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 |
| CYP1A2 | Amiodarone, cimetidine, fluvoxamine, ciprofloxacin |
| CYP1B1 | Some flavonoids and coumarins, flutamide |
| CYP2A6 | Tranylcypromine, methoxsalen, grapefruit juice |
| CYP2B6 | Tranylcypramine, ticlopidine |
| CYP2C8 | Efavirenz |
| CYP2C9 | Isoniazid, fluvastatin, fluvoxamine, lovastatin |
| CYP1C19 | Tranylcypromine, 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, ginkgo biloba |
