AMANTADINE — amantadine hydrochloride tablet
Alembic Pharmaceuticals Inc.
Amantadine hydrochloride, USP is designated generically as amantadine hydrochloride and chemically as 1-adamantanamine hydrochloride.
Amantadine hydrochloride, USP is a stable white or practically white crystalline powder, freely soluble in water, soluble in alcohol and in chloroform.
Amantadine hydrochloride, USP has pharmacological actions as both an anti-Parkinson and an antiviral drug.
Each tablet intended for oral administration contains 100 mg amantadine hydrochloride, USP and has the following inactive ingredients: microcrystalline cellulose, povidone K-30, povidone K-90, sodium starch glycolate, colloidal silicon dioxide, magnesium stearate and FD&C Yellow No. 6 Aluminium Lake.
Mechanism of Action: Antiviral
The mechanism by which amantadine exerts its antiviral activity is not clearly understood. It appears to mainly prevent the release of infectious viral nucleic acid into the host cell by interfering with the function of the transmembrane domain of the viral M2 protein. In certain cases, amantadine is also known to prevent virus assembly during virus replication. It does not appear to interfere with the immunogenicity of inactivated influenza A virus vaccine.
Amantadine inhibits the replication of influenza A virus isolates from each of the subtypes, i.e., H1N1, H2N2 and H3N2. It has very little or no activity against influenza B virus isolates. A quantitative relationship between the in vitro susceptibility of influenza A virus to amantadine and the clinical response to therapy has not been established in man. Sensitivity test results, expressed as the concentration of amantadine required to inhibit by 50% the growth of virus (ED50 ) in tissue culture vary greatly (from 0.1 μg/mL to 25 μg/mL) depending upon the assay protocol used, size of virus inoculum, isolates of influenza A virus strains tested, and the cell type used. Host cells in tissue culture readily tolerated amantadine up to a concentration of 100 μg/mL.
Influenza A variants with reduced in vitro sensitivity to amantadine have been isolated from epidemic strains in areas where adamantane derivatives are being used. Influenza viruses with reduced in vitro sensitivity have been shown to be transmissible and to cause typical influenza illness. The quantitative relationship between the in vitro sensitivity of influenza A variants to amantadine and the clinical response to therapy has not been established.
Mechanism of Action: Parkinson’s Disease
The mechanism of action of amantadine in the treatment of Parkinson’s disease and drug-induced extrapyramidal reactions is not known. Data from earlier animal studies suggest that amantadine hydrochloride may have direct and indirect effects on dopamine neurons. More recent studies have demonstrated that amantadine is a weak, non-competitive NMDA receptor antagonist (Ki = 10μM). Although amantadine has not been shown to possess direct anticholinergic activity in animal studies, clinically, it exhibits anticholinergic-like side effects such as dry mouth, urinary retention, and constipation.
Amantadine hydrochloride is well absorbed orally. Maximum plasma concentrations are directly related to dose for doses up to 200 mg/day. Doses above 200 mg/day may result in a greater than proportional increase in maximum plasma concentrations. It is primarily excreted unchanged in the urine by glomerular filtration and tubular secretion. Eight metabolites of amantadine have been identified in human urine. One metabolite, an N-acetylated compound, was quantified in human urine and accounted for 5 to 15% of the administered dose. Plasma acetylamantadine accounted for up to 80% of the concurrent amantadine plasma concentration in 5 of 12 healthy volunteers following the ingestion of a 200 mg dose of amantadine. Acetylamantadine was not detected in the plasma of the remaining seven volunteers. The contribution of this metabolite to efficacy or toxicity is not known.
There appears to be a relationship between plasma amantadine concentrations and toxicity. As concentration increases, toxicity seems to be more prevalent, however, absolute values of amantadine concentrations associated with adverse effects have not been fully defined.
Amantadine pharmacokinetics were determined in 24 normal adult male volunteers after the oral administration of a single amantadine hydrochloride 100 mg soft gel capsule. The mean ± SD maximum plasma concentration was 0.22 ± 0.03 μg/mL (range: 0.18 to 0.32 μg/mL). The time to peak concentration was 3.3 ± 1.5 hours (range: 1.5 to 8 hours). The apparent oral clearance was 0.28 ± 0.11 L/hr/kg (range: 0.14 to 0.62 L/hr/kg). The half-life was 17 ± 4 hours (range: 10 to 25 hours). Across other studies, amantadine plasma half-life has averaged 16 ± 6 hours (range: 9 to 31 hours) in 19 healthy volunteers.
After oral administration of a single dose of 100 mg amantadine syrup to five healthy volunteers, the mean ± SD maximum plasma concentration Cmax was 0.24 ± 0.04 μg/mL and ranged from 0.18 to 0.28 μg/mL. After 15 days of amantadine 100 mg b.i.d., the Cmax was 0.47 ± 0.11 μg/mL in four of the five volunteers. The administration of amantadine tablets as a 200 mg single dose to 6 healthy subjects resulted in a Cmax of 0.51 ± 0.14 μg/mL. Across studies, the time to Cmax (Tmax ) averaged about 2 to 4 hours.
Plasma amantadine clearance ranged from 0.2 to 0.3 L/hr/kg after the administration of 5 mg to 25 mg intravenous doses of amantadine to 15 healthy volunteers.
In six healthy volunteers, the ratio of amantadine renal clearance to apparent oral plasma clearance was 0.79 ± 0.17 (mean ± SD).
The volume of distribution determined after the intravenous administration of amantadine to 15 healthy subjects was 3 to 8 L/kg, suggesting tissue binding. Amantadine, after single oral 200 mg doses to 6 healthy young subjects and to 6 healthy elderly subjects has been found in nasal mucus at mean ± SD concentrations of 0.15 ± 0.16, 0.28 ± 0.26, and 0.39 ± 0.34 μg/g at 1, 4, and 8 hours after dosing, respectively. These concentrations represented 31 ± 33%, 59 ± 61%, and 95 ± 86% of the corresponding plasma amantadine concentrations. Amantadine is approximately 67% bound to plasma proteins over a concentration range of 0.1 to 2 μg/mL. Following the administration of amantadine 100 mg as a single dose, the mean ± SD red blood cell to plasma ratio ranged from 2.7 ± 0.5 in 6 healthy subjects to 1.4 ± 0.2 in 8 patients with renal insufficiency.
The apparent oral plasma clearance of amantadine is reduced and the plasma half-life and plasma concentrations are increased in healthy elderly individuals age 60 and older. After single dose administration of 25 to 75 mg to 7 healthy, elderly male volunteers, the apparent plasma clearance of amantadine was 0.1 ± 0.04 L/hr/kg (range 0.06 to 0.17 L/hr/kg) and the half-life was 29 ± 7 hours (range 20 to 41 hours). Whether these changes are due to decline in renal function or other age related factors is not known.
In a study of young healthy subjects (n=20), mean renal clearance of amantadine, normalized for body mass index, was 1.5 fold higher in males compared to females (p<0.032).
Compared with otherwise healthy adult individuals, the clearance of amantadine is significantly reduced in adult patients with renal insufficiency. The elimination half-life increases two to three fold or greater when creatinine clearance is less than 40 mL/min/1.73 m2 and averages eight days in patients on chronic maintenance hemodialysis. Amantadine is removed in negligible amounts by hemodialysis.
The pH of the urine has been reported to influence the excretion rate of amantadine hydrochloride. Since the excretion rate of amantadine hydrochloride increases rapidly when the urine is acidic, the administration of urine acidifying drugs may increase the elimination of the drug from the body.
INDICATIONS AND USAGE
Amantadine hydrochloride tablets are indicated for the prophylaxis and treatment of signs and symptoms of infection caused by various strains of influenza A virus. Amantadine hydrochloride tablets are also indicated in the treatment of parkinsonism and drug-induced extrapyramidal reactions.
Influenza A Prophylaxis
Amantadine hydrochloride tablets are indicated for chemoprophylaxis against signs and symptoms of influenza A virus infection. Because amantadine hydrochloride tablets do not completely prevent the host immune response to influenza A infection, individuals who take this drug may still develop immune responses to natural disease or vaccination and may be protected when later exposed to antigenically related viruses. Following vaccination during an influenza A outbreak, amantadine hydrochloride tablets prophylaxis should be considered for the 2- to 4-week time period required to develop an antibody response.
Influenza A Treatment
Amantadine hydrochloride tablets are also indicated in the treatment of uncomplicated respiratory tract illness caused by influenza A virus strains especially when administered early in the course of illness. There are no well-controlled clinical studies demonstrating that treatment with amantadine hydrochloride tablets will avoid the development of influenza A virus pneumonitis or other complications in high risk patients.
There is no clinical evidence indicating that amantadine hydrochloride tablets are effective in the prophylaxis or treatment of viral respiratory tract illnesses other than those caused by influenza A virus strains.
The following points should be considered before initiating treatment or prophylaxis with amantadine hydrochloride tablets:
- Amantadine hydrochloride tablets are not a substitute for early vaccination on an annual basis as recommended by the Centers for Disease Control and Prevention Advisory Committee on Immunization Practices.
- Influenza viruses change over time. Emergence of resistance mutations could decrease drug effectiveness. Other factors (for example, changes in viral virulence) might also diminish clinical benefit of antiviral drugs. Prescribers should consider available information on influenza drug susceptibility patterns and treatment effects when deciding whether to use amantadine hydrochloride tablets.
Amantadine hydrochloride tablets are indicated in the treatment of idiopathic Parkinson’s disease (Paralysis Agitans), postencephalitic parkinsonism, and symptomatic parkinsonism which may follow injury to the nervous system by carbon monoxide intoxication. It is indicated in those elderly patients believed to develop parkinsonism in association with cerebral arteriosclerosis. In the treatment of Parkinson’s disease, amantadine hydrochloride tablets are less effective than levodopa, (-)-3-(3,4- dihydroxyphenyl)-L-alanine, and its efficacy in comparison with the anticholinergic antiparkinson drugs has not yet been established.
Drug-Induced Extrapyramidal Reactions
Amantadine hydrochloride tablets are indicated in the treatment of drug-induced extrapyramidal reactions. Although anticholinergic-type side effects have been noted with amantadine hydrochloride tablets when used in patients with drug-induced extrapyramidal reactions, there is a lower incidence of these side effects than that observed with the anticholinergic antiparkinson drugs.
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