CYP2D6: Doxepin 2015/2016/2017
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CYP2D6: doxepin 2015/2016/2017 AUC = area under the concentration-time curve, Cl or = oral clearance, Css = plasma concentration in steady state, EM = extensive metaboliser (gene dose 1.5-2.5) (normal CYP2D6 enzyme activity), IM = intermediate metaboliser (gene dose 0,5-1) (decreased CYP2D6 enzyme activity), MR = metabolic ratio, NS = non-significant, PM = poor metaboliser (gene dose 0) (absent CYP2D6 enzyme activity), S = significant, SmPC = Summary of Product Characteristics, TCA = tricyclic antidepressant, UM = ultra-rapid metaboliser (gene dose ≥ 3) (increased CYP2D6 enzyme activity) Disclaimer : The Pharmacogenetics Working Group of the KNMP formulates the optimal recommendations for each phenotype group based on the available evidence. If this optimal recommendation cannot be followed due to practical restrictions, e.g. therapeutic drug monitoring or a lower dose is not available, the health care professional should consider the next best option. Brief summary and justification of choices: Doxepin and the active metabolite N-methyldoxepin (nordoxepin) are primarily converted by CYP2D6 to inactive hydroxy metabolites. Genetic variants in CYP2D6 can result in a decreased CYP2D6 enzyme activity (intermediate metabolisers (IM)), an absent CYP2D6 enzyme activity (poor metabolisers (PM)) or an increased CYP2D6 enzyme activity (ultra-rapid meta- bolisers (UM)). Kinetic studies showed (significant) differences in doxepin + nordoxepin exposure for patients with CYP2D6 gene variants (Kirchheiner 2005, Kirchheiner 2002 and Tacke 1992). Two cases suggest an increased risk for adverse events in PM (Koski 2007, Grasmader 2004). Because doxepin has a narrow therapeutic range, changes in exposure are likely to have therapeutic consequences. For these reasons, the KNMP Pharmacogenetics Working Group decides that a gene-drug interaction is present and that dose adjustments are required for PM, IM and UM (yes/yes- interactions). Justification of choices per CYP2D6 phenotype Dose adjustments have been calculated on the basis of the AUC or C ss of doxepin + nordoxepin. PM: A case of death by doxepin intoxication was reported for PM, in which the defective genotype probably contri- buted to the death. Dose adjustment or use is therefore desirable. The weighted mean of the calculated dose adjustment is a dose reduction to 44% of the standard dose (34-46% for two studies, 114% for one case). This was translated to 40% to be more achievable in clinical practice. IM: Only kinetic effects are known for IM, but - analogous to PM and based on the limited therapeutic range of doxepin - dose adjustment is recommended. The weighted mean of the calculated dose adjustment is a dose reduction to 84% of the standard dose (one study with 8 individuals). This was translated to 80% to be more achievable in clinical practice. UM: The weighted mean of the calculated dose adjustment is a dose increase to 221% of the standard dose (one study with 6 individuals). This was translated to 200% to be more achievable in clinical practice. An alternative can be selected as a precaution due to the lack of knowledge about the effects of high concen- trations of the possible cardiotoxic hydroxy metabolites. Note: The reliability of the dose adjustment calculation is currently limited by the fact that it is not known which isomer is the active form, whilst Kirchheiner 2002 found that the metabolism of the E-isomer in particular is influenced by CYP2D6. You can find an overview of the observed kinetic and clinical consequences per phenotype in the background infor- mation text of the gene-drug interactions on the KNMP Kennisbank. You might also have access to this background information text via your pharmacy or physician electronic decision support system. Recommendation concerning pre-emptive genotyping, including justification of choices: The Dutch Pharmacogenetics Working Group considers genotyping before starting doxepin to be potentially beneficial for the prevention of side effects. Genotyping can be considered on an individual patient basis. If, however, the geno- type is available, the Dutch Pharmacogenetics Working Group recommends adhering to the gene-drug guideline. The clinical implication of the gene-drug interaction scores 2 out of the maximum of 10 points (with pre-emptive geno- typing considered to be potentially beneficial for scores ranging from 0 to 2 points) (see also the clinical implication 1 score tables at the end of this risk analysis): A case of fatal doxepin intoxication was reported for PM, in which the defective genotype probably contributed to the death (severity score F, corresponding to CTCAE grade 5). This results in the maximum score of 2 points for the first criterion of the clinical implication score, the clinical effect associated with the gene-drug interaction (2 points for CTCAE grade 5). There were no studies showing an increase in adverse events in patients with CYP2D6 genetic variants. This results in a score of 0 of the maximum of 3 points for the second criterion of the clinical implication score: the level of eviden- ce supporting an associated clinical effect grade ≥ 3 (only points for at least one publication with level of evidence score ≥ 3). A severe clinical effect has only been observed in a case report and not in studies, and only one case report with a severe clinical effect was found. This indicates that the number needed to genotype (NNG) to prevent one clinical effect code ≥ D (grade ≥ 3) cannot be determined, but is likely to be very high. This results in a score of 0 of the maxi- mum of 3 points for the third criterion of the clinical implication score: the number needed to genotype (NNG) in the Dutch population to prevent one clinical effect grade ≥ 3 (only points for NNG ≤ 1000). The American Summary of Product Characteristics (SmPC) of doxepin mentions the CYP2D6 PM phenotype, but the Dutch SmPC does not. This results in 0 out of the maximum of 2 points for the fourth and last criterion of the clinical implication score, the pharmacogenetics information in the SmPC (only points for at least one genotype/phenotype mentioned in the SmPC). The table below follows the KNMP definitions for EM, PM, IM and UM. The definitions of EM, PM, IM and UM used in the table below may therefore differ from the definitions used by the authors in the article. Source Code Effect Comments ref. 1 2 Blood samples were collected following the death of a 43- Koski A et al. year-old male. The plasma concentration of doxepin was A fatal doxepin poiso- 2.4 mg/L (therapeutic range approx. 0.03-0.15 mg/L) and ning associated with a that of nordoxepin was 2.9 ml/L. The cause of death was defective CYP2D6 confirmed as fatal doxepin intoxication. MR doxepin/nor- genotype. doxepin (0.83) was low in comparison to 20 other fatal Am J Forensic Med doxepin intoxications (2.0-75, for 19 of the 20 intoxica- Pathol tions it was 3.8-75). 2007;28:259-61. PM: F The man was found to be CYP2D6 *3/*4 and CYP2C19 *1/*1. As the low MR doxepin/nordoxepin did not correspond to acute intoxication, the authors consider it likely that the defective CYP2D6 genotype contributed to his death, probably as a result of repeated high doses of doxepin. ref. 2 3 A total of 25 healthy volunteers (11x EM (7x gene dose 2, Kirchheiner J et al. 4x gene dose 1.5), 3x PM (gene dose 0), 6x UM (gene Impact of the CYP- dose 3), 5x ‘UM’ (4x gene dose 2.5, 1x gene dose 2) 2D6 ultra-rapid meta- received a single dose of doxepin 75 mg. bolizer genotype on doxepin pharmaco- PM versus EM: kinetics and serotonin PM: A - increase in AUC doxepin + nordoxepin from 1061 to AUC doxepin + in platelets. 2291 nmol.hour/L (S by 116%) nordoxepin versus Pharmacogenet EM: Genomics UM versus EM: PM: 216% 2005;15:579-87. UM: A - decrease in AUC doxepin + nordoxepin from 1061 to UM: 45% 479 nmol.hour/L (S by 55%) ‘UM’: 53% ‘UM’ versus EM: - decrease in AUC doxepin + nordoxepin from 1061 to 562 nmol.hour/L (S by 47%) There was a significant negative correlation between the sum of the plasma concentrations of doxepin + nordoxe- pin and the systolic blood pressure, but no difference in heart rate, blood pressure, QTc or sedation between the various CYP2D6 genotypes. NOTE: The results for ‘UM’ are described separately in this summary, because 20% of these cases involved the 2 ref. 2, continuation genotype n x 0.5 – 1 (gene dose 2). According to the KNMP definition, gene dose 2 is an EM. NOTE: Genotyping was performed for the alleles *3, *4, *5, *6, *9, *10, *35, *41 and for gene duplication of *1, *2, *4, *9, *10 and *41. ref. 3 3 Genotyping was performed on 4 patients on doxepin Grasmader K et al. (dose unknown). Out of the 4 patients, one was CYP2D6 Impact of polymor- PM and the other three were either CYP2D6 IM or EM. phisms of cyto- Relevant co-medication was not excluded. Plasma concen- chrome-P450 isoen- The mean dose-corrected C ss of doxepin + nordoxepin tration doxepin + zymes 2C9, 2C19 and was 0.27 ng/mL per mg of dosed doxepin. For the PM, nordoxepin versus 2D6 on plasma the corrected plasma concentration was 12% lower than EM (+ IM): concentrations and (1) the mean. PM: 88% clinical effects of PM: C The PM had relevant side effects. antidepressants in a naturalistic clinical setting. Eur J Clin Pharmacol 2004;60:329-36. ref. 4 3 A total of 42 healthy volunteers (8x *1/*1, 7x *1/*4, 1x Authors’ conclusion: Kirchheiner J et al. *1/*5, 6x *4/*4, 1x *4/*5, 1x *3/*5; all CYP2C19 EM and “The CYP2D6 poly- Contributions of CYP2C9 EM) received a single dose of doxepin 75 mg.