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  1. #1
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    Post What is Cytochrome P450 & How It Can Cause Drug Interactions

    Cytochrome p450 Drug Metabolism and Interactions

    Oxidative metabolism by cytochrome p450 enzymes is a primary method of drug metabolism. The purpose of drug metabolism is to make drugs more water soluble so they can be more easily excreted from the body. The recent news that Posicor (mibefradil), a newer calcium channel blocker, has been removed from the market due to its high propensity to iteract with other drugs places a spotlight on hepatic drug metabolism.

    Drug interactions involving the cytochrome p450 system are common, and generally result from either enzyme inhibition or induction.

    • Enzyme inhibition generally involves competition with another drug for enzyme binding sites, and usually begins with the first dose of the inhibitor. Duration of inhibition corresponds to the half-lives of the respective drugs.
    • Enzyme induction occurs when one drug stimulates production of more enzymatic metabolism capacity. The effects of enzyme induction are sometimes more difficult to predict because these are dependent on drug half-lives, the rate of enzyme production, and individual genetic variations.


    Genetic differences are the reason one patient might be susceptible to interactions when another may not. Cytochrome p450 enzymes are often designated by the letters CYP followed by a set of letters and numbers that distinguish enzyme isoforms.

    Understanding CYP enzyme interactions might allow prescribers the ability to better anticipate and manage each patient's response to a drug regimen.

    University of Colorado Health Sciences Center provides a very good description of Cytochrome pharmacokinetics HERE Note that the Drug Interaction Table referred to on that page is not available.

    Attached to this posting are two pdf files containing an abbreviated drug interaction table associated with Cytochrome p450 metabolism.

    The abbreviated list shows commonly used drugs affected by CYP interactions. It should be noted that this list is not complete, and that metabolism of some drugs is complex, and may involve more than one enzyme system.

    Drug interactions also occur via mechanisms unrelated to CYP enzymatic activity.

    Additional information on this topic is in This Psychlinks Posting
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  2. #2
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    Cytochrome P450 2D6 Genotype Variation and Venlafaxine Dosage

    It seems that Cytochrome P450 may be also imnplicated in response or non-response to SSRIs:

    Cytochrome P450 2D6 Genotype Variation and Venlafaxine Dosage
    DONALD E. MCALPINE, MD; DENNIS J. O?KANE, PhD; JOHN L. BLACK, MD; DAVID A. MRAZEK, MD

    OBJECTIVE: To determine whether the presence or absence of a fully functioning cytochrome P450 2D6 allele was associated with the dosage of the antidepressant drug venlafaxine in patients who had either adverse effects or absence of a therapeutic response to treatment with the immediate release or extended release form of venlafaxine.

    PATIENTS AND METHODS: We reviewed the electronic medical records of 199 patients enrolled in a previous pharmacogenomic study (June 1, 2002 through April 30, 2004) who had either adverse effects or the absence of a therapeutic response to treatment with psychotropic medications. This review identified 38 patients previously treated with venlafaxine immediate release or extended release and subsequently genotyped for the 2D6 gene with a commercial genotyping assay. Their dosage was examined along with their 2D6 genotype to determine whether the presence or absence of a fully functioning 2D6 allele was associated with their venlafaxine dosage.

    RESULTS: Of the 38 patients, 5 had a 2D6 genotype that consisted of 1 inactive allele and 1 allele associated with decreased activity. None of these 5 patients were able to tolerate treatment with more than 75 mg/d of venlafaxine. The remaining 33 patients had at least 1 fully active 2D6 allele, 26 of whom had been able to tolerate treatment with 150 mg/d or more of venlafaxine (P<.002).

    CONCLUSION: Genetic variations of the P450 2D6 gene may contribute to patient-specific variation in response to treatment with venlafaxine. Physicians should be alert to the possibility that an adverse reaction may indicate a slow metabolizer and consider genotyping such patients.

    Mayo Clin Proc. 2007;82(9):1065-1068 [Full Text]
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  3. #3
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    Post Re: What is Cytochrome P450 & How It Can Cause Drug Interactions

    Genetic variations of the P450 2D6 gene may contribute to patient-specific variation in response to treatment with venlafaxine
    Recent technology has brought an issue to the forefront that was not recognized when venlafaxine (Effexor) was introduced to the market in 1997. At that time, venlafaxine was considered a so-called "clean" drug not implicated in this phase of metabolism.

    Good up to date information!

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