Imipramine demethylation in vivo: impact of CYP1A2, CYP2C19, and CYP3A4

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Precision Medicine in Antidepressants Treatment

Precision medicine uses innovative approaches to improve disease prevention and treatment outcomes by taking into account people's genetic backgrounds, environments, and lifestyles. Treatment of depression is particularly challenging, given that 30-50% of patients do not respond adequately to antidepressants, while those who respond may experience unpleasant adverse drug reactions (ADRs) that decrease their quality of life and compliance. This chapter aims to present the available scientific data that focus on the impact of genetic variants on the efficacy and toxicity of antidepressants. We compiled data from candidate gene and genome-wide association studies that investigated associations between pharmacodynamic and pharmacokinetic genes and response to antidepressants regarding symptom improvement and ADRs. We also summarized the existing pharmacogenetic-based treatment guidelines for antidepressants, used to guide the selection of the right antidepressant and its dose based on the patient's genetic profile, aiming to achieve maximum efficacy and minimum toxicity. Finally, we reviewed the clinical implementation of pharmacogenomics studies focusing on patients on antidepressants. The available data demonstrate that precision medicine can increase the efficacy of antidepressants and reduce the occurrence of ADRs and ultimately improve patients' quality of life.

Pharmacogenetics of drug oxidation via cytochrome P450 (CYP) in the populations of Denmark, Faroe Islands and Greenland

Denmark, the Faroe Islands and Greenland are three population-wise small countries on the northern part of the Northern Hemisphere, and studies carried out here on the genetic control over drug metabolism via cytochrome P450 have led to several important discoveries. Thus, CYP2D6 catalyzes the 2-hydroxylation, and CYP2C19 in part catalyzes the N-demethylation of imipramine. The phenomenon of phenocopy with regard to CYP2D6 was first described when Danish patients changed phenotype from extensive to poor metabolizers during treatment with quinidine. It was a Danish extensive metabolizer patient that became a poor metabolizer during paroxetine treatment, and this was due to the potent inhibition of CYP2D6 by paroxetine, which is also is metabolized by this enzyme. Fluoxetine and norfluoxetine are also potent inhibitors of CYP2D6, and fluvoxamine is a potent inhibitor of both CYP1A2 and CYP2C19. The bioactivation of proguanil to cycloguanil is impaired in CYP2C19 poor metabolizers. The O-demethylation of codeine and tramadol to their respective my-opioid active metabolites, morphine and (+)-O-desmethyltramadol was markedly impaired in CYP2D6 poor metabolizers compared to extensive metabolizers, and this impairs the hypoalgesic effect of the two drugs in the poor metabolizers. The frequency of CYP2D6 poor metabolizers is 2%-3% in Greenlanders and nearly 15% in the Faroese population. The frequency of CYP2C19 poor metabolizers in East Greenlanders is approximately 10%. A study in Danish mono and dizygotic twins showed that the non-polymorphic 3-N-demethylation of caffeine catalyzed by CYP1A2 is subject to approximately 70% genetic control.

Association of graded allele-specific changes in CYP2D6 function with imipramine dose requirement in a large group of depressed patients

The inactivation and clearance of the tricyclic antidepressant imipramine is dependent on CYP2D6 activity. First, CYP2C19 converts imipramine into the active metabolite desipramine, which is then inactivated by CYP2D6. This retrospective single center study aimed to prove whether CYP2C19 and ample CYP2D6 genotyping (taking into consideration four null alleles and three decreased-activity alleles) could be used to predict imipramine and desipramine plasma concentrations in depressed patients, and whether genotype-based drug dose recommendations might assist in the early management of imipramine pharmacotherapy. In 181 subjects with major depressive disorder, drug doses were recorded, imipramine and desipramine plasma concentrations were monitored and CYP2C19 (*2) and CYP2D6 genotype (*3, *4, *5, *6, *9, *10, *41 and gene duplication) were obtained, yielding graded allele-specific CYP2D6 patient groups. Desipramine and imipramine+desipramine plasma concentration per drug dose unit, imipramine dose at steady state, and imipramine dose requirement significantly depended on CYP2D6 genotype (Kruskal-Wallis test, P<0.0001). Mean (+/-s.d.) drug dose requirements were 131 (+/-109), 155 (+/-70), 217 (+/-95), 245 (+/-125), 326 (+/-213), and 509 (+/-292) mg imipramine/day in carriers of 0, 0.5, 1, 1.5, 2, and >2 active CYP2D6 genes, respectively. Our protocol for CYP2D6 genotyping will thus importantly aid in the prediction of imipramine metabolism, allowing for the use of an adjusted starting dose and faster achievement of predefined imipramine+desipramine plasma levels in all genetic patient subgroups. Therefore, therapeutic efficacy and efficiency may be improved, the number of adverse drug reactions decreased, and hospital stay reduced.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

An autopsy case of imipramine poisoning

We present a fatal imipramine poisoning. Quantitative analysis of imipramine and its metabolite, desipramine, was performed by high-performance liquid chromatography. The concentrations of imipramine and desipramine were 18.67 microg/mL and 6.21 microg/mL in heart blood and 6.90 microg/mL and 1.77 microg/mL in the femoral venous blood, respectively. We concluded that the cause of death was due to imipramine poisoning.

Different effects of desipramine on bufuralol 1''-hydroxylation by rat and human CYP2D enzymes

Inhibitory effects of desipramine (DMI) on rat and human CYP2D enzymes were studied using bufuralol (BF) 1''-hydroxylation as an index. Inhibition was examined under the following two conditions: 1) DMI was co-incubated with BF and NADPH in the reaction mixture containing rat or human liver microsomes or yeast cell microsomes expressing rat CYP2D1, CYP2D2 or human CYP2D6 (co-incubation); 2) DMI was preincubated with NADPH and the same enzyme sources prior to adding the substrate (preincubation). When either rat liver microsomes or recombinant CYP2D2 was employed, the preincubation with DMI (0.3 microM) caused a greater inhibition of BF 1''-hydroxylation than the co-incubation did, whereas BF 1''-hydroxylation by rat CYP2D1 was not markedly affected under the same conditions. The inhibitory effect of DMI on BF 1''-hydroxylation by human liver microsomal fractions or recombinant CYP2D6 was much lower than that on the hydroxylation by rat liver microsomes or CYP2D2. Kinetic studies demonstrated that the inhibition-type changed from competitive for the co-incubation to noncompetitive for the preincubation in the case of CYP2D2, whereas the inhibition-type was competitive for both the co-incubation and the preincubation in the case of CYP2D6. Furthermore, the loss of activity of rat CYP2D2 under the preincubation conditions followed pseudo-first-order kinetics. Binding experiments employing the recombinant enzymes and [(3)H]-DMI revealed that CYP2D2 and CYP2D6 were the only prominent proteins to which considerable radioactive DMI metabolite(s) bound. These results indicate that rat CYP2D2 biotransforms DMI into reactive metabolite(s), which covalently bind to CYP2D2, resulting in inactivation of the enzyme. In contrast, human CYP2D6 may also biotransform DMI into some metabolite(s) that covalently bind to CYP2D6, but that do not inactivate the enzyme.

Chip-Based P450 Drug Metabolism Coupled to Electrospray Ionization-Mass Spectrometry Detection

A chip-based P450 in vitro metabolism assay coupled with ESI-MS and ESI-MS/MS detection is described in this paper. The chips were made of a cyclic olefin polymer using a hot embossing process. The introduction of reagent solutions into the chip was carried out using fused-silica capillaries coupled to two syringes with the flow rate controlled by a syringe pump. Initial experiments described here employed a small commercial guard column in an off-chip format to desalt and concentrate the products of the enzymatic reaction prior to ESI-MS analysis. The system was used both to yield the Michaelis constant (K(m)) of the P450 biotransformation of imipramine into desipramine and to determine the IC50 value of a chemical inhibitor (tranylcypromine) for this CYP2C19-mediated reaction. The results demonstrated that the kinetics of the reaction inside the 4-microL volume within the channels of the cyclic olefin polymer chip provided results in agreement with those reported in the literature using conventional assays. The above reactions were carried out using human liver microsomes, and the metabolites were detected by ESI-MS showing the potential of the chip-based P450 reaction for metabolite screening studies as well as for P450 inhibition assays. A porous monolithic column was subsequently integrated into the chip to perform the reaction mixture cleanup process in an integrated fashion on the chip that is necessary for ESI-MS detection. The miniature monolithic SPE column was prepared in situ inside the chip via UV-initiated polymerization. The results obtained using the integrated system demonstrated the possibility of performing P450 enzymatic reactions in a microvolume reaction chamber coupled directly to ESI-MS detection and required less than 4 microg of HLM protein.

Effects of polymorphisms in CYP2D6, CYP2C9, and CYP2C19 on trimipramine pharmacokinetics

Little is known about the impact of cytochrome P450 polymorphisms on the metabolism of trimipramine, which is still widely used as antidepressant due to its positive effect on sleep patterns. A single oral dose of 75 mg trimipramine was given to 42 healthy volunteers selected according to their CYP2D6, CYP2C19, and CYP2C9 genotypes. The reference group included 8 subjects with homozygous active wild-type genotypes of all 3 enzymes (EM). This group was compared with 7 intermediate (IM) with 1 and 7 poor metabolizers (PM) with zero active alleles of CYP2D6 and CYP2C19, respectively, and with 4 subjects with the genotype CYP2C9*3/*3. Pharmacokinetics of trimipramine and its demethylated metabolite strongly depended on the CYP2D6 genotype. Median oral clearance of trimipramine was 276 L/h (range 180-444) in the reference group but only 36 L/h (range 24-48) in CYP2D6 PMs (P < 0.001). These differences could only be explained by an effect of CYP genotypes on both parameters, systemic clearance and bioavailability, the latter being at least 3-fold higher in CYP2D6 PMs than in the reference group. The desmethyltrimipramine area under the concentration-time curve was 40-fold greater in CYP2D6 PMs than in the reference group (1.7 vs. 0.04 mg/L x h in EMs), but below the quantification limit in most carriers of deficiencies of CYP2C19 or CYP2C9. This indicates that both CYP2C enzymes contribute to the demethylation of desmethyltrimipramine and CYP2D6 to further metabolism.

Toxicological interactions involving psychiatric drugs and alcohol: an update

This review focuses on the toxicological interactions between alcohol (ethanol) and psychiatric drugs (antidepressants and antipsychotics), including those leading to fatal poisoning. Acute or chronic ingestion of alcohol when combined with psychiatric drugs may lead to several clinically significant toxicological interactions. The metabolism of these drugs is generally but not always delayed by acute alcohol ingestion. Drugs undergoing metabolism may also show increased metabolic clearance with chronic alcohol ingestion. Therefore, the net effect may be influenced by internal (e.g. disease, age, gender), external (e.g. environment, diet) and pharmacokinetic (e.g. dose, timing of ingestion, gastrointestinal absorption, distribution and elimination) factors. Cases of fatal poisoning involving coadministration of psychiatric drugs, alcohol and other drugs prompted this review.

Clinical significance of the cytochrome P450 2C19 genetic polymorphism

Cytochrome P450 2C19 (CYP2C19) is the main (or partial) cause for large differences in the pharmacokinetics of a number of clinically important drugs. On the basis of their ability to metabolise (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as extensive metabolisers (EMs) or poor metabolisers (PMs). Eight variant alleles (CYP2C19*2 to CYP2C19*8) that predict PMs have been identified. The distribution of EM and PM genotypes and phenotypes shows wide interethnic differences. Nongenetic factors such as enzyme inhibition and induction, old age and liver cirrhosis can also modulate CYP2C19 activity. In EMs, approximately 80% of doses of the proton pump inhibitors (PPIs) omeprazole, lansoprazole and pantoprazole seem to be cleared by CYP2C19, whereas CYP3A is more important in PMs. Five-fold higher exposure to these drugs is observed in PMs than in EMs of CYP2C19, and further increases occur during inhibition of CYP3A-catalysed alternative metabolic pathways in PMs. As a result, PMs of CYP2C19 experience more effective acid suppression and better healing of duodenal and gastric ulcers during treatment with omeprazole and lansoprazole compared with EMs. The pharmacoeconomic value of CYP2C19 genotyping remains unclear. Our calculations suggest that genotyping for CYP2C19 could save approximately 5000 US dollars for every 100 Asians tested, but none for Caucasian patients. Nevertheless, genotyping for the common alleles of CYP2C19 before initiating PPIs for the treatment of reflux disease and H. pylori infection is a cost effective tool to determine appropriate duration of treatment and dosage regimens. Altered CYP2C19 activity does not seem to increase the risk for adverse drug reactions/interactions of PPIs. Phenytoin plasma concentrations and toxicity have been shown to increase in patients taking inhibitors of CYP2C19 or who have variant alleles and, because of its narrow therapeutic range, genotyping of CYP2C19 in addition to CYP2C9 may be needed to optimise the dosage of phenytoin. Increased risk of toxicity of tricyclic antidepressants is likely in patients whose CYP2C19 and/or CYP2D6 activities are diminished. CYP2C19 is a major enzyme in proguanil activation to cycloguanil, but there are no clinical data that suggest that PMs of CYP2C19 are at a greater risk for failure of malaria prophylaxis or treatment. Diazepam clearance is clearly diminished in PMs or when inhibitors of CYP2C19 are coprescribed, but the clinical consequences are generally minimal. Finally, many studies have attempted to identify relationships between CYP2C19 genotype and phenotype and susceptibility to xenobiotic-induced disease, but none of these are compelling.

CYP2D6 and CYP2C19 genotype-based dose recommendations for antidepressants: a first step towards subpopulation-specific dosages

OBJECTIVE

This review aimed to provide distinct dose recommendations for antidepressants based on the genotypes of cytochrome P450 enzymes CYP2D6 and CYP2C19. This approach may be a useful complementation to clinical monitoring and therapeutic drug monitoring.

METHOD

Our literature search covered 32 antidepressants marketed in Europe, Canada, and the United States. We evaluated studies which had compared pharmacokinetic parameters of antidepressants among poor, intermediate, extensive and ultrarapid metabolizers.

RESULTS

For 14 antidepressants, distinct dose recommendations for extensive, intermediate and poor metabolizers of either CYP2D6 or CYP2C19 were given. For the tricyclic antidepressants, dose reductions around 50% were generally recommended for poor metabolizers of substrates of CYP2D6 or CYP2C19, whereas differences were smaller for the selective serotonin reuptake inhibitors.

CONCLUSION

We have provided preliminary average dose suggestions based on the phenotype or genotype. This is a first attempt to apply the new pharmacogenetics to suggest dose-regimens that take the differences in drug metabolic capacity into account.

Steady state plasma levels of the enantiomers of trimipramine and of its metabolites in CYP2D6-, CYP2C19- and CYP3A4/5-phenotyped patients

Steady state plasma concentrations of the (L)- and (D)-enantiomers of trimipramine (TRI), desmethyltrimipramine (DTRI), 2-hydroxytrimipramine (TRIOH) and 2-hydroxydesmethyl-trimipramine (DTRIOH) were measured in 27 patients receiving between 300 and 400 mg/day racemic TRI. The patients were phenotyped with dextromethorphan and mephenytoin, and the 8-hour urinary ratios of dextromethorphan/dextrorphan, dextromethorphan/3-methoxymorphinan, and (S)-mephenytoin/(R)mephenytoin were used as markers of cytochrome P-450IID6 (CYP2D6), CYP3A4/5 and CYP2C19 activities, respectively. One patient was a CYP2D6 and one was a CYP2C19 poor metabolizer. A stereoselectivity in the metabolism of TRI has been found, with a preferential N-demethylation of (D)-TRI and a preferential hydroxylation of (L)-TRI. CYP2D6 appears to be involved in the 2-hydroxylation of (L)-TRI, (L)DTRI and (D)-DTRI, but not of (D)-TRI, as significant correlations were measured between the dextromethorphan/dextrorphan ratios and the (L)-TRI/(L)-TRIOH (r = 0.45, p = 0.019), the (L)-DTRI/(L)-DTRIOH (r = 0.47, p = 0.014), and the (D)-DTRI/(D)-DTRIOH (r = 0.51, p = 0.006), but not with the (D)-TRI/(D)-TRIOH ratios (r = 0.29, NS). CYP2C19, but not CYP2D6, appears to be involved in the demethylation pathway, with a stereoselectivity toward the (D)-enantiomer of TRI, as a significant positive correlation was calculated between the mephenytoin (S)/(R) ratios and the concentrations to dose-to-weight ratios of (D)-TRI (r = 0.69, p = 0.00006). CYP3A4/5 appears to be involved in the metabolism of (L)-TRI to a presently not determined metabolite. The CYP2D6 poor metabolizer had the highest (L)-DTRI and (D)-DTRI concentrations to dose-to-weight ratios, and the CYP2C19 poor metabolizer had the highest (L)-TRI and (D)-TRI concentrations to dose-to-weight ratios of the group.

Clinically significant pharmacokinetic drug interactions with psychoactive drugs: antidepressants and antipsychotics and the cytochrome P450 system

Psychotherapeutic drugs (antipsychotics and antidepressants) are widely used for treating anxiety. Many psychotherapeutic drugs are metabolized mainly by cytochrome P450 (CYP)2C19 and CYP2D6, and are often administered with other drugs. Therefore, it is necessary to be careful when co-administering psychotherapeutic drugs whose metabolism might be inhibited by other drugs. In particular, selective serotonin reuptake inhibitors (SSRIs) inhibit the metabolism of psychotherapeutic drugs mediated by CYP2C19 and CYP2D6. It is useful to phenotype CYP2C19 and CYP2D6 (extensive metabolizers or poor metabolizers) before giving such medication. Knowledge of substrates, inhibitors and inducers of CYP isoenzymes may help clinicians to anticipate and avoid psychotherapeutic drug interactions and improve rational prescribing practices. In addition, genotyping for these drugs may be also useful in preventing side-effects.

Desipramine clearance in children and adolescents: absence of effect of development and gender

OBJECTIVE

To examine the influence of development and gender on the pharmacokinetics of desipramine (DMI) in the pediatric population.

METHOD

DMI pharmacokinetic parameters were calculated from 407 routinely drawn, dose- and weight-normalized serum concentrations in 173 youths receiving DMI (90 children, 83 adolescents; 29 were female, 144 were male).

RESULTS

Mean pharmacokinetic parameters for the entire population included dose (3.78 +/- 1.51 mg/kg), weight- and dose-normalized serum concentration (45.41 +/- 47.39 [micrograms/L]/[mg/kg]), and DMI clearance (0.68 +/- 1.51 [L/kg]/hr). No between-group differences for children and adolescents were detected in dose (child, adolescent) (3.73 +/- 1.40 mg/kg, 3.83 +/- 1.68 mg/kg), weight- and dose-normalized serum concentrations (44.52 +/- 39.6 [micrograms/L]/[mg/kg], 46.34 +/- 34.89 [micrograms/L]/[mg/kg]; p = .62), and clearance (0.680 +/- 0.890 [L/kg]/hr, 0.695 +/- 1.05 [L/kg]/hr; p = .103). No between-group gender differences were detected in dose (male, female) (3.83 +/- 1.55 mg, 3.39 +/- 1.84 mg), weight- and dose-normalized serum concentrations (45.15 +/- 37.76 [micrograms/L]/[mg/kg], 47.14 +/- 34.96 [micrograms/L]/mg/kg]; p = .720), and clearance (0.699 +/- 0.89 [L/kg]/hr, 0.606 +/- 0.535 [L/kg]/hr; p = .390).

CONCLUSIONS

These results suggest that age and gender do not significantly influence DMI clearance or dose-normalized serum concentrations in the pediatric population.

The effect of sertraline on the pharmacokinetics of desipramine and imipramine

OBJECTIVE

To examine the pharmacokinetic interaction between the selective serotonin reuptake inhibitor sertraline and the tricyclic antidepressants desipramine or imipramine in 12 healthy male subjects.

METHODS

Participants received a 50 mg single dose of either desipramine or imipramine under three conditions: alone, after a single 150 mg dose of sertraline, and after the eighth daily 150 mg dose of sertraline. Plasma samples were analyzed for desipramine or imipramine concentration by HPLC with electrochemical detection, and pharmacokinetics were determined with use of noncompartmental analysis of individual data.

RESULTS

Multiple-dose, but not single-dose, treatment with sertraline significantly reduced apparent plasma clearance (CL/F) and prolonged the half-life of desipramine relative to baseline. These changes resulted in higher plasma desipramine concentrations, as indicated by a significant increase in maximum plasma concentration (Cmax) and area under the plasma concentration-time curve extrapolated to infinity [AUC(0-infinity)] (22% and 54%, respectively). Both single- and multiple-dose treatment with sertraline significantly reduced the CL/F of imipramine. This effect was stronger after multiple predoses of sertraline, when imipramine Cmax and AUC(0-infinity) were increased by 39% and 68%, respectively. These treatment effects were consistent between individuals.

CONCLUSIONS

This pharmacokinetic interaction is likely the result of an inhibition of CYP2D6 tricyclic metabolism by sertraline. When a tricyclic antidepressant, such as desipramine or imipramine, is coadministered with sertraline, lower dosages of the tricyclic agents may be necessary to prevent elevated tricyclic levels.