The Therapy of Alzheimerâs Disease: Towards a New Generation of Drugs
Luca Piemontese1, *, Fulvio Loiodice1, SĂlvia Chaves2, Maria AmĂ©lia Santos2 1 Dipartimento di FarmaciaâScienze del Farmaco, UniversitĂ degli Studi di Bari âAldo Moroâ, Via E. Orabona 4, 70125 Bari, Italy
2 Centro de QuĂmica Estrutural, Instituto Superior TĂ©cnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
Abstract
The treatment of neurodegenerative diseases is one of the most urgent challenges for pharmaceutical industry and public institutions. Alzheimerâs disease (AD), in particular, is a severe age-dependent dementia, currently affecting 44.4 million people worldwide, and this number is estimated to rise to 131.5 million in 2050.
Only a few drugs are currently available for AD therapy, but these molecules are just able to temporarily improve the symptoms of the patients. Recently, important advancements have been achieved about the knowledge of this complex disease, even if, unfortunately, every attempt to obtain new efficient drugs for its therapy has failed.
Following the theory of the multifactorial origin of the disease, in the last decade, researchers mainly focused on the development of multi-target agents, acting on the classical features recognized as important against the onset of AD, such as NMDA receptor antagonism, inhibition of cholinesterases (ChEs) and beta-Secretase (BACE), as well as inhibition of beta amyloid peptide (AÎČ) aggregation and antioxidant activity.
More recently, the modulation of dyshomeostasis of metal ions (i.e. copper, zinc and iron) in the AD patient brains, has been proposed as a disease-modifying therapeutic (DMT) strategy, due to their involvement in AÎČ aggregation and in the formation of Reactive Oxygen Species (ROS). Noteworthy is the role of Peroxisome Proliferator-Activated Receptors (PPARs) in the onset of neurodegenerative diseases as well. PPARα expression levels have been reported to significantly decline in central nervous system (CNS) during the aging process. PPARÎł, instead, is reported to have a neuroprotective effect, with a different mechanism that influences the AÎČ precursor protein (APP) cleavage and the inflammatory response. The overexpression of certain types of ApoE also seems to increase the risk of developing AD. Therefore, the modulation of both PPAR subtypes seems to be a new interesting target to be explored.
Other innovative targets as well are currently studied to find the final breakthrough for the therapy of AD: a number of years have passed since the approval of the last active drug in the treatment of this pathology and people now need a new hope.
Keywords: Alzheimerâs Disease, AÎČ Amyloid Plaques, Acetylcholinesterase Inhibitors, Disease-Modifying Agents, Donepezil, Innovative Targets, Insulin, Multi-Functional Drugs, Multitarget Therapy, Neurodegenerative Disease, Pioglitazone, PPARs, Type 2 Diabetes Mellitus, Type 3 Diabetes, Tacrine.
* Corresponding author Luca Piemontese: Dipartimento di FarmaciaâScienze del Farmaco, UniversitĂ degli Studi di Bari âAldo Moroâ, Via E. Orabona 4, 70125 Bari, Italy; Tel: +39-080-5442732; E-mail: [email protected] INTRODUCTION
Alzheimerâs disease (AD) is a widely recognized socioeconomic problem. Indeed, the average annual cost per patient is estimated in $15,000â20,000, firstly due to the assistance expenses for not auto-sufficient people [1], taking into account that the annual incidence is around 34/1000 persons over 60 years old, and the prevalence is up to 42.1% at age > 95 years [1-4]. The impact of the pathology will be higher considering the wide increase of life expectancy in most of the world countries: it is estimated that, in the absence of effective therapies, the number of people with this dementia will reach 131.5 million by 2050 [5].
The pathology has been described for the first time by AloĂŻs Alzheimer in 1907 as an âunusual illness of the cerebral cortexâ. His short communication, published in German language, was recently translated [6] to English and it reported the experience of the physician with a 51 years old woman, who suffered from uncommon cognitive impairment, characterized by disorientation in space and time, hallucinations, and progressive memory loss [6]. The post-mortem exam of the patientâs neuronal tissues showed a uniformly atrophic brain without macroscopic focal degeneration. However, further studies revealed many fibrils located next to each other, corresponding to the first observation of beta amyloid (AÎČ) plaques [6]. This occurred around one century ago, but now this pathology is the most widespread neurodegenerative disease. It has a name (Alzheimerâs disease) but its etiology continues to be unknown [7, 8].
AD is characterized by a progressive deterioration of cognitive functions that can be linked to a significant reduction of the volume of the brain, the degeneration of synapses and the death of neurons, especially in hippocampus (this being the reason of progressive memory loss) [8-10]. The AÎČ plaques are located in the extracellular space while another kind of aggregates, very frequently found inside neuronal cells, are constituted by the so-called neurofibrillary tangles. These aggregates, which are also observed in Parkinsonâs disease (PD), are composed of hyper-phosphorylated Tau proteins [8]. The presence of both structures is very often correlated in AD patients [8].
In the last years, many routes have been suggested for understanding AD pathogenesis and addressing relevant drug strategies to fight this pathology. The most commonly pursued approaches are the cholinergic and the amyloid hypotheses [11].
According to numerous research works, the damage of cholinergic neurons or simply the cholinergic activity deficit, frequently linked with AD onset, leads to an overall loss of acetylcholine activity [12]. Moreover, acetylcholinesterase (AChE) seems to play also a certain role in several secondary non-cholinergic functions and particularly in the deposition of AÎČ in the extracellular environment of AD diagnosed brains [12]. A recent research has shown that, in addition to AChE, also butyrylcholinesterase (BChE) plays an important role in cholinergic neurotransmission, especially in normal central nervous system (CNS) [12, 13]. Based on this consideration, many studies suggest that a non-selective ChE inhibitor should lead to better clinical results [12, 14].
The AÎČ peptides are produced by cleavage operated by ÎČ- and Îł-secretase enzymes in the inter-synaptic environment on the membrane-anchored APP (ÎČ-Amyloid Precursor Protein). The activity of these proteases leads to oligomeric fragments that, following a natural aggregation process, become themselves active neurotoxins, therefore causing neuronal dysfunction, loss of synaptic connections and cell death [15]. AÎČ peptide aggregates lead to the formation of amyloid plaques ...