Oxford Drug Design

CYCLOBENZAPRINE HYDROCHLORIDE

cyclobenzaprine hydrochloride

Post-LOE

ORAL · TABLET

Cyclobenzaprine hydrochloride relieves skeletal muscle spasm of local origin without interfering with muscle function. It is ineffective in muscle spasm due to central nervous system disease. Cyclobenzaprine reduced or abolished skeletal muscle hyperactivity in several animal models. Animal studies indicate that cyclobenzaprine does not act at the neuromuscular junction or directly on skeletal muscle. Such studies show that cyclobenzaprine acts primarily within the central nervous system at brain stem as opposed to spinal cord levels, although its action on the latter may contribute to its overall skeletal muscle relaxant activity. Evidence suggests that the net effect of cyclobenzaprine is a reduction of tonic somatic motor activity, influencing both gamma (γ) and alpha (α) motor systems. Pharmacological studies in animals showed a similarity between the effects of cyclobenzaprine and the structurally related tricyclic antidepressants, including reserpine antagonism, norepinephrine potentiation, potent peripheral and central anticholinergic effects, and sedation. Cyclobenzaprine caused slight to moderate increase in heart rate in animals. Pharmacokinetics Estimates of mean oral bioavailability of cyclobenzaprine range from 33% to 55%. Cyclobenzaprine exhibits linear pharmacokinetics over the dose range 2.5 mg to 10 mg, and is subject to enterohepatic circulation. It is highly bound to plasma proteins. Drug accumulates when dosed 3 times a day, reaching steady-state within 3 to 4 days at plasma concentrations about 4-fold higher than after a single dose. At steady-state in healthy subjects receiving 10 mg t.i.d. (n = 18), peak plasma concentration was 25.9 ng/mL (range, 12.8 to 46.1 ng/mL), and area under the concentration-time (AUC) curve over an 8-hour dosing interval was 177 ng·hr/mL (range, 80 to 319 ng·hr/mL). Cyclobenzaprine is extensively metabolized, and is excreted primarily as glucuronides via the kidney. Cytochromes P450 3A4, 1A2, and, to a lesser extent, 2D6, mediate N-demethylation, one of the oxidative pathways for cyclobenzaprine. Cyclobenzaprine is eliminated quite slowly, with an effective half-life of 18 hours (range 8 to 37 hours; n = 18); plasma clearance is 0.7 L/min. The plasma concentration of cyclobenzaprine is generally higher in the elderly and in patients with hepatic impairment (see PRECAUTIONS: USE IN THE ELDERLY and PRECAUTIONS: IMPAIRED HEPATIC FUNCTION). Elderly In a pharmacokinetic study in elderly individuals (≥ 65 yrs old), mean (n = 10) steady-state cyclobenzaprine AUC values were approximately 1.7-fold (171 ng·hr/mL, range 96.1 to 255.3) higher than those seen in a group of 18 younger adults (101.4 ng·hr/mL, range 36.1 to 182.9) from another study. Elderly male subjects had the highest observed mean increase, approximately 2.4-fold (198.3 ng·hr/mL, range 155.6 to 255.3 vs. 83.2 ng·hr/mL, range 41.1 to 142.5 for younger males) while levels in elderly females were increased to a much lesser extent, approximately

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