Demethylation Of Imipramine Research Articles

The effect of antidepressants on ethylmorphine and imipramine N-demethylation in rat liver microsomes.

The effects of single and multiple doses of desipramine, amitriptyline or citalopram on the rat liver microsomal cytochrome P-450 level and on the rate of ethylmorphine and imipramine demethylation in-vitro have been investigated. Desipramine, amitriptyline or citalopram when given to rats as a single dose, did not affect the level of cytochrome P-450 in the liver microsomes, however, there was a tendency towards acceleration of imipramine, and particularly ethylmorphine, demethylation. Prolonged administration of desipramine and citalopram, but not amitriptyline, elevated the microsomal level of cytochrome P-450 and accelerated the rate of ethylmorphine demethylation. All the drugs investigated, when given chronically, inhibited the rate of imipramine demethylation. Since demethylation of ethylmorphine and imipramine in a CO atmosphere was inhibited by ca 90% for the former and only by 58% for the latter, it can be assumed that prolonged administration of the drugs investigated has two different effects on the oxygenase systems in rat liver microsomes: on the one hand they stimulate the cytochrome P450 oxygenase system involved in ethylmorphine demethylation and, on the other, they inhibit the other microsomal oxygenase system involved in demethylation of imipramine.

Delayed ontogenesis of CYP1A2 in the human liver.

The ontogenesis of CYP1A proteins was investigated in a human liver bank composed of fetal, neonatal and adult samples. In immunoblots, a polyclonal antibody raised against rat CYP1A1, cross-reacted with cDNA-expressed human CYP1A1 and CYP1A2. In adult liver microsomes, this antibody reacted with a single band identified as the CYP1A2 protein, while no CYP1A1 could be detected. CYP1A2 protein was absent in microsomes prepared from fetal and neonatal livers and its levels increased in infants aged 1-3 months to attain 50% of the adult value at one year. Enzymatic activities supported by CYP1A proteins were assayed on these samples. Methoxyresorufin demethylase supported by the CYP1A2 recombinant protein followed the same ontogenic profile as the CYP1A2 protein. In liver microsomes, the demethylation of imipramine was essentially due to CYP1A2 and to a smaller extent to CYP3A. In fetuses and early neonates, CYP3A proteins were responsible for the low demethylation of imipramine (3-4% of the adult activity) before the onset of CYP1A2 and the subsequent rise of activity. Immunodetection and enzymatic activities were consistent with the absence of CYP1A1 and the late expression of CYP1A2 in the human liver, compared to the early rise of CYP3A4, CYP2C, CYP2D6, and CYP2E1 proteins.

Effect of Fluvoxamine on the Pharmacokinetics of Imipramine and Desipramine in Healthy Subjects

The effect of the selective serotonin reuptake inhibitor fluvoxamine (100 mg/day for 10 consecutive days) on the kinetics of a single oral dose of imipramine (50 mg) and desipramine (100 mg) was investigated in 12 healthy subjects. Compared with a control session, treatment with fluvoxamine caused a significant prolongation of imipramine half-life (from 22.8 +/- 6.4 to 40.5 +/- 5.0 h, means +/- SD, p < 0.01) and a marked decrease in imipramine apparent oral clearance (from 1.02 +/- 0.19 to 0.28 +/- 0.06 L/h/kg, p < 0.0001). No significant changes in desipramine kinetics were observed during fluvoxamine treatment. These findings indicate that, at the dosage tested, fluvoxamine markedly inhibits the demethylation of imipramine without affecting significantly the CYP2D6-mediated hydroxylation of desipramine.

The mephenytoin oxidation polymorphism is partially responsible for the N-demethylation of imipramine

The metabolism of imipramine in six poor metabolizers of mephenytoin was compared with the metabolism of 16 extensive metabolizers of mephenytoin from an earlier study. Each subject was given single doses of 100 mg imipramine hydrochloride and 100 mg desipramine hydrochloride on separate occasions. Imipramine demethylation clearance was 0.74 L.min-1 (mean; range, 0.31-1.24) in poor metabolizers of mephenytoin compared with 1.43 L.min-1 (mean; range, 0.61-3.81) in extensive metabolizers of mephenytoin (p = 0.01, Mann-Whitney U test). It has previously been shown that the imipramine clearance by way of other pathways and desipramine oral clearance, both largely representing 2-hydroxylation, are considerably lower in poor metabolizers of sparteine than in extensive metabolizers of sparteine. In contrast, five subjects who were poor metabolizers of mephenytoin and extensive metabolizers of sparteine and a control group of 11 subjects who were extensive metabolizers of mephenytoin and sparteine showed no statistically significant difference with regard to these parameters. One subject who was a poor metabolizer of mephenytoin and sparteine had the lowest imipramine oral clearance of all 22 subjects studied. In conclusion, this and an earlier study show that the oxidation of imipramine is mediated by means of two different polymorphic P450 isozymes, 2-hydroxylation by way of the sparteine oxygenase (P450IID6) and demethylation by way of the mephenytoin oxygenase (P450IIC8).

Effects of lithium on alterations of pharmacokinetics of imipramine and on the related changes of monoamines in rat brain

Rats were given chronically i.p. imipramine (20 mg/kg), LiCl (1 mg/kg) or both drugs to examine the effects of lithium (Li) on the alterations of imipramine pharmcokinetics in the whole brain and on the imipramine-related changes of norepinephrine (NE) and serotonin (5-HT) levels in the brain. When rats were given Li for 3 days, followed by a single injection of imipramine, the concentrations of desipramine in the brain and serum were higher than those in the vehicle-treated rat, although the imipramine concentrations in both tissues did not differ in Li- and in vehicle-treated rats. In rats receiving both drugs for 10 days, the steady state levels of imipramine and desipramine in the brain were the same as those in the vehicle-treated rats but the steady state level of desipramine was reached earlier with Li treatments presumably because of the enhanced demethylation of imipramine. Consequently, the desipramine-dependent alterations of NE and 3-methoxy-4-hydroxyphenylglycol levels in the brain appeared to be induced earlier and more markedly when Li was given simultaneously. As the 5-hydroxyindole acetic acid (5-HIAA) levels were elevated in the brains of Li-treated rats and reduced in brains of imipramine-treated rats as compared with the level in vehicle-treated rats, the 5-HIAA level in rats receiving both drugs was equivalent to that in vehicle-treated rats.

Quinidine inhibits the 2-hydroxylation of imipramine and desipramine but not the demethylation of imipramine.

On separate occasions 6 extensive metabolizers of sparteine took a single oral dose of 100 mg imipramine and desipramine before and during the intake of quinidine sulphate 200 mg/day. During quinidine the total oral clearance of imipramine on average was reduced by 35%, and that of desipramine by 85%. The clearance of imipramine via demethylation was not significantly reduced during quinidine administration, whereas its clearance by other pathways, largely 2-hydroxylation, was reduced by more than 50%. 2-OH-Imipramine and 2-OH-desipramine were detected in plasma before (maximum concentrations 30-100 nmol.l-1) but not during quinidine. It appears that quinidine is a potent inhibitor of the sparteine/debrisoquine oxygenase, P450dbl, which is responsible for the 2-hydroxylation of imipramine and desipramine, but not of the P450 isozyme responsible for the demethylation of imipramine.

Measurement of the fractional ratio of demethylation of imipramine in rat by using a co-administration technique.

Desipramine (DMI), a metabolite of imipramine (IP), has approximately the same antidepressant activity as the mother compound. It is important to clarify the fractional ratio of metabolism from IP to DMI (fmIP-DMI). We have developed a new technique to estimate fmIP-DMI by the use of a co-administration technique. After co-administration of a mixture of IP-d4 and DMI-d0 to rats, area under the plasma concentration-time curve (AUC) and renal excretion of DMI-d4 and DMI-d0 were examined. fmIP-DI estimated from the ratio of AUC of DMI-d4 to that of DMI-d0 was almost the same as fmIP-DMI calculated from the ratio of the amount of excreted DMI-d4 to that of DMI-d0. This means that fmIP-DMI can be calculated by using the renal excretion data instead of AUC. The relationship between fmIP-DMI and IP-d4 dose is also discussed.

Metabolic interaction between imipramine and carbamazepine in vivo and in vitro in rats.

The pharmacokinetic consequences of the combination of carbamazepine with imipramine in male Wistar rats have been investigated. It was found that a 2-week treatment with the combination resulted in the increase of the concentrations of the parent compounds and a simultaneous decrease in their metabolites in blood plasma i.e. carbamazepine inhibited imipramine demethylation in the side chain while imipramine inhibited carbamazepine 10,11-epoxidation. The velocity of imipramine 2-hydroxylation and 10,11-epoxy-carbamazepine hydration did not seem to be changed by the combination. On the basis of studies in vitro it is concluded that the observed metabolic interaction between carbamazepine and imipramine is due to the competition of the drugs for the active centre of cytochrome P 450 and to a certain qualitative alteration of the enzyme by imipramine as can be deducted from the decrease of carbamazepine binding to the cytochrome.

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.

The effect of neuroleptics on imipramine demethylation in rat liver microsomes and imipramine and desipramine level in the rat brain

A study of the cytochrome P-450 level and imipramine (IMI) demethylase activity in liver microsomes of rats treated concurrently with IMI and chlorpromazine (CPZ) or IMI and chlorprothixene (CPX) for two weeks were carried out. Concomitant administration of IMI and CPZ or IMI and CPX elevated the cytochrome P-450 level and accelerated IMI demethylation in in vitro study. Kinetic study of IMI demethylation carried out in the absence or in the presence of CPZ or CPX revealed that those neuroleptics inhibited IMI demethylation via competitive mechanism. Simultaneously with the enzymatic study the brain level of IMI and its demethylated metabolite desipramine (DMI) was assessed. It was found that 1 hr after withdrawal of IMI and CPZ or IMI and CPX the brain level of IMI was elevated in comparison with that of IMI treated animals, and the ratio between DMI/IMI brain concentration was decreased. When the assessment of IMI and DMI brain level was performed 24 hr after withdrawal of IMI and CPZ or IMI and CPX, there was no difference between the concentration of IMI and DMI in both, experimental and control animals.

Differential effects of cimetidine and ranitidine on imipramine demethylation and desmethylimipramine hydroxylation by human liver microsomes.

The effect of cimetidine and ranitidine on the demethylation of imipramine (IMI) and on the hydroxylation of desmethylimipramine (DMI) was studied in microsomes from four human livers. Cimetidine inhibited both demethylation of IMI and 2-hydroxylation of DMI, whilst the effect of ranitidine was not statistically significant. 2-hydroxylation of DMI is probably mediated by debrisoquine hydroxylase, a cytochrome P-450 isozyme that is monogenically controlled. The results suggest that cimetidine inhibits this enzyme.

Oxidation of tricyclic antidepressant drugs, debrisoquine and 7-ethoxyresorufin, by human liver preparations.

Data obtained from human studies in vivo show that the dispositions of the tricyclic antidepressant drugs desmethylimipramine (DMI) and nortriptyline are related to the debrisoquine hydroxylation phenotype. To obtain insight into the enzymic mechanisms behind this, the metabolism of debrisoquine and antidepressant drugs by human liver preparations have been studied. The 2-hydroxylation of DMI in vitro correlates with the 4-hydroxylation of debrisoquine among various livers (rs = 0.90). Debrisoquine inhibits DMI hydroxylation competitively, and DMI inhibits debrisoquine hydroxylation, suggesting that DMI hydroxylation is catalysed by the debrisoquine hydroxylase in human liver. By monitoring the hydroxylation of DMI in various fractions during separation and purification of cytochrome P-450 from human liver microsomes we have purified a cytochrome P-450 which efficiently hydroxylates this drug. The apparently electrophoretically homogeneous enzyme had a molecular weight of 51,500 and hydroxylated DMI and debrisoquine at rates of up to 0.95 and 0.45 nmol/min . nmol P-450, respectively. This is probably the major debrisoquine hydroxylating cytochrome P-450 in man. Nortriptyline 10-hydroxylation correlates strongly (r = 0.96) with debrisoquine hydroxylation in human liver microsomes. Nortriptyline inhibits DMI-hydroxylation competitively, and the drug also inhibits the 4-hydroxylation of debrisoquine. Thus it is probable that nortriptyline is hydroxylated by debrisoquine hydroxylase. Imipramine N-demethylation did not correlate significantly (P greater than 0.1) with debrisoquine hydroxylation among microsomes from nine livers. However, if a liver from a subject, which was a poor metabolizer of debrisoquine in vivo, was included, a correlation was obtained (r = 0.79, P less than 0.01, N = 10). Imipramine demethylation also correlated with DMI-hydroxylation only if the 'poor metabolizer' liver was included (r = 0.75, P less than 0.05, N = 10). Debrisoquine inhibited imipramine demethylation competitively. The data indicate that imipramine can interact with debrisoquine- and DMI-hydroxylase, but it is uncertain if this enzyme plays an important quantitative role in its demethylation. Ethoxyresorufin O-deethylation correlated with DMI hydroxylation (r = 0.80) in human liver preparations, and DMI inhibited the former reaction in what is probably a mixed competitive-non-competitive inhibition. Liver preparations from a subject who was a poor oxidizer of debrisoquine both in vivo and in vitro had unusually low capacity to metabolize ethoxyresorufin. Thus ethoxyresorufin, at least partly, seems to interact with an enzyme that can metabolize DMI in human liver.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of imipramine and desipramine on mixed function oxidase in rats.

The effects of imipramine and desipramine on hepatic mixed function oxidase were measured in male Wistar rats. Both antidepressants increased hepatic cytochrome P-450 when given b.i.d. for 7 or 14 days. In vitro demethylation of imipramine was rather depressed than increased. 14CO2 exhalation from [N-methyl-14C] benzphetamine was increased by pretreatment with the antidepressants as well as with phenobarbital, but imipramine had no influence on the exhalation of the label from [6-methoxy-14C]- or [7-methoxy-14C]-scoparone. No difference was observed between treatment with imipramine or desipramine. It is concluded that changes of the demethylation rate of imipramine are not responsible for its delayed elimination observed after chronic treatment (Daniel et al. 1981).

Demethylation of Imipramine by Enteric Bacteria

The ability of a number of aerobic and anaerobic bacteria to N-demethylate imipramine (I) to desipramine (II) has been investigated. Of the bacteria investigated, almost half were known inhabitants of the human GI tract. More than half of the enteric bacteria studied were capable of N-demethylating imipramine (I) to desipramine (II) to some extent in at least one medium. It was found that the medium in which the organism was grown had a significant effect on the N-demethylase activity observed.

The demethylation of imipramine and clomipramine as apparent from their plasma kinetics

The demetylation of imipramine and clomipramine was studied after administration by different routes of single doses of clomipramine hydrochloride and multiple doses of clomipramine as well as imipramine hydrochloride. Five healthy volunteers received 1 mg of clomipramine hydrochloride/kg body weight as single oral and intramuscular doses on different occasions for the purpose of studying the plasma levels of clomipramine and the desmethylclomipramine formed. Desmethylclomipramine was found in the plasma in four of the subjects after oral intake but only in one subject after intramuscular injection. The peak levels of clomipramine were considerably higher after intramuscular than after oral administration. The half-lives of clomipramine after oral administration ranged from 11.6-35.8 h (M = 20.8 +/- 4.0) and after intramuscular administration from 20.1--39.6 h (M = 24.7 +/- 3.7). Twenty subjects received either imipramine or clomipramine both orally and intramuscularly during a period of 3 weeks in a crossover design. The plasma levels of imipramine and clomipramine and their demethylated metabolites desipramine and desmethylclomipramine were determined during the treatment. The ratio between the plasma level of the parent drug and its demethylated metabolite was on average twice as high during intramuscular as during oral treatment.

Effect of dietary dieldrin on the liver and drug metabolism in the female Swiss-Vancouver mouse.

Female Swiss-Vancouver (SWV) mice, 13 weeks old, were exposed to dietary dieldrin for up to 10 weeks. Liver mass, hepatic microsomal protein and cytochrome P-450, and the in vitro metabolism of imipramine were determined at intervals. Dieldrin (5, 10, 15, and 20 ppm) caused hepatomegaly and increases in P-450; both effects were dose-related. All doses increased microsomal protein by about 30% (control value was 15.3 mg of protein per gram of liver). Maximal responses were attained by 2 weeks of exposure and maintained thereafter. Plateau liver masses at these respective doses were 111, 119, 133, and 162% of the control value (57.9 mg of liver per gram of body weight). Cytochrome P-450 was, respectively, 124, 142, 185, and 173% of the control value (0.93 nmol per milligram of microsomal protein). These changes decreased pentobarbital sleeping times by an average of 540% in animals fed 5, 15, or 25 ppm for 4 weeks. Similarly, the latency to the onset of anesthesia was increased by 26% at all doses. The N-oxidation and aryl-hydroxylation of imipramine increased with age, while demethylation decreased. The hydroxylation and demethylation of imipramine was increased after 2 and 4 weeks, respectively, of exposure to 20 ppm; N-oxidation was decreased. Longer exposure to lower doses caused similar changes. The changes in liver parameters and imipramine metabolism induced by 4 weeks exposure to 20 ppm were absent 6 weeks after exposure ceased.

The effects of pretreatment of mice with norethindrone on the metabolism of 14C-imipramine by the liver microsomal drug-metabolizing enzymes.

An assay procedure for the metabolism of 14C-imipramine in vitro is described. Using female mouse liver as the enzyme source, the conditions of the assay have been determined for the formation of 2-hydroxyimipramine, desmethylimipramine, and imipramine-N-oxide. Demethylation was linear up to a substrate concentration of 120 μg/3.5 ml, N-oxidation was linear up to a concentration of imipramine of 70 μg/3.5 ml, and hydroxylation up to 20 μg/3.5 ml of reaction mixture. Desmethylimipramine competitively inhibited both hydroxylation and demethylation, whereas imipramine-N-oxide had no effect. Pretreatment of mice with norethindrone decreased hydroxylation, and increased demethylation. Cytochrome P-450 was also increased by this progestin; N-oxidation was not changed. The effects of concurrent administration of norethindrone with known inducers or inhibitors of drug metabolism have been determined. From these experiments, it was concluded that the induction of cytochrome P-450 by norethindrone is not responsible for the increased demethylation of imipramine. Rather, it appeared that competitive inhibition of hydroxylation of imipramine by the norethindrone allowed more of the drug to be demethylated.

Effect of Vehicles on Metabolism of Serotonin and Imipramine

Effects of different vehicles on excretion and metabolism of serotonin and imipramine were studied.Rats receiving serotonin-14C in dimethyl sulfoxide or propylene glycol excreted radioactivity in the urine slower and in lesser amounts than when water was the vehicle.However, the urinary excretion of radioactivity by rats administered a single dose of imipramine-14C was the same for the three vehicles over a period of 72 hr.Dimethyl sulfoxide and propylene glycol caused not only an increase in the amount of urinary 5-hydroxytryptophol and N-acetylserotonin, but also an increase in the conjugated form of these two metabolites as well as that of 5-hydroxyindoleacetic acid.These two nonaqueous vehicles caused a diminished demethylation of imipramine when compared to the water vehicle.

Imipramine demethylation and norepinephrine storage in brain

Imipramine administered i.p. in rats following intraventricular administration of DL-NE-7 3H decreases the amount of unchanged NE in the brain and leads to a significant increase of normetanephrine, methylated acid and methylated glycol. The results are the same if demethylation of imipramine is inhibited by pretreatment with SKF 525-A. According to current findings, imipramine itself has an effect on the storage capacity of brain tissue for NE.

Demethylation of imipramine in the rat as influenced by SKF 525-A and by different routes of administration

Intraperitoneal injection of imipramine (IP) to normal rats of two strains is followed by the appearance of desmethylimipramine (DMI) in brain and other tissues. In rats pretreated with SKF 525-A the DMI concentrations are considerably decreased but still detectable. The inhibition of IP metabolism by SKF 525-A can also be demonstrated in vitro . After injection of IP by various routes of administration considerable differences in the IP and DMI concentrations in brain or other tissues are observed. E.g. practically no DMI can be detected in brain after s.c. or i.m. injection. It is concluded from the results that the mode of administration highly influences both the rates of drug absorption and of its metabolism, the latter according to systematic (s.c., i.m.) or portal (i.p., p.o.) absorption. The results are also discussed in respect to the recently published (1) absence of brain DMI in rats treated with IP.