Myonal Mechanism of Action

Pharmacology: Skeletal muscle relaxation: Inhibition of experimentally-induced muscle rigidity: Eperisone hydrochloride suppresses intercollicular section induced decerebrate rigidity (γ-rigidity) and ischemic decerebrate rigidity (α-rigidity) in rats dose-dependently.
Suppression of spinal reflexes: In spinal cats, eperisone hydrochloride suppresses mono- and poly-synaptic reflex potentials induced through spinal nerve efferent root stimulation to a similar degree.
Reduction of muscle spindle sensitivity via γ-motor neurons: Eperisone hydrochloride suppresses the activity of afferent nerve fibers (Ia fibers) from human muscle spindles at 20 min after administration. Eperisone hydrochloride suppresses the spontaneous discharge of γ-motor neurons, but does not act directly on muscle spindles in animals. Accordingly, eperisone hydrochloride reduces muscle spindle sensitivity via the γ-motor neurons.
Vasodilatation and Augmentation of blood flow: Vasodilatory action: Eperisone hydrochloride dilates the blood vessels due to Ca⁺⁺-antagonistic action (in guinea pigs) on the vascular smooth muscle and muscular sympatholytic activity (in humans).
Augmentation of blood flow: Eperisone hydrochloride increases the volume of blood flow in skin, muscle, external and internal carotid arteries and vertebral arteries in humans, monkeys and dogs.
Analgesic action and inhibition of the pain reflex in the spinal cord: When eperisone hydrochloride is perfused into the spinal cord of rats, a tail pinch-induced pain reflex is suppressed, but the reflex returns with the withdrawal of eperisone hydrochloride. This suggests that eperisone hydrochloride possesses an analgesic action at the spinal cord level.
Facilitation of voluntary movement: When eperisone hydrochloride is used in the treatment of spastic paralysis in patients with cerebral apoplexy, it improves the cybex torque curve and electromyogram and facilitates voluntary movements, such as extension and flexion of the extremities, without reducing the muscular force.
Clinical Studies: Neck-shoulder-arm syndrome, scapulohumeral periarthritis and low back pain: In open labeled clinical trials and a double blind controlled clinical trial undertaken to determine the effects of MYONAL on myotonic symptoms associated with these diseases, an efficacy rate of 52.1% (234/449) was achieved. (When fairly effective responses are included, the efficacy rate was as high as 80.4 %.)
Spastic paralysis: In open labeled clinical trials and a double blind clinical trial, the usefulness of MYONAL has been established for spastic paralysis associated with diseases such as cerebrovascular disturbances, spastic spinal paralysis or cervical spondylosis. Improvement rates for rigidity and stiffness in patients with spastic paralysis were 42.3% (197/466) and 45.1% (174/386), respectively.
Pharmacokinetics: Blood concentration: Eperisone hydrochloride was administered orally to 8 healthy adult male volunteers at a single dose of 150 mg/day^note) for 14 consecutive days and the plasma concentration was determined at days 1, 8 and 14. The time to reach the peak plasma concentration (t_max) ranged from 1.6 to 1.9 hr, the peak plasma concentration (C_max) was 7.5 to 7.9 ng/mL, elimination half-life (t_1/2) was 1.6 to 1.8 hr, and the area under the plasma concentration- time curve (AUC) was 19.7 to 21.1 ng・hr/mL. The plasma concentration profiles of eperisone hydrochloride determined at days 8 and 14 did not significantly vary from those of the first day. (See figure.)

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