indication

For the treatment of moderate to severe dementia of the Alzheimer's type.

pharmacology

Memantine, an amantadine derivative, is an NMDA receptor antagonist used in the treatment of Alzheimer's disease. It differs from traditional agents used in Alzheimer's disease by acting on glutamatergic neurotransmission, rather than cholinergic. There is some evidence that dysfunction of glutamatergic neurotransmission, manifested as neuronal excitotoxicity, is involved in the aetiology of Alzheimer's disease (Cacabelos et al., 1999). As such, targeting the glutamatergic system, specifically NMDA receptors, was a novel approach to treatment in view of the limited efficacy of existing drugs targeting the cholinergic system. A systematic review of randomised controlled trials found that memantine has a positive effect on cognition, mood, behaviour, and the ability to perform daily activities. There is no evidence that memantine prevents or slows neurodegeneration in patients with Alzheimer's disease.

mechanism of action

Memantine exerts its action through uncompetitive NMDA receptor antagonism, binding preferentially to the NMDA receptor-operated cation channels. Prolonged increased levels of glutamate in the brain of demented patients are sufficient to counter the voltage-dependent block of NMDA receptors by Mg²⁺ ions and allow continuous influx of Ca²⁺ ions into cells, ultimately resulting in neuronal degeneration. Studies suggest that memantine binds more effectively than Mg²⁺ ions at the NMDA receptor, and thereby effectively blocks this prolonged influx of Ca²⁺ ions through the NMDA channel whilst preserving the transient physiological activation of the channels by higher concentrations of synaptically released glutamate. Thus memantine protects against chronically elevated concentrations of glutamate. Memantine also has antagonistic activity at the type 3 serotonergic (5-HT₃) receptor with a potency that is similar to that at the NMDA receptor, and lower antagonistic activity at the nicotinic acetylcholine receptor. This drug has no affinity for γ-aminobutyric acid (GABA), benzodiazepine, dopamine, adrenergic, histamine, or glycine receptors or for voltage-dependent calcium, sodium, or potassium channels.

toxicity

Side effects include pain, abnormal crying, leg pain, fever, increased apetite. Adverse drug reactions include: dizziness, confusion, headache, hallucinations, tiredness. Less common side effects include: vomiting, anxiety, hypertonia, cystitis, and increased libido. Doses of up to 400 mg have been tolerated.

biotransformation

Excreted largely unchanged. About 20% is metabolized to 1-amino-3-hydroxymethyl-5-methyl-adamantane and 3-amino-1-hydroxy-5,7-dimethyl-adamantane.

absorption

Well absorbed orally with a bioavailability of approximately 100%. Peak plasma concentrations are reached in 3-7 hours. Food has no effect on absorption.

half life

60-100 hours

route of elimination

Memantine undergoes partial hepatic metabolism. 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. It is excreted predominantly in the urine, unchanged.

drug interactions

Acetazolamide: Possible increased levels of memantine

Amantadine: Increased risk of CNS adverse effects with this association

Dichlorphenamide: Possible increased levels of memantine

Ketamine: Increased risk of CNS adverse effects with this association

Methazolamide: Possible increased levels of memantine

Sodium bicarbonate: Possible increased levels of memantine