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The precise mechanism of action of baclofen is not fully known. Baclofen is capable of inhibiting both monosynaptic and polysynaptic reflexes at the spinal level, possibly by hyperpolarization of afferent terminals, although actions at supraspinal sites may also occur and contribute to its clinical effect. Although baclofen is an analog of the putative inhibitory neurotransmitter gamma-amino-butyric acid (GABA), there is no conclusive evidence that actions on GABA systems are involved in the production of its clinical effects. In studies with animals, baclofen has been shown to have general CNS depressant properties as indicated by the production of sedation with tolerance, somnolence, ataxia, and respiratory and cardiovascular depression.
Baclofen is rapidly and extensively absorbed. Absorption may be dose-dependent, being reduced with increasing doses. KEMSTRO™ (baclofen) given with or without water is bioequivalent to the baclofen conventional tablet. Thus KEMSTRO™ (baclofen) can be placed on the tongue until it disintegrates and then be swallowed with or without water. Following a single 20 mg oral dose of KEMSTRO™ (baclofen) , the peak plasma concentration was reached about 1½ hours after administration.
The apparent volume of distribution is 59 liters. Baclofen does not readily cross the blood-brain barrier. Plasma protein binding is approximately 30%.
In a study using radiolabeled baclofen, approximately 85% of the dose was excreted unchanged in the urine and feces. About 15% of the dose was metabolized, primarily by deamination. The y-hydroxy metabolite, 3-(p-chlorophenyl)-4-hydroxybutyric acid, is formed after deamination of baclofen.
Baclofen is rapidly and extensively eliminated. There is a relatively large intersubject variation in elimination. Baclofen is excreted primarily by the kidney as unchanged drug; 70 - 80% of a dose appears in the urine as unchanged drug. The remainder is excreted as unchanged drug in the feces or as metabolites in the urine and feces. Excretion is complete within 72 hours after administration. The elimination half-life of KEMSTRO™ (baclofen) is approximately 5½ hours. Total systemic clearance is 180 mL/min and renal clearance is 103 mL/min.
The pharmacokinetics of baclofen tablets were evaluated in elderly patients (69-81 years) and in healthy younger subjects (23-53 years) after a single 10 mg dose. The Cmax was lower (119 ng/mL vs. 178 ng/mL) and the Tmax was longer (3 hours vs. 1 hour) in the elderly patients compared to the younger subjects. The AUCs were similar in the two groups. In this study, the elimination half-life was slightly prolonged in the elderly patients compared to the younger subjects, 4.43 hours vs. 3.75 hours, respectively.
Last reviewed on RxList: 11/20/2007
This monograph has been modified to include the generic and brand name in many instances.
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