Enzyme Inhibition, continued

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The page below is a sample from the LabCE course Drug Metabolism. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Enzyme Inhibition, continued

CYP3A4 is involved in about 50% of drug metabolism, so there is a high risk of drug-to-drug interactions with this enzyme. Perhaps the most well-known inhibitor of CYP3A4 is Ritonavir (trade name Norvir - Abbott Laboratories). Ritonavir is an antiretroviral drug from the protease inhibitor class that is used to treat HIV and AIDS and is a potent inhibitor of CYP3A4.
Substrates of CYP2D6 include tricyclic antidepressants, antipsychotics (e.g., haloperidol), MDMA, SSRIs, beta-blockers, and opiates. Drug-to-drug interactions (i.e., competitive inhibition) can occur with all these drugs when combined with CYP2D6 inhibitors.
A pharmacist's key role is to notice and prevent drugs that can negatively interact and thus prevent adverse drug reactions in patients. Enzyme induction and inhibition, as well as drugs competing for phase I and II reactions, are the most common drug-drug interactions that pharmacists review.
Inhibitors of Cytochrome P450 Enzymes
CYP1A1 a-naphtoflavone
CYP1A2Amiodarone, cimetidine, fluvoxamine, ciprofloxacin
CYP1B1 Some flavonoids and coumarins, flutamide
CYP2A6Tranylcypromine, methoxsalen, grapefruit juice
CYP2B6Tranylcypramine, ticlopidine
CYP2C8Efavirenz
CYP2C9Isoniazid, fluvastatin, fluvoxamine, lovastatin
CYP1C19Tranylcypromine, cimetidine, fluoxetine, ketoconazole
CYP2D6Amiodarone, cimetidine, ranitidine, chlorpheniramine, quinidine, SSRIs, St. John's wort, Goldenseal
CYP2E1 Suplhide, disulfiram, capsaicin
CYP3A4/5Amiodarone, azoles (anti-fungal medication), cimetidine, grapefruit juice, macrolides, steroids, protease inhibitors, ginkgo biloba