How is Flexeril (cyclobenzaprine) metabolized?

Cyclobenzaprine Metabolism

Cyclobenzaprine (Flexeril) is extensively metabolized in the liver primarily via cytochrome P450 enzymes CYP3A4, CYP1A2, and to a lesser extent CYP2D6, with N-demethylation being the major metabolic pathway. 1

Primary Metabolic Pathways

The drug undergoes hepatic metabolism through multiple cytochrome P450 isoenzymes:

• CYP3A4: Primary enzyme responsible for N-demethylation
• CYP1A2: Co-primary enzyme for N-demethylation
• CYP2D6: Minor role only 2

Research demonstrates that CYP3A4 and CYP1A2 are the predominant enzymes, while CYP2D6 plays only a minimal role. This is clinically significant because genetic polymorphisms of CYP2D6 should not be a major concern in cyclobenzaprine therapy, unlike with many tricyclic compounds 2.

Metabolite Formation and Excretion

Cyclobenzaprine is extensively metabolized with less than 1% excreted unchanged in urine 3. The major metabolites include:

• Desmethylcyclobenzaprine (primary metabolite via N-demethylation)
• Cyclobenzaprine glucuronide (major metabolite in humans)
• 10,11-dihydroxynortriptyline 4

Excretion occurs primarily as glucuronides via the kidneys 1. The drug undergoes significant first-pass metabolism, resulting in oral bioavailability of only 33-55% 1.

Pharmacokinetic Characteristics

Key metabolic features include:

• Effective half-life: 18 hours (range 8-37 hours)
• Plasma clearance: 0.7 L/min
• High plasma protein binding: 90-99%
• Enterohepatic circulation occurs 1

The drug exhibits linear pharmacokinetics over the 2.5-10 mg dose range 1.

Clinical Implications for Special Populations

Hepatic Impairment

Patients with hepatic impairment show approximately double the AUC and Cmax compared to healthy controls 1, 5. For mild hepatic impairment, start with 5 mg and titrate slowly. Cyclobenzaprine is not recommended in moderate to severe hepatic impairment 1.

Elderly Patients

Elderly patients (≥65 years) demonstrate 1.7-fold higher steady-state AUC values, with elderly males showing the greatest increase at 2.4-fold 1, 5. Initiate therapy at 5 mg in elderly patients and titrate slowly upward 6.

Renal Impairment

Renal excretion is a minor pathway, so renal insufficiency has minimal effect on plasma kinetics 7.

Drug Interactions

Because cyclobenzaprine is structurally related to tricyclic antidepressants, it shares similar metabolic pathways and potential interactions:

• Strong CYP3A4 inducers (rifampin, phenobarbital, carbamazepine, phenytoin) may decrease efficacy 8
• MAO inhibitors: Concomitant use increases risk of serotonin syndrome 6
• The drug has peripheral and central anticholinergic activity with potential for norepinephrine potentiation 6