D. M. Stein, M. Sano
INTRODUCTION
Alzheimer’s disease (AD) represents one of the most challenging healthcare problems world-wide. While the incidence and prevalence grows with life expectancy there are few treatment options and little knowledge of effective prevention strategies. Oxidative damage in the brain has been implicated in the pathophysiology of AD, suggesting a possible role for anti-oxidant interventions in treatment and prevention. Current clinical practice involves the use of anti-oxidant treatment in AD, and a number of clinical trials and observational studies have in the past and are currently exploring the efficacy and safety of the use of anti- oxidants in preventing and/or delaying progression of neurodegenerative processes. The scientific basis of this approach is grounded in in vitroand in vivostudies, which have demonstrated that oxidative stress is at least a significant player in what is likely a multi- factorial pathogenesis of neurodegenerative disorders such as AD. Epidemiological and clinical studies provide some support for a beneficial effect in AD. Recently meta-analyses and post hocexamination of large clinical trials using vitamin E suggest possible risks par- ticularly within clinically relevant subgroups. We will review the information regarding our current understanding of anti-oxidants as an intervention for AD.
BIOLOGICAL BASIS FOR ANTI-OXIDANT USE IN AD
The idea that oxidative injury plays a role in AD likely originated from the free radical hypothesis of ageing, proposed formally in the 1950s [1]. This theory states that a ‘single common process, modifiable by genetic and environmental factors . . . [is] responsible for the ageing and death of all living things’ [2]. The process implicated is that of the initiation of free radical reactions, which are innate to biological systems and produce random, and dele- terious changes to cellular structures and chemical composition. That AD is significantly correlated with ageing led to a natural question of whether oxidative stress is a contributing factor to the disease. The fact that brain tissue is a particularly susceptible target of reactive oxygen species (ROS), due to its relatively low content of anti-oxidants, its high content of polyunsatureated fatty acids, and its high metabolic activity, underscores this possibility [3].
A number of studies show evidence of the role of oxidative stress in the pathogenesis of AD. Early studies showed evidence of a relative increase of ROS release. This was thought
Daniel M. Stein, MD, Research Associate, Department of Psychiatry, The Mount Sinai Medical Center, New York, Research and Development Program, James J. Peters VA Medical Center, Bronx, New York, USA
Mary Sano, PhD, Director of the Alzheimer’s Disease Research Center, Professor of Psychiatry, Department of Psychiatry, The Mount Sinai Medical Center, New York, Research and Development Program, James J. Peters VA Medical Center, Bronx, New York, USA
©Atlas Medical Publishing Ltd 2007
to be secondary to one or more of the following: a deficiency of cytochrome oxidase activity (involved in mitochondrial electron transfer), an alteration of iron homeostasis, a reduced potency of normal anti-oxidative systems, and an unbalanced activity of physiological anti- oxidants such as superoxide dismutase and monamine oxidase isoenzyme B [4].
‘Surrogate markers’ of free radical injury include lipid peroxidation, protein oxidation, glyco-oxidation, and oxidation of nucleic acids [5, 6]. The past several years of study in this area have shown that patients with AD have increased levels of oxidation of all of these bio- logical macromolecules, and have led researchers to conclude that ‘oxidative imbalance is a prominent feature of AD’ [7]. One of the most commonly studied of these markers in AD is brain lipid peroxidation. Patients with a clinical diagnosis of AD have been shown to have increased levels of isoprostanes (IPs), markers of in vivolipid peroxidation, in blood, urine and cerebrospinal fluid (CSF) [8, 9]. Evidence of increased oxidative stress in AD patients has been found in peripheral cells and tissues. Lymphocytes were used to detect increased oxidation peripheral cells of AD patients [10, 11]. Levels of the end processes of lipid per- oxidation were measured in skin fibroblasts and lymphoblasts in patients with AD and in healthy controls. This study demonstrated an increase in lipid peroxides in AD patients, suggesting an attack by free radicals on cell membrane phospholipids [12]. Animal studies involving transgenic/knockout mice that model AD support this theory, as these models have been shown to display signs of increased oxidative stress [13–15].
While it is apparent that increased oxidative stress is present in AD, it is not as clear whether it is a cause or an effect of the pathology that leads to the disease. There are several findings, however, that hint that it may play a causal role rather than a secondary one.
Signs of oxidative damage have been shown to be present in the regions of vulnerable neurones associated with amyloid-plaques and neurofibrillary tangles [16, 17]. These find- ings suggest a link in the pathophysiology of AD between plaques and tangles, and the aforementioned oxidative imbalance. Several other studies have suggested amyloid-itself can lead to increased levels of ROS, and vice versa [18–21].
Patients with mild cognitive impairment (MCI), a precursor or early stage of AD, have been shown to have signs of increased oxidative stress. As with AD patients, MCI patients were shown to have increased levels of IPs [22]. A recent study showed increased DNA damage secondary to oxidized purines and pyrimidines in peripheral leukocytes of both patients with AD and those with MCI [23]. Another recent study showed a pattern of increased homocysteine (another risk factor for AD, and one that contributes to oxidative stress) and decreased total anti-oxidant capacity in both AD and MCI patients [24]. These findings of high levels of oxidation in MCI patients as compared to normal controls stress the role of oxidative damage in neurodegenerative disease even in its very early stages.
Such early involvement suggests that oxidative stress could play a primary role in the neurodegenerative process of AD.
Another interesting source of evidence stems from studies in Down’s syndrome (DS), in which nearly all individuals develop a pathology that is akin to AD. Oxidative damage plays a significant role in the pathology and DS patients demonstrate similar amyloid-
pathology. One study suggested the causal role of oxidative injury in DS by actually linking apoptotic neuronal death to lipid peroxidation and showing that this could be inhibited by the use of free radical scavengers [25]. Another group showed that increased oxidative stress in DS temporally precedes amyloid-deposition, suggesting it is not merely sec- ondary to this process [26]. This group subsequently showed that in AD itself, oxidative damage to RNA and amino acids is perhaps the earliest pathological sign even in AD, and that it occurs more in the early stages of the disease than in the later stages [27].
The abundance of evidence that oxidative stress plays a role in AD, coupled with the evi- dence suggesting that the role is causative or primary in nature and not merely a side-effect of other pathological mechanisms, leads one to seek out ways to determine whether inter- ventions to reduce oxidative damage in the brains of patients with AD is a viable therapeutic
or even preventive strategy. While there are a number of laboratory/experimental studies that suggest this [28–36], the clinical evidence is taken from both epidemiological studies and clinical trials which are reviewed below.
CLINICAL EVIDENCE OF ANTI-OXIDANT EFFECT IN AD
Many observational studies support the notion that anti-oxidants may have a beneficial effect in AD either by reducing the risk or by modifying disease severity. Table 6.1 summa- rizes several of the observational studies supporting this notion.
Six of these are reports from ageing, population-based studies which have longitudinally followed subjects measuring anti-oxidant use with food frequency questionnaires and vari- ous forms of self-reported vitamin and supplement intake [37–42]. The most commonly studied agents are vitamins E and C, both from food and from vitamin supplementation.
The results of these studies range from mild beneficial effect to no effect. For example, high intake of vitamins C and E from foods has been associated with a decreased risk of demen- tia in three studies [40–42] with one of these only finding a benefit among those who took both vitamins C and E. These samples had relatively long follow-up time permitting suffi- cient cases to convert to dementia thereby ensuring sufficient power to see an effect if it was there. Two other studies were unable to observe a benefit using similar methodologies [38–39]. In one the observation period was briefer, although the conversion rate was rela- tively high suggesting that the risk of low power may not have interfered with the results [39]. In the Honolulu Asia Aging Study cohort, a beneficial effect was seen for supplements of vitamins E and C on vascular dementia [38], the more common diagnosis in this cohort, but not on AD. Since this cohort depends on clinical diagnosis and there is little autopsy, it is not possible to determine in this data set if AD pathology may be affected by the use of these anti-oxidants.
Other studies have used clinical convenience samples such as the report of Fillenbaum et al. [43], which describes a secondary analysis of a sub-sample of cases from a 10-year prospective cohort study of community-dwelling elderly. The presence or absence of vita- min supplement use was recorded, although dose and frequency were not available. This sample, in which only 10% used any vitamin supplement, was unable to identify any benefit to vitamins E or C. Klatte et al. [44] reported on a chart review which identified 130 patients from memory disorders clinic, with a diagnosis of probable AD by NINCDS-ADRDA crite- ria who were taking donepezil (at least 5mg daily) and vitamin E (at least 1000 U daily). He examined the change in Mini-mental State Examination Score among those who were followed for at least 1 year and found a slower rate of decline than among historical controls not exposed to these treatments.
Overall the strongest observational data for a beneficial effect of vitamin E use comes from studies of dietary intake, which in general yields lower amounts than reported in stud- ies of supplement use. It is also important to consider that dietary vitamin E is not in isol- ation and may reflect other aspects of dietary habits that contribute to positive outcomes.
While studies have controlled for some of these, such as caloric intake there may be other aspects of food and lifestyle associated with these intake patterns that could be considered in multifactorial models.
Few clinical trials have been conducted examining the effect of vitamin E use as a treat- ment for dementia. Table 6.2 summarizes those trials in which dementia or cognitive loss was a primary or secondary outcome. One multicentre randomized trial examined vitamin E use (2000 IU/day) among community-residing patients with AD of moderate severity [45]. The trial examined vitamin E, selegiline and the combination compared to placebo. The primary outcome was the time to reach any of several clinical endpoints (nursing home placement, loss of basic activities of daily living, advancement to severe dementia or death).
After adjustment for baseline imbalances in cognition, a beneficial effect was observed with
Therapeutic Strategies in Dementia Morris et al.[37] Population and Design:633 random sample of Results:91 cases of AD in 4.3 years. Lower rate of AD
healthy, aged ⱖ65 years. Vitamin supplements in those taking vitamin E (both unadjusted and adjusted) taken in ‘last 2 weeks’ determined by and lower adjusted rate of AD in those taking vitamin C direct inspection. Prospective follow-up Comment:No effect of multi-vitamin on rate of AD Outcome:Clinical diagnosis of AD
Masaki et al.[38] Population and Design:3385 community- Results:Reduced risk of vascular dementia in those taking based males aged 71–93 from the both vitamins E and C supplements. No protective effect Honolulu-Asia Aging Study, prospectively for AD. Vitamin E or C alone had better cognitive test followed. Vitamins E and C supplements performance at follow-up
were ascertained by interview Comment:Vitamin use reported at one time-point only.
Outcomes:Presence of dementia and type No measures of cognitive function at baseline (Vascular, AD, Mixed) bases on CASI
Commenges et al.[53] Population and Design:1367 subjects Results:Reduced risk of incident dementia randomly selected from 5554 of the Paquid with increased flavenoid intake
Study, SW France, age 65 years followed Comment:Small number of incident cases (n⫽65) prospectively, for cognitive loss and dementia and large dropout (16% before 5 years, 20% before 8 Outcome:DSM-IIIR Diagnosis of years)
dementia assessed with psychometric testing confirmed by neurologist
Luchsinger et al.[39] Population and Design:980 subjects Results:242 incident cases of AD. No effect of intake of
⬎65 years old, random sample of healthy carotenes and vitamin C, or vitamin E in supplemental or Medicare beneficiaries from North Manhattan dietary form or in both forms, on risk of AD
(WHICAP) with at least 1 year follow-up. Comment:Relatively high intake of dietary anti-oxidants, Observational prospective cohort. Food high conversion rate and short observation period frequency questionnaire used to assess
vitamin intake
Outcome:Incident AD by NINCDS-ADRDA
Engelhart et al.[40] Population and Design:5395 subjects Results:High dietary intake of vitamins C and E age ⬎55 years, non-demented from food was associated with lower risk of AD from population based prospective, Comment:Positive effect of diet may require long cohort Rotterdam Study. Evaluated observation period which is difficult to achieve with dietary assessment mean follow-up in a randomized clinical trial
period 6 years
Outcome:Incident AD (DSM-IIIR; NINCDS-ADRDA)
xidant drugs61 random sample from biracial community reduced when dietary differences were controlled (fats, in Chicago (CHAPS). Observational other anti-oxidants. Supplement use (vitamins C prospective cohort with mean follow-up of and E, -carotene) had no effect
3.9 years. Dietary assessment with food Comment:No dietary vitamin E effect in ApoE-4 subjects.
frequency and reported supplement use Increased vitamin E supplement use during study, Outcome:Incident AD based on with possibility of inadequate exposure to see an effect NINCDS-ADRDA
Klatte et al.[44] Population and Design:130 subjects Results:Subjects taking vitamin E declined at from memory disorders clinic, significantly lower rate than those in CERAD cohort probable AD by NINCDS-ADRDA Comment:Many limitations of CERAD cohort (dropouts criteria, taking donepezil not included and vitamin E use not available and (at least 5 mg daily) and vitamin E age differences)
(at least 1000 U daily) with at least 1 year follow-up on these meds
Outcome:Mini-mental State Examination average cumulative score change compared to those of historical control (CERAD) before these treatments were available
Zandi et al.[42] Population and Design:4740 subjects, Results:Vitamins C and E combination supplement use was ⱖ65 years old, from Cache County Study, associated with reduced prevalence and risk of incidence a population-based study of prevalence of AD. No evidence of a protective effect when vitamin and incidence of AD and other C or E used alone or with other vitamin complexes dementias. Observational cross-sectional Comment:Vitamin users were younger, more educated, and prospective data review. In-home and in better general health
direct observation of current supplement use Outcome:Diagnosis of AD by means of multistage assessment procedures
Fillenbaum et al.[43] Population and Design:616 subjects from a Results:8% subjects used vitamins; 141 subjects secondary analysis of subsample of cases had dementia (93 AD). Use of low- or high- from Duke EPSE project, aged 65–105. dose vitamin supplement of C and/or Prospective cohort study of community-dwelling E did not delay incidence of dementia or AD
elderly. Vitamin use by subject report Comment:Did not record dose or duration of vitamin Outcome:Time to dementia/AD on consensus use. Few used vitamin but high conversion rate conference using NINCDS-ADRDA criteria
Therapeutic Strategies in Dementia Sano et al.[45] Subjects and Study Design:342 subjects with Results:Delay in the time to the primary outcome for
moderate probable AD aged 55–90. Randomized, patients treated with selegiline, ␣-tocopherol, double-blind, placebo-controlled 2-year, multi- or both, as compared to placebo
centre (23 sites) study of 4 treatment Secondary Outcome:Slower decline in functional groups: selegiline (10 mg), ␣-tocopherol outcomes. No improvement on cognitive, behavioural or
(2000 IU)l, both, placebo extra-pyramidal measures
Primary Outcome:Time to any endpoint Safety:More falls in combined treatment group, (death, institutionalization, loss of activities unclear aetiology
of daily living or severe dementia) Comment:Adjusted analysis due to imbalance in baseline Secondary Outcome:Cognition, function, MMSE scores
behaviour, extrapyramidal signs
Le Bars et al.[54] Subjects and Study Design: 309 subjects, with Results:EGb group did significantly better on the mild to severe dementia (AD or multi-infarct). ADAS-Cog (1.4 points) and better in GERRI
Randomized, double-blind, placebo-controlled (0.14) than placebo. CGIC not significantly different multicentre study of two groups: EGb 761 Comment:Significant dropout rate
(extract of ginkgo) 120 mg/day or placebo for l52 weeks
Primary Outcome:Cognitve (ADAS-Cog), and two Clinical Global (GERRI) and CGIC
Adair et al.[59] Subjects and Study Design:43 mild to moderate Results:No significant effect of treatment with NAC probable AD (MMSE of 12–26). Randomized, on primary outcome. Significant benefit on some subtests double-blind, placebo-controlled trial of NAC of the cognitive battery
(50 mg/kg/day) with assessments at 3 and Safety:Fatigue, headache and appetite loss were the most
6 months common side-effects
Primary Outcome:Change in MMSE and ADL scale Comment:Trend toward a positive result in some other Secondary Outcome:Performance on cognitive outcomes
battery components
Gutzmann et al.[60] Subjects and Study Design:203 subjects aged Results:Higher benefit from treatment in patients 40–90 with mild to moderate primary dementia randomized to idebenone, very high dropout rates in (DSM-IIIR) and probable AD (NINCDS-ADRDA). both groups
Double-blind, parallel-group, multicentre study Comment:Did not focus on drug vs. placebo, but randomized to either idebenone (360 mg/day) on improvement after set 60-week time period or tacrine (up to 160 mg/day) and treated for
60 weeks
xidant drugs63 psychologist, patient’s relative
Secondary Outcome:ADAS-Cog, NOSGER-IADL, CGIC
HPSCG [47] Subjects and Study Design:20 536 UK adults, Results:No benefit of anti-oxidant supplements on aged 40–80 with coronary/occlusive cognitive performance or incident dementia
artery disease, or diabetes. Randomized Comment:Cognition was an ‘add-on’ to the cardiovascular placebo-controlled factorial design 5-year study study and no baseline evaluation of cognition or incident of anti-oxidant supplementation (600 mg vitamin E, dementia was conducted
250 mg vitamin C, 20 mg -carotene/day) and simvastatin (40 mg/day)
Primary Outcome (non-cognitive): Major coronary or vascular events
Secondary Outcome (cognitive):Cognitive impairment measured by a telephone interview at study end and incident dementia by record review of report
Thal et al.[58] Subjects and Study Design:536 subjects, age Results:There were no significant differences between
⬎50 years with mild to moderate (MMSE 12–25) any dose group and placebo for the primary or probable AD (NINCDS-ADRDA), randomized, secondary outcomes
placebo-controlled trial for 1 year with Comment:In an exploratory two-group analysis 3 treatment groups: idebenone 120, 240, or comparing all three treated groups combined to 360 mg t.i.d., each of which was compared with placebo, drug-treated patients performed
placebo better on the ADAS-Cog but not the CGIC
Primary Outcome: ADAS-Cog,ADCS-CGIC Secondary Outcome:ADCS-ADL (activities of daily living scale), MMSE, BEHAVE-AD (behavioural rating scale)
Petersen et al.[46] Subjects and Study Design:769 subjects with Results:No effect of vitamin E on primary outcome.
amnestic MCI, aged 55–90. Multicentre A few significant differences in secondary outcomes (69 sites), randomized, double-blind (executive, language, overall cog scores) for first placebo-controlled, 3-year study of vitamin E 18 months only
(200 IU/day), donepezil (10 mg/day) each Comment:Incident dementia occurred primarily in the
compared to placebo ApoE-4 group
Primary Outcome:Incident AD
Secondary Outcome:Cognitive battery and IADL