MICA: ET-1-mediated reduction of cerebral blood flow in Alzheimer's disease: therapeutic potential of zibotentan

Blood flow through the brain is reduced in patients with Alzheimer's disease (AD). The reduction occurs before the onset of dementia. In people who already have AD, the more marked the reduction in blood flow through the brain the more severe their dementia. Patients with AD do tend to develop structural changes to the blood vessels in the brain - degenerative small vessel disease (SVD) and cerebral amyloid angiopathy (CAA). However, the distribution of the reduction in cerebral blood flow differs from that of the structural changes to cerebral blood vessels in AD, and there is increasing evidence that it is largely mediated by functional rather than structural abnormalities of the vessels.

The brain damage and memory disturbances that occur in AD are thought to result from the accumulation of a substance known as amyloid beta (Abeta) in the brain. This occurs in two main forms, a shorter one (Abeta40) and a longer one (Abeta42). In AD there is an increase in the proportion of Abeta that is of the longer form (Abeta42). The longer form of Abeta causes nerve cells to produce more of an enzyme called ACE that, in turn, produces the chemical Ang II. In addition, both forms of Abeta increase the production of another chemical called endothelin-1 (ET-1): Abeta42 does so by acting on an enzyme (called ECE) that synthesizes ET-1 in nerve cells, and Abeta40 by stimulating the production of ET-1 in the lining of blood vessels in the brain. Both ET-1 and Ang II cause blood vessels to constrict, narrowing their lumen. This reduces blood flow through the brain in patients with AD, and thereby impairs the delivery of oxygen and nutrients to nerve cells and slows the removal of waste products, adversely affecting nerve cell function. In addition, experimental studies suggest that the reduction in blood supply may increase the production of Abeta and thereby accelerate the progression of the AD. In a rat model of AD, other researchers showed that injection of ET-1 led to more accumulation of Abeta, more marked secondary degenerative changes and impaired performance in memory tests.

ET-1 mediates its reduction in blood flow by acting on specific receptors in the vessel walls (ET-A receptors). Ang II mediates its effects by acting on other receptors (Ang II receptors). By blocking either or both sets of receptors, it should be possible to improve blood flow through the brain. We now wish to explore the utility of (i) zibotentan, a specific ET-A blocker produced by AstraZeneca, and (ii) losartan, a licensed Ang II receptor blocker, for improving blood flow through the brain in a rat model that combines ET-1-mediated reduction in blood flow and Abeta accumulation in the brain. The rat studies will allow us to examine the effects of zibotentan, losartan and a combination of the two in great detail, including how these drugs affect maintenance of flow when there are changes in blood pressure or brain activity, and what the effects are on Abeta metabolism (including on the activity of ECE and ACE, both of which help the brain to remove Abeta).

Both zibotentan and losartan are well tolerated when given over long periods of time to elderly people. These studies will clarify their separate and combined potential for restoring blood flow in AD - and potentially therefore improving memory and slowing the progression of dementia.

Cerebral blood flow (CBF) is reduced in Alzheimer's disease (AD) before the onset of dementia. More severe reduction predicts more rapid cognitive decline. In mouse models the reduction in CBF greatly exceeds the decline in neuronal metabolic activity. The distribution differs from that of structural vessel disease and is probably mediated by functional vascular abnormalities.
AD is thought to be initiated by the intracerebral accumulation of amyloid beta (Abeta). We have shown that Abeta42 upregulates neuronal endothelin-converting enzyme (ECE)-2 and angiotensin-converting enzyme (ACE). ACE catalyses production of angiotensin II (Ang II) and ECE that of endothelin-1 (ET-1), both potent vasoconstrictors. ET-1 concentration is significantly elevated in AD cortex. In addition, Abeta40 increases ECE-1 activity in endothelial cells, and both ECE-1 activity and ET-1 level are elevated in blood vessels from AD brains; this is likely to contribute to impairment of functional hyperaemia in AD.

Cerebral vasoconstriction reduces CBF, impairs the delivery of oxygen and nutrients and slows the removal of waste products (including Abeta). Brain ischaemia also increases Abeta production. We and others reported the use of Ang II blockers (ARBs) to be associated with a reduced incidence of AD. No comparable human data are available for ET-1 receptor blockers. In a rat model of AD, injection of ET-1 exacerbated Abeta accumulation and neurodegeneration and impaired memory test performance.
We suggest that Abeta40- and Abeta42-mediated increases in ET-1 and/or ACE are at least partly responsible for reduced CBF in AD. ET-1 mediates its cerebrovascular effects through ET-A receptors. We propose to compare the effects of zibotentan, a specific ET-A blocker, and losartan, an ARB, on CBF, autoregulation, functional hyperaemia and other vasoregulatory pathways, in a rat model of Abeta-mediated cerebral vasoconstriction, with a view to zibotentan usage in AD.

Any therapy that improves or even simply slows the decline in cognitive abilities and functional capacity in AD has the potential to have major impact not only on the well-being of individual patients (of whom there are about 0.5 million in the UK) but also family members and other carers, and on the cost of social care and healthcare delivery. AD is much the most common cause of dementia, which is estimated cost to cost the UK £23 billion each year. In the US, the current annual cost of caring for patients with AD is ~$200 billion and the value of unpaid care provided by family and friends is estimated at $210.5 billion, suggesting that the lost economic opportunity costs are similar to the direct costs. However, this is not solely a problem of affluent countries. Earlier this year the WHO issued a report citing the estimated worldwide cost of dementia in 2010 as $604 billion but noting that the worldwide prevalence of dementia (currently approximately 35.6 million) is estimated to reach 65.7 million by 2030 and 115.4 million by 2050, an increase that will disproportionately affect low and middle-income countries, placing a massive burden on their health and social systems and economies. Any treatment that even simply slows the rate of progression of AD in individual patients, that prolongs independence and delays the need for full-time care, will have a hugely positive impact on the social and economic burden of this disease.

Whilst we have learnt a great deal about the pathology and cellular biology of AD, the genetic and environmental factors that influence the risk of developing the disease, and how to diagnose it more accurately and earlier, that knowledge has not yet been translated into improvements in therapy. In this application we describe an approach that is novel, likely to be safe, already available for clinical use and, we believe, has a good chance of proving effective. It has the potential to be translated into clinical practice relatively rapidly, within the next few years. Innovative approaches to the treatment of AD are urgently needed and even if the results of this project prove disappointing, we suggest that the findings are likely to stimulate other research aimed at improving the CBF with a view to ensuring not only that neurons have an appropriate supply of oxygen, glucose and other nutrients, but also adequate haematogenous removal of potentially toxic products of neural metabolism.