT and the metabolic clearance by E-10-hydroxylation shows that this metabolic reaction is important in the disposition of the drug.

Oral (50 mg) and intramuscular (25 mg) amitriptyline (AT) was given to six normal subjects and the area under the plasma concentration-time curve (AUC) for nortriptyline (NT) formed was calculated. There was no difference between the AUCs (corrected for dose) after the two routes of administration. The ratio between the AUCs (corrected for dose) after the two routes of administration. The ratio between AUCoral and AUCim averaged 0.95 (range 0.69 to 1.13). After intramuscular AT maximum NT plasma concentration was reached after 24 to 48 hr, whereas it was 8 to 24 after oral dosing.

The active and major metabolite of nortriptyline (NT), E-10-hydroxynortriptyline (E-10-OH-NT), was taken orally as the hydrogen maleate in single doses by nine healthy subjects. The doses (10 to 100 mg) were completely absorbed, as shown by the high urinary recovery of 86.1% +/- 9.9%. Of the given dose, 51.2% +/- 8.7% was recovered as conjugated E-10-OH-NT and 23.9% +/- 4.3% was recovered as unchanged compound. The plasma t1/2 of E-10-OH-NT was 8.0 +/- 1.2 hours and total plasma clearance was 47.5 +/- 10.3 L/hr. The rate of elimination varied little between individuals. There was no indication of dose-dependent elimination. The mean apparent volume of distribution was 7.7 +/- 2.1 L/kg. Single oral doses of 50 mg E-10-OH-NT significantly increased the plasma levels of norepinephrine in both the supine and standing positions (P less than 0.01). Pulse rate increased in the standing but not the supine position. These effects might result from inhibition of neuronal uptake of norepinephrine by E-10-OH-NT. Coupled with its low affinity for muscarinic receptors, these kinetic and pharmacodynamic features of E-10-OH-NT call for further phase I studies.

Eleven Ghanaian and 12 Swedish subjects phenotyped with a debrisoquine (D) hydroxylation test were given a single oral dose of nortriptyline (NT). Much the same percentage of the given NT dose was excreted as 10-hydroxy-NT (10-OH-NT) by Ghanaians (43.1%) and Swedes (49.2%). There was a close correlation between plasma clearance of NT by 10-hydroxylation and the D metabolic ratio (D/4-OH-D in urine) in the Ghanaians (rs = -0.95; P less than 0.01) and Swedes (rs = -0.84; P less than 0.01). The E-isomer of 10-OH-NT is the major isomer in both Ghanaians (76% to 92% of total 10-OH-NT) and Swedes (78% to 95%). It is suggested that the E-10-hydroxylation of NT and the 4-hydroxylation of D are similarly coregulated in Ghanaians and Swedes.

Eleven healthy nonsmokers with wide variation in the ability to hydroxylate debrisoquin (D) were given single oral doses of amitriptyline and nortriptyline on different occasions. The urinary D/4-hydroxy-D ratio correlated significantly (P less than 0.01) with all three parameters of amitriptyline disposition measured (total plasma clearance, clearance by demethylation, and clearance by pathways other than demethylation), with rs = -0.89, -0.78, and -0.83, respectively. In contrast, we failed to demonstrate such correlations in a previous sample of smokers. Our data suggest that there may be a common regulation of the hydroxylation of D and the oxidative metabolism of amitriptyline in nonsmokers. It is hypothesized that an additional demethylase/hydroxylase is induced in smokers that is not involved in D hydroxylation.

Conjugation of racemic E-10-hydroxynortriptyline (E-10-OH-NT) with glucuronic acid was studied in the liver microsomal fraction of rats and humans. The diastereomeric glucuronides of E-10-OH-NT were resolved and quantitated by HPLC. Only the (+)-enantiomer was glucuronidated in liver microsomes from humans. Rat liver microsomes catalyzed the formation of both glucuronides. Phenobarbital pretreatment of rats increased the glucuronidation of both enantiomers about five-fold. The formation rate of (+)-E-10-OH-NT glucuronide varied from 5.5 to 33.2 pmol/mg x min, in microsomes from 13 humans. High activity was found in individuals previously treated with pentobarbital. Inhibition experiments with human liver microsomes showed that amitriptyline is a potent competitive inhibitor of (+)-E-10-OH-NT glucuronidation. p-Nitrophenol, paracetamol and 2-hydroxydesipramine also inhibited this reaction.

Six healthy male subjects were randomly given nortriptyline (NT) (25 and 50 mg) and placebo in a double-blind, crossover study. An NT dose of 50 mg (but not 25 mg) clearly reduced saliva flow (P less than 0.05) and was therefore used for comparison with the major and active metabolite of NT, E-10-hydroxynortriptyline (E-10-OH-NT). Equimolar doses of both compounds (and placebo) were randomly given to eight healthy male subjects in another double-blind, crossover study aimed to assess the reduction of saliva flow. NT significantly depressed saliva flow compared with both placebo (P less than 0.01) and E-10-OH-NT (P less than 0.05). By contrast, there was no difference between E-10-OH-NT and placebo. This study confirms previous indications that the anticholinergic effect of E-10-OH-NT is considerably less than that of the parent drug NT.

The disposition and elimination kinetics of the enantiomers of E-10-hydroxynortriptyline (E-10-OH-NT) were studied in six rapid and four slow hydroxylators of debrisoquin after a single oral dose of 75 mg racemic E-10-OH-NT hydrogen maleate. The plasma levels and the AUC of unconjugated (-)E-10-OH-NT were two to five times higher than those of (+)E-10-OH-NT. The plasma half-lives of both enantiomers were 8 to 9 hours. A significantly higher proportion of the given dose of (+)E-10-OH-NT (64.4% +/- 12.1%) than of (-)E-10-OH-NT (35.3% +/- 9.7%) was recovered in urine as glucuronide conjugate, but more (-)E-10-OH-NT was recovered unchanged in urine. The total oral plasma clearance and the metabolic clearance by glucuronidation were significantly (p less than 0.0001) higher for (+)E-10-OH-NT than for (-)E-10-OH-NT. The findings indicate that first-pass glucuronidation of E-10-OH-NT is enantioselective in human beings in vivo, with preference for (+)E-10-OH-NT. The renal clearance of unbound (-)E-10-OH-NT (0.57 +/- 0.16 L.kg-1.hr-1), on the other hand, exceeded that of (+)E-10-OH-NT (0.44 +/- 0.14 L.kg-1.hr-1) (p less than 0.005), which suggests enantioselective tubular secretion. The debrisoquin hydroxylation status was not associated with any of the investigated kinetic processes that relate to E-10-OH-NT.

In 10 patients treated with nortriptyline, the steady-state plasma concentrations of the parent drug and the active 10-hydroxy metabolite were very similar before and during concomitant treatment with alprazolam (0.5 mg orally t.i.d.). In human liver microsomes the 2-hydroxylation of desipramine was not inhibited by alprazolam. The absence of an inhibition by alprazolam on the hydroxylations of nortriptyline in vivo and of desipramine in vitro indicates that alprazolam is not metabolized by the debrisoquine hydroxylase.

To enable in vitro characterization of nortriptyline 10-hydroxylase activity of microsomal fractions of human liver, methods for the determination of E- and Z-10-hydroxynortriptyline were developed. High performance liquid chromatography (HPLC) with UV-detection or HPLC separation followed by quantitation by gas chromatography-mass spectrometry were used. The microsomal 10-hydroxylation of nortriptyline was linear to 60 min with a protein concentration of 1 mg x ml-1. In 15 human livers a 5-fold variation in activity was found. The formation of the Z-isomer comprised 13 +/- 3% (mean +/- SD) of the total 10-hydroxylase activity. The 10-hydroxylation was inhibited by debrisoquine, sparteine, propranolol, and encainide. No effect or a stimulation of the activity was seen with p-nitroanisole and acetanilide.

After 3 weeks' nortriptyline (NT) treatment the mean plasma concentration of its 10-hydroxy metabolite (10-OH-NT) (599 +/- 207 nmol l-1) was higher than that of the parent drug (433 +/- 199 nmol l-1) in 25 depressed patients. Also in the cerebrospinal fluid (CSF) the mean level of 10-OH-NT (67 +/- 20 nmol l-1) was higher than that of NT (39 +/- 23 nmol l-1). There was a strong correlation (P less than 0.001) between the CSF and plasma concentration of both NT (r = 0.92) and 10-OH-NT (r = 0.77). The interindividual variation in the CSF/plasma ratio of both compounds was small, compared to the variation in plasma levels. These results show that 10-OH-NT passes the blood-brain barrier as it is present in concentrations higher than those of NT in the CSF. 10-OH-NT has previously been shown to be a potent blocker of noradrenaline uptake and to have much less affinity for muscarinic receptors than NT itself. This active metabolite might therefore be a potential antidepressant with less disturbing anticholinergic side-effects.

Plasma concentrations of nortriptyline (NT) and its major metabolite 10-hydroxy-NT (10-OH-NT) were measured in 30 patients with depression, treated with NT for 3 weeks. Nine patients who recovered completely had plasma concentrations of NT and 10-OH-NT ranging from 358 to 728 nmol/L and from 428 to 688 nmol/L, respectively. Of the 21 patients who did not recover completely, only four had plasma concentrations within the window limited by these two plasma concentration ranges. A correlation was found between the degree of amelioration and the plasma concentration of NT (rs = 0.469; P less than 0.01). Lumbar punctures were performed in 26 patients before and after 3 weeks of NT treatment. During treatment there was a 30.9% mean decrease in the noradrenaline metabolite 4-hydroxy-3-methoxyphenylglycol (HMPG) in cerebrospinal fluid (CSF). We could not evaluate the extent to which this decrease was caused by NT or 10-OH-NT, respectively, because both are strong inhibitors of noradrenaline uptake. The ratio between the concentration of NT and 10-OH-NT in CSF correlated to the reduction of HMPG in CSF (r = 0.397; P less than 0.05) and to the amelioration of depression (rs = 0.623; P less than 0.001). This might indicate that NT and 10-OH-NT interact on the noradrenaline system in a nonadditive way. During treatment there was a 15.2% decrease in CSF concentration of the serotonin metabolite 5-hydroxyindoleacetic acid. The reduction was significantly correlated to the CSF concentration of NT but not to that of 10-OH-NT. This is in accordance with the fact that NT is a more potent inhibitor of serotonin uptake than is 10-OH-NT. The dopamine metabolite homovanillic acid in CSF decreased significantly by 10.0%. The biochemical data indicate that noradrenergic, serotoninergic, and dopaminergic systems are affected by NT treatment and that 10-OH-NT might be more selective on noradrenergic neurons than the parent drug.

Treatment of endogenous depression with nortriptyline (NT), at a daily dose of 150 mg, resulted in a pronounced improvement of seven of ten patients investigated. The concentration of the norepinephrine metabolite HMPG in cerebrospinal fluid (CSF) decreased by 29% (P less than 0.01) after 3 weeks of treatment. There was no significant effect of treatment on the serotonin and dopamine metabolites 5-HIAA and HVA. In previous larger materials, however, a decrease of 5-HIAA in CSF has been demonstrated. Platelets from the patients showed an increase in Km for serotonin uptake in response to NT treatment. The IC50 value of NT for serotonin uptake inhibition was 940 nM, while the corresponding value of the major metabolite of NT, i.e. E-10-OH-NT, was much higher (6700 nM). Thus, during treatment, the parent drug and not the metabolite was responsible for the serotonin uptake inhibition in platelets. There was a close correlation between Km and the plasma concentration of NT after 1 week of treatment (r = 0.88, P less than 0.01) but not after 3 weeks of treatment (r = 0.48; ns). There was no uniform effect of NT treatment on Vmax. It is concluded that clinical NT treatment results in uptake inhibition not only in norepinephrine but also in serotonin neurons.

The relative affinities of nortriptyline (NT) and its 10-hydroxymetabolites to rat brain muscarinic acetylcholine receptors have been determined by competition with 3H-quinuclidinyl benzilate binding. It is shown that the major NT metabolite, E-10-OH-NT, has only 1/18 the affinity of NT for the muscarinic receptor. Since this metabolite is equipotent to NT in inhibiting neuronal noradrenaline uptake, it is suggested that it might be of clinical value as an antidepressant by virtue of having less anticholinergic side-effects than NT itself.

In 20 depressed patients treated with nortriptyline (NT) there was a significant relationship between the plasma concentration of NT and the debrisoquine metabolic ratio (rs = 0.77; P less than 0.01). (The debrisoquine test was performed after stopping NT treatment). This is in agreement with the hypothesis that the hydroxylations of NT and debrisoquine are mediated by similar enzymatic mechanisms. In contrast there was no significant relationship between the debrisoquine metabolic ratio and the plasma concentrations of the active metabolite 10-hydroxy-nortriptyline. In 11 of the patients the debrisoquine metabolic ratio was significantly higher during than after NT treatment. This may be due to an inhibition of the debrisoquine hydroxylation by NT.