Cytochrome P450 Enzymes

(Table 7-11)
  • Major drug-metabolizing enzymes that are most predominant in the liver but can also be found in other organs (i.e., intestine, lung)
  • Drug interaction involving p450 enzymes typically occur from one of two processes: enzyme inhibition or enzyme induction
  • Enzyme inhibition: competition with another drug for the enzyme-binding site
  • Enzyme induction: drug stimulates synthesis of more enzyme protein, enhancing metabolizing capacity of the enzyme
  • Different isoforms: CYP2D6, CYP3A4 (most abundant cytochrome enzymes in humans and involved in many clinically important drug interactions), CYP1A2, CYP2C9, CYP2E1, CYP2C19
  • Many drugs are substrates (metabolized by specific p450 isoenzyme) but unable to inhibit or induce the p450 enzymes

   
 
Table 7-11 Cytochrome p450 Drugs
 Cytochrome p450 Substrates3A4 Inducers3A4 Inhibitors
 
Anti-arrhythmics
Digoxin
Quinidine
Amiodarone

Antibiotics
Erythromycin
Clarithromycin
Rifampin

Anticonvulsants
Phenytoin
Carbamazepine
Ethosuximide

Antidepressants
Amitryptaline
Doxepin
Sertraline
Imipramine

Antihistamines
Fexofenadine
Terfenadine
Astemizole
Calcium Channel Blocker (CCB)
Nifedipine
Diltiazem
Verapamil
Felodipine

Immunosuppressants
Tacrolimus
Cyclosporine
Dapsone
Cyclophosphamide

Protease Inhibitors
Indinavir
Ritonavir
Saquinavir

Statins
Atorvastatin
Lovastatin
Simvastatin

Miscellaneous
Glyburide
Pimozide
Theophylline
Warfarin
Anticonvulsants
Phenytoin
Phenobarbital
Carbamazepine

Anti-TB drugs
Isoniazid
Rifampin

HIV antivirals
Ritonavir

Miscellaneous
Propranolol
St. John’s Wort
Omeprazole
Griseofulvin
Antidepressants
Fluoxetine
Paroxetine
Sertraline

Azole Antifungals
Ketoconazole
Itraconazole
Fluconazole

Antibiotics
Erythromycin
Clarithromycin
Ciprofloxacin
Metronidazole

CCBs
Verapamil
Nifedipine
Diltiazem

Miscellaneous
Cimetidine
Cyclosporine
Quinine
Grapefruit Juice
Warfarin

Protease Inhibitors
Ritonavir
 Warfarin + phenobarbital/rifampin/phenytoin → decreased levels of warfarin
 Warfarin + cimetidine/erythromycin/itraconazole/ketoconazole → increased levels of warfarin
 Cyclosporine + itraconazole → increased levels of cyclosporine
        
 
   


   
 
Table 7-12 Drug Reactions
 ReactionCulprit drugs
 
Acneiform lesions
Corticosteroids, androgenic steroids, phenytoin, INH, lithium, halogens, (bromides, iodides), epidermal growth factor receptor inhibitors (EGFRI) such as erlotinib, cetuximab, gefitinib
 
Acral erythema (erythrodysesthesia)
Doxorubicin, daunorubicin, 5-fluorouracil, cytarabine, docetaxel, methotrexate (MTX)
 
Acral sclerosis
Bleomycin
 
AGEP (acute generalized exanthematous pustulosis)
β-lactam antibiotics, macrolides, calcium channel blockers (i.e., diltiazem), chloroquine, terbinafine
 
Alopecia
Alkylating agents (i.e., cyclophosphamide), anthracyclines (doxorubicin, daunarubicin), taxanes (paclitaxel, docetaxel), vincristine, vinblastine, actinomycin-D, etoposide
 
Autoimmune hepatitis
Minocycline (MCN)
 
Bullous pemphigoid
Furosemide, PCN, captopril, β-blockers, sulfonamides, terbinafine, penicillamine
 
Dermatomyositis-like eruption
Hydroxyurea, penicillamine, statins
 
Eccrine squamous syringometaplasia
Cytarabine, cyclosphosphamide, paclitaxel, docetaxel, busulfan, carmustine
 
Elastosis perforans serpiginosa (EPS)
D-penicillamine
 
Erythema nodosum
OCPs, antibiotics (sulfonamides, tetracyclines), NSAIDs
 
Extravasation reaction
(ulcer, chemical cellulitis)
Anthracyclines (doxorubicin/daunorubicin), actinomycin-D, docetaxel, paclitaxel, vinblastine, vincristine, etoposide, 5-fluorouracil (5-FU)
 
Fixed drug eruption (FDE)
NSAIDS (naproxen), sulfonamides, TCNs, acetaminophen, aspirin, OCP, phenolphthalein, pseudoephedrine (non-pigmenting variant), barbiturates
 
Flag sign (of chemo)
MTX (horizontal hyperpigmented bands of hair alternating w/ normal color)
 
Gingival hyperplasia
Phenytoin, cyclosporine, CCBs (i.e., nifedipine, amlodipine)
 
Hyperpigmentation
Antimalarials, clofazamine (violet-brown to blue in lesional skin), imipramine, amiodarone (blue-gray), phenothiazines (chlorpromazine, promethazine, prochlorperazine), chemotherapeutics (busulfan, cyclophosphamide, hydroxyurea, dactinomycin, MTX, 5-FU), and minocycline (see below):
 
Type 1: blue-black color within scars, + iron (hemosiderin) with Perls stain {MCN pigmentation}
 
Type 2: blue-gray color on shins, + melanin (Fontana-Masson stain), + iron (Perls stain) {MCN pigmentation}
 
Type 3: generalized “muddy brown” hyperpigmentation in sun-exposed sites (↑ melanin within epidermis, no iron deposition)
 
Hypersensitivity syndrome
Aromatic anticonvulsants (phenytoin, phenobarbital, carbamazepine, lamotrigine, oxcarbazepine), sulfonamides, allopurinol, dapsone, gold, olanzapine, saquinavir
 
Hypertrichosis
Cyclosporine, phenytoin, minoxidil, danazol, anabolic steroids
 
Inflamed AKs
5-fluorouracil, capecitabine, pentostatin
 
Inflamed SCC
Fludarabine
 
Inflamed SKs
Cytarabine, docetaxel
 
Leg ulcers
Hydroxyurea
 
Leukocytoclastic vasculitis
Antibiotics (especially β-lactam antibiotics), NSAIDs, diuretics
 
Lichenoid eruption
Penicillamine, gold, hydrochlorothiazide (HCTZ), furosemide, β-blockers (propanolol, labetalol), ACEI (captopril, enalapril), chlorpromazine, antimalarials (chloroquine, hydroxychloroquine, quinacrine), quinidine
 
Linear IgA bullous dermatosis (LABD)
Vancomycin, β-lactam antibiotics, NSAIDs, captopril, phenytoin, sulfonamides, furosemide, amiodarone, lithium
 
Localized hyperpigmentation
Flagellate hyperpigmentation: bleomycin
 
Supravenous serpentine hyperpigmentation: 5-FU
 
Sun-exposed hyperpigmentation: daunorubicin, 5-FU, MTX
 
Mucosal: busulfan, cyclophosphamide, 5-FU, hydroxyurea, doxorubicin
 
Occluded areas: thiotepa, ifosfamide, topical carmustine, docetaxel, cisplatin
 
Lupus
SCLE: HCTZ, griseofulvin, terbinafine, CCBs
 
SLE: hydralazine, procainamide, isoniazid, MCN, phenytoin, penicillamine
 
Melanonychia
Chemotherapeutics (melphalan, hydroxyurea, bleomycin, capecitabine, doxorubicin), lamuvidine, MCN, zidovudine (AZT)
 
Mucositis
Cyclophosphamide, daunorubicin, doxorubicin, MTX (high dose), 5-FU
 
Neutrophilic eccrine hidradenitis
Doxorubicin, daunorubicin, cytarabine, 5-FU, MTX, bleomycin, cyclosphosphamide, busulfan, taxanes
 
Onycholysis
Paclitaxel, docetaxel, etoposide, retinoids, doxorubicin, captopril
 
Photo-onycholysis: TCN, OCPs, fluoroquinolones, psoralens
 
Orange-red body fluid
Rifampin
 
Pemphigus vulgaris
Thiol drugs (penicillamine, captopril, lisinopril, piroxicam, gold sodium thiomalate), penicillins, cephalosporines, quinolones, rifampicin, phenylbutazone, propranolol, carbamazepine
 
Penile ulcers
Foscarnet
 
Photosensitivity (includes allergic and toxic reactions)
Griseofulvin, NSAIDs, phenothiazines, sulfonamides, thiazides, dapsone, MTX, hydroxyurea, 5-FU, fluoroquinolones, TCNs, furosemide, diltiazem, isotretinoin, imipramine, chlorpromazine, phenothiazines
 
Pseudoporphyria
NSAIDs (naproxen, piroxicam), nalidixic acid, furosemide, HCTZ, isotretinoin, TCNs, sulfonamides
 
Pseudotumor cerebri
TCN, isotretinoin
 
Pseudoxanthoma elasticum (PXE)
Penicillamine
 
Psoriasis (induce/worsen)
Terbinafine, NSAIDs, antimalarials, ACEI, lithium, β-blockers
 
Pulmonary fibrosis
MTX, bleomycin, busulfan, amiodarone, gold, penicillamine
 
Radiation recall
Doxorubicin, daunorubicin, paclitaxel, docetaxel, MTX, actinomycin-D, capecitabine, gemcitabine, bleomycin
 
Raynaud’s phenomenon
Combination of bleomycin and vinblastine
 
Sweet’s syndrome
Granulocyte colony-stimulating factor (GCSF), carbamazepine, trimethoprim-sulfamethoxazole (TMP-SMZ)
 
Toxic epidermal necrolysis (TEN)
Allopurinol, PCNs, anticonvulsants (carbamazepine, lamotrigine, phenytoin), sulfonamides, antiretrovirals, barbiturates, NSAIDs