Infex (amantadine hcl)

Infex (amantadine hcl) (amantadine hydrochloride) is indicated for the prophylaxis and treatment of signs and symptoms of infection caused by various strains of influenza A virus. Infex (amantadine hcl) (amantadine hydrochloride) is also indicated in the treatment of parkinsonism and drug-induced extrapyramidal reactions.

Influenza A Prophylaxis

Infex (amantadine hcl) (amantadine hydrochloride) is indicated for chemoprophylaxis against signs and symptoms of influenza A virus infection. Because Infex (amantadine hcl) (amantadine hydrochloride) does 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, Infex (amantadine hcl) (amantadine hydrochloride) prophylaxis should be considered for the 2- to 4-week time period required to develop an antibody response.

Influenza A Treatment

Infex (amantadine hcl) (amantadine hydrochloride) is 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 Infex (amantadine hcl) (amantadine hydrochloride) will avoid the development of influenza A virus pneumonitis or other complications in high risk patients.

There is no clinical evidence indicating that Infex (amantadine hcl) (amantadine hydrochloride) is 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 Infex (amantadine hcl) (amantadine hydrochloride) :

• Infex (amantadine hcl) (amantadine hydrochloride) is 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 Infex (amantadine hcl) (amantadine hydrochloride).

Parkinson's Disease/Syndrome

Infex (amantadine hcl) (amantadine hydrochloride) is 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, Infex (amantadine hcl) (amantadine hydrochloride) is 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

Infex (amantadine hcl) (amantadine hydrochloride) is indicated in the treatment of drug-induced extrapyramidal reactions. Although anticholinergic-type side effects have been noted with Infex (amantadine hcl) (amantadine hydrochloride) 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.

The dose of Infex (amantadine hcl) (Amantadine Hydrochloride, USP) may need reduction in patients with congestive heart failure, peripheral edema, orthostatic hypotension, or impaired renal function (see Dosage for Impaired Renal Function).

Dosage for Prophylaxis and Treatment of Uncomplicated Influenza A Virus Illness

Adult

The adult daily dosage of Infex (amantadine hcl) (amantadine hydrochloride) is 200 mg; two 100 mg tablets (or four teaspoonfuls of syrup) as a single daily dose. The daily dosage may be split into one tablet of 100 mg (or two teaspoonfuls of syrup) twice a day. If central nervous system effects develop in once-a-day dosage, a split dosage schedule may reduce such complaints. In persons 65 years of age or older, the daily dosage of Infex (amantadine hcl) (amantadine hydrochloride) is 100 mg.

A 100 mg daily dose has also been shown in experimental challenge studies to be effective as prophylaxis in healthy adults who are not at high risk for influenza-related complications. However, it has not been demonstrated that a 100 mg daily dose is as effective as a 200 mg daily dose for prophylaxis, nor has the 100 mg daily dose been studied in the treatment of acute influenza illness. In recent clinical trials, the incidence of central nervous system (CNS) side effects associated with the 100 mg daily dose was at or near the level of placebo. The 100 mg dose is recommended for persons who have demonstrated intolerance to 200 mg of Infex (amantadine hcl) (amantadine hydrochloride) daily because of CNS or other toxicities.

Pediatric Patients: 1 yr.-9 yrs. of age

The total daily dose should be calculated on the basis of 2 to 4 mg/lb/day (4.4 to 8.8 mg/kg/day), but not to exceed 150 mg per day.

9 yrs.-12 yrs. of age

The total daily dose is 200 mg given as one tablet of 100 mg (or two teaspoonfuls of syrup) twice a day. The 100 mg daily dose has not been studied in this pediatric population. Therefore, there are no data which demonstrate that this dose is as effective as or is safer than the 200 mg daily dose in this patient population.

Prophylactic dosing should be started in anticipation of an influenza A outbreak and before or after contact with individuals with influenza A virus respiratory tract illness.

Infex (amantadine hcl) (amantadine hydrochloride) should be continued daily for at least 10 days following a known exposure. If Infex (amantadine hcl) (amantadine hydrochloride) is used chemoprophylactically in conjunction with inactivated influenza A virus vaccine until protective antibody responses develop, then it should be administered for 2 to 4 weeks after the vaccine has been given. When inactivated influenza A virus vaccine is unavailable or contraindicated, Infex (amantadine hcl) (amantadine hydrochloride) should be administered for the duration of known influenza A in the community because of repeated and unknown exposure.

Treatment of influenza A virus illness should be started as soon as possible, preferably within 24 to 48 hours after onset of signs and symptoms, and should be continued for 24 to 48 hours after the disappearance of signs and symptoms.

Dosage for Parkinsonism

Adult

The usual dose of Infex (amantadine hcl) (amantadine hydrochloride) is 100 mg twice a day when used alone. Infex (amantadine hcl) (amantadine hydrochloride) has an onset of action usually within 48 hours.

The initial dose of Infex (amantadine hcl) (amantadine hydrochloride) is 100 mg daily for patients with serious associated medical illnesses or who are receiving high doses of other antiparkinson drugs. After one to several weeks at 100 mg once daily, the dose may be increased to 100 mg twice daily, if necessary.

Occasionally, patients whose responses are not optimal with Infex (amantadine hcl) (amantadine hydrochloride) at 200 mg daily may benefit from an increase up to 400 mg daily in divided doses. However, such patients should be supervised closely by their physicians.

Patients initially deriving benefit from Infex (amantadine hcl) (amantadine hydrochloride) not uncommonly experience a fall-off of effectiveness after a few months. Benefit may be regained by increasing the dose to 300 mg daily. Alternatively, temporary discontinuation of Infex (amantadine hcl) (amantadine hydrochloride) for several weeks, followed by reinitiation of the drug, may result in regaining benefit in some patients. A decision to use other antiparkinson drugs may be necessary.

Dosage for Concomitant Therapy

Some patients who do not respond to anticholinergic antiparkinson drugs may respond to Infex (amantadine hcl) (amantadine hydrochloride). When Infex (amantadine hcl) (amantadine hydrochloride) or anticholinergic antiparkinson drugs are each used with marginal benefit, concomitant use may produce additional benefit.

When Infex (amantadine hcl) (amantadine hydrochloride) and levodopa are initiated concurrently, the patient can exhibit rapid therapeutic benefits. Infex (amantadine hcl) (amantadine hydrochloride) should be held constant at 100 mg daily or twice daily while the daily dose of levodopa is gradually increased to optimal benefit.

When Infex (amantadine hcl) (amantadine hydrochloride) is added to optimal well-tolerated doses of levodopa, additional benefit may result, including smoothing out the fluctuations in improvement which sometimes occur in patients on levodopa alone. Patients who require a reduction in their usual dose of levodopa because of development of side effects may possibly regain lost benefit with the addition of Infex (amantadine hcl) (amantadine hydrochloride).

Dosage for Drug-Induced Extrapyramidal Reactions

Adult

The usual dose of Infex (amantadine hcl) (amantadine hydrochloride) is 100 mg twice a day. Occasionally, patients whose responses are not optimal with Infex (amantadine hcl) (amantadine hydrochloride) at 200 mg daily may benefit from an increase up to 300 mg daily in divided doses.

Dosage for Impaired Renal Function

Depending upon creatinine clearance, the following dosage adjustments are recommended:

The recommended dosage for patients on hemodialysis is 200 mg every 7 days.

Infex (amantadine hcl) (amantadine hydrochloride) is contraindicated in patients with known hypersensitivity to amantadine hydrochloride or to any of the other ingredients in Infex (amantadine hcl) (amantadine hydrochloride).

REFERENCES

2. D.F. Casey, N. Engl. J. Med. 298:516, 1978.

3. C.D. Berkowitz, J. Pediatr. 95:144, 1979.

WARNINGS

Deaths

Deaths have been reported from overdose with Infex (amantadine hcl) (amantadine hydrochloride). The lowest reported acute lethal dose was 1 gram. Acute toxicity may be attributable to the anticholinergic effects of amantadine. Drug overdose has resulted in cardiac, respiratory, renal or central nervous system toxicity. Cardiac dysfunction includes arrhythmia, tachycardia and hypertension (see OVERDOSAGE).

Suicide Attempts

Suicide attempts, some of which have been fatal, have been reported in patients treated with Infex (amantadine hcl) (amantadine hydrochloride) , many of whom received short courses for influenza treatment or prophylaxis. The incidence of suicide attempts is not known and the pathophysiologic mechanism is not understood. Suicide attempts and suicidal ideation have been reported in patients with and without prior history of psychiatric illness. Infex (amantadine hcl) (amantadine hydrochloride) can exacerbate mental problems in patients with a history of psychiatric disorders or substance abuse. Patients who attempt suicide may exhibit abnormal mental states which include disorientation, confusion, depression, personality changes, agitation, aggressive behavior, hallucinations, paranoia, other psychotic reactions, and somnolence or insomnia. Because of the possibility of serious adverse effects, caution should be observed when prescribing Infex (amantadine hcl) (amantadine hydrochloride) to patients being treated with drugs having CNS effects, or for whom the potential risks outweigh the benefit of treatment.

CNS Effects

Patients with a history of epilepsy or other “seizures” should be observed closely for possible increased seizure activity.

Patients receiving Infex (amantadine hcl) (amantadine hydrochloride) who note central nervous system effects or blurring of vision should be cautioned against driving or working in situations where alertness and adequate motor coordination are important.

Other

Patients with a history of congestive heart failure or peripheral edema should be followed closely as there are patients who developed congestive heart failure while receiving Infex (amantadine hcl) (amantadine hydrochloride).

Patients with Parkinson's disease improving on Infex (amantadine hcl) (amantadine hydrochloride) should resume normal activities gradually and cautiously, consistent with other medical considerations, such as the presence of osteoporosis or phlebothrombosis.

Because Infex (amantadine hcl) (amantadine hydrochloride) has anticholinergic effects and may cause mydriasis, it should not be given to patients with untreated angle closure glaucoma.

PRECAUTIONS

Infex (amantadine hcl) (amantadine hydrochloride) should not be discontinued abruptly in patients with Parkinson's disease since a few patients have experienced a parkinsonian crisis, i.e., a sudden marked clinical deterioration, when this medication was suddenly stopped. The dose of anticholinergic drugs or of Infex (amantadine hcl) (amantadine hydrochloride) should be reduced if atropine-like effects appear when these drugs are used concurrently. Abrupt discontinuation may also precipitate delirium, agitation, delusions, hallucinations, paranoid reaction, stupor, anxiety, depression and slurred speech.

Neuroleptic Malignant Syndrome (NMS)

Sporadic cases of possible Neuroleptic Malignant Syndrome (NMS) have been reported in association with dose reduction or withdrawal of Infex (amantadine hcl) (amantadine hydrochloride) therapy. Therefore, patients should be observed carefully when the dosage of Infex (amantadine hcl) (amantadine hydrochloride) is reduced abruptly or discontinued, especially if the patient is receiving neuroleptics.

NMS is an uncommon but life-threatening syndrome characterized by fever or hyperthermia; neurologic findings including muscle rigidity, involuntary movements, altered consciousness; mental status changes; other disturbances such as autonomic dysfunction, tachycardia, tachypnea, hyper- or hypotension; laboratory findings such as creatine phosphokinase elevation, leukocytosis, myoglobinuria, and increased serum myoglobin.

The early diagnosis of this condition is important for the appropriate management of these patients. Considering NMS as a possible diagnosis and ruling out other acute illnesses (e.g., pneumonia, systemic infection, etc.) is essential. This may be especially complex if the clinical presentation includes both serious medical illness and untreated or inadequately treated extrapyramidal signs and symptoms (EPS). Other important considerations in the differential diagnosis include central anticholinergic toxicity, heat stroke, drug fever and primary central nervous system (CNS) pathology.

The management of NMS should include: 1) intensive symptomatic treatment and medical monitoring, and 2) treatment of any concomitant serious medical problems for which specific treatments are available. Dopamine agonists, such as bromocriptine, and muscle relaxants, such as dantrolene are often used in the treatment of NMS, however, their effectiveness has not been demonstrated in controlled studies.

Renal disease

Because Infex (amantadine hcl) (amantadine hydrochloride) is mainly excreted in the urine, it accumulates in the plasma and in the body when renal function declines. Thus, the dose of Infex (amantadine hcl) (amantadine hydrochloride) should be reduced in patients with renal impairment and in individuals who are 65 years of age or older (see DOSAGE AND ADMINISTRATION; Dosage for Impaired Renal Function).

Liver disease

Care should be exercised when administering Infex (amantadine hcl) (amantadine hydrochloride) to patients with liver disease. Rare instances of reversible elevation of liver enzymes have been reported in patients receiving Infex (amantadine hcl) (amantadine hydrochloride) , though a specific relationship between the drug and such changes has not been established.

Melanoma

Epidemiological studies have shown that patients with Parkinson's disease have a higher risk (2-to approximately 6-fold higher) of developing melanoma than the general population. Whether the increased risk observed was due to Parkinson's disease or other factors, such as drugs used to treat Parkinson's disease, is unclear.

For the reasons stated above, patients and providers are advised to monitor for melanomas frequently and on a regular basis when using Infex (amantadine hcl) (amantadine hydrochloride) for any indication. Ideally, periodic skin examinations should be performed by appropriately qualified individuals (e.g., dermatologists).

Other

The dose of Infex (amantadine hcl) (amantadine hydrochloride) may need careful adjustment in patients with congestive heart failure, peripheral edema, or orthostatic hypotension. Care should be exercised when administering Infex (amantadine hcl) (amantadine hydrochloride) to patients with a history of recurrent eczematoid rash, or to patients with psychosis or severe psychoneurosis not controlled by chemotherapeutic agents.

Serious bacterial infections may begin with influenza-like symptoms or may coexist with or occur as complications during the course of influenza. Infex (amantadine hcl) (amantadine hydrochloride) has not been shown to prevent such complications.

Carcinogenesis and Mutagenesis

Long-term in vivo animal studies designed to evaluate the carcinogenic potential of Infex (amantadine hcl) (amantadine hydrochloride) have not been performed. In several in vitro assays for gene mutation, Infex (amantadine hcl) (amantadine hydrochloride) did not increase the number of spontaneously observed mutations in four strains of Salmonella typhimurium (Ames Test) or in a mammalian cell line (Chinese Hamster Ovary cells) when incubations were performed either with or without a liver metabolic activation extract. Further, there was no evidence of chromosome damage observed in an in vitro test using freshly derived and stimulated human peripheral blood lymphocytes (with and without metabolic activation) or in an in vivo mouse bone marrow micronucleus test (140-550 mg/kg; estimated human equivalent doses of 11.7-45.8 mg/kg based on body surface area conversion).

Impairment of Fertility

The effect of amantadine on fertility has not been adequately tested, that is, in a study conducted under Good Laboratory Practice (GLP) and according to current recommended methodology. In a three litter, non-GLP, reproduction study in rats, Infex (amantadine hcl) (amantadine hydrochloride) at a dose of 32 mg/kg/day (equal to the maximum recommended human dose on a mg/m² basis) administered to both males and females slightly impaired fertility. There were no effects on fertility at a dose level of 10 mg/kg/day (or 0.3 times the maximum recommended human dose on a mg/m² basis); intermediate doses were not tested.

Failed fertility has been reported during human in vitro fertilization (IVF) when the sperm donor ingested amantadine 2 weeks prior to, and during the IVF cycle.

Pregnancy Category C

The effect of amantadine on embryofetal and peri-postnatal development has not been adequately tested, that is, in studies conducted under Good Laboratory Practice (GLP) and according to current recommended methodology. However, in two non-GLP studies in rats in which females were dosed from 5 days prior to mating to Day 6 of gestation or on Days 7-14 of gestation, Infex (amantadine hcl) (amantadine hydrochloride) produced increases in embryonic death at an oral dose of 100 mg/kg (or 3 times the maximum recommended human dose on a mg/m² basis). In the non-GLP rat study in which females were dosed on Days 7-14 of gestation, there was a marked increase in severe visceral and skeletal malformations at oral doses of 50 and 100 mg/kg (or 1.5 and 3 times, respectively, the maximum recommended human dose on a mg/m² basis). The no-effect dose for teratogenicity was 37 mg/kg (equal to the maximum recommended human dose on a mg/m² basis). The safety margins reported may not accurately reflect the risk considering the questionable quality of the study on which they are based. There are no adequate and well-controlled studies in pregnant women. Human data regarding teratogenicity after maternal use of amantadine is scarce. Tetralogy of Fallot and tibial hemimelia (normal karyotype) occurred in an infant exposed to amantadine during the first trimester of pregnancy (100 mg P.O. for 7 days during the 6th and 7th week of gestation). Cardiovascular maldevelopment (single ventricle with pulmonary atresia) was associated with maternal exposure to amantadine (100 mg/d) administered during the first 2 weeks of pregnancy. Infex (amantadine hcl) (amantadine hydrochloride) should be used during pregnancy only if the potential benefit justifies the potential risk to the embryo or fetus.

Nursing Mothers

Infex (amantadine hcl) (amantadine hydrochloride) is excreted in human milk. Use is not recommended in nursing mothers.

Pediatric Use

The safety and efficacy of Infex (amantadine hcl) (amantadine hydrochloride) in newborn infants and infants below the age of 1 year have not been established.

Usage in the Elderly

Because Infex (amantadine hcl) (amantadine hydrochloride) is primarily excreted in the urine, it accumulates in the plasma and in the body when renal function declines. Thus, the dose of Infex (amantadine hcl) (amantadine hydrochloride) should be reduced in patients with renal impairment and in individuals who are 65 years of age or older. The dose of Infex (amantadine hcl) (amantadine hydrochloride) may need reduction in patients with congestive heart failure, peripheral edema, or orthostatic hypotension (see DOSAGE AND ADMINISTRATION).

The adverse reactions reported most frequently at the recommended dose of Infex (amantadine hcl) (amantadine hydrochloride) (5-10%) are: nausea, dizziness (lightheadedness), and insomnia.

Less frequently (1-5%) reported adverse reactions are: depression, anxiety and irritability, hallucinations, confusion, anorexia, dry mouth, constipation, ataxia, livedo reticularis, peripheral edema, orthostatic hypotension, headache, somnolence, nervousness, dream abnormality, agitation, dry nose, diarrhea and fatigue.

Infrequently (0.1-1%) occurring adverse reactions are: congestive heart failure, psychosis, urinary retention, dyspnea, skin rash, vomiting, weakness, slurred speech, euphoria, thinking abnormality, amnesia, hyperkinesia, hypertension, decreased libido, and visual disturbance, including punctate subepithelial or other corneal opacity, corneal edema, decreased visual acuity, sensitivity to light, and optic nerve palsy.

Rare (less than 0.1%) occurring adverse reactions are: instances of convulsion, leukopenia, neutropenia, eczematoid dermatitis, oculogyric episodes, suicidal attempt, suicide, and suicidal ideation (see WARNINGS).

Other adverse reactions reported during postmarketing experience with Infex (amantadine hcl) (amantadine hydrochloride) usage include:

Nervous System/Psychiatric

coma, stupor, delirium, hypokinesia, hypertonia, delusions, aggressive behavior, paranoid reaction, manic reaction, involuntary muscle contractions, gait abnormalities, paresthesia, EEG changes, and tremor. Abrupt discontinuation may also precipitate delirium, agitation, delusions, hallucinations, paranoid reaction, stupor, anxiety, depression and slurred speech;

Cardiovascular

cardiac arrest, arrhythmias including malignant arrhythmias, hypotension, and tachycardia;

Respiratory

acute respiratory failure, pulmonary edema, and tachypnea;

Gastrointestinal

dysphagia;

Hematologic

leukocytosis; agranulocytosis

Special Senses

keratitis and mydriasis;

Skin and Appendages

pruritus and diaphoresis;

Miscellaneous

neuroleptic malignant syndrome (see WARNINGS), allergic reactions including anaphylactic reactions, edema, fever, pathological gambling, increased libido including hypersexuality, and impulse control symptoms.

Laboratory Test

elevated: CPK, BUN, serum creatinine, alkaline phosphatase, LDH, bilirubin, GGT, SGOT, and SGPT.

Deaths have been reported from overdose with Infex (amantadine hcl) (amantadine hydrochloride). The lowest reported acute lethal dose was 1 gram. Because some patients have attempted suicide by overdosing with amantadine, prescriptions should be written for the smallest quantity consistent with good patient management.

Acute toxicity may be attributable to the anticholinergic effects of amantadine. Drug overdose has resulted in cardiac, respiratory, renal or central nervous system toxicity. Cardiac dysfunction includes arrhythmia, tachycardia and hypertension. Pulmonary edema and respiratory distress (including adult respiratory distress syndrome - ARDS) have been reported; renal dysfunction including increased BUN, decreased creatinine clearance and renal insufficiency can occur. Central nervous system effects that have been reported include insomnia, anxiety, agitation, aggressive behavior, hypertonia, hyperkinesia, ataxia, gait abnormality, tremor, confusion, disorientation, depersonalization, fear, delirium, hallucinations, psychotic reactions, lethargy, somnolence and coma. Seizures may be exacerbated in patients with prior history of seizure disorders. Hyperthermia has also been observed in cases where a drug overdose has occurred.

There is no specific antidote for an overdose of Infex (amantadine hcl) (amantadine hydrochloride). However, slowly administered intravenous physostigmine in 1 and 2 mg doses in an adult² at 1- to 2-hour intervals and 0.5 mg doses in a child³ at 5- to 10-minute intervals up to a maximum of 2 mg/hour have been reported to be effective in the control of central nervous system toxicity caused by amantadine hydrochloride. For acute overdosing, general supportive measures should be employed along with immediate gastric lavage or induction of emesis. Fluids should be forced, and if necessary, given intravenously. The pH of the urine has been reported to influence the excretion rate of Infex (amantadine hcl) (amantadine hydrochloride). Since the excretion rate of Infex (amantadine hcl) (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. The blood pressure, pulse, respiration and temperature should be monitored. The patient should be observed for hyperactivity and convulsions; if required, sedation, and anticonvulsant therapy should be administered. The patient should be observed for the possible development of arrhythmias and hypotension; if required, appropriate antiarrhythmic and antihypotensive therapy should be given. Electrocardiographic monitoring may be required after ingestion, since malignant tachyarrhythmias can appear after overdose.

Care should be exercised when administering adrenergic agents, such as isoproterenol, to patients with a Infex (amantadine hcl) (amantadine hydrochloride) overdose, since the dopaminergic activity of Infex (amantadine hcl) (amantadine hydrochloride) has been reported to induce malignant arrhythmias.

The blood electrolytes, urine pH and urinary output should be monitored. If there is no record of recent voiding, catheterization should be done.

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.

Antiviral Activity

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 25.0 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.

Drug Resistance

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 Infex (amantadine hcl) (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 (K_i = 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.