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 (eg, haloperidol), MDMA, SSRIs, beta-blockers, and opiates. Drug-to-drug interactions (i.e. competitive inhibition) can occur with all of these drugs when taken in combination with CYP2D6 inhibitors.
One of the key roles of a pharmacist 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 that compete for phase I and phase II reactions, are the most common drug-drug interactions that are reviewed by pharmacists.
| Inhibitors of Cytochrome P450 Enzymes |
| CYP1A1 | a-naphtoflavone |
| CYP1A2 | Amiodarone, cimetidine, fluvoxamine, ciprofloxacin |
| CYP1B1 | Some flavonoids and coumarins, flutamide |
| CYP2A6 | Tranylcypramine, methoxsalen, grapefruit juice |
| CYP2B6 | Tranylcypramine, ticlopidine |
| CYP2C8 | Efavirenz |
| CYP2C9 | Isoniazid, fluvastatin, fluvoxamine, lovastatin |
| CYP1C19 | Tranylcypramine, cimetidine, fluoxetine, ketoconazole |
| CYP2D6 | Amiodarone, cimetidine, ranitidine, chlorpheniramine, quinidine, SSRIs, St. John's wort, Goldenseal |
| CYP2E1 | Suplhide, disulfiram, capsaicin |
| CYP3A4/5 | Amiodarone, azoles (anti-fungal medication), cimetidine, grapefruit juice, macrolides, steroids, protease inhibitors, ginko biloba |
