CLINICAL PHARMACOLOGY
Mechanism Of Action
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.
Pharmacodynamics
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.
Pharmacokinetics
Absorption
Following oral administration memantine is highly
absorbed with peak concentrations reached in about 3-7 hours. Memantine has
linear pharmacokinetics over the therapeutic dose range. Food has no effect on
the absorption of memantine.
Distribution
The mean volume of distribution of memantine is 9-11 L/kg
and the plasma protein binding is low (45%).
Metabolism
Memantine undergoes partial hepatic metabolism. The
hepatic microsomal CYP450 enzyme system does not play a significant role in the
metabolism of memantine.
Elimination
Memantine is excreted predominantly (about 48%) unchanged
in urine and has a terminal elimination half-life of about 60-80 hours.
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 Nglucuronide conjugate. Renal clearance involves active
tubular secretion moderated by pH dependent tubular reabsorption.
Pharmacokinetics in Specific Populations
Gender
Following multiple dose administration of NAMENDA 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.
Elderly
The pharmacokinetics of NAMENDA in young and elderly
subjects are similar.
Renal Impairment
Memantine pharmacokinetics were evaluated following
single oral administration of 20 mg memantine HCl in 8 subjects with mild renal
impairment (creatinine clearance, CLcr, > 50 – 80 mL/min), 8 subjects with
moderate renal impairment (CLcr 30 – 49 mL/min), 7 subjects with severe renal
impairment (CLcr 5 – 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.
No dosage adjustment is recommended for patients with
mild and moderate renal impairment. Dosage should be reduced in patients with
severe renal impairment [see DOSAGE AND ADMINISTRATION].
Hepatic Impairment
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 7-9) 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. No dose adjustment is
recommended for patients with mild and moderate hepatic impairment. Memantine
should be administered with caution to patients with severe hepatic impairment
as the pharmacokinetics of memantine have not been evaluated in that
population.
Drug-Drug Interactions
Use with Cholinesterase Inhibitors
Coadministration of memantine with the AChE inhibitor
donepezil HCl 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 event profile observed with a combination of
NAMENDA and donepezil was similar to that of donepezil alone.
Effect of NAMENDA 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 buproprion. Memantine did not
affect the pharmacokinetics of the CYP2B6 substrate buproprion or its
metabolite hydroxybuproprion. Furthermore, memantine did not affect the
pharmacokinetics or pharmacodynamics of warfarin as assessed by the prothrombin
INR.
Effect of Other Drugs on NAMENDA
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 NAMENDA 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 HCl) 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.
Animal Toxicology And/Or Pharmacology
Memantine induced neuronal lesions (vacuolation and
necrosis) in the multipolar and pyramidal cells in cortical layers III and IV
of the posterior cingulate and retrosplenial neocortices in rats, similar to
those which are known to occur in rodents administered other NMDA receptor
antagonists. Lesions were seen after a single dose of memantine. In a study in
which rats were given daily oral doses of memantine for 14 days, the no-effect
dose for neuronal necrosis was 6 times the maximum recommended human dose of 20
mg/day on a mg/m² basis
In acute and repeat-dose neurotoxicity studies in female
rats, oral administration of memantine and donepezil in combination resulted in
increased incidence, severity, and distribution of neurodegeneration compared
with memantine alone. The no-effect levels of the combination were associated
with clinically relevant plasma memantine and donepezil exposures.
The relevance of these findings to humans is unknown.
Clinical Studies
The clinical efficacy studies described below were
conducted with NAMENDA tablets and not with NAMENDA oral solution; however,
bioequivalence of NAMENDA oral solution with NAMENDA tablets has been
demonstrated.
The effectiveness of NAMENDA as a treatment for patients
with moderate to severe Alzheimer's disease was demonstrated in 2 randomized,
double-blind, placebo-controlled clinical studies (Studies 1 and 2) conducted
in the United States that assessed both cognitive function and day to day
function. The mean age of patients participating in these two trials was 76
with a range of 50-93 years. Approximately 66% of patients were female and 91%
of patients were Caucasian. A third study (Study 3), carried out in Latvia,
enrolled patients with severe dementia, but did not assess cognitive function
as a planned endpoint. Study Outcome Measures: In each U.S. study, the effectiveness
of NAMENDA was determined using both an instrument designed to evaluate overall
function through caregiver-related assessment, and an instrument that measures
cognition. Both studies showed that patients on NAMENDA experienced significant
improvement on both measures compared to placebo.
Day-to-day function was assessed in both studies using
the modified Alzheimer's disease Cooperative Study -Activities of Daily Living
inventory (ADCS-ADL). The ADCS-ADL consists of a comprehensive battery of ADL questions
used to measure the functional capabilities of patients. Each ADL item is rated
from the highest level of independent performance to complete loss. The
investigator performs the inventory by interviewing a caregiver familiar with
the behavior of the patient. A subset of 19 items, including ratings of the
patient's ability to eat, dress, bathe, telephone, travel, shop, and perform
other household chores has been validated for the assessment of patients with
moderate to severe dementia. This is the modified ADCS-ADL, which has a scoring
range of 0 to 54, with the lower scores indicating greater functional
impairment.
The ability of NAMENDA to improve cognitive performance
was assessed in both studies with the Severe Impairment Battery (SIB), a multi-item
instrument that has been validated for the evaluation of cognitive function in
patients with moderate to severe dementia. The SIB examines selected aspects of
cognitive performance, including elements of attention, orientation, language,
memory, visuospatial ability, construction, praxis, and social interaction. The
SIB scoring range is from 0 to 100, with lower scores indicating greater
cognitive impairment.
Study 1 (Twenty-Eight-Week Study)
In a study of 28 weeks duration, 252 patients with
moderate to severe probable Alzheimer's disease (diagnosed by DSM-IV and
NINCDS-ADRDA criteria, with Mini-Mental State Examination scores ≥ 3 and
≤ 14 and Global Deterioration Scale Stages 5-6) were randomized to
NAMENDA or placebo. For patients randomized to NAMENDA, treatment was initiated
at 5 mg once daily and increased weekly by 5 mg/day in divided doses to a dose
of 20 mg/day (10 mg twice a day).
Effects on the ADCS-ADL
Figure 1 shows the time course for the change from
baseline in the ADCS-ADL score for patients in the two treatment groups
completing the 28 weeks of the study. At 28 weeks of treatment, the mean
difference in the ADCS-ADL change scores for the NAMENDA-treated patients
compared to the patients on placebo was 3.4 units. Using an analysis based on
all patients and carrying their last study observation forward (LOCF analysis),
NAMENDA treatment was statistically significantly superior to placebo.
Figure 1: Time course of the
change from baseline in ADCS-ADL score for patients completing 28 weeks of
treatment.
Figure 2 shows the cumulative
percentages of patients from each of the treatment groups who had attained at
least the change in the ADCS-ADL shown on the X axis. The curves show that both
patients assigned to NAMENDA and placebo have a wide range of responses and
generally show deterioration (a negative change in ADCS-ADL compared to
baseline), but that the NAMENDA group is more likely to show a smaller decline
or an improvement. (In a cumulative distribution display, a curve for an effective
treatment would be shifted to the left of the curve for placebo, while an
ineffective or deleterious treatment would be superimposed upon or shifted to
the right of the curve for placebo).
Figure 2: Cumulative
percentage of patients completing 28 weeks of double-blind treatment with
specified changes from baseline in ADCS-ADL scores.
Effects on the SIB
Figure 3 shows the time course
for the change from baseline in SIB score for the two treatment groups over the
28 weeks of the study. At 28 weeks of treatment, the mean difference in the SIB
change scores for the NAMENDA-treated patients compared to the patients on
placebo was 5.7 units. Using an LOCF analysis, NAMENDA treatment was
statistically significantly superior to placebo.
Figure 3: Time course of the
change from baseline in SIB score for patients completing 28 weeks of
treatment.
Figure 4 shows the cumulative
percentages of patients from each treatment group who had attained at least the
measure of change in SIB score shown on the X axis. The curves show that both
patients assigned to NAMENDA and placebo have a wide range of responses and
generally show deterioration, but that the NAMENDA group is more likely to show
a smaller decline or an improvement.
Figure 4: Cumulative
percentage of patients completing 28 weeks of double-blind treatment with
specified changes from baseline in SIB scores.
Study 2 (Twenty-Four-Week
Study)
In a study of 24 weeks
duration, 404 patients with moderate to severe probable Alzheimer's disease
(diagnosed by NINCDS-ADRDA criteria, with Mini-Mental State Examination scores
≥ 5 and ≤ 14) who had been treated with donepezil for at least 6
months and who had been on a stable dose of donepezil for the last 3 months
were randomized to NAMENDA or placebo while still receiving donepezil. For
patients randomized to NAMENDA, treatment was initiated at 5 mg once daily and
increased weekly by 5 mg/day in divided doses to a dose of 20 mg/day (10 mg
twice a day).
Effects on the ADCS-ADL
Figure 5 shows the time course
for the change from baseline in the ADCS-ADL score for the two treatment groups
over the 24 weeks of the study. At 24 weeks of treatment, the mean difference
in the ADCS-ADL change scores for the NAMENDA/donepezil treated patients
(combination therapy) compared to the patients on placebo/donepezil
(monotherapy) was 1.6 units. Using an LOCF analysis, NAMENDA/donepezil
treatment was statistically significantly superior to placebo/donepezil.
Figure 5: Time course of the
change from baseline in ADCS-ADL score for patients completing 24 weeks of
treatment.
Figure 6 shows the cumulative
percentages of patients from each of the treatment groups who had attained at
least the measure of improvement in the ADCS-ADL shown on the X axis. The
curves show that both patients assigned to NAMENDA/donepezil and
placebo/donepezil have a wide range of responses and generally show
deterioration, but that the NAMENDA/donepezil group is more likely to show a
smaller decline or an improvement.
Figure 6: Cumulative
percentage of patients completing 24 weeks of double-blind treatment with specified
changes from baseline in ADCS-ADL scores.
Effects on the SIB
Figure 7 shows the time course
for the change from baseline in SIB score for the two treatment groups over the
24 weeks of the study. At 24 weeks of treatment, the mean difference in the SIB
change scores for the NAMENDA/donepezil-treated patients compared to the
patients on placebo/donepezil was 3.3 units. Using an LOCF analysis,
NAMENDA/donepezil treatment was statistically significantly superior to
placebo/donepezil.
Figure 7: Time course of the
change from baseline in SIB score for patients completing 24 weeks of
treatment.
Figure 8 shows the cumulative
percentages of patients from each treatment group who had attained at least the
measure of improvement in SIB score shown on the X axis. The curves show that
both patients assigned to NAMENDA/donepezil and placebo/donepezil have a wide
range of responses, but that the NAMENDA/donepezil group is more likely to show
an improvement or a smaller decline.
Figure 8: Cumulative
percentage of patients completing 24 weeks of double-blind treatment with
specified changes from baseline in SIB scores.
Study 3 (Twelve-Week Study)
In a double-blind study of 12
weeks duration, conducted in nursing homes in Latvia, 166 patients with
dementia according to DSM-III-R, a Mini-Mental State Examination score of <
10, and Global Deterioration Scale staging of 5 to 7 were randomized to either
NAMENDA or placebo. For patients randomized to NAMENDA, treatment was initiated
at 5 mg once daily and increased to 10 mg once daily after 1 week. The primary
efficacy measures were the care dependency subscale of the Behavioral Rating
Scale for Geriatric Patients (BGP), a measure of day-to-day function, and a
Clinical Global Impression of Change (CGI-C), a measure of overall clinical
effect. No valid measure of cognitive function was used in this study. A
statistically significant treatment difference at 12 weeks that favored NAMENDA
over placebo was seen on both primary efficacy measures. Because the patients
entered were a mixture of Alzheimer's disease and vascular dementia, an attempt
was made to distinguish the two groups and all patients were later designated
as having either vascular dementia or Alzheimer's disease, based on their
scores on the Hachinski Ischemic Scale at study entry. Only about 50% of the
patients had computerized tomography of the brain. For the subset designated as
having Alzheimer's disease, a statistically significant treatment effect
favoring NAMENDA over placebo at 12 weeks was seen on both the BGP and CGI-C.