Currently no cure is available for AD or other chronic neurodegenerative disorders. Therapies and systemic medication for AD patients help to maintain the health-related quality of life. Due to numerous approaches, in this section only those strategies are described, which are related to the Aβ hypothesis and the recently applied therapy using an acetylcholinesterase inhibitor.
Aß-peptide: Aß (1-40/42)
DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA β-secretase/?
N-truncated peptides 1) Aß(3-40/42)
DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA 2) Aß(11-40/42)
DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA glutaminyl cyclase
1*) Aß(pE3-40/42)
pEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA 2*) Aß(pE11-40/42)
pEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA dominant
fraction of total Aß-peptides in toxic plaques in AD-affected brain
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1.3.3.1 Acetylcholinesterase inhibitor
As a consequence of the degeneration of basal forebrain in the course of AD, the level of some neurotransmitters e.g. acetylcholine is reduced. This lack of signaling molecules leads to impaired signal transmission accompanied by the progressive loss of cognitive abilities [38, 39].
To improve the cholineric neurotransmission different therapeutic approaches were investigated in the past. The aim of these approaches was to increase the presynaptic release of acetylcholine and the deceleration of acetylcholine degradation by acetylchloinesterase within the synaptic space [38]. This approach shows only small beneficial effects on cognitive, functional and behavioral symptoms [40-42]. However, a minor beneficial effect was found for four acetylcholinesterase inhibitors: tacrine (1,2,3,4-tetrahydroacridin-9-amine), donezipil ( 1-Benzyl-4-[(5,6- dimethoxyindan-1-on-2-yl) methyl]piperidin), rivastigmine ( {3-[α-(Dimethylamino)ethyl]phenyl}-N-ethyl-N-methylcarbamat) and galantamine (3-Methoxy-11- benzofurobenzazepin-6-ol). These are frequently applied in the therapy of AD patients with mild and moderate symptoms [43].
1.3.3.2 Inhibition of β- and γ-secretases can reduce Aβ peptide formation
The release of full-length Aβ peptides is mediated, as mentioned above, by the secretases β and γ (cf. Section 1.3.2, Figure 1.1) that catalyze the sequential proteolysis of APP. The β- and γ-secretase inhibition would lead to a reduced production of full-length Aβ peptide and consequently to a reduced formation of the truncated and pyroGlu-modified Aβ peptide (cf.
Section 1.3.2).
The current state of drug development regarding β-secretase inhibitors is still in an early phase.
Several companies e.g. Merck (Germany) and Eisai (Japan) are evaluating potential drug compounds in different clinical phases. In 2012 Merck could show that an inhibitor named MK-8931 can reduce the Aβ accumulation in the cerebral spinal fluid (CSF) of healthy (human) subjects by more than 90 % without dose-limiting side effects. Currently, clinical phases II and III are in progress to evaluate the efficacy of the mentioned drug candidate [published on Merck Newsroom Home (http://www.mercknewsroom.com) in March 2012]. A similar outcome of clinical phase I was reported by Eisai or the β-secretase inhibitor E2609 that reduced the overall
23 http://www.eisai.com/news/news201247.html].
The investigation of γ-secretase as target protein in AD treatment is, due to its complexity, less advanced compared to the β-secretase. The inhibition of γ-secretase activity can reduce the Aβ amount in brain, cerebrospinal fluid and plasma [44, 45]. The application of γ-secretase inhibitors in transgenic mice with AD-like neuropathology reveals an improvement of Aβ induced cognitive deficits [44, 45]. Among others, one physiological function of the γ-secretase is the processing of the Notch protein. The unspecific inhibition of γ-secretase affects also the maturation of Notch protein that causes a dysregulated cellular homeostasis, organ side effects in the gastrointestinal tract. The development of specific γ-secretase inhibitors is still in the preclinical phase and requires further investigations [45].
1.3.3.3 Enhancing α-secretase activity can reduce Aβ formation in the human brain
A further therapeutic approach in the treatment of AD is the upregulation of α-secretase. As demonstrated in 1.3.2 (cf. Figure 1.1) this enzyme cleaves APP within the Aβ-domain. α-secretase is the body´s own tool to diminish the Aβ production by reducing the amount of substrate for β-secretase.
It could be demonstrated that α-secretase cleaves APP constitutively and can be stimulated by different signaling molecules [46]. The stimulation of α-secretase is not yet fully understood. It is likely that the non-constitutional activation is linked to the activity of protein kinase C [47]. For clinical applications this matter has to be investigated and remains for now a theoretical approach in the treatment of AD.
1.3.3.4 Immunotherapy for AD leads to deaggreagation of Aβ plaques
Extensive studies deal with the removal of already manifested Aβ depositions from neuronal tissue by immunoreactive mechanisms. Studies in 1999 and 2000 confirmed that the vaccination of mice (depicting Aβ induced AD-like pathology) with solubilized Aβ(1-42) or Aβ(1-40) lowers the Aβ accumulation in the brain by up to 50 – 60 % [48, 49]. Also passive immunization with monoclonal and polyclonal anti-Aβ antibodies leads to improvement of AD pathology in mouse models [50, 51].
Recent studies with humanized monoclonal anti-Aβ antibodies are actually in clinical phase III.
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1.3.3.5 Inhibition of N-terminal pyroGlu-modification of truncated Aβ peptides reduces the accumulation of neurotoxic Aβ-species
As mentioned above, the formation of pyroGlu-modified Aβ peptides (e.g. Aβ(pE3-40/42) and Aβ(pE11-40/42) is a crucial step in the cascade of neuropathological progress. In this step N-terminal glutamate of previously truncated Aβ(1-40/42) into Aβ(E3-40/42) and Aβ(E11-40/42) is converted into pyroglutamic acid (pyroGlu or pE). The generated N-terminal lactam ring of the peptides Aβ(pE3-40/42) and Aβ(pE11-40/42) hallmarks Aβ peptides with high amyloidogeneity (propensity to form insoluble aggregates) and neurotoxicity (cf. Section 1.3.2). This chemical reaction is catalyzed by the enzyme glutaminyl cyclase (cf. Figure 1.3).
Figure 1.3 Conversion of N-terminal glutamate into pyroglutamic acid of N-truncated Aβ peptides Glutaminyl cyclase (QC) catalyzes the conversion of glutamate into pyroglutamic acid in the N-terminal position of truncated Aβ peptides. This pathophysiological side reaction of the QC provides highly amyloidogenic and neurotoxic Aβ species.
The interjection of this step constitutes a further strategy for the treatment of AD. The spontaneous cyclization reaction of glutamate into pyroglutamic acid at physiological pH exhibits a rate constant of 10-9 s-1 in vitro. QC mediated reaction enhances this reaction by a factor of five in vitro [52]. In experiments with rats it was demonstrated that an injection of Aβ(1-40) and Aβ(3-40) leads to a significant increase of pyroGlu-modified Aβ peptides in the cortex. Rats which received intracortical microinjection of QC inhibitors simultaneously with Aβ(1-40) and Aβ(3-40) showed significantly decreased formation of pyroGlu-modified Aβ peptides [53, 54].
Thus, these findings indicated that QC is involved in the initial pathoneurophysiological progress of the development of AD and represents a target enzyme concerning AD treatment.
The general physiological function of QC that is described in detail in the following section is the maturation of peptide hormones and several chemokines e.g. thyrotropin releasing hormone (TRH) or chemokine(C-C motif) ligand 2 (CCL2). The down-regulation of a constitutive enzyme contains severe risks with respect to regulatory dysfunctions. Interestingly, Schilling et al.
O
25 no distinguishable changes in fertility, cognition and ingestion behavior compared to wild-type mice. Even blood glucose and glucose tolerance are comparable to wild-type animals. This is a significant contrast to TRH knock-out mice. Also (hypothalamic) pGlu-hormones are not significantly changed, implying the disposability of alternative hormone maturation (pyroGlu-modification) mechanisms. It is likely that an isoform of the QC fulfill hormone maturation processes (cf. Section 1.4.5) [55]. These results reveal that QC inhibition might be a promising strategy to reduce the formation of pyroGlu-modified Aβ peptides in the brain. Recent investigations have led to the development of a potent inhibitor compound (PQ912) whose harmlessness is proven. The efficacy of PQ912 has now to be tested in a phase 2 clinical study.
[http://www.probiodrug.de].
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