Absorption, Distribution and Excretion
This drug undergoes significant first-pass metabolism. After the administration of an oral dose, tizanidine is mostly absorbed. The absolute oral bioavailability of tizanidine is measured to be about 40%. **Effect of food on absorption** Food has been shown to increase absorption for both the tablets and capsules. The increase in absorption with the tablet (about 30%) was noticeably higher than the capsule (~10%). When the capsule and tablet were administered with food, the amount absorbed from the capsule was about 80% of the amount absorbed from the tablet. It is therefore advisable to take this drug with food for increased absorption, especially in tablet form.
This drug is mainly eliminated by the kidney.
Extensively distributed throughout the body. The average steady-state volume of distribution is 2.4 L/kg.
**A note on renal impairment** Tizanidine clearance is found to be decreased by more than 50% in elderly patients with renal insufficiency (creatinine clearance < 25 mL/min) compared to healthy elderly subjects; this would be expected to lead to a longer duration of clinical effect. This drug should be used with caution in patients with renal impairment.

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Metabolism / Metabolites
About 95% of the ingested dose of tizanidine is metabolized. The main enzyme involved in the hepatic metabolism of tizanidine is CYP1A2.
Tizanidine has known human metabolites that include 2-[(5-chloro-2,1,3-benzothiadiazol-4-yl)amino]-4,5-dihydro-1H-imidazol-4-ol.

Route of Elimination: Approximately 95% of an administered dose is metabolized.
Half Life: 2.5 hours

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Biological Half-Life
Approximately 2.5 hours.

Toxicity Summary
Tizanidine reduces spasticity by increasing presynaptic inhibition of motor neurons through agonist action at a2-adrenergic receptor sites.

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Hepatotoxicity
Transient and asymptomatic elevations in serum ALT greater than 3 times the upper limit of normal occur in ~5% of patients taking tizanidine compared to 0.4% of subjects on placebo. Reports of severe hepatotoxicity, acute liver failure and death have been mentioned in review articles on tizanidine, but few case reports have been published. In these reports, the latency to onset of jaundice has ranged from 2 to 14 weeks and the enzyme elevations have been both cholestatic and hepatocellular (Case 1). Immunoallergic and autoimmune features have not been mentioned. Recovery was complete after 1 to 2 months.
Likelihood score: C (Probable rare cause of clinically apparent liver injury).

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Protein Binding
About 30% bound to plasma proteins.

[1]. A practical overview of tizanidine use for spasticity secondary to multiple sclerosis, stroke, and spinal cord injury. Curr Med Res Opin. 2008 Feb;24(2):425-39.

Tizanidine is 2,1,3-Benzothiadiazole substituted at C-4 by a Delta(1)-imidazolin-2-ylamino group and at C-4 by a chloro group. It is an agonist at alpha2-adrenergic receptor sites. It has a role as an alpha-adrenergic agonist and a muscle relaxant. It is a benzothiadiazole and a member of imidazoles.
Tizanidine is a fast-acting drug used for the management of muscle spasm, which may result from the effects of multiple sclerosis, stroke, an acquired brain injury, or a spinal cord injury. It may also be caused by musculoskeletal injury. Regardless of the cause, muscle spasticity can be an extremely painful and debilitating condition. Initially approved by the FDA in 1996, tizanidine is an Alpha-2 adrenergic receptor agonist reducing spasticity by the presynaptic inhibition of excitatory neurotransmitters that cause firing of neurons promoting muscle spasm.
Tizanidine is a Central alpha-2 Adrenergic Agonist. The mechanism of action of tizanidine is as an Adrenergic alpha2-Agonist.
Tizanidine is a commonly used muscle relaxant that has been linked to rare instances of acute liver injury, a few of which have been fatal.
Tizanidine is a short-acting drug for the management of spasticity. Tizanidine is an agonist at a2-adrenergic receptor sites and presumably reduces spasticity by increasing presynaptic inhibition of motor neurons. In animal models, tizanidine has no direct effect on skeletal muscle fibers or the neuromuscular junction, and no major effect on monosynaptic spinal reflexes. The effects of tizanidine are greatest on polysynaptic pathways. The overall effect of these actions is thought to reduce facilitation of spinal motor neurons.
See also: Tizanidine Hydrochloride (has salt form).

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Drug Indication
Tizanidine is indicated for the relief of muscle spasticity, which can interfere with daily activities. The general recommendation is to reserve tizanidine use for periods of time when there is a particular need for relief, as it has a short duration of action.
FDA Label

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Mechanism of Action
Tizanidine reduces spasticity by causing presynaptic inhibition of motor neurons via agonist actions at Alpha-2 adrenergic receptor sites. This drug is centrally acting and leads to a reduction in the release of excitatory amino acids like glutamate and aspartate, which cause neuronal firing that leads to muscle spasm. The above reduction and excitatory neurotransmitter release results in presynaptic inhibition of motor neurons. The strongest effect of tizanidine has been shown to occur on spinal polysynaptic pathways. The anti-nociceptive and anticonvulsant activities of tizanidine may also be attributed to agonist action on Alpha-2 receptors. Tizanidine also binds with weaker affinity to the Alpha-1 receptors, explaining its slight and temporary effect on the cardiovascular system.

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Pharmacodynamics
**A note on spasticity** Spasticity is an increase in muscle accompanied by uncontrolled, repetitive contractions of skeletal muscles which are involuntary. The patient suffering from muscle spasticity may have reduced mobility and high levels of pain, contributing to poor quality of life and problems performing activities of personal hygiene and care. **General effects** Tizanidine is a rapidly acting drug used for the relief of muscle spasticity when it is required for performing specific activities. It acts as an agonist at Alpha-2 adrenergic receptor sites and relieves symptoms of muscle spasticity, allowing the continuation of normal daily activities. In animal models, tizanidine has not been shown to exert direct effects on skeletal muscle fibers or the neuromuscular junction, and has shown no significant effect on monosynaptic spinal reflexes (consisting of the communication between only 1 sensory neuron and 1 motor neuron). The frequency of muscle spasm and clonus are shown to be decreased by tizanidine. Tizanidine shows a stronger action on polysynaptic reflexes, which involve several interneurons (relay neurons) communicating with motor neurons stimulating muscle movement. **Effects on blood pressure and heart rate** This drug decreases heart rate and blood pressure in humans. Despite this, rebound hypertension and tachycardia along with increased spasticity can occur when tizanidine is abruptly discontinued.

C9H8N5SCL

253.71132

253.019

51322-75-9

Tizanidine hydrochloride;64461-82-1;Tizanidine-d4;1188331-19-2

5487

Light yellow to yellow solid powder

1.82 g/cm3

391.2ºC at 760 mmHg

221 - 223ºC

190.4ºC

1.553

2

4

2

16

300

0

XFYDIVBRZNQMJC-UHFFFAOYSA-N

InChI=1S/C9H8ClN5S/c10-5-1-2-6-8(15-16-14-6)7(5)13-9-11-3-4-12-9/h1-2H,3-4H2,(H2,11,12,13)

5-chloro-N-(4,5-dihydro-1H-imidazol-2-yl)-2,1,3-benzothiadiazol-4-amine

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

May dissolve in DMSO (in most cases), if not, try other solvents such as H2O, Ethanol, or DMF with a minute amount of products to avoid loss of samples

Note: Listed below are some common formulations that may be used to formulate products with low water solubility (e.g. < 1 mg/mL), you may test these formulations using a minute amount of products to avoid loss of samples.

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Injection Formulation 1: DMSO : Tween 80： Saline = 10 : 5 : 85 (i.e. 100 μL DMSO stock solution → 50 μL Tween 80 → 850 μL Saline)
*Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH ₂ O to obtain a clear solution.
Injection Formulation 2: DMSO : PEG300 ：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL DMSO → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)
Injection Formulation 3: DMSO : Corn oil = 10 : 90 (i.e. 100 μL DMSO → 900 μL Corn oil)
Example: Take the Injection Formulation 3 (DMSO : Corn oil = 10 : 90) as an example, if 1 mL of 2.5 mg/mL working solution is to be prepared, you can take 100 μL 25 mg/mL DMSO stock solution and add to 900 μL corn oil, mix well to obtain a clear or suspension solution (2.5 mg/mL, ready for use in animals).

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Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

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Oral Formulation 1: Suspend in 0.5% CMC Na (carboxymethylcellulose sodium)
Oral Formulation 2: Suspend in 0.5% Carboxymethyl cellulose
Example: Take the Oral Formulation 1 (Suspend in 0.5% CMC Na) as an example, if 100 mL of 2.5 mg/mL working solution is to be prepared, you can first prepare 0.5% CMC Na solution by measuring 0.5 g CMC Na and dissolve it in 100 mL ddH2O to obtain a clear solution; then add 250 mg of the product to 100 mL 0.5% CMC Na solution, to make the suspension solution (2.5 mg/mL, ready for use in animals).

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Oral Formulation 3: Dissolved in PEG400
Oral Formulation 4: Suspend in 0.2% Carboxymethyl cellulose
Oral Formulation 5: Dissolve in 0.25% Tween 80 and 0.5% Carboxymethyl cellulose
Oral Formulation 6: Mixing with food powders

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Note: Please be aware that the above formulations are for reference only. InvivoChem strongly recommends customers to read literature methods/protocols carefully before determining which formulation you should use for in vivo studies, as different compounds have different solubility properties and have to be formulated differently.

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 (Please use freshly prepared in vivo formulations for optimal results.)