Epidemiological data hint that they might just do that. Previously, researchers led by Benjamin Wolozin at Boston University, Massachusetts, had studied the U.S. Veterans Affairs database to show that in a large population of elderly men with cardiovascular disease, those taking ARBs had a lower incidence and slower progression of dementia than those on other anti-hypertensives (see ARF related news story on Li et al., 2010). Another human neuropathology study reported that people with high blood pressure who were on hypertension meds had about half the amyloid plaques in their brains as did non-hypertensive people, although this study did not break out classes of drugs (see Hoffman et al., 2009). Likewise, animal data indicate that at least some ARBs can slash Aβ pathology and improve learning (see ARF related news story; Danielyan et al., 2010).

Hajjar and colleagues hypothesized that treatment with ARBs might also reduce Aβ pathology in people. To test this idea, the authors used the National Alzheimer’s Coordinating Center (NACC) database, which collects clinical and neuropathological data from the 29 Alzheimer’s Disease Centers across the U.S. Their database includes detailed clinical data on more than 26,000 people and is freely available to researchers worldwide. For a detailed description of the NACC, see ARF related news story. From among about 1,300 people who had donated their brains for autopsy, Hajjar and colleagues analyzed the nearly 900 who had had hypertension and either cognitive impairment or abnormal pathology. About 15 percent of this population took ARBs, 20 percent were unmedicated for their hypertension, and the rest took other anti-hypertensives. The authors evaluated pathology by several criteria, including the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) score, Alzheimer's Disease and Related Disorders Association (ADRDA) score, and Braak and Braak staging. By all three measures, people who had taken ARBs had significantly less β amyloid pathology, with odds ratios of around 0.5.

“I was surprised by the strength of the effect,” Wolozin told Alzforum, noting that odds ratios of 0.5 for pathology data are “quite striking.” Wolozin was not involved in the current study, but said it agrees well with his own epidemiological findings. He pointed out that, in his study, the ARBs that penetrated the brain the best also had the greatest benefit for cognition. This suggests that the drugs might act directly on the brain.

How these drugs might lower β amyloid remains mysterious. The effect seems to be unrelated to their ability to control blood pressure, as other anti-hypertensives did not show it. Hajjar suggested the answer might lie in the drugs’ unique mechanism of action. ARBs block the hormone angiotensin II from binding to the angiotensin II receptor type 1 (AT1) on blood vessel walls. AT1 mediates vasoconstriction, which raises blood pressure. ARBs do not interfere with the binding of angiotensin II to the AT2 receptor. This receptor generally opposes the effects of AT1, causing blood vessels to dilate, lowering inflammation, and possibly increasing amyloid clearance, Hajjar said. By blocking AT1 while leaving the beneficial AT2 pathway intact, ARBs may improve the health of the brain. In contrast, a similar class of anti-hypertensives, angiotensin converting enzyme (ACE) inhibitors, prevents the maturation of angiotensin II and therefore inhibits both pathways.

Wolozin agreed the AT2 receptor pathway could be a key factor in the positive effects of ARBs. “ARBs increase blood flow to the brain,” he pointed out. More blood means more oxygen, which is good for the aging brain. Stressed brains have been shown to make more amyloid, he added. Indeed, several studies link hypoxia and vascular disease to a higher risk of dementia or faster cognitive decline (see, e.g., ARF related news story and ARF news story). Cardiovascular disease is a risk factor for dementia (see ARF related news story), though the data on hypertension are equivocal (see AlzRisk).

Hajjar and colleagues also saw a difference in cognition. People taking ARBs scored significantly better on several clinical measures, such as the Mini-Mental State Exam and the Global Clinical Dementia Rating scale, and were less likely to have a clinical diagnosis of AD than people on other anti-hypertensives or no medication. Hajjar cautioned, however, that this type of data is subject to selection bias. In theory, doctors could have prescribed ARBs to people with better cognition because of other health factors present in this population. One factor arguing against this idea, however, is that ARBs and ACE inhibitors tend to be prescribed to the same sort of patients, and ACE inhibitors had no effects on pathology in this study, Hajjar noted.

Commentators David Knopman at the Mayo Clinic in Rochester, Minnesota, and Deborah Blacker at Massachusetts General Hospital, Boston, agreed that selection bias could be a problem with this type of data. Blacker, who also oversees the AlzRisk database on Alzforum, noted that this is a particularly troublesome issue with neuropathology data, as few people agree to donate their brains after death. Therefore, autopsy data may not represent the whole population.

For his part, Knopman pointed out that the study looked at whether people took anti-hypertensive drugs, but included no information on dosage or duration of use. “It’s hard to interpret a beneficial effect of the medication when those things are unknown,” Knopman said. He considers the evidence to date too preliminary to recommend that physicians change how they prescribe anti-hypertensives. (Currently ARBs, which are newer and less studied than ACE inhibitors, are an alternative therapy for people who do not tolerate the latter.) It may make sense to look further at ARBs in a clinical trial setting, Knopman suggested. “The question is, Is there any value in giving them to people who are symptomatic?” he asked, pointing out that current thinking and recent clinical trials (see, e.g., ARF related news story and ARF news story) suggest that amyloid-targeting therapies may need to be initiated at presymptomatic stages to have a clinical benefit. ARBs might form a component of early-stage or preventive combination therapy in the future.

Hajjar believes that the drugs might help those already showing signs of impairment. In a small, one-year pilot trial, cognitively impaired people with hypertension who took ARBs improved their executive function and memory compared to people on other anti-hypertensives (see Hajjar et al., 2012). Based on these findings and the neuropathology data, Hajjar is beginning a five-year clinical trial in about 150 cognitively impaired people to see if ARBs can protect them from further decline. Hajjar will use structural MRI and arterial spin labeling (ASL) perfusion MRI to look for an effect of the drugs on structure and blood flow, respectively, in the brain. ASL-MRI acts as a surrogate measure of brain metabolism, similar to FDG-PET (see ARF related news story). Brain hypoperfusion as measured by ASL-MRI can predict cognitive decline (see Chao et al., 2010). Hajjar does not plan to include brain amyloid imaging in the trial. If ARBs prove to have a cognitive benefit, they might become part of the treatment regimen for older adults with hypertension, Hajjar suggested.

The AlzRisk meta-analysis of the current scientific literature on hypertension and dementia validates that high blood pressure, particularly in midlife, has been associated with a higher risk of dementia in some studies (see ARF related news story). The AlzRisk discussion noted that few studies have examined how treating hypertension affects AD risk.. “This [paper] adds a novel dimension” to the discussion of hypertension and AD, Blacker said.

Several clinical trials of other classes of anti-hypertensives are in the works, including a Phase 3 trial of the calcium channel blocker nilvadipine (see ARF related news story), and a pilot trial of the β blocker carvedilol (see ARF related news story), both of which also appear to lower β amyloid. Like ARBs, however, their effects on amyloid are believed to be independent of their ability to regulate blood pressure.—Madolyn Bowman Rogers.

Reference:
Hajjar I, Brown L, Mack WJ, Chui H. Impact of angiotensin receptor blockers on Alzheimer disease neuropathology in a large brain autopsy series. Arch Neurol. 2012 Sep 10:1-7. Abstract