Alzheimer's disease (AD) is a progressive degenerative disease of the central nervous system characterized by progressive and increasing memory loss, followed by loss of control of limbs and bodily functions with a fatal ending. It is by far the most common cause of dementia affecting 1 to 6% of people over the age of 65 years and between 10 to 20% of those over 80.
AD is distinguished from other types of dementia by several pathological features, including the progressive appearance in the brain of the patients of senile plaques in the extracellular space between neurons. The plaques have central cores of amyloid deposits formed mainly by fibrils of a 40-42 amino acids peptide referred to β amyloid peptide (Aβ) surrounded by degenerated neurites and glial cells. This peptide results from the proteolytic processing of a precursor protein called a amyloid precursor protein (βAPP).
The National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA) established the most commonly used NINCDS-ADRDA Alzheimer's Criteria for diagnosis in 1984. According to the NINCDS/ADRDA. These criteria are the following:                Definitive Alzheimer's disease: The patient meets the criteria for probable Alzheimer's disease and shows histopathologic evidence of AD via autopsy or biopsy.        Probable or prodromal Alzheimer's disease: Dementia has been established by clinical and neuropsychological examination. Cognitive impairments also have to be progressive and be present in two or more areas of cognition. The onset of the deficits has been between the ages of 40 and 90 years and finally there must be an absence of other diseases capable of producing a dementia syndrome.        Possible or non-prodromal Alzheimer's disease: There is a dementia syndrome with an atypical onset, presentation or progression; and without a known etiology; but no co-morbid diseases capable of producing dementia are believed to be in the origin of it.        Unlikely Alzheimer's disease: The patient presents a dementia syndrome with a sudden onset, focal neurologic signs, or seizures or gait disturbance early in the course of the illness.        
Mild cognitive impairment (MCI, also known as incipient dementia, or isolated memory impairment) is a diagnosis given to individuals who have cognitive impairments beyond that expected for their age and education, but that do not interfere significantly with their daily activities (Petersen R C et al. (1999) Arch. Neurol. 56 (3): 303-8). It is considered to be the boundary or transitional stage between normal aging and dementia. Although MCI can present with a variety of symptoms, when memory loss is the predominant symptom it is termed “amnestic MCI” and is frequently seen as a risk factor for Alzheimer's disease (Grundman M et al. (2004). Arch. Neurol. 61 (1): 59-66).
Studies suggest that these individuals tend to progress to probable or prodromal Alzheimer's disease at a rate of approximately 10% to 15% per year (Grundman M et al ad supra.) Additionally, when individuals have impairments in domains other than memory it is classified as non-amnestic single- or multiple-domain MCI and these individuals are believed to be more likely to convert to other dementias (Tabert M H et al. (2006). Arch. Gen. Psychiatry 63 (8): 916-24).
The diagnosis of MCI requires considerable clinical judgement, and as such a comprehensive clinical assessment including clinical observation, neuroimaging, blood tests and neuropsychological testing are best in order to rule out an alternate diagnosis. A similar assessment is usually done for diagnosis of Alzheimer's disease. MCI is diagnosed when there is (Morris J C et al. (2001). Arch. Neurol. 58 (3): 397-405):                Evidence of memory impairment        Preservation of general cognitive and functional abilities        Absence of diagnosed dementia        
In the last decade, several attempts have been performed to identify peripheral markers by using plasma, serum or circulating cells. In particular, because amyloid plaques are a defining feature of Alzheimer disease neuropathology, and Aβ can be detected in plasma, its measure is a compelling candidate biomarker for Alzheimer disease.
In clinical praxis, diagnosis of AD is carried out using clinical criteria based on the presence of typical clinical hallmarks and the exclusion of other types of dementia using neuroimaging techniques and blood analysis. Using these criteria, diagnostic reliability is acceptable although, according to studies done using brain autopsy, between 10-20% of the patients diagnosed with AD suffered from a different disease. Moreover, the current diagnostic methods can only be carried out when the neurodegenerative process is so advanced that the patient suffers from severe dementia and the brain damages are so extensive that the number of therapeutic measures is limited. Definitive diagnosis requires pathologic examination of post-mortem brain tissue.
In view of the fact that Aβ accumulates in the brain of AD patients and is a central element in the pathogenesis of AD, this protein has been considered as the most suitable candidate as AD biomarker. However, the use of AD as plasma biomarker for AD faces the problem that the concentrations of the Aβ peptides (Aβ(1-40) and Aβ(1-42)) in serum are extremely low, so that there are no assays which are sensitive enough so as to allow reliable detection of said peptide species.
Many different assays have been used to determine levels of amyloid beta peptides in biological samples (see e.g. the methods described by Scheuner et al (Nature Med., 1996, 2:864-870); Tamaoka A et al. (J Neurol Sci., 1996, 141, 65-68); Suzuki, N. et al. (Science, 1994, 264:1336-1340); WO200722015, Vanderstichele H et al. (Amyloid, 2000. 7, 245-258); Fukomoto y col. (Arch. Neurol. 2003, 60, 958-964); Mehta et al. (Arch. Neurol. 57, 2000, 100-105); Mayeux, R. et al. (Ann Neurol. 1999, 46, 412-416); Lanz, T. A and Schacthter, J. B. (J. Neuroscience Methods, 2006, 157:71-81), WO200750359, WO0162801, WO0315617, WO0246237, WO0413172. However, all the ELISA-based assays known to date have a lower detection limit which is not in the range of single digit pg/mL at the most, which is sufficient for detecting Aβ40 and Aβ42 in CSF as well as for detecting said species in plasma in patients suffering from familiar AD, but are unsuitable for detecting Aβ42 in the plasma of patients suffering from sporadic AD, wherein the Aβ42 plasma concentration is much lower.
To date, the only Aβ peptide assays showing a lower detection limit lower than the single digit pg/mL correspond to the assays described in WO200646644 and in WO2009015696.
WO200646644 describes an electrochemiluminiscent (ECL) sandwich assay wherein the mAb 21F12 (which recognises amino acids 33-42 of Aβ42) is coupled to magnetic beads, which are then used to capture the Aβ42 peptide in the sample containing Aβ42 and further contacted with 3D6 mAb coupled to a ruthenium complex. The amount of 3D6 antibody bound is then detected by the luminescence emitted by the ruthenium complex when electrical energy is applied. Using this assay, the inventors are capable of detecting as low as 0.5 pg/mL of a Aβ42 standard. However, when the same assay is used to compare Aβ42 in plasma samples from AD patients and healthy controls, no significant differences could be observed between the two sets of patients, which led the inventors to conclude that the amount of intact Aβ42 in serum is very low due to degradation and turned to a competitive ELISA assay using 21F12 mAb which provides lower sensitivity levels in the range of ng/mL.
WO2009015696 describes a high-sensitivity ELISA sandwich assay wherein the detection antibody is contacted with a biotin-labeled reagent showing specificity for said antibody. The reagent is contacted with streptavidin which is coupled to peroxidase. Peroxidase activity is then detected by colorimetry using TMB or fluorescently using QuantaBlue.
WO2006053251 describes a method for the determination of amyloid beta peptide species in a sample comprising contacting a sample with a denaturing agent, extracting the peptide pool from the sample-denaturing agent mixture, separating the amyloid beta peptide species from the pool and determining the amount of amyloid beta peptide species. This method requires a step of separation of the peptides prior to the determination, which results in increased processing time and increased costs.
Methods are known in the prior art to diagnose AD by detecting the levels of biomarkers present in the brain or CSF of patients. Different biomarkers have been characterised whose determination is carried out in CSF. CSF reflects directly the composition of the extracellular space of the central nervous system and thus, provides higher concentrations as biomarkers. However, CSF can only be retrieved by means of lumbar punction, which is not a routine diagnostic method easily accepted by patients suffering from dementia, let alone in patients with memory disorders. Thus, there is a need for AD biomarkers which can be detected in samples which can be non-invasively retrieved from the body.
Suitable AD biomarkers described in the prior art and which can be detected in plasma include (i) markers derived from the amyloid plaque, (ii) autoantibodies against Aβ o βAPP, (iii) inflammatory markers such IL-6, its receptor or gp130, C-reactive protein or oxidative stress (isoprostanes), (iv) markers of lipidic metabolism (apoE, oxysterols) and (v) vascular disease markers (homocysteine, lipoprotein b C1q) (Scheuner D et al. (1996) Nature Med 2, 864-870).
However, in view of the fact that Aβ accumulates in the brain of AD patients and is a central element in the pathogenesis of AD, this protein has been considered as the most suitable candidate as AD biomarker. However, the use of AB as plasma biomarker for AD faces the problem that the concentrations of the Aβ peptides (Aβ(1-40) and Aβ(1-42)) in serum are extremely low, so that there are no assays which are sensitive enough so as to allow reliable detection of said peptide species.
Furthermore, several antibodies have been described to detect Aβ peptides and to be used in immunological assays. For example, the monoclonal anti-Aβ(1-17) (6E10) is an antibody directed to the N-terminal region of the Aβ peptide, generated against the peptide Aβ(1-17) (Kim K S, et al. Neurosci. Res. Comm. 7; 1988) and recognizing the Aβ peptides including said region or the monoclonal antibody generated against the peptide Aβ(1-28) (Pierce).
However, there is a need in the art for improved immunological assays and kits to detect Aβ-derived peptides which overcome the problems of the methods and kits known in the art, in particular, which are sensitive enough to detect Aβ peptides in a reliable manner in plasma of patients suffering from sporadic AD. There is also a need for identifying biomarkers for the early diagnosis of AD which are sensitive and specific and which allow distinguishing cognitive impairment due to age from those associated with the early symptoms of the process, as well as to distinguish changes due to AD and due to other degenerative conditions. According to Growdon et al. (Neurobiol. Aging, 1998, 19:109-116), the ideal marker for AD should meet the following requirements:                It should detect a fundamental feature of the neuropathology        It should be validated in neuropathologically-confirmed cases of the disease        It should show a sensitivity of at least 80% for detecting AD        It should show a specificity of at least 80% to distinguish AD from other types of dementia and        It should be reliable, reproducible, non-invasive, simple to perform and inexpensive.        