A poor metabolizer for cytochromes P450 2D6 and 2C19: a case report on antidepressant treatment.

Scientific literature has never described a poor metabolizer for both the cytochrome P450 (CYP) 2D6 and the CYP 2C19. They are expected to be rare (<1% in different ethnic groups) and prone to adverse drug reactions with many antidepressants. In an ongoing pharmacogenetic study, after genotyping 1,576 subjects in three Kentucky state hospitals we have found one poor metabolizer for both CYP 2D6 and CYP 2C19, which corresponds to a prevalence of 0.06% (95% CI 0.01 to 0.36). The naturalistic antidepressant treatment of this poor metabolizer for both enzymes is described in this article. As genotyping reaches clinical practice, it will be interesting to prospectively establish whether mirtazapine is a reasonable choice as an antidepressant for these patients, as the data and this case suggest.

The AmpliChip CYP450 genotyping test: Integrating a new clinical tool.

The AmpliChip CYP450 Test, which analyzes patient genotypes for cytochrome P450 (CYP) genes CYP2D6 and CYP2C19, is a major step toward introducing personalized prescribing into the clinical environment. Interest in adverse drug reactions (ADRs), the genetic revolution, and pharmacogenetics have converged with the introduction of this tool, which is anticipated to be the first of a new wave of such tools to follow over the next 5-10 years. The AmpliChip CYP450 Test is based on microarray technology, which combines hybridization in precise locations on a glass microarray and a fluorescent labeling system. It classifies individuals into two CYP2C19 phenotypes (extensive metabolizers [EMs] and poor metabolizers [PMs]) by testing three alleles, and into four CYP2D6 phenotypes (ultrarapid metabolizers [UMs], EMs, intermediate metabolizers [IMs], and PMs) by testing 27 alleles, including seven duplications. CYP2D6 is a metabolic enzyme with four activity levels (or phenotypes): UMs with unusually high activity; normal subjects, known as EMs; IMs with low activity; and PMs with no CYP2D6 activity (7% of Caucasians and 1-3% in other ethnic groups). Levels of evidence for the association between CYP2D6 PMs and ADRs are relatively reasonable and include systematic reviews of case-control studies of some typical antipsychotics and tricyclic antidepressants (TCAs). Evidence for other phenotypes is considerably more limited. The CYP2D6 PM phenotype may be associated with risperidone ADRs and discontinuation due to ADRs. Venlafaxine, aripiprazole, duloxetine, and atomoxetine are newer drugs metabolized by CYP2D6 but studies of the clinical relevance of CYP2D6 genotypes are needed. Non-psychiatric drugs metabolized by CYP2D6 include metoprolol, tamoxifen, and codeine-like drugs. CYP2C19 PMs (3-4% of Caucasians and African Americans, and 14-21% of Asians) may require dose adjustment for some TCAs, moclobemide, and citalopram. Other drugs metabolized by CYP2C19 are diazepam and omeprazole. The future of pharmacogenetics depends on the ability to overcome serious obstacles, including the difficulties of conducting and publishing studies in light of resistance from grant agencies, pharmaceutical companies, and some scientific reviewers. Assuming more studies are published, pharmacogenetic clinical applications may be compromised by economic factors and the lack of physician education. The combination of a US FDA-approved test, such as the AmpliChip CYP450 Test, and an FDA definition of CYP2D6 as a 'valid biomarker' makes CYP2D6 genotyping a prime candidate to be the first successful pharmacogenetic test in the clinical environment. One can use microarray technology to test for hundreds of single nucleotide polymorphisms (SNPs) but, taking into account the difficulties for single gene approaches such as CYP2D6, it is unlikely that very complex pharmacogenetic approaches will reach the clinical market in the next 5-10 years.

Response to hydrocodone, codeine and oxycodone in a CYP2D6 poor metabolizer.

Codeine is metabolized by the cytochrome P450 2D6 (CYP2D6) to morphine. Codeine is a much weaker agonist at mu opioid receptors than morphine. Therefore, codeine analgesia is highly dependent on CYP2D6 activity. Large prospective studies in the clinical environment do not exist, but it appears reasonable to avoid codeine use in CYP2D6 poor metabolizers (PMs). CYP2D6 metabolizes other opioid analgesics, including tramadol, dihydrocodeine, oxycodone and hydrocodone, although they have been less systematically studied. It is unclear whether these other pro-drugs may be as completely dependent on CYP2D6 for their analgesia as codeine. We describe a patient identified as a CYP2D6 PM with a history of problems with opioid analgesics. The patient was an 85-year-old female Caucasian who had hip surgery. The patient had a long-standing intolerance to codeine. In her first admission, she couldn't tolerate the regimen of oxycodone combined with tramadol prns (as needed). She was genotyped as a CYP2D6 PM and after the information was provided to the treating physician in her second admission, she seemed to have a better response to hydrocodone. Large case-control naturalistic studies followed by randomized trials in patients taking opioid analgesics may be needed to definitively establish that CYP2D6 genotyping has clinical relevance in the use of several opioid analgesics.