Search

To this day, the pharmacological treatment of Alzheimer's disease remains limited to the temporary stabilisation of cognitive decline and the reduction of neuropsychiatric symptoms. It is moreover with great difficulty to predict and select promising drug candidates in the early stages of the discovery and developmental process. In this context, scientists have developed new experimental paradigms to artificially induce transient cognitive impairments in healthy volunteers akin to those observed in Alzheimer's disease, i.e. the Cognitive Challenge Models. In the last decade, a great amount of literature on Sleep Deprivation was published which mainly focused on the consequences of sleep loss for public health. However, sleep deprivation paradigm may also be regarded as a cognitive challenge model. It is commonly accepted that sleep deprivation induces cognitive impairments related to a global decrease in vigilance, while in fact, there is a controversial approach related to the selective effects on cognitive functions. The identification and validation of cognitive challenge models in healthy volunteers are suitable in early clinical development of drugs to determine the 'hint of efficacy' of drug candidates. The present review aims at exploring in detail the methods, designs and cognitive paradigms used in non pharmacological sleep deprivation studies. Sleep deprivation can be induced by different methods. Probing the four main cognitive functions will allow identifying the extent to which different sleep deprivation designs selectively compromise executive function, working memory, episodic memory and attention. Findings will be discussed in line with cognitive processing levels that are required according to the tasks.

The early assessment of new symptomatic drugs against Alzheimer's disease remains difficult because of the lack of a predictive end-point. The use of a battery including different parameters could improve this early development. In order to test the reverse effect of symptomatic drugs in healthy volunteers, scientists have developed new experimental paradigms to artificially induce transient cognitive impairments in healthy volunteers akin to those observed in Alzheimer's disease, i.e. Cognitive Challenge Models. In this context, transient hypoxia could be a relevant Cognitive Challenge Model. The deleterious effects of hypoxia on cognition, as described in the literature, should be considered carefully since they are usually assessed with different populations that do not have the same hypoxic sensitivity. Hypoxia can be obtained by the means of two different methods: normobaric and hypobaric hypoxia. In both designs, cognitive changes can be directly modulated by the severity of hypoxic levels. The purpose of this review is to gather existing support on the application of hypoxia within different cognitive domains and to highlight the scientific interests of such a model to predict and select promising drug candidates. We aimed at reviewing in detail the methods, designs and cognitive paradigms used in non-pharmacological hypoxia studies. Probing the four main cognitive functions will allow identifying the extent to which different hypoxia designs selectively compromise cognitive functioning. For each cognitive process, the convergent and divergent results are discussed in terms of paradigm differences whereas we will focus on defining the optimal methodology for obtaining the desired effects.

Pharmacological therapies currently marketed for Alzheimer's disease (AD) are only symptomatic and show limited effects in terms of clinical benefit. Thus, the development of new symptomatic drugs remains essential. However the dramatic increase in costs associated with drug development together with the poor number of emerging drugs highlights how crucial it is to accelerate the findings aiming to bringing new drugs to market. In this respect, optimization of the development process by integrating, at early stage, reliable biomarkers able to predict clinical benefit in phase III clinical trials may help. The improvement of certain techniques such as imaging and electrophysiological methods has led to a more accurate assessment of the brain's physiological impact of pharmacological treatments used to alleviate symptoms in AD patients. This review aims to gather the main findings from clinical studies where the effect of anti-dementia drugs were assessed in healthy volunteers and AD patients through one or several such biomarkers (electroencephalography (EEG), magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT)). Overall, evidence presented in this review suggests that various biomarkers associated with key impairments observed in AD were sensitive to acetylcholinesterase inhibitors (AChE-I) medication and memantine with a good correlation with enhancement of cognitive performance. In most of the reviewed studies, only one kind of biomarker was used. Among these, deficits in quantitative EEG profile, P300 latency, and regional brain activity measured by either functional MRI (fMRI) during face encoding and working memory task or by PET/SPECT have been shown to be reversed by anti-dementia drugs. It is therefore suggested that a single biomarker approach would be limited and not be sufficiently predictive to extensively assess the potential of a new symptomatic drug. Hence, it appears that a combination approach with the use of a panel of biomarkers rather than a single biomarker may be more appropriate to establish a good correlation between the disease and therapeutic intervention.

International audience