The impact of Cytochrome P450-2D6 genotype on the use and interpretation of therapeutic drug monitoring in long-stay patients treated with antidepressant and antipsychotic drugs in daily psychiatric practice

An economic model of the cost-utility of pre-emptive genetic testing to support pharmacotherapy in patients with major depression in primary care

The pharmacokinetics of many antidepressants (tricyclic antidepressants (TCA) or selective serotonin re-uptake inhibitors (SSRI)) are influenced by the highly polymorphic CYP2D6 enzyme. Therefore, pharmacogenetics could play an important role in the treatment of depressive patients. The potential cost-utility of screening patients is however still unknown. Therefore, a Markov model was developed to compare the strategy of screening for CYP2D6 and subsequently adjust antidepressant treatment according to a patient's metabolizer profile of poor, extensive, or ultra metabolizer, with the strategy of no screening ('one size fits all' principle). Each week a patient had a probability of side effects, which was followed by dosage titration or treatment switching. After 6 weeks treatment effect was evaluated followed by treatment adjustments if necessary, with a total time horizon of the model of 12 weeks. The analysis was performed from a societal perspective. The strategy of screening compared with no screening resulted in incremental costs of €91 (95 percentiles: €39; €152) more expensive but also more effect with 0.001 quality adjusted life years (QALYs) (95 percentiles: 0.001; 0.002) gain. The incremental cost-effectiveness ratio (ICER) was therefore €77,406 per QALY gained, but varied between €22,500 and €377,500 depending on the price of screening and productivity losses. According to our model, we cannot unequivocally conclude that screening for CYP2D6 in primary care patients using antidepressants is be cost-effective, as the results are surrounded by large uncertainty. Therefore, information from ongoing studies should be used to reduce these uncertainties.

Availability of CYP2D6 genotyping results in general practitioner and community pharmacy medical records

Aim: To investigate the availability of CYP450–2D6 (CYP2D6) genotyping results in general practitioner (GP) and/or community pharmacy records, and the influence thereof on psychotropic CYP2D6 substrate dosing. Materials & methods: Primary outcome was the percentage of patients genotyped for CYP2D6 with their genotype/phenotype registered in GP and/or pharmacy records. Secondary outcome was the number of defined daily doses of psychotropic CYP2D6 substrates prescribed after genotyping. Results: For 216 out of 1307 eligible patients, medication overviews could be obtained. Genotyping results were available at GPs for 3.1% and at pharmacies for 5.9%. The average psychotropic CYP2D6 substrate dose was not different between any non-extensive metabolizer group and extensive metabolizer group (all p ≥ 0.486). Conclusion: Valuable information for individualizing psychiatric pharmacotherapy is lost on a large scale.

Prevalence of Patients Using Drugs Metabolized by Cytochrome P450 2D6 in Different Populations: a Cross-Sectional Study

Background:

Despito a large number of studies investigating the potential clinical relevance of CYP2D6 genotyping in preventing treatment failure (eg, insufficient efficacy and/or unacceptable adverse effects), the prevalence of patients using drugs metabolized by that isoenzyme is relatively unknown.

Objective:

To investigate the prevalence of patients in different populations using drugs metabolized by CYP2D6.

Methods:

In this cross-sectional study. 6 different patient populations were investigated: general, general hospital, geriatric, psychogoriatric, psychiatric, and mentally retarded. From every population, 150 adults using at least one drug were randomly selected. Primary outcome was the prevalence of patients using at least one drug metabolized by CYP2D6. The prevalence of patients using at least one CYP2D6 substrate in different populations was compared with the general population using χ2 statistics. Data were expressed as a relative risk with a 95% confidence interval.

Results:

Patients from the general hospital (RR 1.81; 95% CI 1.26 to 2.62), geriatric patients (RR 2.16; 95% CI 1.26 to 2.62), psychogeriatric patients (RR 2.31; 95% CI 1.63 to 3.27), and psychiatric patients (RR 2.44; 95% CI 1.73 to 3.44) were treated more frequently with at least one drug metabolized by CYP2D6 compared with patients in the general population. Approximately 50% of psychiatric (52%), psychogeriatric (49%), and geriatric (46%) patients used at least one drug metabolized by CYP2D6. In total, 416 drugs metabolized by CYP2D6 were prescribed, with 257 (62%) of these classified as an antidepressant (Anatomical and Therapeutic Chemical [ATC] category N06A) or antipsychotic (ATC N05A).

Conclusions:

Several patient populations (eg, psychiatric, psychogeriatric, geriatric) have a high prevalence of patients treated with at least one drug metabolized by CYP2D6. This study does not provide evidence regarding the clinical evidence of CYP2D6 genotyping, but shows that, if CYP2D6 genotyping is relevant for patient care, the highest probability of cost-effectiveness will, most likely, be in specific populations.

Comparison of delivery strategies for pharmacogenetic testing services

The number and use of pharmacogenetic tests to assess a patient's likelihood of response or risk of an adverse event is expanding across medical specialties and becoming more prevalent. During this period of development and translation, different approaches are being investigated to optimize delivery of pharmacogenetic services. In this paper, we review pre-emptive and point-of-care delivery approaches currently implemented or being investigated and discuss the advantages and disadvantages of each approach. The continued growth in knowledge about the genetic basis of drug response combined with development of new and less expensive testing technologies and electronic medical records will impact future delivery systems. Regardless of delivery approach, the currently limited knowledge of health professionals about genetics generally or PGx specifically will remain a major obstacle to utilization.

Clinical validity of cytochrome P450 metabolism and serotonin gene variants in psychiatric pharmacotherapy

Adverse events, response failures and medication non-compliance are common in patients receiving medications for the treatment of mental illnesses. A systematic literature review assessed whether pharmacokinetic (PK) or pharmacodynamic (PD) responses to 26 commonly prescribed antipsychotic and antidepressant medications, including efficacy or side effects, are associated with nucleotide polymorphisms in eight commonly studied genes in psychiatric pharmacotherapy: CYP2D6, CYP2C19, CYP2C9, CYP1A2, CYP3A4, HTR2C, HTR2A, and SLC6A4. Of the 294 publications included in this review, 168 (57%) showed significant associations between gene variants and PK or PD outcomes. Other studies that showed no association often had insufficient control for confounding variables, such as co-medication use, or analysis of medications not substrates of the target gene. The strongest gene-outcome associations were for the PK profiles of CYP2C19 and CYP2D6 (93% and 90%, respectively), for the PD associations between HTR2C and weight gain (57%), and for SLC6A4 and clinical response (54%), with stronger SLC6A4 response associations for specific drug classes (60-83%). The preponderance of evidence supports the validity of analyzing nucleotide polymorphisms in CYP and pharmacodynamic genes to predict the metabolism, safety, or therapeutic efficacy of psychotropic medications commonly used for the treatment of depression, schizophrenia, and bipolar illness.

Do we need pharmacogenetics to personalize antidepressant therapy?

This review examines the role of drug metabolism and drug target polymorphism in determining the clinical response to antidepressants. Even though antidepressants are the most effective available treatment for depressive disorders, there is still substantial need for improvement due to the slow onset of appreciable clinical improvement and the association with side effects. Moreover, a substantial group of patients receiving antidepressant therapy does not achieve remission or fails to respond entirely. Even if the large variation in antidepressant treatment outcome across individuals remains poorly understood, one possible source of this variation in treatment outcome are genetic differences. The review focuses on a few polymorphisms which have been extensively studied, while reporting a more comprehensive reference to the existing literature in table format. It is relatively easy to predict the effect of polymorphisms in drug metabolizing enzymes, such as cytochromes P450 2D6 (CYP2D6) and cytochrome P450 2C19 (CYP2C19), which may be determined in the clinical context in order to explain or prevent serious adverse effects. The role of target polymorphism, however, is much more difficult to establish and may be more relevant for disease susceptibility and presentation rather than for response to therapy.

Frequency of certain single-nucleotide polymorphisms and duplication of CYP2D6 in the Jordanian population

The CYP2D6 isozymes are responsible for metabolism of 7-10% of clinically available drugs. Genetic polymorphism in CYP2D6 may have an impact on drug efficacy and toxicity. The aim of this study was to determine the allelic frequency of CYP2D6*4, *10, and *17 and CYP2D6*2×N duplication allele in 192 healthy unrelated male and female Jordanian volunteers. Polymerase chain reaction (PCR)-restriction fragment length polymorphism-based methods were used to identify the CYP2D6*4, *10, and *17 genotypes; and allele-specific long PCR was used to determine the CYP2D6*2×N allelic frequency. The CYP2D6*10 allele was the most frequent mutant allele in Jordanians (14.8%) followed by CYP2D6*4 and *17 at 12.8%, and 8.3%, respectively. The duplication allele was found in 13.5% of the studied sample. The CYP2D6*4 G-A heterozygote genotype frequency was 20.3%, and the homozygous mutant genotype was 2.6%. In case of CYP2D6*10 C-T and CYP2D6*17 G-C heterozygote genotypes, the frequencies were 21.4% and 12.5%, respectively, while the homozygous mutant genotype frequencies of T-T and C-C were 4.2% and 2.1%, respectively. In conclusion, the allelic distributions of the CYP2D6 gene among Jordanians are different from other Mediterranean groups, especially the *10 and *17 single-nucleotide polymorphisms, and more importantly the CYP2D6*2×N duplication allele, which seems to follow a gradient reduction in prevalence from Ethiopia to Northern Europe.

Genetic polymorphisms of cytochrome P450 enzymes and antidepressant metabolism

INTRODUCTION

The cytochrome P450 (CYP) enzymes are the major enzymes responsible for Phase I reactions in the metabolism of several substances, including antidepressant medications. Thus, it has been hypothesized that variants in the CYP network may influence antidepressant efficacy and safety. Nonetheless, data on this field are still contradictory. The authors aim to give an overview of the published studies analyzing the influence of CYP highly polymorphic loci on antidepressant treatment in order to translate the acquired knowledge to a clinical level.

AREAS COVERED

The authors collected and compared experimental works and reviews published from the 1980s to the present and included in the Medline database. The included studies pertain to the effects of CYP gene polymorphisms on antidepressant pharmacokinetic parameters and clinical outcomes (response and drug-related adverse effects), with a focus on applications in clinical practice. The authors focused mainly on in vivo studies in humans (patients or healthy volunteers).

EXPERT OPINION

Great variability in antidepressant metabolism among individuals has been demonstrated. Thus, with the current interest in individualized medicine, several genetic tests to detect CYP variants have been produced. They provide a potentially useful way to anticipate some clinical outcomes of antidepressant treatment, although they will only be extensively used in clinical practice if precise and specific treatment options and guidelines based on genetic tests can be provided.

Do guidelines recommending pharmacogenetic testing of psychiatric patients affect treatment costs and the use of healthcare services?

Aim: To identify the effects of local recommendations of pharmacogenetic testing in psychiatry with respect to treatment costs. Methods: Based on Danish patient registers, individual treatment costs within a 365-day period at three psychiatric hospitals recommending and using pharmacogenetic testing is compared retrospectively with treatment costs at other Danish psychiatric hospitals using alternate treatment strategies. Primary outcome of interest is total direct costs analyzed by multilevel modelling. Secondary outcome measures are healthcare consumption within specific sectors analyzed by Tobitregressions. Results: Costs among patients treated at hospitals recommending and using pharmacogenetic testing were not found to be statistically significantly different from costs among patients treated at sites using alternate treatment strategies. In spite of recommendations to test all patients the uptake of the test was, however, low (26—31 %). Treatment practice using routine therapeutic drug monitoring (in Ãrhus) shows a trend towards lower costs. Conclusions: Based on this natural experiment we were not able to document statistically significant differences in total costs between treatment sites that had guidelines recommending pharmacogenetic testing, relative to sites without such guidelines, over a period of one year. However, guidelines of pharmacogenetic testing and possibly also therapeutic drug monitoring seem to lead to reductions in costs for primary care services. In the case of the former, reductions do, however, seem to be outweighed by increases in costs for psychiatric and non-psychiatric inpatient stays. In conclusion, no statistically significant differences in total direct costs across sites with different treatment strategies were found.

Estimating the Danish populations' preferences for pharmacogenetic testing using a discrete choice experiment. The case of treating depression

OBJECTIVE

The objective is to estimate willingness-to-pay (WTP) for pharmacogenetic testing in the treatment of depression.

METHODS

In a web-based discrete choice questionnaire, four attributes were included: 1) number of changes in antidepressants before symptom relief; 2) time with dosage adjustments due to adverse side effects and/or lack of effects; 3) cost of pharmacogenetic testing; 4) probability of benefits from pharmacogenetic testing. Respondents were asked to choose between two scenarios; 1) pharmacogenetic testing; and 2) an opt-out option reflecting a scenario without pharmacogenetic testing. The indirect utility model was assumed to be multiplicative in probability of benefits and reduced time with dosage adjustments as well as reduced number of antidepressant changes.

RESULTS

Most coefficients had the expected signs and were statistically significant. WTP for avoidance of one change in antidepressant medication is 1571 Danish Krone (DKK), whereas WTP for reducing the period with dosage-adjustments by 1 month is DKK604. Both were statistically significantly different from zero.

CONCLUSION

If diagnosed with depression, peoples' WTP for pharmacogenetic testing appears to exceed its price as long as there is a reasonable probability for improvements in treatment (in the present case 10%). Utility is associated with outcomes only. Hence, other modes of provision of similar improvements in treatment may be valued equally highly. WTP estimates and the associated policy implications appear to be robust because they were unaffected by estimation model.

Patient preferences for pharmacogenetic screening in depression

Objectives:The aims of this study were to estimate preferences and willingness-to-pay (WTP) for genetic screening for CYP2D6 polymorphisms among a group of former and currently depressed patients.

Methods:A Web-based discrete choice questionnaire was sent to 89 respondents, age 18–65. Four attributes were included: (i) shifts in antidepressant medication before symptom relief, (ii) time with antidepressant medication without symptom relief, (iii) time with antidepressant medication without symptoms but with adverse side-effects, (iv) cost of genetic screening. We used a switching model with two scenarios, one representing patients’ own treatment history and the other a treatment scenario with genetic screening.

Results:In a main-effects model involving the four attributes all coefficients had the expected sign, indicating that as the number of shifts, price or time without symptom relief, and/or dosage-adjustments increased, the likelihood of choosing the screening test decreased. Price and number of shifts in medicine were significant. Marginal WTP for 5 percent probability of a reduction of one in antidepressant shifts was DKK2,599 (€350).

Conclusions:Patients value reductions in shifts in antidepressants and price when choosing between genetic screening and no screening. They do not focus on how the reductions are provided, nor do they value the genetic information the test provides irrespective of its effect on outcome. Given, that the test is able to provide a reduction of one shift in the number of antidepressant shifts with a probability of 5 percent, WTP for the test exceeds its cost.

Influence of the CYP2D6*4 polymorphism on dose, switching and discontinuation of antidepressants

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

* Most antidepressants are metabolized by CYP2D6. The variant allele CYP2D6*4 is the main polymorphism resulting in reduced enzyme activity in Caucasians. * Reduced enzyme activity potentially leads to increased toxicity of antidepressants, but the relevance of genotyping for clinical practice is unclear. Most clinical studies suffer from small numbers of patients.

WHAT THIS STUDY ADDS

* This large population-based cohort study in 1198 elderly Dutch patients examines the influence of the CYP2D6*4 polymorphism on intolerability of antidepressants. * The risk of switching to another antidepressant in tricyclic antidepressant users is higher in poor metabolizers (PMs), but not in SSRI users. PMs require a lower maintenance dose of antidepressants compared with extensive metabolizers (EMs). * Antidepressants were initiated in a relatively low dose, with gradual dose increments thereafter, reducing the risk of adverse drug reactions. Therefore, the question remains whether genotyping prior to the start of antidepressant therapy contributes substantially to the optimization of pharmacotherapy.

AIMS

To study the effect of CYP2D6*4 on antidepressant dose, switching and discontinuation of therapy.

METHODS

The study consisted of all subjects in the Rotterdam Study, who received a first antidepressant prescription between April 1st 1991 and July 1st 2005 and for whom data on CYP2D6 genotype were available. Binary logistic regression was performed to study the association between CYP2D6*4 and switching to any other antidepressant or discontinuation of therapy within 45 days. The difference in mean antidepressant dose was compared between CYP2D6 genotypes using t-tests and repeated measurements analyses.

RESULTS

In users of tricyclic antidepressants (TCAs) the risk of switching to another antidepressant was significantly higher in poor metabolizers (PMs:*4/*4) compared with extensive metabolizers (EMs:*1/*1), with an adjusted OR of 5.77 (95% CI 1.59, 21.03; P = 0.01). In SSRI users there was no significant difference (OR 0.91; 95% CI 0.20, 4.15; P = 0.90). Heterozygous patients did not have an increased risk of switching in both TCA and SSRI users. The mean TCA dose was significantly lower in PMs than in EMs at the third and fourth prescription (difference 0.11 DDD, P = 0.03). In SSRI users the difference in mean dose between PMs and EMs was significant at the third prescription (0.17 DDD; P = 0.02).

CONCLUSIONS

The risk of switching to another antidepressant in TCA users is higher in PMs than in EMs. The maintenance doses of antidepressants were significantly lower in PMs. However, the question whether genotyping prior to the start of antidepressant therapy contributes substantially to the optimization of pharmacotherapy, requires further study.

Effect of haloperidol on transient outward potassium current in rat ventricular myocytes

Although sigma ligand haloperidol is known to affect repolarization in heart, its effect on potassium currents in cardiomyocytes has not yet been studied. We analyzed the effect of 1 micromol/l haloperidol on transient outward K(+) current (I(to)) in enzymatically isolated rat right ventricular cardiomyocytes using the whole-cell patch-clamp technique at room temperature. Haloperidol induced a decrease of amplitude and an acceleration of apparent inactivation of I(to), both in a voltage-independent manner. The averaged inhibition of I(to), evaluated as a change of its time integral, was 23.0+/-3.2% at stimulation frequency of 0.1 Hz. As a consequence of slow recovery of I(to) from the haloperidol-induced block (time constant 1482+/-783 ms), a cumulation of the block up to about 40% appeared at 3.3 Hz. We conclude that haloperidol causes a voltage-independent block of I(to) that cumulates at higher stimulation frequencies. Based on the computer reconstruction of experimental data, a block of I(to)-channels in both open and open-inactivated states appears to be likely mechanism of haloperidol-induced inhibition of I(to).