Author: John R. Horn, PharmD, FCCP, and Philip D. Hansten, PharmD
Drs. Horn and Hansten are both professors
of pharmacy at the University of
Washington School of Pharmacy. For
an electronic version of this article,
including references if any, visit
www.hanstenandhorn.com.
In previous issues of Pharmacy
Times, we have discussed the cytochrome
P450 (CYP450) enzymes
CYP1A2, CYP2C9, CYP2C19, and CYP2D6
(see www.PharmacyTimes.com/Drug Interactions). In the spirit of saving the
best for last, in this issue, we will discuss
the most important of all CYP450
enzymes: CYP3A4. It has been estimated
that CYP3A4 metabolizes about half of
all drugs on the market. Because many
other commonly used drugs are moderate-to-potent inhibitors of CYP3A4, it
is not surprising that drug toxicity of
CYP3A4 substrates due to inhibition of
CYP3A4 is relatively common.
CYP3A4 also is sensitive to enzyme
induction, and a number of drugs are
known to be CYP3A4 inducers. CYP3A4
inducers tend to lower plasma concentrations
of CYP3A4 substrates, resulting
in reduced efficacy of the substrate.
This type of drug interaction is probably
more frequent than commonly realized,
because reduced drug effect may simply
be attributed to lack of patient response.
Many drugs that are CYP3A4 substrates,
inhibitors, and inducers are
also substrates, inhibitors, or inducers
of the ABC transport protein known
as P-glycoprotein. Many drug interactions,
therefore, involve additive effects
of both CYP3A4 and P-glycoprotein.
Table |
CYP3A4 Substrates Producing Potentially Serious Toxicity When Combined with CYP3A4 Inhibitors |
Drug | Potential Toxicity |
Alfuzosin (Uroxatral) | Severe hypotension |
Alprazolam (Xanax) | Excessive CNS depression |
Budesonide | Cushing's syndrome |
Carbamazepine (Tegretol) | Vomiting, headache, dizziness, drowsiness |
Colchicine | Fever, diarrhea, muscle pain, paresthesias (may be fatal) |
Cyclosporine (eg, Neoral) | Cyclosporine toxicity |
Dexamethasone | Cushing's syndrome |
Disopyramide (Norpace) | Cardiac arrhythmias |
Ergotamine (and other ergot alkaloids) | Ergotism (peripheral ischemia, cyanosis, hypertension) |
Fluticasone (Flovent) | Cushing's syndrome |
Lovastatin (Mevacor) | Rhabdomyolysis |
Methylprednisolone | Cushing's syndrome |
Midazolam (oral) | Excessive CNS depression |
Pimozide (Orap) | Torsades de pointes |
Quinidine | Cardiac arrhythmias |
Repaglinide (Prandin) | Hypoglycemia |
Rifabutin (Mycobutin) | Uveitis, bone marrow suppression, rash |
Sildenafil (Viagra) | Hypotension, syncope |
Simvastatin (Zocor) | Rhabdomyolysis |
Tadalafil (Cialis) | Hypotension, syncope |
Triazolam (Halcion) | Excessive CNS depression |
Vardenafil (Levitra) | Hypotension, syncope |
Vinblastine (Velban) | Bone marrow suppression |
Vincristine (Oncovin) | Peripheral neuropathy, paralytic ileus |
|
CNS = central nervous system. |
CYP3A4 Substrates
Drugs metabolized by CYP3A4 are
called CYP3A4 substrates. Keep in mind
that many drugs are metabolized by
more than one CYP450 enzyme, and
CYP3A4 may represent only one pathway.
Unfortunately, many CYP3A4 substrates
have substantial toxicity, and
some patients may develop severe toxicity
when CYP3A4 inhibitors are taken
concurrently. A selected list of such
interactions appears in the Table.
CYP3A4 Substrates |
|
Alfentanil (Alfenta)
Alfuzosin (Uroxatral)
Almotriptan (Axert)
Alprazolam (Xanax)
Amiodarone (Cordarone)
Amlodipine (Norvasc)
Aprepitant (Emend)
Atazanavir (Reyataz)
Atorvastatin (Lipitor)
Bepridil (Vascor)
Bexarotene (Targretin)
Bosentan (Tracleer)
Bromocriptine (Parlodel)
Budesonide (Entocort)
Buprenorphine (Subutex)
Bupropion (Buspar)
Carbamazepine (eg, Tegretol)
Cevimeline (Evoxac)
Cilostazol (Pletal)
Cisapride (Propulsid)
Clarithromycin (Biaxin)
Clonazepam (Klonopin)
Clopidogrel (Plavix)
Colchicine
Cyclophosphamide (Cytoxan)
Cyclosporine (Neoral)
Dapsone (Avlosulfon)
Darunavir (Prezista)
Dasatinib (Sprycel)
Delavirdine (Rescriptor)
Dexamethasone (Decadron)
Dihydroergotamine
Diltiazem (Cardizem)
Disopyramide (Norpace)
Docetaxel (Taxotere)
Donepezil (Aricept)
Doxorubicin (Adriamycin)
Droperidol
Dutasteride (Avodart)
Ebastine (Kestine)
Efavirenz (Sustiva)
Eletriptan (Relpax)
Eplerenone (Inspra)
Ergotamine (Ergomar)
Erlotinib (Tarceva)
Erythromycin
Estazolam (ProSom)
Eszopiclone (Lunesta)
Ethinyl Estradiol
Ethosuximide (Zarontin)
Etoposide (Vepesid)
Exemestane (Aromasin)
Felodipine (Plendil)
Fentanyl (Sublimaze)
Finasteride (Proscar)
Flurazepam (Dalmane)
Fosamprenavir (Lexiva)
Galantamine (Reminyl)
Gefitinib (Iressa)
Granisetron (Kytril)
Halofantrine (Halfan)
Ifosfamide (Ifex)
Imatinib (Gleevec)
Indinavir (Crixivan)
Irinotecan (Camptosar)
Isradipine (DynaCirc)
Itraconazole (Sporanox)
|
Ixabepilone (Ixempra)
Ketoconazole (Nizoral)
Lapatinib (Tykerb)
Levomethadyl (Orlaam)
Loperamide (Imodium)
Lopinavir (Kaletra)
Loratadine (Claritin)
Lovastatin (Mevacor)
Maraviroc (Selzentry)
Mefloquine (Lariam)
Methylprednisolone
Midazolam (Versed)
Mifepristone (Mifeprex)
Modafinil (Provigil)
Nefazodone
Nevirapine (Viramune)
Nicardipine (Cardene)
Nifedipine (Adalat)
Nimodipine (Nimotop)
Nisoldipine (Sular)
Nitrendipine (Baypress)
Oxybutynin (Ditropan)
Oxycodone (Percodan)
Paclitaxel (Taxol)
Paricalcitol (Zemplar)
Pimozide (Orap)
Pioglitazone
Praziquantel (Biltricide)
Prednisolone
Prednisone
Propoxyphene (Darvon)
Quazepam (Doral)
Quetiapine (Seroquel)
Quinacrine
Quinidine
Quinine
Ranolazine (Ranexa)
Repaglinide (Prandin)
Rifabutin (Rimactane)
Ritonavir (Norvir)
Saquinavir (Invirase)
Sibutramine (Meridia)
Sildenafil (Viagra)
Simvastatin (Zocor)
Sirolimus (Rapamune)
Solifenacin (Vesicare)
Sufentanil (Sufenta)
Sunitinib (Sutent)
Tacrolimus (Prograf)
Tadalafil (Cialis)
Tamoxifen (Nolvadex)
Tamsulosin (Flomax)
Teniposide (Vumon)
Testosterone
Tiagabine (Gabitril)
Tinidazole (Tindamax)
Tipranavir (Aptivus)
Topiramate (Topamax)
Triazolam (Halcion)
Vardenafil (Levitra)
Verapamil (Calan)
Vinblastine (Velbane)
Vincristine (Oncovin)
Ziprasidone (Geodon)
Zolpidem (Ambien)
Zonisamide (Zonegran)
Zopiclone (Imovane) |
|
CYP3A4 Inhibitors
Drugs that inhibit CYP3A4 activity will
almost always increase the plasma concentrations
of the CYP3A4 substrate medications.
Some drugs, such as clarithromycin,
itraconazole, and ketoconazole, are
particularly potent inhibitors of CYP3A4;
patients on these drugs may have markedly
reduced CYP3A4 activity.
CYP3A4 Inhibitors |
|
Amiodarone
Amprenavir
Aprepitant
Atazanavir
Chloramphenicol
Clarithromycin
Conivaptan
Cyclosporine
Darunavir
Dasatinib
Delavirdine
Diltiazem
Erythromycin
Fluconazole
Fluoxetine
Fluvoxamine
Fosamprenavir
Grapefruit juice
|
Imatinib
Indinavir
Isoniazid
Itraconazole
Ketoconazole
Lapatinib
Miconazole
Nefazodone
Nelfinavir
Posaconazole
Ritonavir
Quinupristin
Saquinavir
Tamoxifen
Telithromycin
Troleandomycin
Verapamil
Voriconazole
|
|
CYP3A4 Inducers
CYP3A4 inducers are drugs that increase
the activity of CYP3A4. Note that the
CYP3A4 enzyme is particularly susceptible
to enzyme inducers, and marked
reductions in the plasma concentrations
of CYP3A4 substrates may occur. For
example, a patient taking the potent
CYP3A4 inducer rifampin may have a
roughly 90% reduction in serum concentrations
of CYP3A4 substrates, such as
buspirone, triazolam, and verapamil.
CYP3A4 Inducers |
|
Aminoglutethimide
Bexarotene
Bosentan
Carbamazepine
Dexamethasone
Efavirenz
Fosphenytoin
Griseofulvin
Modafinil
Nafcillin
|
Nevirapine
Oxcarbazepine
Phenobarbital
Phenytoin
Primidone
Rifabutin
Rifampin
Rifapentine
St. John's wort
|
|