The majority of people with Alzheimer’s disease are 65 years of age and older. In the clinical study of memantine hydrochloride extended-release, the mean age of patients was approximately 77 years; over 91% of patients were 65 years and older, 67% were 75 years and older, and 14% were at or above 85 years of age. The efficacy and safety data presented in the clinical trial sections were obtained from these patients. There were no clinically meaningful differences in most adverse reactions reported by patient groups ≥ 65 years old and < 65 years old.
No dosage adjustment is needed in patients with mild or moderate renal impairment. A dosage reduction is recommended in patients with severe renal impairment [see Dosage and Administration (2.3) and Clinical Pharmacology (12.3)].
No dosage adjustment is needed in patients with mild or moderate hepatic impairment. Memantine hydrochloride extended-release capsules were not studied in patients with severe hepatic impairment [see Clinical Pharmacology (12.3)].
Signs and symptoms most often accompanying overdosage with other formulations of memantine in clinical trials and from worldwide marketing experience, alone or in combination with other drugs and/or alcohol, include agitation, asthenia, bradycardia, confusion, coma, dizziness, ECG changes, increased blood pressure, lethargy, loss of consciousness, psychosis, restlessness, slowed movement, somnolence, stupor, unsteady gait, visual hallucinations, vertigo, vomiting, and weakness. The largest known ingestion of memantine worldwide was 2 grams in a patient who took memantine in conjunction with unspecified antidiabetic medications. This patient experienced coma, diplopia, and agitation, but subsequently recovered.
One patient participating in a memantine hydrochloride extended-release capsules clinical trial unintentionally took 112 mg of memantine hydrochloride extended-release capsules daily for 31 days and experienced an elevated serum uric acid, elevated serum alkaline phosphatase, and low platelet count.
Fatal outcome has been very rarely reported with memantine, and the relationship to memantine was unclear.
Because strategies for the management of overdose are continually evolving, it is advisable to contact a poison control center to determine the latest recommendations for the management of an overdose of any drug. As in any cases of overdose, general supportive measures should be utilized, and treatment should be symptomatic.
Elimination of memantine can be enhanced by acidification of urine.
Memantine hydrochloride is an orally active NMDA receptor antagonist. The chemical name for memantine hydrochloride is 1-amino-3,5-dimethyladamantane hydrochloride with the following structural formula:
The molecular formula is C12 H21 N•HCl and the molecular weight is 215.76. Memantine hydrochloride occurs as a white to off-white, colored powder and is slightly soluble in water, soluble in methanol and practically insoluble in acetone.
Memantine hydrochloride extended-release capsules are supplied for oral administration as 7 mg, 14 mg, 21 mg, and 28 mg capsules. Each capsule contains extended-release pellets with the labeled amount of memantine hydrochloride USP and the following inactive ingredients: ethylcellulose, hypromellose, polyethylene glycol, sugar spheres [which contains starch (maize) and sucrose] and talc. The hard gelatin capsule shell contains gelatin, iron oxide yellow and titanium dioxide. In addition the 14 mg, 21 mg and 28 mg hard gelatin capsule shell contains FD&C blue 1 and FD&C yellow 6. The 7 mg, 14 mg and 21 mg capsules are imprinted with black ink containing black iron oxide, potassium hydroxide and shellac. The 28 mg capsules are imprinted with white ink containing potassium hydroxide, shellac and titanium dioxide.
Persistent activation of central nervous system N-methyl-D-aspartate (NMDA) receptors by the excitatory amino acid glutamate has been hypothesized to contribute to the symptomatology of Alzheimer’s disease. Memantine is postulated to exert its therapeutic effect through its action as a low to moderate affinity uncompetitive (open-channel) NMDA receptor antagonist which binds preferentially to the NMDA receptor-operated cation channels. There is no evidence that memantine prevents or slows neurodegeneration in patients with Alzheimer’s disease.
Memantine showed low to negligible affinity for GABA, benzodiazepine, dopamine, adrenergic, histamine and glycine receptors and for voltage-dependent Ca2+ , Na+ , or K+ channels. Memantine also showed antagonistic effects at the 5HT3 receptor with a potency similar to that for the NMDA receptor and blocked nicotinic acetylcholine receptors with one-sixth to one-tenth the potency.
In vitro studies have shown that memantine does not affect the reversible inhibition of acetylcholinesterase by donepezil, galantamine, or tacrine.
Memantine is well absorbed after oral administration and has linear pharmacokinetics over the therapeutic dose range. It is excreted predominantly unchanged in urine and has a terminal elimination half-life of about 60 to 80 hours. In a study comparing 28 mg once daily memantine hydrochloride extended-release capsules to 10 mg twice daily memantine hydrochloride tablets, the Cmax and AUC0-24 values were 48% and 33% higher for the extended-release dosage regimen, respectively.
After multiple dose administration of memantine hydrochloride extended-release capsules, memantine peak concentrations occur around 9 to 12 hours post-dose. There is no difference in the absorption of memantine hydrochloride extended-release capsules when the capsule is taken intact or when the contents are sprinkled on applesauce.
There is no difference in memantine exposure, based on Cmax or AUC, for memantine hydrochloride extended-release capsules whether that drug product is administered with food or on an empty stomach. However, peak plasma concentrations are achieved about 18 hours after administration with food versus approximately 25 hours after administration on an empty stomach.
The mean volume of distribution of memantine is 9 to 11 L/kg and the plasma protein binding is low (45%).
Memantine undergoes partial hepatic metabolism. The hepatic microsomal CYP450 enzyme system does not play a significant role in the metabolism of memantine.
Memantine is excreted predominantly unchanged in the urine and has a terminal elimination half-life of about 60 to 80 hours. About 48% of administered drug is excreted unchanged in urine; the remainder is converted primarily to three polar metabolites which possess minimal NMDA receptor antagonistic activity: the N-glucuronide conjugate, 6-hydroxy-memantine, and 1-nitroso-deaminated memantine. A total of 74% of the administered dose is excreted as the sum of the parent drug and the N-glucuronide conjugate. Renal clearance involves active tubular secretion moderated by pH dependent tubular reabsorption.
The pharmacokinetics of memantine in young and elderly subjects are similar.
Following multiple dose administration of memantine hydrochloride 20 mg daily, females had about 45% higher exposure than males, but there was no difference in exposure when body weight was taken into account.
Memantine pharmacokinetics were evaluated following single oral administration of 20 mg memantine hydrochloride in 8 subjects with mild renal impairment (creatinine clearance, CLcr, > 50 to 80 mL/min), 8 subjects with moderate renal impairment (CLcr 30 to 49 mL/min), 7 subjects with severe renal impairment (CLcr 5 to 29 mL/min) and 8 healthy subjects (CLcr > 80 mL/min) matched as closely as possible by age, weight and gender to the subjects with renal impairment. Mean AUC0-∞ increased by 4%, 60%, and 115% in subjects with mild, moderate, and severe renal impairment, respectively, compared to healthy subjects. The terminal elimination half-life increased by 18%, 41%, and 95% in subjects with mild, moderate, and severe renal impairment, respectively, compared to healthy subjects.
Memantine pharmacokinetics were evaluated following the administration of single oral doses of 20 mg in 8 subjects with moderate hepatic impairment (Child-Pugh Class B, score 7 to 9) and 8 subjects who were age-, gender-, and weight-matched to the hepatically-impaired subjects. There was no change in memantine exposure (based on Cmax and AUC) in subjects with moderate hepatic impairment as compared with healthy subjects. However, terminal elimination half-life increased by about 16% in subjects with moderate hepatic impairment as compared with healthy subjects.
Use with Cholinesterase Inhibitors
Coadministration of memantine with the AChE inhibitor donepezil did not affect the pharmacokinetics of either compound. Furthermore, memantine did not affect AChE inhibition by donepezil. In a 24-week controlled clinical study in patients with moderate to severe Alzheimer’s disease, the adverse reaction profile observed with a combination of memantine immediate-release and donepezil was similar to that of donepezil alone.
Effect of Memantine on the Metabolism of Other Drugs
In vitro studies conducted with marker substrates of CYP450 enzymes (CYP1A2, -2A6, -2C9, -2D6, -2E1, -3A4) showed minimal inhibition of these enzymes by memantine. In addition, in vitro studies indicate that at concentrations exceeding those associated with efficacy, memantine does not induce the cytochrome P450 isozymes CYP1A2, -2C9, -2E1 and -3A4/5. No pharmacokinetic interactions with drugs metabolized by these enzymes are expected.
Pharmacokinetic studies evaluated the potential of memantine for interaction with warfarin and bupropion. Memantine did not affect the pharmacokinetics of the CYP2B6 substrate bupropion or its metabolite hydroxybupropion. Furthermore, memantine did not affect the pharmacokinetics or pharmacodynamics of warfarin as assessed by the prothrombin INR.
Effect of Other Drugs on Memantine
Memantine is predominantly renally eliminated, and drugs that are substrates and/or inhibitors of the CYP450 system are not expected to alter the metabolism of memantine.
Drugs Eliminated via Renal Mechanisms
Because memantine is eliminated in part by tubular secretion, coadministration of drugs that use the same renal cationic system, including hydrochlorothiazide (HCTZ), triamterene (TA), metformin, cimetidine, ranitidine, quinidine, and nicotine, could potentially result in altered plasma levels of both agents. However, coadministration of memantine and HCTZ/TA did not affect the bioavailability of either memantine or TA, and the bioavailability of HCTZ decreased by 20%. In addition, coadministration of memantine with the antihyperglycemic drug Glucovance® (glyburide and metformin hydrochloride) did not affect the pharmacokinetics of memantine, metformin and glyburide. Furthermore, memantine did not modify the serum glucose lowering effect of Glucovance® , indicating the absence of a pharmacodynamic interaction.
Drugs Highly Bound to Plasma Proteins
Because the plasma protein binding of memantine is low (45%), an interaction with drugs that are highly bound to plasma proteins, such as warfarin and digoxin, is unlikely.
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