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Substantial evidence has accumulated in support of the hypothesis that elevated cholesterol levels increase the risk of developing Alzheimer’s disease (AD). As a result, much work has been done investigating the potential use of lipid-lowering agents (LLAs), particularly statins, as preventive or therapeutic agents for AD. While epidemiology and preclinical statin research (described in Part 1 of this review) have generally supported an adverse role of high cholesterol regarding AD, human studies of statins (reviewed here) show highly variable outcomes, making it difficult to draw firm conclusions. We identify several confounding factors among the human studies, including differing blood-brain barrier permeabilities among statins, the stage in AD at which statins were administered, and the drugs’ pleiotropic metabolic effects, all of which contribute to the substantial variability observed to date. We recommend that future human studies of this important therapeutic topic 1) take the blood-brain barrier permeabilities of statins into account when analyzing results, 2) include specific analyses of effects on low-density and high-density lipoprotein cholesterol, and most importantly, 3) conduct statin treatment trials solely in mild AD patients, who have the best chance for disease modification.
In Part 1 of this review, we examined the epidemiological and neuropathological literature relevant to the relationship between cholesterol levels and AD risk, as well as preclinical studies of LLAs as potential therapeutic or preventive agents. In this second segment, we appraise the human studies of this topic and find substantial variability in outcomes. We then address whether the complex and inconsistent trial results to date may be explained in part by differential abilities of statins to cross the blood-brain barrier (BBB) as well as several other confounding factors. Our review reveals that the principal basis for reported differences in the effects of cholesterol-lowering drugs across studies is the highly variable relationship between the time of statin initiation and the time of onset and the severity of AD. These and other findings herein lead us to recommend a set of specific criteria for conducting much-needed clinical trials of the potentially important effects of cholesterol regulation on AD incidence and progression.
The authors performed an unbiased search of the PubMed database for relevant studies in the English language, without regard to publication date. Additional studies were identified by citations in the resultant papers, and also by the recommendation of the co-authors or consultants (identified in Acknowledgments). We included all articles that provided well-controlled studies and clearly interpretable conclusions about this topic. Studies of the effects of statins in human subjects were required to include at least fifty persons, and human studies examining LLAs were required to specify which one(s) were investigated.
A number of observational studies in human subjects bolster the hypothesis that statins may be able to mitigate the course of AD or reduce the probability of developing it (Table 1). A case-control study by Jick and colleagues1 found that people over 50 who were taking statins had a lower risk of developing dementia. An examination of patients older than 60 years demonstrated that patients taking lovastatin and/or pravastatin had a lower AD risk compared to the general population or patients taking other medications for hypertension or heart disease2. In a study of postmenopausal women younger than 80 years with heart disease, subjects taking statins did better on the Modified MMSE and were less likely to be cognitively impaired3. Wolozin and colleagues4 found that simvastatin treatment significantly reduced the risk of both dementia and Parkinson’s Disease. A study of elderly Mexican-Americans reported that statin treatment was associated with a reduced risk of both dementia and cognitive impairment without frank dementia5. Similarly, participants in the Rotterdam epidemiological study were less likely to develop AD if they were taking statins, an effect that was not dependent on ApoE genotype6.
Other studies have examined the effects of cholesterol-lowering treatment on subjects who had already been diagnosed with AD. A neuropathological assessment by Li et al.7 concluded that while statin users did not have lower CERAD scores (a quantitative scale of amyloid plaque pathology) than nonusers, they did have lower Braak stages (a quantitative scale of neurofibrillary tangle and neuritic pathology). Furthermore, subjects using statins were less likely to have “typical AD-type neuropathology,” which the authors defined as a Braak stage of at least IV (maximum score is VI) and a CERAD score of at least “moderate” (maximum score is “severe”).
Not all observational studies conclude that statins are beneficial in the setting of AD (Table 1). When a large cohort (4895 for prevalent dementia, 3308 for incident dementia) of elderly people was surveyed for overall dementia or AD and the use of statins (an aggregate of lovastatin, simvastatin, cerivastatin, atorvastatin, pravastatin, and fluvastatin) in 1995-1997 and again in 1998-2000, neither statin use at the initial visit nor at follow-up visits correlated with the risk of developing any dementia or AD during the period between the two visits, although statin use was less common among those who already had dementia at the initial visit8. Another study found that statin treatment of patients 65 years and older did not affect their risk of developing AD or other forms of dementia, and this result did not change when more lipophilic (lovastatin, simvastatin, cerivastatin) and less lipophilic (atorvastatin, pravastatin, fluvastatin) drugs were analyzed separately9. A study mentioned above4 that showed a protective effect with simvastatin failed to find such an effect with lovastatin. Arvanitakis et al. reported that while statin users were less likely to be demented at time of death, statin use didn’t affect AD risk or cognitive ability10.
As with the human observational studies above, the results of RCTs in AD patients have been contradictory (Table 2). Administering a controlled-release form of lovastatin to nondemented subjects has been shown to decrease serum Aβ levels in a dose-dependent manner11 (Figure 1). Sparks et al.12 observed differences between atorvastatin users and nonusers on the Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-cog) at 6 months and on the Geriatric Depression Scale (GDS) at 12 months. However, large double-blind RCTs of pravastatin13 and simvastatin14 lasting 3.2-5 years were unable to demonstrate a protective effect, and the Sparks study12 did not find an effect on the Mini Mental State Exam (MMSE), Neuropsychiatric Inventory (NPI), or Alzheimer’s Disease Cooperative Study-Activities of Daily Living scale (ADCS-ADL). Most recently, an RCT found that cognitive decline in patients already having mild to moderate AD was unaltered by a 72-week administration of atorvastatin15. Overall, we interpret these various studies to indicate that statins have not yet been shown to influence the course of AD in controlled trials.
Some observational studies have examined the effects of non-statin compounds that decrease cholesterol levels (Table 1). Yaffe et al.3 examined a combination of niacin, cholestyramine resin, and other non-specified compounds and did not find these to be effective at preventing cognitive impairment in postmenopausal women without dementia. Fibrates (a group of amphipathic carboxylic acids used for hypercholesterolemia treatment) were also reported to be ineffective for dementia or AD protection1,6,8,9 (although one study16 found them to be efficacious), as were cholestyramine and nicotinic acid8 or niacin, bile acid sequestrants and probucol9.
The substantial variability in outcome of the diverse human studies reviewed above makes it difficult to ascertain whether statins could have a beneficial role in preventing or treating AD (Tables 1 and and2).2). One possible reason for the inconsistency is that in order to potentially affect the course of AD, any prospective treatment must be able to pass through the BBB and enter the brain. In this respect, all statins are not created equal: some have a much greater capacity to cross the BBB than others, but this is not always commented upon by authors in interpreting their studies.
Differences in BBB permeability among the various statins (Table 3) were recognized while investigating why some statins, when prescribed as a treatment for hypercholesterolemia, had a higher incidence of neurological side effects than others. Lovastatin, but not fluvastatin, efficiently crossed a monolayer of bovine brain microvessel endothelial cells (a model for the BBB) and was detected in brain extracts of rats receiving the compound17. In another study, both lovastatin and simvastatin had much higher BBB permeability coefficients than pravastatin18 (Figure 2). Like pravastatin, rosuvastatin’s ability to cross the BBB is limited19. The evidence on atorvastatin and cerivastatin is mixed: some researchers reported that they cross the BBB effectively (reviewed for atorvastatin20 and cerivastatin21), whereas others did not (reviewed for atorvastatin21 and cerivastatin20).
What is responsible for this disparity in BBB permeability among the statins? One key determinant is lipophilicity. Lovastatin and simvastatin are much more lipophilic than pravastatin22, while fluvastatin falls somewhere in between23. Also, fluvastatin is negatively charged and so would be repelled by the anionic microdomains in the plasma membranes of BBB endothelial cells17. Different transport mechanisms may also play a role: simvastatin and lovastatin appear to cross the BBB via passive diffusion, whereas pravastatin relies on an active transport system, for which it may have a low affinity24.
This variation in the biochemical properties of statins provides an important clue as to why some studies may have shown a beneficial effect of statin treatment on dementia and AD whereas others have not. However, as several investigators found that the lipophilicity of statins did not affect their usefulness as preventive agents for AD6,9,10, other factors must be taken into account as well. Among these are the methodology and design of statin studies, as well as the pleiotropic effects of statins.
The above data on the variability of BBB penetration across statins (Table 3) provide one potentially important explanation for the inconsistency in outcome. Some studies focus on a statin that doesn’t cross the BBB well, or else they aggregate data from penetrant and non-penetrant drugs (Tables 1 and and2).2). With substantial variation in BBB permeability among statins, it becomes difficult to reconcile the conflicting findings in the literature. Also, some studies base their conclusions for statins as a whole on the analysis of a single drug.
Various methodological issues are also likely to contribute to the inconsistency. In their review, Kandiah and Feldman20 identified the following as particularly important: 1) the distinction between pre-existing and incident AD; 2) the time of statin use vs. the time of cognitive assessment; 3) the dose of statin; 4) the duration of statin use; 5) the duration of observation; 6) the conflation of AD with other forms of dementia (a major concern); 7) patient compliance with the statin prescription; and 8) whether or not vascular risk factors were controlled.
In accord with that review20, the authors of primary papers have often discussed such methodological concerns relevant to the evaluation of their experimental results. Solomon et al.25, for example, discussed survival bias as one of the difficulties in studying the relationship between cholesterol and AD. Statin dosage and brain penetration were alluded to by Meske et al.26 as potential confounding factors in various epidemiological studies: negative results may be due to statins not reaching a therapeutic concentration in the brain. The importance of dosage was stressed in an elegant study conducted by Ostrowski et al.27, which found that while simvastatin and lovastatin can affect both Rho and Rab family proteins, applying a physiologically relevant concentration of the statin would influence some members of these protein families but not others. The authors postulate that Rab- and Rho-dependent effects overlap at high statin doses, whereas Rho-dependent effects dominate at lower doses.
Indication bias (in which a drug is prescribed to treat a condition that is associated with the variable of interest) and cessation bias (in which some of the observed protective effects of a drug may be due to patients stopping the drug after being diagnosed with a condition) have also been suggested as problems that may occur in statin studies28. Another potential pitfall is the question of whether those who are in general good health are more likely to be prescribed statins, resulting in a misleading apparent effect of these drugs on disease prevention or treatment. This was addressed by Rockwood et al.28, who found that there was no association between self-reported health and LLA use. On the contrary, there was an association between LLA use and other vascular risk factors, indicating that LLAs are not more likely to be used by healthier people.
The time period during which a study was conducted may be a particularly important factor. After finding protective effects for lovastatin and pravastatin but not simvastatin, Wolozin et al.2 pointed out that simvastatin was (at the time) a relatively new drug, so prescribing patterns may have been different than for the other two compounds studied. In another study, Wolozin and colleagues4 chose not to assess fluvastatin because the number of people taking it increased drastically over the time course of the study, which they felt introduced a confounding variable. The time in the patient’s life at which measurements are taken may also be critical: Yaffe et al.3 mentioned that dementia might alter a patient’s diet or metabolism, thus having an effect on lipid levels, and Part 1 of this paper emphasizes that studies conducted in middle- vs. old-age reach different conclusions about the relationship between cholesterol levels and dementia risk.
The statistical power of a study is a major issue to consider. Li et al.7 expressed concern that the “crude, semiquantitative” CERAD scale may not have very high power to detect relationships between statin use and AD pathology. Arvanitakis et al.10 indicate that their study is limited in statistical power, which could account for the observed lack of correlation between statin use and overall AD brain pathology or tangle immunoreactivity. Difficulties can arise when only a small percentage of even a large initial cohort develop AD over the course of the study, making it problematic to detect potential effects of therapeutic interventions on outcome.
Another confounding factor is the fact that an AD patient’s ApoE genotype may affect not only AD risk but also the effectiveness of statins as an AD prevention or treatment. In one study, people with the ApoE4 allele saw less benefit (in terms of cholesterol levels) from statin treatment than those with the E2 or E3 alleles29. Although a detailed discussion of ApoE’s potential effects is not within the purpose of this review, some trials of statins have taken ApoE genotype into account6, and the mechanism by which ApoE genotype might influence AD risk continues to be the subject of intense research (e.g. 30,31).
Statins have other effects on physiology and metabolism besides lowering cholesterol levels, some of which could independently affect AD risk. For example, statins can alter the expression of genes related to cell growth, signaling, trafficking, and apoptosis32. Here, too, we see differences among the statins: in this study32, 38 genes were affected by simvastatin, 26 by lovastatin, and only 21 by pravastatin.
Rosuvastatin, atorvastatin, and simvastatin have all been found to increase expression and activity of endothelial nitric oxide synthase (eNOS) in mice33. Furthermore, treatment of normocholesterolemic, spontaneously hypertensive rats with a physiologically relevant dose of atorvastatin mediated a host of beneficial effects: it lowered blood pressure, enhanced carbachol-induced vasodilation, inhibited angiotensin II-induced vasoconstriction, decreased the production of superoxide in blood vessel walls, lowered mRNA and protein expression of angiotensin Type 1 receptor, and reduced mRNA expression of the NAD(P)H oxidase subunit p22phox34. Although the mechanisms behind these effects are largely unknown, the authors speculated that they may involve downstream isoprenoids, some of which are important for post-translational modification of other proteins. In any event, the broad spectrum of actions of some or most statins means that any effect they have on AD incidence or course (or on preclinical measures such as degree of Aβ pathology) may not necessarily relate to their cholesterol-lowering effects per se.
One potential effect of statin treatment deserves special mention. Inhibition of HMG-CoA reductase activity interferes with the isoprenylation of other proteins, impairing their functions and causing a wide variety of downstream effects. Isoprenylation is important in protein trafficking and signaling, and in cytoskeletal structure. If statins are added to cells in the presence of mevalonate, cholesterol synthesis will still be blocked, but isoprenylation can continue unimpeded. This gives researchers a way to separately analyze the cholesterol- and isoprenoid-dependent effects of statins. Using this strategy, Cole et al.35 concluded that low isoprenoid levels inhibit the passage of APP through the secretory pathway, leading various processing products to accumulate intracellularly. Low cholesterol levels, meanwhile, may inhibit receptor-mediated endocytosis of APP. The authors hypothesized that there are two largely separate pools of Aβ: an intracellular pool that is mainly affected by isoprenoids, and a secreted pool that is mainly influenced by cholesterol (Figure 3).
In summary, the diverse potential roles for cholesterol in aspects of AD pathogenesis have led to numerous studies evaluating cholesterol-lowering drugs such as statins for potential therapeutic effects. Based on all available studies, statins appear to hold greater promise than other classes of LLAs. However, individual statins differ in ways that could affect their efficacy with respect to AD. The conflation of statins with other LLAs, the disparities among statins in their respective BBB permeabilities, and the differences in biochemical effects among these drugs are all likely to contribute to the unfortunate variability in study outcomes observed to date.
Most of the laboratory studies discussed in Part 1 of this review converge on a model suggesting that increased cholesterol promotes Aβ formation and accumulation in the brain. However, the human epidemiological studies (also Part 1) are difficult to bring together with similar conviction. Throughout this second part of our review, we have found reconciling the many human LLA studies difficult because of 1) the different biochemical properties of the statins and other LLAs used in the studies, 2) the myriad time points of cholesterol measurements (see Part 1) and statin administration (Tables 1 and and2),2), 3) the inconsistent use of biomarker and cognitive endpoints, and 4) the methodological challenges of observing complex human populations, especially as regards the conflation of AD with vascular dementia.
Based on these observations, we can make several urgent recommendations for future studies investigating the potential link between statin use and AD risk or progression. First, the characteristics of the LLAs should be more explicitly defined (particularly their permeability across the BBB) and BBB-permeant compounds assessed separately from non-permeant ones. This step will help clarify whether BBB permeability governs how a statin modifies the incidence and progression of AD. Second, future cohort studies should focus on investigating the effects of cholesterol levels and statin use in either the presymptomatic or very mild symptomatic stages of AD and their contribution to modifying the risk of developing frank AD, and they should assess LDL and HDL separately. We consider this point of evaluating statins in mild (or earlier) AD subjects to be particularly salient. Due to the progressive nature of AD neuropathology, measurement and modulation of cholesterol in moderate or late stages of the disease, when significant neurological injury has accrued, may show little or no effect. Third, an agreed set of biomarkers in serum and CSF (particularly Aβ42, tau and phospho-tau levels), brain imaging (including amyloid imaging by positron emission tomography, which has only recently become available), and cognitive measures (such as ADAS-cog and, preferably, more sophisticated and specific tests of verbal and episodic memory) should be uniformly applied to future epidemiology studies and clinical trials. Fourth, given the complex spectrum of non-vascular dementias that includes MCI and AD, the common binary distinction between demented and non-demented subjects is insufficient. Available diagnostic modalities (including CSF Aβ42 and tau assays and amyloid imaging by PET) should generally be sufficient to define the probable pathological basis of a dementia. Fifth, there are many complexities in this area of clinical research that could also be explored in well-controlled animal models, to give the field a clearer picture of what occurs mechanistically in the brain when statins are administered. These issues include the question of whether the concentration of statins typically administered to human patients is sufficient to interfere with isoprenylation and, importantly, to what extent the BBB is damaged in mild AD subjects and how this might affect the penetration of both cholesterol and statins into the brain.
To evaluate more rigorously than heretofore the potential efficacy of statins for AD, we propose a prospective epidemiological cohort study -- designed and conducted by several experts coming together – that begins with subject recruitment in late middle age and includes comprehensive assessment and follow-up using the biomarkers and clinical assessments we list just above. It is our hope that such research can untangle the complex factors discussed here that have precluded a clear answer to the overarching question of whether statins, such widely prescribed and generally safe drugs in our society, represent viable therapeutic or preventive agents for the most common cause of progressive cognitive failure in older humans.
The authors thank Francine Grodstein, ScD, of the Harvard School of Public Health and Benjamin Wolozin, MD, PhD, of the Boston University School of Medicine for providing insightful comments and suggestions on an earlier draft of this manuscript. Neither received financial compensation for their assistance. DJS is a founding scientist and consultant of Elan PLC; NES and GMS declare no competing interests as regards this work. Supported by NIH grant AG06173 (DJS). The funding organization had no role in design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review or approval of the manuscript.
Supported by NIH grant AG06173 to DJS.
Author Contributions: Dr. Selkoe had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study Concept and Design: Shankar
Acquisition of Data: Shepardson, Shankar
Analysis and Interpretation of Data: Shepardson, Selkoe
Drafting of the Manuscript: Shepardson, Shankar
Critical Revision of the Manuscript for Important Intellectual Content: Selkoe, Shepardson, Shankar
Obtained funding: Selkoe
Study Supervision: Selkoe