Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs

Pharmacokinetics and Safety of Atogepant Co-administered with Quinidine Gluconate in Healthy Participants: A Phase 1, Open-Label, Drug-Drug Interaction Study

Atogepant, an oral calcitonin gene-related peptide receptor antagonist, is approved for the preventive treatment of migraine. Atogepant is a substrate of P-glycoprotein (P-gp), breast cancer resistance protein, organic anion transporting polypeptide transporters, and cytochrome P450 (CYP)3A4 and 2D6. Quinidine is a strong P-gp and CYP2D6 inhibitor. A phase 1 open-label study evaluated the effect of P-gp and CYP2D6 inhibition by quinidine on the pharmacokinetics of atogepant, and the safety and tolerability of atogepant and quinidine gluconate (QG) when co-administered and when given alone in 33 healthy adults. There was no significant change in the atogepant maximum plasma concentration with QG co-administration. The overall systemic exposure, the area under the plasma concentration-time curve (from time 0 to time t or to infinity), of atogepant increased by 25% when co-administered with QG. However, such an increase was not considered clinically relevant. Atogepant did not alter the mean plasma concentration of quinidine at steady state. The incidence of treatment-emergent adverse events (TEAEs) was highest when QG was administered alone (42.4%), which was primarily due to QT prolongation. Most TEAEs reported were mild in severity and resolved within 1-2 days. Co-administration of atogepant with QG did not result in any unexpected tolerability findings in this phase 1 study in healthy participants. The increase in atogepant exposure during QG co-administration could be due to inhibition of CYP2D6 (a minor contributor to atogepant clearance) as well as inhibition of P-gp.

An Overview of Various Rifampicin Analogs against Mycobacterium tuberculosis and their Drug Interactions

The success of the TB control program is hampered by the major issue of drug-resistant tuberculosis (DR-TB). The situation has undoubtedly been made more difficult by the widespread and multidrug-resistant (XDR) strains of TB. The modification of existing anti-TB medications to produce derivatives that can function on resistant TB bacilli is one of the potential techniques to overcome drug resistance affordably and straightforwardly. In comparison to novel pharmaceuticals for drug research and progress, these may have a better half-life and greater bioavailability, be more efficient, and serve as inexpensive alternatives. Mycobacterium tuberculosis, which is drugsusceptible or drug-resistant, is effectively treated by several already prescribed medications and their derivatives. Due to this, the current review attempts to give a brief overview of the rifampicin derivatives that can overcome the parent drug's resistance and could, hence, act as useful substitutes. It has been found that one-third of the global population is affected by M. tuberculosis. The most common cause of infection-related death can range from latent TB to TB illness. Antibiotics in the rifamycin class, including rifampicin or rifampin (RIF), rifapentine (RPT), and others, have a special sterilizing effect on M. tuberculosis. We examine research focused on evaluating the safety, effectiveness, pharmacokinetics, pharmacodynamics, risk of medication interactions, and other characteristics of RIF analogs. Drug interactions are especially difficult with RIF because it must be taken every day for four months to treat latent TB infection. RIF continues to be the gold standard of treatment for drug-sensitive TB illness. RIF's safety profile is well known, and the two medicines' adverse reactions have varying degrees of frequency. The authorized once-weekly RPT regimen is insufficient, but greater dosages of either medication may reduce the amount of time needed to treat TB effectively.

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.

How Science Is Driving Regulatory Guidances

This chapter provides regulatory perspectives on how to translate in vitro drug metabolism findings into in vivo drug-drug interaction (DDI) predictions and how this affects the decision of conducting in vivo DDI evaluation. The chapter delineates rationale and analyses that have supported the recommendations in the U.S. Food and Drug Administration (FDA) DDI guidances in terms of in vitro-in vivo extrapolation of cytochrome P450 (CYP) inhibition-mediated DDI potential for investigational new drugs and their metabolites as substrates or inhibitors. The chapter also describes the framework and considerations to assess UDP-glucuronosyltransferase (UGT) inhibition-mediated DDI potential for drugs as substrates or inhibitors. The limitations of decision criteria and further improvements needed are also discussed. Case examples are provided throughout the chapter to illustrate how decision criteria have been utilized to evaluate in vivo DDI potential from in vitro data.

Kinetic Analysis of Lidocaine Elimination by Pig Liver Cells Cultured in 3D Multi-Compartment Hollow Fiber Membrane Network Perfusion Bioreactors

Liver cells cultured in 3D bioreactors is an interesting option for temporary extracorporeal liver support in the treatment of acute liver failure and for animal models for preclinical drug screening. Bioreactor capacity to eliminate drugs is generally used for assessing cell metabolic competence in different bioreactors or to scale-up bioreactor design and performance for clinical or preclinical applications. However, drug adsorption and physical transport often disguise the intrinsic drug biotransformation kinetics and cell metabolic state. In this study, we characterized the intrinsic kinetics of lidocaine elimination and adsorption by porcine liver cells cultured in 3D four-compartment hollow fiber membrane network perfusion bioreactors. Models of lidocaine transport and biotransformation were used to extract intrinsic kinetic information from response to lidocaine bolus of bioreactor versus adhesion cultures. Different from 2D adhesion cultures, cells in the bioreactors are organized in liver-like aggregates. Adsorption on bioreactor constituents significantly affected lidocaine elimination and was effectively accounted for in kinetic analysis. Lidocaine elimination and cellular monoethylglicinexylidide biotransformation featured first-order kinetics with near-to-in vivo cell-specific capacity that was retained for times suitable for clinical assist and drug screening. Different from 2D cultures, cells in the 3D bioreactors challenged with lidocaine were exposed to close-to-physiological lidocaine and monoethylglicinexylidide concentration profiles. Kinetic analysis suggests bioreactor technology feasibility for preclinical drug screening and patient assist and that drug adsorption should be accounted for to assess cell state in different cultures and when laboratory bioreactor design and performance is scaled-up to clinical use or toxicological drug screening.

Evaluation of Phenotypic and Genotypic Variations of Drug Metabolising Enzymes and Transporters in Chronic Pain Patients Facing Adverse Drug Reactions or Non-Response to Analgesics: A Retrospective Study

This retrospective study evaluates the link between an adverse drug reaction (ADR) or a non-response to treatment and cytochromes P450 (CYP), P-glycoprotein (P-gp) or catechol-O-methyltransferase (COMT) activity in patients taking analgesic drugs for chronic pain. Patients referred to a pain center for an ADR or a non-response to an analgesic drug between January 2005 and November 2014 were included. The genotype and/or phenotype was obtained for assessment of the CYPs, P-gp or COMT activities. The relation between the event and the result of the genotype and/or phenotype was evaluated using a semi-quantitative scale. Our analysis included 243 individual genotypic and/or phenotypic explorations. Genotypes/phenotypes were mainly assessed because of an ADR (n = 145, 59.7%), and the majority of clinical situations were observed with prodrug opioids (n = 148, 60.9%). The probability of a link between an ADR or a non-response and the genotypic/phenotypic status of the patient was evaluated as intermediate to high in 40% and 28.2% of all cases, respectively. The drugs in which the probability of an association was the strongest were the prodrug opioids, with an intermediate to high link in 45.6% of the cases for occurrence of ADRs and 36.0% of the cases for non-response. This study shows that the genotypic and phenotypic approach is useful to understand ADRs or therapeutic resistance to a usual therapeutic dosage, and can be part of the evaluation of chronic pain patients.

Complex Drug-Drug-Gene-Disease Interactions Involving Cytochromes P450: Systematic Review of Published Case Reports and Clinical Perspectives

Drug pharmacokinetics (PK) is influenced by multiple intrinsic and extrinsic factors, among which concomitant medications are responsible for drug-drug interactions (DDIs) that may have a clinical relevance, resulting in adverse drug reactions or reduced efficacy. The addition of intrinsic factors affecting cytochromes P450 (CYPs) activity and/or expression, such as genetic polymorphisms and diseases, may potentiate the impact and clinical relevance of DDIs. In addition, greater variability in drug levels and exposures has been observed when such intrinsic factors are present in addition to concomitant medications perpetrating DDIs. This variability results in poor predictability of DDIs and potentially dramatic clinical consequences. The present review illustrates the issue of complex DDIs using systematically searched published case reports of DDIs involving genetic polymorphisms, renal impairment, cirrhosis, and/or inflammation. Current knowledge on the impact of each of these factors on drug exposure and DDIs is summarized and future perspectives for the management of such complex DDIs in clinical practice are discussed, including the use of advanced Computerized Physician Order Entry (CPOE) systems, the development of model-based dose optimization strategies, and the education of healthcare professionals with respect to personalized medicine.

In vitro assessment of competitive and time-dependent inhibition of the nevirapine metabolism by nortriptyline in rats

Nevirapine (NVP) is a non-nucleoside reverse transcriptase inhibitor of human immunodeficiency virus type 1 (HIV-1) widely used as a component of High Active Antiretroviral Therapy (HAART) since it is inexpensive, readily absorbed after oral administration and non-teratogenic. In the present work, the mechanism of a previously described pharmacokinetic interaction between NVP and the antidepressant drug nortriptyline (NT) was studied using rat hepatic microsomes. The obtained results showed a competitive inhibition of the NVP metabolism by NT. The three main NVP metabolites (2-OH-NVP, 3-OH-NVP and 12-OH-NVP) where competitively inhibited with similar inhibitory constant values (K_i = 4.01, 3.97 and 4.40 μM, respectively). Time-dependent inhibition of the NVP metabolism was also detected, with a 2.5-fold reduction in the IC₅₀ values of NT for 2-, 3-, and 12-OH-NVP formation when NT was preincubated with the microsomal suspension in the presence of an NADPH-generating system. A concentration-dependent inhibition of the formation of NVP metabolites by the main NT metabolite (10-OH-NT) was also observed, however, the inhibitory potency of 10-OH-NT was much lower than that of the parent drug. The apparent hepatic intrinsic clearance of NVP determined in these in vitro experiments was used to predict the in vivo clearance of NVP using the "well-stirred" and the "parallel-tube" models, resulting in values close to those previously observed in vivo clearance. Finally, a good prediction of the increase in the plasma concentrations of NVP when co-administered with NT was obtained employing the inhibitory constant of NT determined in vitro and the estimated plasma concentration of NT entering the liver.

Effects of Strong CYP3A Inhibition and Induction on the Pharmacokinetics of Ixazomib, an Oral Proteasome Inhibitor: Results of Drug-Drug Interaction Studies in Patients With Advanced Solid Tumors or Lymphoma and a Physiologically Based Pharmacokinetic Analysis

At clinically relevant ixazomib concentrations, in vitro studies demonstrated that no specific cytochrome P450 (CYP) enzyme predominantly contributes to ixazomib metabolism. However, at higher than clinical concentrations, ixazomib was metabolized by multiple CYP isoforms, with the estimated relative contribution being highest for CYP3A at 42%. This multiarm phase 1 study (Clinicaltrials.gov identifier: NCT01454076) investigated the effect of the strong CYP3A inhibitors ketoconazole and clarithromycin and the strong CYP3A inducer rifampin on the pharmacokinetics of ixazomib. Eighty-eight patients were enrolled across the 3 drug-drug interaction studies; the ixazomib toxicity profile was consistent with previous studies. Ketoconazole and clarithromycin had no clinically meaningful effects on the pharmacokinetics of ixazomib. The geometric least-squares mean area under the plasma concentration-time curve from 0 to 264 hours postdose ratio (90%CI) with vs without ketoconazole coadministration was 1.09 (0.91-1.31) and was 1.11 (0.86-1.43) with vs without clarithromycin coadministration. Reduced plasma exposures of ixazomib were observed following coadministration with rifampin. Ixazomib area under the plasma concentration-time curve from time 0 to the time of the last quantifiable concentration was reduced by 74% (geometric least-squares mean ratio of 0.26 [90%CI 0.18-0.37]), and maximum observed plasma concentration was reduced by 54% (geometric least-squares mean ratio of 0.46 [90%CI 0.29-0.73]) in the presence of rifampin. The clinical drug-drug interaction study results were reconciled well by a physiologically based pharmacokinetic model that incorporated a minor contribution of CYP3A to overall ixazomib clearance and quantitatively considered the strength of induction of CYP3A and intestinal P-glycoprotein by rifampin. On the basis of these study results, the ixazomib prescribing information recommends that patients should avoid concomitant administration of strong CYP3A inducers with ixazomib.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Tales from the war on error: the art and science of curating QSAR data

Curating the data underlying quantitative structure-activity relationship models is a never-ending struggle. Some curation can now be automated but much cannot, especially where data as complex as those pertaining to molecular absorption, distribution, metabolism, excretion, and toxicity are concerned (vide infra). The authors discuss some particularly challenging problem areas in terms of specific examples involving experimental context, incompleteness of data, confusion of units, problematic nomenclature, tautomerism, and misapplication of automated structure recognition tools.

Pharmacokinetic interaction between nevirapine and nortriptyline in rats: inhibition of nevirapine metabolism by nortriptyline

One of the most frequent comorbidities of HIV infection is depression, with a lifetime prevalence of 22 to 45%. Therefore, it was decided to study a potential pharmacokinetic interaction between the nonnucleoside reverse transcriptase inhibitor nevirapine (NVP) and the tricyclic antidepressant nortriptyline (NT). NVP and NT were administered to rats either orally, intraduodenally, or intravenously, and the changes in plasma levels and pharmacokinetic parameters were analyzed. Experiments with rat and human hepatic microsomes were carried out to evaluate the inhibitory effects of NT on NVP metabolism. NVP plasma concentrations were significantly higher when this drug was coadministered with NT. The maximum plasma concentrations of NVP were increased 2 to 5 times and the total plasma clearance was decreased 7-fold in the presence of NT. However, statistically significant differences in the pharmacokinetic parameters of NT in the absence and presence of NVP were not found. In vitro studies with rat and human hepatic microsomes confirmed the inhibition of NVP hepatic metabolism by NT in a concentration-dependent way, with the inhibition being more intense in the case of rat microsomes. In conclusion, a pharmacokinetic interaction between NVP and NT was detected. This interaction was a consequence of the inhibition of hepatic metabolism of NVP by NT. In vivo human studies are required to evaluate the effects of this interaction on the pharmacokinetics of NVP before it can be taken into account for patients receiving NVP.

Psychotropic drug-drug interactions involving P-glycoprotein

Multidrug resistance P-glycoprotein (P-gp; also known as MDR1 and ABCB1) is expressed in the luminal membrane of the small intestine and blood-brain barrier, and the apical membranes of excretory cells such as hepatocytes and kidney proximal tubule epithelia. P-gp regulates the absorption and elimination of a wide range of compounds, such as digoxin, paclitaxel, HIV protease inhibitors and psychotropic drugs. Its substrate specificity is as broad as that of cytochrome P450 (CYP) 3A4, which encompasses up to 50 % of the currently marketed drugs. There has been considerable interest in variations in the ABCB1 gene as predictors of the pharmacokinetics and/or treatment outcomes of several drug classes, including antidepressants and antipsychotics. Moreover, P-gp-mediated transport activity is saturable, and is subject to modulation by inhibition and induction, which can affect the pharmacokinetics, efficacy or safety of P-gp substrates. In addition, many of the P-gp substrates overlap with CYP3A4 substrates, and several psychotropic drugs that are P-gp substrates are also CYP3A4 substrates. Therefore, psychotropic drugs that are P-gp substrates may cause a drug interaction when P-gp inhibitors and inducers are coadministered, or when psychotropic drugs or other medicines that are P-gp substrates are added to a prescription. Hence, it is clinically important to accumulate data about drug interactions through studies on P-gp, in addition to CYP3A4, to assist in the selection of appropriate psychotropic medications and in avoiding inappropriate combinations of therapeutic agents. There is currently insufficient information available on the psychotropic drug interactions related to P-gp, and therefore we summarize the recent clinical data in this review.

Risk assessment of accidental nortriptyline poisoning: the importance of cytochrome P450 for nortriptyline elimination investigated using a population-based pharmacokinetic simulator

It is not possible to make a prospective clinical study that reveals the importance of the nortriptyline metabolising cytochrome P450 (CYP) isoforms (CYP1A2, CYP2C19, CYP2D6, and CYP3A4) in relation to attaining potential toxic nortriptyline concentrations with a possibly fatal outcome. Therefore to study this we have applied the population based pharmacokinetic simulator Simcyp. The objective was to estimate how important CYP2C19 and CYP2D6 phenotype status, hepatic activity of CYP3A4, body weight, CYP2D6 phenotype dose adjustment, and drug-drug interactions are with regard to accidental poisoning in a virtual population receiving a daily dose of 100mg nortriptyline. Accidental poisoning is here defined as intake of a normal dose which because of slow metabolism may lead to potentially toxic concentrations. The input parameters values for Simcyp were based on average literature in vitro and in vivo data. The Simcyp simulations of nortriptyline pharmacokinetics reflected reported clinical concentration-time profiles, therapeutic drug monitoring data, and the consequence of CYP2D6 poor metaboliser (PM) and ultrarapid metaboliser status. Of the investigated factors, the simulations indicate that having CYP2D6 PM status is a major risk factor for attaining high concentrations and thereby possibly becoming poisoned by nortriptyline. Of the CYP2D6 PM subjects 16% would attain plasma concentrations exceeding the toxic limit. Individuals with the combination of CYP2D6 PM status and 10% of the average liver CYP3A4 expression had a 90% risk of becoming poisoned. The results point towards the combination of low CYP3A4 activity and CYP2D6 PM status of major importance for attaining possibly toxic nortriptyline concentrations. In a forensic toxicological context, the results indicate that both the activity of CYP3A4, information on possible drug-drug interactions, and the genotype of CYP2D6 are needed in order to elucidate whether an individual might have been accidentally poisoned because of slow metabolism. In a clinical context, the simulations suggest that precise individual dose adjustment of nortriptyline requires information regarding the activity of both CYP3A4 and CYP2D6. This underlines the value of therapeutic drug monitoring for nortriptyline. Population based pharmacokinetic simulations are considered useful tools for risk assessment in clinical and forensic toxicology.

Antidepressant-induced akathisia-related homicides associated with diminishing mutations in metabolizing genes of the CYP450 family

PURPOSE

To examine the relation between variant alleles in 3 CYP450 genes (CYP2D6, CYP2C9 and CYP2C19), interacting drugs and akathisia in subjects referred to a forensic psychiatry practice in Sydney, Australia.

PATIENTS AND METHODS

This paper concerns 10/129 subjects who had been referred to the first author's practice for expert opinion or treatment. More than 120 subjects were diagnosed with akathisia/serotonin toxicity after taking psychiatric medication that had been prescribed for psychosocial distress. They were tested for variant alleles in CYP450 genes, which play a major role in Phase I metabolism of all antidepressant and many other medications. Eight had committed homicide and many more became extremely violent while on antidepressants. Ten representative case histories involving serious violence are presented in detail.

RESULTS

Variant CYP450 allele frequencies were higher in akathisia subjects compared with random primary care patients tested at the same facility. Ten subjects described in detail had variant alleles for one or more of their tested CYP450 genes. All but two were also on interacting drugs, herbals or illicit substances, impairing metabolism further. All those described were able to stop taking antidepressants and return to their previously normal personalities.

CONCLUSION

THE PERSONAL, MEDICAL, AND LEGAL PROBLEMS ARISING FROM OVERUSE OF ANTIDEPRESSANT MEDICATIONS AND RESULTING TOXICITY RAISE THE QUESTION: how can such toxicity events be understood and prevented? The authors suggest that the key lies in understanding the interplay between the subject's CYP450 genotype, substrate drugs and doses, co-prescribed inhibitors and inducers and the age of the subject. The results presented here concerning a sample of persons given antidepressants for psychosocial distress demonstrate the extent to which the psychopharmacology industry has expanded its influence beyond its ability to cure. The roles of both regulatory agencies and drug safety "pharmacovigilantes" in ensuring quality and transparency of industry information is highlighted.

Pharmacogenetics and gender association with psychotic episodes on nortriptyline lower doses: patient cases

The variation in individual responses to psychotropic drug treatment remains a critical problem in the management of psychotic disorders. Although most patients will experience remission, some patients may develop drug-induced adverse effects that may range from troublesome to life threatening. Antidepressants are freely prescribed by general practitioners, and there should be constant awareness in the medical community about possible serious side effects. We describe two cases of adverse drug reactions on low dosage treatment that led to extreme psychotic episodes as examples of the potential for dangerous side effects. The patients developed adverse reactions on the normal recommended dosage of nortriptyline, a tricyclics antidepressant (TCA). Both were females, with no history of antidepressant treatment, unsocial behaviour, nor any family history of psychosis, but both experienced severe psychiatric symptoms. Pharmacogenetic tests can easily be performed and interpreted according to the likelihood of adverse reactions and should be included in toxicity interpretation.

Safe and effective medicines for all: is personalized medicine the answer?

An improvement in drug treatment and clinical outcome is one of the major challenges in clinical medicine. The development of evidence-based standards of care has led to a significant improvement, but, by definition, strictly standardized cohorts in clinical trials have to ignore individual differences. Personalized medicine is defined as the application of genomic and molecular data to better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a person's predisposition to a particular disease or condition. After the deciphering of the human genome, however, the high expectations in individualized medicine were not always fulfilled. However, personalized medicine has become indispensable in the treatment of malignant diseases and there is increasing evidence for its benefit in other areas. This article outlines the impact of pharmacogenetics and pharmacogenomics, especially with regard to personalized medicine, in major medical indications and reflects the obstacles and chances taken in current daily practice.

Nortriptyline Cardiac Toxicity Resulting from a Probable Interaction with Telithromycin

Objective

To report a case of probable nortriptyline toxicity associated with a combination of telithromycin and nortriptyline.

Case Summary

A 50-year-old white woman was admitted to the hospital with new-onset palpitations after taking 5 days of telithromycin in combination with nortriptyline. On admission her electrocardiograph showed atrioventricular nodal reentry tachycardia. The patient was also hypotensive, with a blood pressure of 90/45 mm Hg. She had been taking nortriptyline 150 mg daily at bedtime for 10 years without complications. The final day of telithromycin therapy was completed, and nortriptyline was discontinued on admission. The tachycardia subsided with a dose of diltiazem.

Discussion

It is suspected that the cardiovascular symptoms observed in the patient are the result of nortriptyline toxicity caused by inhibition of CYP2D6 by telithromycin. Limited evidence shows that telithromycin may be an inhibitor of CYP2D6. An assessment of causation of the adverse effects using the Drug Interaction Probability Scale suggests a probable interaction between telithromycin and nortriptyline.

Conclusion

It is recommended that clinicians remain aware of possible interactions between telithromycin and known CYP2D6 substrates such as nortriptyline. Should telithromycin therapy be required, close monitoring of the potential adverse effects and/or plasma drug levels of patients taking interacting drugs is warranted.

Usefulness of hepatocytes for evaluating the genetic polymorphism of CYP2D6 substrates

The usefulness of human hepatocytes for assessing CYP2D6-related genetic polymorphisms was investigated. Propranolol and propafenone, which undergo phase I and II biotransformations, were used as model substrates alongside metoprolol, which is only metabolized via oxidative pathways. The contributions of CYP2D6 to the primary metabolisms of the substrates were estimated from the quinidine-mediated inhibition of their depletion rate constants in human hepatocytes and liver microsomes. The contributions in hepatocytes were 19.2% for propranolol at 0.05 microM and 36.7--76.3% for propafenone at 0.05--1.0 microM, and smaller than the contribution in microsomes, unlike the case for metoprolol. The differences between microsomes and hepatocytes were attributable to conjugate formation. The CYP2D6 contributions in hepatocytes reflected the in vivo data. The relevance of the concentration-dependent involvement of CYP2D6 in propafenone metabolism in hepatocytes to the in vivo polymorphic profile and the applicability of hepatocytes for evaluating these polymorphisms are discussed.

Comparative metabolic capabilities and inhibitory profiles of CYP2D6.1, CYP2D6.10, and CYP2D6.17

Polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene are a major cause of pharmacokinetic variability in human. Although the poor metabolizer phenotype is known to be caused by two null alleles leading to absence of functional CYP2D6 protein, the large variability among individuals with functional alleles remains mostly unexplained. Thus, the goal of this study was to examine the intrinsic enzymatic differences that exist among the several active CYP2D6 allelic variants. The relative catalytic activities (enzyme kinetics) of three functionally active human CYP2D6 allelic variants, CYP2D6.1, CYP2D6.10, and CYP2D6.17, were systematically investigated for their ability to metabolize a structurally diverse set of clinically important CYP2D6-metabolized drugs [atomoxetine, bufuralol, codeine, debrisoquine, dextromethorphan, (S)-fluoxetine, nortriptyline, and tramadol] and the effects of various CYP2D6-inhibitors [cocaine, (S)-fluoxetine, (S)-norfluoxetine, imipramine, quinidine, and thioridazine] on these three variants. The most significant difference observed was a consistent but substrate-dependent decease in the catalytic efficiencies of cDNA-expressed CYP2D6.10 and CYP2D6.17 compared with CYP2D6.1, yielding 1.32 to 27.9 and 7.33 to 80.4% of the efficiency of CYP2D6.1, respectively. The most important finding from this study is that there are mixed effects on the functionally reduced allelic variants in enzyme-substrate affinity or enzyme-inhibitor affinity, which is lower, higher, or comparable to that for CYP2D6.1. Considering the rather high frequencies of CYP2D6*10 and CYP2D6*17 alleles for Asians and African Americans, respectively, these data provide further insight into ethnic differences in CYP2D6-mediated drug metabolism. However, as with all in vitro to in vivo extrapolations, caution should be applied to the clinical consequences.

Metabolism of N-Methyl, N-propargylphenylethylamine: Studies with Human Liver Microsomes and cDNA Expressed Cytochrome P450 (CYP) Enzymes

1. We used an in vitro screening procedure and studies with individual human liver microsomes and cDNA-expressed CYP enzymes to investigate the metabolism of the putative neuroprotective drug N-methyl,N-propargyl-2-phenylethylamine (MPPE) to N-methylphenylethylamine (N-methylPEA) and N-propargylphenylethylamine (N-propargylPEA). 2. An electron-capture gas chromatographic procedure previously developed in our laboratories was used to measure the quantities of N-methylPEA and N-propargylPEA formed in the experiments with a single donor human liver microsome panel and cDNA expressed single CYP enzyme systems. The data were fitted to nonlinear regressions using Prism to determine kinetic constants. The results from a fluorogenic screen determined which cDNA-expressed single CYP enzymes were investigated. 3. CYP2B6, CYP2C19, and CYP2D6 all contributed to the formation of N-methylPEA, while only CYP2B6 catalyzed the formation of N-propargylPEA. The K (M) and V (max) values for N-propargylPEA formation were 290 +/- 70 microM and 139+/-16 ng/mL/min. The values for formation of N-methylPEA were not determined from these experiments due to the complexity of fitting the data to a three-variable equation, but data on the time course of N-methylPEA formation are presented. 4. Catabolism of MPPE to N-methylPEA and N-propargylPEA is catalyzed by CYP enzymes. CYP2B6, 2C19 and 2D6 all contribute to the depropargylation of the parent compound, but only CYP2B6 also catalyzes demethylation. CYP2C19 was found to be the most active with respect to generation of N-methylPEA.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

MDR- and CYP3A4-mediated drug–herbal interactions

According to recent epidemiological reports, almost 40% of American population use complimentary and alternative medicine (CAM) during their lifetime. Patients detected with HIV or cancer often consume herbal products especially St. John's wort (SJW) for antidepressants in combination with prescription medicines. Such self-administered herbal products along with prescribed medicines raise concerns of therapeutic activity due to possible drug-herbal interactions. P-glycoprotein (P-gp) and cytochrome P450 3A4 (CYP3A4) together constitute a highly efficient barrier for many orally absorbed drugs. Available literature, clinical reports and in vitro studies from our laboratory indicate that many drugs and herbal active constituents are substrates for both P-gp and CYP3A4. Results from clinical studies and case reports indicate that self-administered SJW reduce steady state plasma concentrations of amitriptyline, cyclosporine, digoxin, fexofenadine, amprenavir, indonavir, lopinavir, ritonavir, saquinavir, benzodiazepines, theophyline, irinotecan, midazolan and warfarin. This herbal agent has been also reported to cause bleeding and unwanted pregnancies when concomitantly administered with oral contraceptives. Most of these medicinal agents and SJW are substrates for P-gp and/or CYP3A4. In vitro studies from our laboratory suggest that short-term exposure with pure herbal agents such as hypericin, kaempferol and quercetin or extract of SJW resulted in higher uptake or influx of ritonavir and erythromycin. Hypericin, kaempferol and quercetin also caused a remarkable inhibition of cortisol metabolism with the percent intact cortisol values of 64.58%, 89.6% and 90.1%, respectively, during short-term in vitro experiments. Conversely, long-term exposure of herbal agents (hyperforin, kaempferol and quercetin) showed enhanced expression of CYP3A4 mRNA in Caco-2 cells. In another study, we observed that long-term exposure of hypericin, kaempferol, quercetin and silibinin resulted in higher MDR-1 mRNA expression in Caco-2 cells. Therefore, herbs can pharmacokinetically act as inhibitors or inducers. Medicinal agents that are substrates P-gp-mediated efflux and/or CYP-mediated metabolism are likely to be potential candidates for drug-herbal interactions. The duration of exposure of cells/healthy volunteers/animals to herbals appears to be critical for drug-herbal interaction. An increase in plasma drug concentration is possible during concomitant administration of SJW and prescribed drugs. In contrast, prolonged intake of herbal supplement followed by drug administration may result in subtherapeutic concentrations. Therefore, clinical implications of such drug herbal interactions depend on a variety of factors such as dose, frequency and timing of herbal intake, dosing regimen, route of drug administration and therapeutic range. In vitro screening techniques will play a major role in identifying possible herb-drug interactions and thus create a platform for clinical studies to emerge. Mechanisms of drug-herbal interaction have been discussed in this review article.

Association between plasma interleukin-18 levels and liver injury in chronic hepatitis C virus infection and non-alcoholic fatty liver disease

There is significant upregulation of interleukin-18 (IL-18) expression in viral infectious diseases and in some chronic hepatic diseases, especially (i) hepatitis C virus (HCV) infection, (ii) HCV infection with persistently normal ALT levels (PNAL), and (iii) non-alcoholic fatty liver disease (NAFLD). The aim of this study was a better understanding of the implications of plasma IL-18 levels in the above-mentioned liver diseases. Thirty-four patients with HCV infection, 13 with NAFLD, and 10 controls were enrolled. The HCV-RNA and HCV-genotypes and the serum or plasma levels of IL-18, aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyltranspeptidase (gamma-GT), alkaline phosphatase, total cholesterol, triglycerides, alpha(1)-fetoprotein, and ferritin were evaluated. Patients with HCV showed higher levels of IL-18 than the NAFLD patients (p <0.01) and the controls (p <0.005). Patients with NAFLD showed higher values of body mass index and liver disease parameters, compared to HCV-infected subjects or controls. These data confirm previous reports of enhanced expression of IL-18 in patients with HCV and NAFLD, compared to healthy subjects, and suggest that IL-18 is important as a marker of liver diseases.

Drug disposition of chiral and achiral drug substrates metabolized by cytochrome P450 2D6 isozyme: case studies, analytical perspectives and developmental implications

The concepts of drug development have evolved over the last few decades. Although number of novel chemical entitities belonging to varied classes have made it to the market, the process of drug development is challenging, intertwined as it is with complexities and uncertainities. The intention of this article is to provide a comprehensive review of novel chemical entities (NCEs) that are substrates to cytochrome P450 (CYP) 2D6 isozyme. Topics covered in this review aim: (1) to provide a framework of the importance of CYP2D6 isozyme in the biotransformation of NCEs as stand-alones and/or in conjunction with other CYP isozymes; (2) to provide several case studies of drug disposition of important drug substrates, (3) to cover key analytical perspectives and key assay considerations to assess the role and involvement of CYP2D6, and (4) to elaborate some important considerations from the development point of view. Additionally, wherever applicable, special emphasis is provided on chiral drug substrates in the various subsections of the review.

An overview of psychotropic drug-drug interactions

The psychotropic drug-drug interactions most likely to be relevant to psychiatrists' practices are examined. The metabolism and the enzymatic and P-glycoprotein inhibition/induction profiles of all antidepressants, antipsychotics, and mood stabilizers are described; all clinically meaningful drug-drug interactions between agents in these psychotropic classes, as well as with frequently encountered nonpsychotropic agents, are detailed; and information on the pharmacokinetic/pharmacodynamic results, mechanisms, and clinical consequences of these interactions is presented. Although the range of drug-drug interactions involving psychotropic agents is large, it is a finite and manageable subset of the much larger domain of all possible drug-drug interactions. Sophisticated computer programs will ultimately provide the best means of avoiding drug-drug interactions. Until these programs are developed, the best defense against drug-drug interactions is awareness and focused attention to this issue.

The Metabolic Fate of Amitriptyline, Nortriptyline and Amitriptylinoxide in Man

Amitriptyline (AT), the most widely used tricyclic antidepressant, undergoes oxidative metabolism in the side chain with production of the secondary amine nortriptyline (NT), a primary amine, and the N-oxide amitriptylinoxide (AT-NO); in addition, direct conjugation leads to a quaternary ammonium-linked glucuronide. Hydroxylation of AT or NT at the ethylene bridge of the central seven-membered ring results in four isomeric alcohols and occurs with high stereo- and enantioselectivity, the (-)-(E)-10-hydroxy compounds usually being the major products. The disposition of the alcohols is also partially enantioselective, for instance with regard to glucuronidation and reversible oxidation to ketones. Introduction of a second hydroxy group results in isomeric glycols. Oxidative attack at an aromatic ring is a minor pathway leading to dihydrodiols and phenols. Numerous metabolites originate by combinations of reactions in the ring system and the side chain. AT-NO is by about one-third excreted in unchanged form or as 10-hydroxy derivative; the major part is reduced to AT and metabolized further. The review covers current knowledge on the enzymes participating in the individual pathways. Their quantitative importance is inferred from kinetic studies in volunteers and patients and from experiments in vitro. Clinical consequences of biochemical findings mainly derive from the impact of the polymorphic CYP2D6 mediating (-)-(E)-10-hydroxylation and from its potential inhibition by other psychoactive drugs.

Neurobiological differences on populational level

At present, 13% of world population are suffering from neuropsychiatric disorders. Prevention and treatment of these pathological conditions are very important. New research strategies are needed for the creation of effective methods for controlling of neuropsychiatric disorders. International collaborative research projects within WHO's activities in psychopharmacology and biological psychiatry with the participation of 12 countries representing different world regions have been organized and successfully finalized. The author discusses some of these in the Festschrift for Dr. Corneille Radouco-Thomas.

Pharmacokinetic interactions of drugs with St John's wort

There is a worldwide increasing use of herbs which are often administered in combination with therapeutic drugs, raising the potential for herb-drug interactions. St John's wort (Hypericum perforatum) is one of the most commonly used herbal antidepressants. A literature search was performed using Medline (via Pubmed), Biological Abstracts, Cochrane Library, AMED, PsycINFO and Embase (all from their inception to September 2003) to identify known drug interaction with St John's wort. The available data indicate that St John's wort is a potent inducer of CYP 3A4 and P-glycoprotein (PgP), although it may inhibit or induce other CYPs, depending on the dose, route and duration of administration. Data from human studies and case reports indicate that St John's wort decreased the blood concentrations of amitriptyline, cyclosporine, digoxin, fexofenadine, indinavir, methadone, midazolam, nevirapine, phenprocoumon, simvastatin, tacrolimus, theophylline and warfarin, whereas it did not alter the pharmacokinetics of carbamazepine, dextromethorphan, mycophenolic acid and pravastatin. St John's wort decreased the plasma concentration of the active metabolite SN-38 in cancer patients receiving irinotecan treatment. St John's wort did not alter the pharmacokinetics of tolbutamide, but increased the incidence of hypoglycaemia. Several cases have been reported that St John's wort decreased cyclosporine blood concentration leading to organ rejection. St John's wort caused breakthrough bleeding and unplanned pregnancies when used concomitantly with oral contraceptives. It also caused serotonin syndrome when coadministered with selective serotonin-reuptake inhibitors (e.g. sertaline and paroxetine). Both pharmacokinetic and pharmacodynamic components may play a role in these interactions. Because the potential interaction of St John's wort with other drugs is a major safety concern, additional systematic research on herb-drug interactions and appropriate regulation in herbal safety and efficacy is needed.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

High-throughput screening to estimate single or multiple enzymes involved in drug metabolism: microtitre plate assay using a combination of recombinant CYP2D6 and human liver microsomes

1. The purpose of this study was to estimate readily involvement of single or multiple enzymes in the metabolism of a drug through inhibitory assessment. Inhibitory effects of various compounds on CYP2D6 activity assayed by formation of fluorescent metabolite from 3-[2-(N,N-diethyl-N-methyl-ammonium)ethyl]-7-methoxy-4-methyl-coumarin (AMMC) were assessed using microtitre plate (MTP) assays with a combination of recombinant CYP2D6 and human liver microsomes (HLM). 2. Among various compounds studied, antipsychotic drugs extensively inhibited recombinant CYP2D6 activity and the IC50 values were generally lower than those of antidepressants and antiarrhythmic drugs. 3. After pre-incubation, the IC50 values of mianserin, chlorpromadine, risperidone, thioridazine, alprenolol, propafenone and dextromethorphan increased but the values of timolol, S-metoprolol and propranolol substantially decreased compared with those in case of co-incubation. 4. The IC50 values of typical substrates of CYP2D6 (bufuralol and dextromethorphan at lower substrate concentration) in inhibition studies using HLM, were similar to those in the case of recombinant CYP2D6, but the values of the compounds that are metabolized by multiple CYP forms (perphenazine and chlorpromazine) in HLM were much larger. 5. If the ratio (HLM/rCYP ratio) of IC50 values between HLM and recombinant CYP2D6 exceeds approximately 2, it suggests that other CYP forms in addition to CYP2D6 might be involved in the metabolism of the test compounds. From the advantage such as speed, high throughput and ease of the technique, the MTP assay using a combination of the recombinant CYP2D6 and HLM is useful to estimate the involvement of single or multiple enzymes in the metabolism of drugs at the stage of drug discovery.

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.

Drug–phytochemical interactions

Changes in dietary habits favouring diets rich in fruits and vegetables, and a meteoric rise in the consumption of dietary supplements and herbal products have substantially increased human exposure to phytochemicals. It is, therefore, not surprising that diet and herbal remedies can modulate drug-metabolising enzyme systems, such as cytochromes P450, leading to clinically relevant drug-phytochemical interactions. Phytochemicals have the potential to both elevate and suppress cytochrome P450 activity. Such effects are more likely to occur in the intestine, where high concentrations of phytochemicals may be achieved, and alteration in cytochrome P450 activity will influence, in particular, the fate of drugs that are subject to extensive first-pass metabolism as a result of intestinal cytochrome P450-mediated biotransformation. Moreover, it is becoming increasingly apparent that phytochemicals can also influence the pharmacological activity of drugs by modifying their absorption characteristics through interaction with drug transporters. Clearly, phytochemicals have the potential to alter the effectiveness of drugs, either impairing or exaggerating their pharmacological activity.

Differences in cytochrome P450 forms involved in the metabolism of N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine monohydrochloride (NE-100), a novel sigma ligand, in human liver and intestine

N,N-Dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine monohydrochloride (NE-100) has been developed to treat subjects with schizophrenia. This drug is mainly excreted in the form of oxidative metabolites. In the present study, identification of p450 forms involved in the metabolism was carried out using human livers and intestinal microsomes (HLM and HIM). Eadie-Hofstee plots for NE-100 disappearance in HLM were biphasic, thus indicating the involvement of at least two p450 forms. The metabolism of NE-100 was mediated with recombinant CYP1A1, CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. A significant correlation was observed between activities of NE-100 metabolism and dextromethorphan O-demethylation (a specific activity for CYP2D6) or testosterone 6beta-hydroxylation (a specific activity for CYP3A4) in HLM. The activity of NE-100 metabolism was inhibited by approximately 80% by an anti-CYP2D6 antibody and only by quinidine among the p450-selective inhibitors at a low substrate concentration (0.1 microM). In contrast, with a high substrate concentration (10 microM), the activity was inhibited by an anti-CYP3A4 antibody and by ketoconazole. On the other hand, in HIM, the Eadie-Hofstee plots for NE-100 disappearance were monophasic, and the metabolism was strongly inhibited by an anti-CYP3A4 antibody and by ketoconazole but not by other inhibitors used. These results strongly suggest that NE-100 has different profiles regarding metabolism between liver and intestine. During absorption, NE-100 is mainly metabolized by CYP3A4 in the intestine and thereafter by CYP2D6 in the liver in the presence of therapeutic doses.

Preclinical factors influencing the relative contributions of Phase I and II enzymes to the metabolism of the experimental anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid

It is important to determine the relative contribution of each metabolic pathway (f(p)) and of enzymes to the net metabolism of a drug. The aim of this study was to investigate, using a human liver bank, the f(p) of the anti-cancer drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and the effects of various inhibitors and inducers on f(p). The mean apparent K(m) and V(max) values (N=14) were 21+/-5 microM and 0.04+/-0.02 nmol/min/mg, respectively, for 6-methylhydroxylation, and 143+/-79 microM and 0.71+/-0.52 nmol/min/mg, respectively, for acyl glucuronidation in human liver microsomes. 6-Methylhydroxylation and acyl glucuronidation contributed 26 and 74%, respectively, to DMXAA metabolism at 5 microM; values were 7 and 93% at 350 microM DMXAA. There was a significant relationship between the ratio of metabolic activity by Phase II and I reactions (R(II/I)) and uridine diphosphate glucuronosyltransferase (UGT2B7) protein level (r=0.605, P=0.022), whereas a reverse correlation between R(II/I) and cytochrome P450 (CYP1A) protein level was observed (r=-0.540, P=0.046). Various compounds inhibited either DMXAA glucuronidation or 6-methylhydroxylation, or both pathways. Pretreatment of rats with beta-naphthoflavone, but not phenobarbitone and cimetidine, increased the percentage of the contribution by 6-methylhydroxylation to 17% from 4% of control at 5 microM DMXAA. Our results indicate that the f(p) of DMXAA is subject to substrate concentration, inhibition, induction, and the protein levels of enzymes that biotransform DMXAA. However, clinical studies are important to verify the conclusions drawn from in vitro data.

Predicting pharmacokinetics and drug interactions in patients from in vitro and in vivo models: the experience with 5,6-dimethylxanthenone-4-acetic acid (DMXAA), an anti-cancer drug eliminated mainly by conjugation

The novel anti-tumor agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) was developed in the Auckland Cancer Society Research Center. Its pharmacokinetic properties have been investigated using both in vitro and in vivo models, and the resulting data extrapolated to patients. The metabolism of DMXAA has been extensively studied mainly using hepatic microsomes, which indicated that UGT1A9 and UGT2B7-catalyzed glucuronidation on its acetic acid side chain and to a lesser extent CYP1A2-catalyzed hydroxylation of the 6-methyl group are the major metabolic pathways, resulting in DMXAA acyl glucuronide (DMXAA-G) and 6-hydroxymethyl-5-methylxanthenone-4-acetic acid. The predominant metabolite in human urine (up to 60% of total dose) was identified as DMXAA-G, which was chemically reactive, undergoing hydrolysis, intramolecular rearrangement, and covalent binding to plasma proteins. In vivo formation of DMXAA-protein adducts were also observed in cancer patients receiving DMXAA treatment. The comparison of the in vitro human hepatic microsomal metabolism and inhibition of DMXA by UGT and/or CYP substrates with animal species indicated species differences. Renal microsomes from all animal species examined had glucuronidation activity for DMXAA, but lower than the liver. In vitro-in vivo extrapolations based on human microsomal data indicated a 7-fold underestimation of plasma clearance in patients. In contrast, allometric scaling using in vivo data from the mouse, rat, and rabbit predicted a plasma clearance of 3.5 mL/min/kg, similar to that observed in patients (3.7 mL/min/kg). Based on in vitro metabolic inhibition studies, it appears possible to predict the effects on the plasma kinetic profile of DMXAA of drugs such as diclofenac, which are mainly metabolized by UGT2B7. However, it did not appear possible to predict the effect of thalidomide on the pharmacokinetics of DMXAA in patients based on in vitro inhibition and animal studies. These data indicate that preclincial pharmacokinetic studies using both in vitro and in vivo models play an important but different role in predicting pharmacokinetics and drug interactions in patients.

Pharmacokinetic interaction between nortriptyline and terbinafine

OBJECTIVE

To clarify the mechanism of interaction between nortriptyline and terbinafine.

CASE SUMMARY

Nortriptyline intoxication secondary to terbinafine treatment was observed in a woman with a major depressive disorder. This is the second report of this interaction. A rechallenge was performed during which serum concentrations were measured of nortriptyline and the 2 hydroxy metabolites. To document the case, genotyping of CYP2D6 was also performed.

DISCUSSION

Metabolism by CYP2D6 is of major importance for the hydroxylation of nortriptyline, making it susceptible to competitive inhibition by terbinafine.

CONCLUSIONS

The pharmacokinetic interaction between nortriptyline and terbinafine is probably due to inhibition of CYP2D6 of the nortriptyline metabolism by terbinafine. The interaction is not restricted to a subpopulation, but may occur even in persons without deviations in CYP2D6.

Effect of nortriptyline and paroxetine on CYP2D6 activity in depressed elderly patients

This study was performed in elderly patients (1) to assess the degree to which CYP2D6 mediated metabolism of debrisoquine at baseline determines plasma concentration to dose quotients for nortriptyline or paroxetine after 4 weeks of treatment, and (2) to compare the effects of nortriptyline and paroxetine on debrisoquine metabolism after 6 weeks of treatment. CYP2D6 activity was estimated in 66 subjects (71.4 +/- 7.2 years) before initiating treatment and again after 6 weeks of treatment with either nortriptyline or paroxetine under randomized, double-blind conditions according to a standard protocol. CYP2D6 activity was estimated by the debrisoquine recovery ratio in a 6- to 8-hour urine sample collected after oral administration of 10 mg debrisoquine sulfate. Nortriptyline and paroxetine plasma concentrations were obtained weekly. Baseline debrisoquine recovery ratio values were significantly correlated with the plasma concentration to dose quotient at 4 weeks for both nortriptyline ( = -0.75, = 0.0001, N = 29) and paroxetine ( = -0.50, = 0.003, N = 33). Treatment with either nortriptyline or paroxetine was associated with a significant decrease in the median debrisoquine recovery ratio, reflecting inhibition of CYP2D6 metabolism. The percent decrease associated with nortriptyline was significantly smaller than that with paroxetine ( < 0.0001). None of the patients treated with nortriptyline but 19 of the 32 extensive metabolizers treated with paroxetine were converted to phenotypic poor metabolic status. Our observations of CYP2D6 inhibition are consistent with data and results obtained in younger healthy volunteers. The significant correlations between baseline debrisoquine recovery ratio and the plasma concentrations to dose quotients at 4 weeks for both nortriptyline and paroxetine are consistent with CYP2D6 playing a major role in the metabolism of both drugs. CYP2D6 inhibition by paroxetine, which effectively converted 59% of patients to phenotypic PMs, may be especially relevant for elderly patients given their generally higher concentration of paroxetine.