Sequential metabolism of secondary alkyl amines to metabolic-intermediate complexes: opposing roles for the secondary hydroxylamine and primary amine metabolites of desipramine, (s)-fluoxetine, and N-desmethyldiltiazem.

Three secondary amines desipramine (DES), (S)-fluoxetine [(S)-FLX], and N-desmethyldiltiazem (MA) undergo N-hydroxylation to the corresponding secondary hydroxylamines [N-hydroxydesipramine, (S)-N-hydroxyfluoxetine, and N-hydroxy-N-desmethyldiltiazem] by cytochromes P450 2C11, 2C19, and 3A4, respectively. The expected primary amine products, N-desmethyldesipramine, (S)-norfluoxetine, and N,N-didesmethyldiltiazem, are also observed. The formation of metabolic-intermediate (MI) complexes from these substrates and metabolites was examined. In each example, the initial rates of MI complex accumulation followed the order secondary hydroxylamine > secondary amine >> primary amine, suggesting that the primary amine metabolites do not contribute to formation of MI complexes from these secondary amines. Furthermore, the primary amine metabolites, which accumulate in incubations of the secondary amines, inhibit MI complex formation. Mass balance studies provided estimates of the product ratios of N-dealkylation to N-hydroxylation. The ratios were 2.9 (DES-CYP2C11), 3.6 [(S)-FLX-CYP2C19], and 0.8 (MA-CYP3A4), indicating that secondary hydroxylamines are significant metabolites of the P450-mediated metabolism of secondary alkyl amines. Parallel studies with N-methyl-d(3)-desipramine and CYP2C11 demonstrated significant isotopically sensitive switching from N-demethylation to N-hydroxylation. These findings demonstrate that the major pathway to MI complex formation from these secondary amines arises from N-hydroxylation rather than N-dealkylation and that the primary amines are significant competitive inhibitors of MI complex formation.

The effects of desvenlafaxine and paroxetine on the pharmacokinetics of the cytochrome P450 2D6 substrate desipramine in healthy adults.

The potential for cytochrome P450 (CYP) 2D6 substrates to interact with desvenlafaxine (administered as desvenlafaxine succinate) and paroxetine was evaluated. In an open-label, crossover study, 20 healthy volunteers (aged 21-50) were randomized to 2 series of 9 days each of desvenlafaxine (100 mg/d) or paroxetine (20 mg/d), separated by a 5-day washout. The CYP2D6 substrate desipramine (50 mg) was administered alone on day 1 and coadministered on day 6 of dosing with either desvenlafaxine or paroxetine. CYP2D6 genotype was determined at baseline. Based on least squares geometric mean ratios between reference (desipramine alone) and test treatments, desvenlafaxine produced minor increases in desipramine area under the plasma concentration versus time curve (AUC; 36%) and peak plasma concentration (C(max); 30%) (vs paroxetine: 419%, 90%, respectively; both P < .001). Desvenlafaxine produced little change in 2-hydroxydesipramine AUC (16% increase) and C(max) (0%) versus paroxetine (18% and 82% decreases, respectively; P = .008, P < .001, respectively), indicating that desvenlafaxine, especially at the recommended therapeutic dose of 50 mg/d for major depressive disorder in the United States, has little potential to interact with CYP2D6 substrates.

An assessment of drug-drug interactions: the effect of desvenlafaxine and duloxetine on the pharmacokinetics of the CYP2D6 probe desipramine in healthy subjects.

A number of antidepressants inhibit the activity of the cytochrome P450 2D6 enzyme system, which can lead to drug-drug interactions. Based on its metabolic profile, desvenlafaxine, administered as desvenlafaxine succinate, a new serotonin-norepinephrine reuptake inhibitor, is not expected to have an impact on activity of CYP2D6. This single-center, randomized, open-label, four-period, crossover study was undertaken to evaluate the effect of multiple doses of desvenlafaxine (100 mg/day, twice the recommended therapeutic dose for major depressive disorder in the United States) and duloxetine (30 mg b.i.d.) on the pharmacokinetics (PK) of a single dose of desipramine (50 mg). A single dose of desipramine was given first to assess its PK. Desvenlafaxine or duloxetine was then administered, in a crossover design, so that steady-state levels were achieved; a single dose of desipramine was then coadministered. The geometric least-square mean ratios (coadministration versus desipramine alone) for area under the plasma concentration versus time curve (AUC) and peak plasma concentrations (C(max)) of desipramine and 2-hydroxydesipramine were compared using analysis of variance. Relative to desipramine alone, increases in AUC and C(max) of desipramine associated with duloxetine administration (122 and 63%, respectively) were significantly greater than those associated with desvenlafaxine (22 and 19%, respectively; P < 0.001). Duloxetine coadministered with desipramine was also associated with a decrease in 2-hydroxydesipramine C(max) that was significant compared with the small increase seen with desvenlafaxine and desipramine (-24 versus 9%; P < 0.001); the difference between changes in 2-hydroxydesipramine AUC did not reach statistical significance (P = 0.054). Overall, desvenlafaxine had a minimal impact on the PK of desipramine compared with duloxetine, suggesting a lower risk for CYP2D6-mediated drug interactions.

Desipramine and 2-hydroxydesipramine plasma levels in endogenous depressed patients. Lack of correlation with therapeutic response.

Studies of the relationship between plasma concentrations of desipramine hydrochloride and clinical response have shown contradictory results, and only one prior study examined 2-hydroxydesipramine and its relationship to treatment. We therefore performed a study in a large, carefully diagnosed group of depressed patients taking fixed maintenance doses of desipramine to elucidate a potential relationship between clinical response and plasma concentrations of desipramine and 2-hydroxydesipramine. There was no significant correlation between clinical response and steady-state plasma levels of desipramine, 2-hydroxydesipramine, or the sum of desipramine plus 2-hydroxydesipramine. Although some commercial laboratories suggest a specific therapeutic plasma level "range" for desipramine, our data provide no support for such a range, nor for the routine measurement of plasma desipramine and 2-hydroxydesipramine concentrations in depressed patients.

Major adverse reactions during desipramine treatment: relationship to plasma drug concentrations, concomitant antipsychotic treatment, and patient characteristics.

Major adverse reactions interrupting drug therapy during treatment of 84 patients with desipramine hydrochloride were studied to determine their relationship to desipramine plasma concentrations and other clinical variables. The frequency of adverse reactions was higher in patients over 60 years old (39%), and in patients also receiving antipsychotic medications (32%), but low in patients under 60 years old (7%). Desipramine plasma concentrations in patients having side effects did not differ significantly from those in patients without side effects. Steady state desipramine plasma concentrations did not increase with age. Symptomatic orthostatic hypotension, the most common side effect encountered, occurred early in treatment at low desipramine plasma concentrations. Other side effects, usually described as anticholinergic, occurred exclusively in the 34 patients receiving both desipramine and antipsychotic drugs. The concentration of 2-hydroxy-desipramine, the total concentration of 2-hydroxy-desipramine and desipramine, and the ratio of 2-hydroxy-desipramine to desipramine were not higher in 11 patients having side effects than in a comparison group without side effects.

Inhibition of desipramine 2-hydroxylation by quinidine and quinine.

Urinary excretion of desipramine (DMI) and 2-hydroxydesipramine (2-OH-DMI) after single oral doses of 25 mg DMI was investigated in seven rapid and three slow debrisoquin hydroxylators, before and after pretreatment with either quinidine or its diastereoisomer quinine. After treatment with 800 mg quinidine daily for 2 days, excretion of 2-OH-DMI decreased by 96% in rapid hydroxylators and 68% in slow hydroxylators. After treatment with 750 mg quinine/day for 2 days, excretion of 2-OH-DMI in rapid hydroxylators was 54% lower than during the control experiment, whereas in slow hydroxylators no significant changes in the excretion pattern were observed. Unchanged DMI constituted only a minor fraction of recovered drug and no significant changes in its recovery were observed in either phenotypic group after pretreatment with quinidine or quinine. Thus both quinidine and quinine decreased the excretion of 2-OH-DMI. At similar does the effect of quinidine was much stronger than that of quinine, virtually transforming rapid hydroxylators into slow hydroxylators. The mechanism probably involves a stereoselective inhibition of DMI 2-hydroxylation.

Clinical implications of 2-hydroxydesipramine plasma concentrations.

The clinical utility of 2-hydroxydesipramine (2-OH-DMI) measurements was evaluated by examining the relationship of 2-OH-DMI concentrations in plasma to clinical outcome and side effects in depressed inpatients treated with desipramine (DMI). Studies were performed in responders and nonresponders to treatment and in patients experiencing subjective side effects or major adverse reactions necessitating interruption of treatment. Unlike DMI concentrations, 2-OH-DMI concentrations did not correlate with response. Summing the concentrations of parent drug and metabolite (DMI + 2-OH-DMI) did not improve the correlation over that achieved with DMI alone. Neither DMI, 2-OH-DMI, nor their sum correlated with subjective side effect totals or major adverse reactions. While our data do not permit any conclusions regarding the clinical activity of 2-OH-DMI, they suggest that its routine measurement in plasma is not likely to be useful in the management of depression.

Nonlinear desipramine kinetics: prevalence and importance.

Tricyclic antidepressant plasma levels have been used to guide dose adjustment in nonresponding patients, and recently 24-hour drug levels have been advocated for predicting therapeutic doses. Both methods of dose adjustment assume linear drug kinetics. Recent reports have suggested that desipramine kinetics are nonlinear, but the samples described were small, six subjects or fewer. In the current study, plasma desipramine concentrations were examined in 42 inpatients who were depressed who had achieved steady-state conditions with a low initial dose and subsequently with a higher dose. Desipramine concentrations increased significantly more than that predicted by the dose increase; however, only one third of the sample had substantial nonlinear changes (an increase in the concentration 50% greater than expected). In the remainder of the sample, disproportionate increases in plasma concentrations were not likely to be of clinical consequence.

Imipramine demethylation and hydroxylation: impact of the sparteine oxidation phenotype.

Eighteen healthy volunteers, selected according to their ability to oxidize sparteine, took single oral doses of 100 mg imipramine and desipramine. For imipramine the following clearances (L X min-1) were found in six rapid extensive metabolizers (EM), six slow EM, and six poor metabolizers (PM), respectively (mean and range): apparent oral clearance: 2.55 (1.39 to 3.47), 2.28 (1.18 to 4.26), and 1.35 (0.96 to 1.64). Clearance via demethylation was: 1.42 (0.61 to 2.01), 1.60 (0.78 to 3.81), and 1.09 (0.76 to 1.64); clearance via other pathways was: 1.13 (0.74 to 1.75), 0.69 (0.40 to 1.59), and 0.26 (0 to 0.46). For desipramine the apparent oral clearance (L X min-1) was 0.19 (0.12 to 0.24) in PM compared with 1.64 (1.46 to 1.80) and 1.03 (0.77 to 1.13) in rapid EM and slow EM. Extremely long elimination t1/2s of desipramine were seen in PM: 81 to 131 hours compared with 13 to 23 hours in EM. 2-OH-imipramine and 2-OH-desipramine were detectable in plasma of only the 12 EM, where the ratio-to-parent compound was higher in rapid EM than in slow EM. This study confirms that 2-hydroxylation of imipramine and desipramine depends almost exclusively on the sparteine oxygenase, whereas the demethylation of imipramine depends mainly on a different isozyme.

Antidepressant activity of 2-hydroxydesipramine.

We describe the relationship of 2-hydroxydesipramine (OH-DMI) plasma levels and response in a prospective DMI study in which dosage was rapidly adjusted to achieve a relatively uniform DMI plasma level. In prior studies, OH-DMI plasma levels were not related to response, but in these fixed-dose protocols the effects of OH-DMI are easily obscured by the higher concentrations of the parent drug. We hypothesized that in this study a contribution of OH-DMI to response might become apparent because DMI levels were relatively constant. Inpatients with nonpsychotic, unipolar DSM-III major depression who remained depressed (Hamilton score greater than 18) after 1 week of hospitalization without medication received a 4-week DMI trial. Twenty-four-hour drug plasma levels were used to adjust dose to reach a target DMI steady-state plasma level. Twenty-seven patients completed the trial. On every measure of response, total drug levels (DMI + OH-DMI) were more strongly correlated with outcome than were DMI levels alone. With multiple regression, both DMI and OH-DMI levels were independently and significantly associated with response. These findings suggest that OH-DMI has antidepressant activity.

Imipramine metabolites in blood of patients during therapy and after overdose.

Plasma or serum concentrations of imipramine and five of its nonconjugated metabolites (desipramine, 2-OH-imipramine, 2-OH-desipramine, imipramine-N-oxide, and didesipramine) were followed in three cases of imipramine overdose and during steady state in 24 patients on continuous imipramine treatment. In the overdose cases the imipramine and desipramine concentrations declined monoexponentially with t 1/2s of 12 to 21 and 31 to 37 hr. The 2-OH-imipramine and 2-OH-desipramine levels were lower and declined in parallel with their corresponding parent compounds. In the patients on continuous imipramine treatment, the steady-state levels of 2-OH-imipramine and 2-OH-desipramine were very low or immeasurable (less than 15 nmol/l) in five patients. In most patients (n = 18) the hydroxymetabolite levels were much higher with 2-OH-imipramine/imipramine ratios of 0.09 to 0.45 and 2-OH-desipramine/desipramine ratios of 0.36 to 0.86. In one patient there were particularly high ratios (2-OH-imipramine/imipramine, 0.85; 2-OH-desipramine/desipramine, 1.30). The patients with very low hydroxymetabolite levels had considerably higher desipramine levels than the others, indicating that the low metabolite levels were due to poor hydroxylation. In one of these poor hydroxylators a desipramine t 1/2 of about 120 hr was estimated after imipramine discontinuation. With increased imipramine dose the 2-OH-imipramine levels tended to rise little or not at all. Imipramine-N-oxide could only be detected in the overdose cases during the first 6 to 12 hr and didesipramine was generally present only when the desipramine levels were above 200 nmol/l.

Desipramine hydroxylation: variability and effect of antipsychotic drugs.

Steady-state plasma concentrations of desipramine (DMI), unconjugated 2-OH DMI, and total 2-OH DMI were measured in 82 depressed inpatients, 35 of whom were concurrently receiving a phenothiazine or butyrophenone antipsychotic drug. In the patients not on an antipsychotic, the ratio of unconjugated metabolite to parent varied from 0.01 to 1.5, with a median of 0.48, and was inversely related to the parent drug level. Antipsychotic drug was associated with higher DMI levels and a lower proportion of OH-metabolite (median, 0.23). In both groups the unconjugated form accounted for only about 10% of the total metabolite. No relationship of age, sex, drinking history, or smoking to DMI or 2-OH DMI levels was found.

Altered hydroxydesipramine concentrations in elderly depressed patients.

Recent reports demonstrate that hydroxy metabolites of desipramine (DMI) have pharmacologic activity and do not just produce side effects. In patients treated with DMI, we determined the ratio of 2-hydroxydesipramine (2-OH-DMI) to drug at steady state. The ratios in the elderly patients were higher than in younger patients, and whereas plasma levels of 2-OH-DMI increased with age, urinary clearances decreased. The known decrease in glomerular filtration with age may explain the selective increase of 2-OH-DMI concentration in the elderly.

Phenotypic consistency in hydroxylation of desmethylimipramine and debrisoquine in healthy subjects and in human liver microsomes.

The 2-hydroxylation of desmethylimipramine (DMI) and the 4-hydroxylation of debrisoquine (D) were studied in healthy subjects and in human liver microsomes. A single oral dose of DMI (25 mg) was given to 18 healthy subjects previously phenotyped with D (13 rapid and five slow hydroxylators). Urine was collected for 24 hr and DMI and total 2-hydroxydesmethylimipramine (2-OH-DMI) levels were determined by HPLC. The urinary ratio DMI/2-OH-DMI correlated strongly (r = 0.92) with the urinary ratio of D to 4-hydroxydebrisoquine (D/4-OH-D). The two hydroxylations were also studied in human liver microsomes from 10 different subjects. Formation rates of the hydroxylated metabolites correlated strongly (r = 0.869). Moreover, D competitively inhibited the 2-hydroxylation of DMI. These findings suggest that both are hydroxylated by the same cytochrome P-450 isozyme.

The nonlinear kinetics of desipramine and 2-hydroxydesipramine in plasma.

Plasma levels of desipramine (DMI) and the unconjugated form of its principal metabolite 2-hydroxydesipramine (OH-D) were measured under steady-state conditions in nine depressed inpatients during treatment with 75 mg DMI every 12 hr and after at least 1 wk of an increased dose of DMI (after steady state). When DMI dosage was raised after an initial steady state had been reached, the rise in plasma DMI level was proportionately greater than the increase in dosage, suggesting saturation of DMI elimination pathways. Levels of OH-D rose in proportion to dose, suggesting saturation of DMI elimination by 2-hydroxylation could not explain DMI plasma level changes. In contrast, there were no dose-dependent effects on the disposition of amitriptyline or its metabolite nortriptyline in subjects receiving the same amitriptyline dose.

Active metabolites of imipramine and desipramine in man.

Active hydroxy metabolites of imipramine (IMI) and desipramine (DMI) have been quantified in plasma and cerebrospinal fluid (CSF) from patients at steady-state. In plasma of prepubescent boys and adults the concentration of unconjugated 2-hydroxyimipramine is only 15% to 25% that of IMI; 2-hydroxydesipramine (OH-DMI) concentration, however, is usually 50% that of DMI and in some cases OH-DMI is the predominant compound. In CSF from adult patients the ratio of concentrations of OH-DMI/DMI is higher than in plasma. Judging from the CSF/plasma ratio 12% of DMI exists in the free form at steady state, whereas 16% of OH-DMI is free (P less than 0.02). There is no evidence for saturation of hydroxylation within the therapeutic dose and concentration ranges investigated. On the basis of a steady-state OH-DMI/DMI ratio of less than 1/30 in plasma 5% of the population studied could be classified as deficient DMI hydroxylators. This in the same as the incidence of deficient debrisoquine hydroxylators reported in other populations.

Desipramine and 2-hydroxydesipramine in human breast milk and the nursing infant's serum.

Desipramine and its metabolite 2-hydroxydesipramine were measured in the milk of a nursing mother and in the plasma of the mother and infant during administration of 300 mg/day of desipramine to the mother. Similar concentrations of both compounds were found in maternal plasma and milk. A pharmacokinetic plot of the milk over 24 hours showed the expected rise and fall of the substances following a single nighttime dose. Neither parent compound nor metabolite could be detected in the infant's serum even though the measurements were made shortly after peak ingestion by the infant. Furthermore, no clinical signs of toxicity were observed in the infant after 3 weeks of treating the mother with desipramine.

Effects of desipramine on clinical liver function tests.

Results of liver function tests in 46 depressed patients changed little during treatment with desipramine and were uncorrelated with drug plasma levels. The findings suggest that tricyclic-associated hepatitis, rather than being dose dependent, is an uncommon, idiosyncratic phenomenon.

Inhibition of microsomal cytochromes P450 in rat liver by the tricyclic antidepressant drug desipramine and its primary oxidized metabolites.

N-Monoalkyl substituted tricyclic antidepressants like desipramine (DES) undergo cytochrome P450 (P450)-mediated biotransformation in liver to produce inhibitory metabolite-intermediate (MI) complexes with the enzyme. However, additional oxidation pathways that generate isolable metabolites have also been identified, so that the relationship between MI complexation and total oxidative metabolism is unclear. The present study investigated the capacity of DES and three putative metabolites (2-hydroxy- and 10-hydroxy-DES and N,N-didesmethylimipramine; DIDES) to elicit MI complexation and inhibit P450-dependent activities in rat liver. MI complexation of P450 was produced by DES, but not with the three metabolites, in NADPH-supplemented microsomes. Consistent with this finding, inhibition of testosterone hydroxylation pathways was enhanced markedly by prior incubation of DES with NADPH and microsomes. Direct addition of DIDES to incubations resulted in significant inhibition of P450 activities (IC50s of 35 and 29 microM against estradiol 6 beta- and 16 alpha-hydroxylation mediated by P450s 3A2 and 2C11, respectively). Neither 2-hydroxy- nor 10-hydroxy-DES directly inhibited testosterone hydroxylation (IC50s > 100 microM). However, after a preincubation step between these metabolites and NADPH-fortified microsomes, enhanced inhibition of reactions mediated by P450 3A2 and P450 2C11/2A1 was produced by 2-hydroxy-DES and 10-hydroxy-DES, respectively. Metabolism of DES to DIDES and 2-hydroxy-DES was estimated as 7.77 +/- 0.48 nmol/mg protein/hr (10-hydroxy-DES was not detected). It is likely that secondary oxidized metabolites derived from 2-hydroxy-DES, as well as the primary metabolite DIDES, may contribute to the inhibition of P450 activity during DES biotransformation. These results indicate that the 2-hydroxy-, 10-hydroxy-, and N-desmethyl-metabolites of DES are not involved in MI complexation, but complexation is not the sole mechanism by which DES inhibits microsomal drug oxidation that may lead to pharmacokinetic drug interactions.

Kinetic properties of the metabolism of imipramine and desipramine in isolated rat hepatocytes.

The metabolism of imipramine and desipramine was examined by using isolated rat hepatocytes. The enzyme systems having high-affinity-and-low-capacity and low-affinity-and-high-capacity kinetic properties were found to catalyze aromatic 2-hydroxylations of imipramine and desipramine, and aliphatic N-demethylation of imipramine, respectively. The Km and Vmax values for N-demethylation of imipramine (which formed desipramine) were about 5-10 and 5 times larger than those of both 2-hydroxylations respectively. A competitive inhibition between the 2-hydroxylations of imipramine and desipramine ("parallel pathway interaction") (Chiba M, Fujita S and Suzuki T, J Pharm Sci 77: 944-947, 1988), observed using liver microsomes, was found also in isolated hepatocytes. It was concluded that the characteristics of imipramine metabolism observed in liver microsomes were well reproduced in isolated rat hepatocytes.