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Memantine New Option for Advanced Alzheimer’S

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Memantine New Option for Advanced Alzheimer’S Current p SYCHIATRY O ut of the pipeline > Memantine New option for advanced Alzheimer’s NMDA receptor antagonist targets functioning, is well-tolerated, and may be combined with cholinesterase inhibitors. Jeffrey L. Cummings, MD Director, UCLA Alzheimer’s Disease Center Professor, department of neurology David Geffen School of Medicine University of California-Los Angeles s America’s population ages, the need to Table find new treatments for Alzheimer’s Memantine: Fast facts A disease (AD) is increasingly urgent. Agents that have reached the medical mainstream Drug brand name: in recent years target the disease in its mild to Namenda moderate stages. Memantine recently gained FDA Class: approval for treating moderate to severe AD. NMDA receptor antagonist FDA-approved indication: Moderate to severe Alzheimer’s disease HOW IT WORKS Memantine is an uncompetitive N-methyl-D- Approval date: aspartate (NMDA) receptor antagonist. NMDA Oct. 17, 2003 receptors mediate the effects of the excitatory Manufacturer: amino acid glutamate, promote entry of calcium Forest Pharmaceuticals through ion channel pores, and are essential for Dosing forms: normal learning and memory.1 Prolonged exces- 5 mg, 10 mg (“titration packets” containing 5-mg and 10-mg tablets are available) sive glutamate stimulation, however, can lead to excitotoxicity and nerve cell death. Recommended dosage: Begin at 5 mg/d for 1 week; increase to 5 mg bid High-affinity NMDA receptor antagonists the second week, then to 10 mg in the morning cause unacceptable side effects in humans and and 5 mg in the evening for the third week; have not been well tolerated in clinical trials. By increase to 10 mg bid for continued dosing contrast, memantine—a moderate- to low-affini- continued on page 73 64 VOL. 3, NO. 1 / JANUARY 2004 continued from page 64 ty NMDA receptor antagonist with rapid block- present as the parent compound. These kinetics ing/unblocking kinetics—has been well tolerated justify once-daily dosing, although memantine in clinical trials. The agent is readily displaced by usually is given bid. presynaptic stimuli to allow normal channel Three metabolites have been identified, none function, but it reduces calcium influx from of which exhibit NMDA receptor antagonist activ- chronic low-amplitude glutamate stimulation.2 ity. Memantine minimally inhibits cytochrome P- Memantine’s voltage-dependent characteris- 450 enzymes, so interactions with drugs metabo- tics allow it to block low-level tonic pathologic lized by these enzymes are unlikely.5 activation of NMDA receptors caused by low glu- Memantine may potentiate the effects of barbi- tamate concentrations. This property also allows turates, neuroleptics, anticholinergics, L-dopa, ket- physiologic activation of receptors after synaptic amine, amantadine, dextromethorphan, and release of larger glutamate concentrations that dopaminergic agonists. Concomitant use of produce membrane depolarization.2 Memantine memantine and amantadine should be avoided has demonstrated neuroprotection of neurons because the compounds are chemically related and exposed to glutamate in a variety of in-vitro both are NMDA antagonists. Memantine may hin- preparations.3 In experimental models, Memantine is not likely to interact with drugs memantine has been shown to prolong long-term potenti- metabolized by cytochrome P-450 enzymes ation, a neurophysiologic correlate of learning and memory. Rats treated der the effects of dantrolene or baclofen, so doses of with memantine show enhanced learning recov- these agents may need to be adjusted upward. ery following entorhinal cortex lesions.3 Memantine is eliminated almost completely Memantine has been shown to protect via renal cation transport proteins. Drugs that use cholinergic cells in both acute and chronic animal the same transport system—such as cimetidine, models. It also prevents pathologic changes in the ranitidine, procainamide, quinine, and nico- hippocampus produced by direct injection of beta- tine—may interact with memantine, possibly amyloid protein.3 These findings suggest that leading to increased plasma levels of these agents. memantine may improve learning and memory Hydrochlorothiazide activity is reduced by 20% and may have neuroprotective properties in AD. when memantine is co-administered. Sodium bicarbonate, carbonic anhydrous inhibitors, and PHARMACOKINETICS other drugs that alkalinize the urine may reduce Memantine is absorbed completely from the GI memantine clearance and increase its serum levels.4 tract and reaches maximum serum concentration In healthy elderly volunteers with normal in 6 to 8 hours. It is widely distributed and passes and reduced renal function, researchers observed the blood-brain barrier with CSF concentrations a significant correlation between creatine clear- approximately one-half those of serum levels. ance and total renal clearance of memantine, sug- Dosages between 5 and 30 mg/d result in serum gesting that patients with renal disease may levels of 0.025 to 0.529 mmol. Relatively little bio- require lower dosages.5 transformation occurs. The agent’s half-life ranges between 75 and EFFICACY 100 hours.4 Memantine is 10% to 45% protein In a preliminary, placebo-controlled study7 of bound, and 80% of circulating memantine is patients with vascular- or Alzheimer’s-type VOL. 3, NO. 1 / JANUARY 2004 Current 73 p SYCHIATRY O ut of the pipeline >Memantine dementia, memantine was associated with tine-group patients and 87% of the placebo group improved Clinical Global Impression of Change experienced adverse effects. More placebo-group and Behavioral Rating Scale for Geriatric Patients than memantine-group patients (17% vs. 10%) scores. Mini-Mental State Examination (MMSE) discontinued the study because of adverse events. scores for all patients entering the study were Agitation was the most commonly cited reason <10, indicating severe cognitive impairment. for discontinuation (7% of the placebo group and 5% of those taking Data show combination cholinesterase inhibitor/ memantine). No adverse event was significantly memantine therapy to be clinically safe more common in the memantine group. Global measures improved in 61 of 82 (73%) Tariot et al9 noted that confusion and patients taking memantine, 10 mg/d, and in 38 of headache were somewhat more common among 84 (45%) patients taking placebo. Care depen- those receiving memantine versus placebo. In dence improved 3.1 points in the memantine other studies, symptoms possibly related to group and 1.1 points in the placebo group. memantine included headache, akathisia, insom- Reisberg et al8 gave memantine, 20 mg/d, or nia, increased motor activity, and excitement.6, 10-12 placebo to 252 patients with AD across 28 weeks. The memantine group performed at significant- CO-ADMINISTRATION ly higher functional levels than the placebo WITH CHOLINESTERASE INHIBITORS group on the Alzheimer’s Disease Cooperative The range of AD severity targeted by memantine Study ADL Scale and the Severe Impairment overlaps that addressed by the cholinesterase Battery (SIB). The differences on the Clinical inhibitors donepezil, galantamine, and rivastig- Interview-Based Impression of Change with mine, which are indicated for mild to moderate caregiver input (CIBIC-plus) scale were nearly AD. Many patients will receive both memantine significant (p = 0.06). Patients entering the study and a cholinesterase inhibitor. had MMSE scores between 3 and 14. The mag- Data show this combination therapy to be nitude of drug-placebo difference was modest clinically safe. Tariot et al9 found no increase in (approximately 6 points on the SIB). adverse events when memantine was co-adminis- In a third pivotal trial, 403 patients with AD tered with donepezil. Post-marketing surveillance were randomly assigned to memantine, 20 mg/d, studies in Germany indicate low rates of adverse or placebo across 24 weeks. All patients were also events among patients receiving a cholinesterase taking the cholinesterase inhibitor donepezil, 10 inhibitor and memantine.13 In-vitro laboratory mg/d.9 The memantine/donepezil group scored data indicate that memantine does not affect or higher than the placebo/donepezil group on sev- interact with cholinesterase inhibition.14 eral scales. MMSE scores at entry ranged from 5 Memantine is not metabolized by liver to 14. Drug-placebo differences were similar in enzymes. No interaction with antidepressants magnitude to those observed in earlier studies. or antipsychotics commonly used in AD is anticipated. TOLERABILITY Controlled trials of memantine in patients with CLINICAL IMPLICATIONS AD demonstrated few adverse effects. Memantine, with a mechanism of action differ- Reisberg et al8 reported that 84% of meman- ent from that of existing agents, offers a new continued on page 77 74 Current VOL. 3, NO. 1 / JANUARY 2004 p SYCHIATRY Current p SYCHIATRY continued from page 74 avenue of therapeutic intervention and expands the spectrum of patients who may benefit from Related resources FDA-approved drug therapy. Alzheimer’s Association. www.alz.org Research is needed to determine whether Mendez M, Cummings JL. Dementia: a clinical approach (3rd ed). Boston: Butterworth Heinemann, 2003. memantine is useful in earlier stages of AD and in treating mild cognitive impairment. The role DRUG BRAND NAMES of glutamate excitotoxicity in AD also needs to Amantadine • Symmetrel Galantamine • Reminyl Cimetidine • Tagamet Memantine • Namenda be defined. Dantrolene • Dantrium Procainamide • Procanbid Donepezil • Aricept Procainamide
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