Trihexyphenidyl

Trihexyphenidyl, also known as Benzhexol, (sold under the brandnames Aparkan and Artane) is an antiparkinsonian drug of the antimuscarinic class. Chemically, it is a tertiary amine. It has been in clinical usage for decades. The drug is available as the hydrochloride salt.

Pharmacology
The exact mechanism of action in parkinsonian syndromes is not precisely understood, but it is known that trihexyphenidyl blocks efferent impulses in parasympathetically innervated structures like smooth muscles (spasmolytic activity), salivary glands, and eyes (mydriasis). In higher doses direct central inhibition of cerebral motor centers may contribute. In very high doses central toxicity as seen in atropine overdose is noted.

Pharmacokinetics
Trihexyphenidyl is rapidly absorbed from the GI-Tract. The onset of action is within 1 hour after oral dosing. The peak activity is noted after 2 to 3 hours. The duration of action of one single dose is 6 to 12 hours in a dose dependent manner. It is excreted in the urine, probably as unchanged drug. More precise data in animals and humans have so far not been determined.

Established uses
Trihexyphenidyl is used for the symptomatic treatment of Parkinson's disease in mono- and combination therapy. It is active in postencephalitic, arteriosclerotic, and idiopathic forms. The drug is also commonly used to treat extrapyramidal side effects occurring during antipsychotic treatment. It reduces the frequency and duration of oculogyric crises as well as of dyskinetic movements and spastic contractions. Excessive salivation may also respond. Trihexyphenidyl may improve psychotic depression and mental inertia frequently associated with Morbus Parkinson and symptomatic problems caused by antipsychotic treatment.

Therapeutic prospects
The drug is not able to cure Morbus Parkinson, but may provide substantial alleviation of symptoms. An estimated 50 to 75% of patients with M. Parkinson will react positively and experience a 20 to 30% symptomatic improvement. To increase therapeutic activity trihexyphenidyl is often given concomitantly with levodopa, other antimuscarinic or antihistaminic (e.g. diphenhydramine) agents. Combination treatment with dopaminergic agonists such as cabergoline is also possible. This is often termed a 'multidimensional approach'.

Investigational uses
Equivocal preleminary results from small studies exist for: Trihexyphenidyl does not improve cerebral palsy and hemiplegia.
 * Other Dyskinesias
 * Huntington's Chorea
 * Spasmodic Torticollis
 * Dystonia

Contraindications and cautions
See Biperiden.
 * Patients under 18 yrs. of age should not be treated due to a lack of clinical experience.

Trihexyphenidyl has been reported as a drug of abuse, and while this is uncommon it may be prudent to be cautious in prescribing this drug to patients with a history of drug addiction.

Pregnancy and lactation
The safe use of Trihexyphenidyl during pregnancy and lactation has so far not been assured.

Side-effects
See Biperiden. Tolerance may develop during therapy which requires dose adjustments.

Interactions
See Biperiden.

Dosage

 * Morbus Parkinson : One mg is given on the first day. Increments are usually 2mg every 3 days until 6 to 10mg are reached. In postencephalitic cases up to 15 mg might be necessary, but then excessive dryness of mouth or nausea could be a problem. To increase tolerability Trihexyphenidyl may be given in 3 divided doses.


 * Extrapyramidal side effects : Usually, 5 to 15 mg daily are needed in 2 or 3 divided doses. Some patients, however, are successfully treated with as little as 1 mg daily.

Overdose
See Biperiden.

Brand names and dose forms

 * Artane : elixir 2mg/5ml, tabletts 2mg and 5mg
 * Generics (e.g. Cypress, URL, Watson, West-Ward) : elixir and/or tabletts
 * Pacitane "http://www.wyethindia.com/products/pacitane.asp?prodid=7"