Pharmacokinetics and Dosing

Amantadine is an aliphatic primary amine formulated as a hydrochloride salt for clinical use, as an oral preparation. It is a relatively inexpensive drug available as a 100-mg tablet or capsule and a 50-mg/mL liquid. Other than some anticholinergics and apomorphine, it is also one of the few PD medications available in a parenteral formulation (amantadine-sulfate). This intravenous preparation, however, is not available for use in the United States (8).

The bioavailability of amantadine is 86% in the elderly and more than 90% in young adults in oral form (9). It is excreted virtually unmetabolized via the kidneys and has a large volume of distribution. In fasting, healthy patients, peak plasma concentration was found 1 to 4 hours after a single oral dose of 2.5 to 5mg/kg. Plasma half-life in healthy elderly men has been reported between 18 and 45 hours, suggesting that steady state may take up to nine days (10). It crosses the blood-brain barrier and the placenta, and is excreted into the breast milk. Serum amantadine levels are not routinely drawn and are probably of limited clinical utility. Pharmacological studies have reported serum levels between 0.2 to 0.9 p,g/mL at dosages of 200mg/day (11). Fahn et al. (12) reported a patient with psychosis following acute intoxication with amantadine who had a level of 2.37 ^g/mL.

Few drug interactions have been reported with amantadine. Ethanol in combination with amantadine can increase central nervous system (CNS) side effects, such as dizziness, confusion, and orthostatic hypotension (13). Antimuscarinics and medication with significant anticholinergic activity may increase the anticholinergic side effects of amantadine (13). A potential interaction of amantadine with bupropion has been reported (13,14). Affected patients develop restlessness, agitation, gait disturbance, and dizziness, and may require hospitalization depending upon severity. There is also a case report, suggesting amantadine toxicity from an interaction with hydrochlorothiazide-triamterene (15).

Routine dosing starts at 100 mg twice daily. Doses up to 500 mg have been reported for controlling motor complications in PD patients (16). The maximum tolerable doses are suggested at 400 to 500 mg each day, in patients with normal renal function (17); however, doses over 400 mg have no added benefit and have an increased incidence of side effects.

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