Incretin-based drugs as potential therapy for neurodegenerative diseases: current status and perspectives

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most frequent neurodegenerative disorders. Despite their pathophysiological and clinical differences, they share several mechanistic similarities at cellular and sub-cellular levels. The current treatments of AD and PD are only symptomatic, since many clinically-tested drugs failed to prevent or halt their progression. There is now evidence that type 2 diabetes mellitus is among the main risk factors for AD and PD and that the insulin resistance in the brain plays a crucial role in their neuropathological processes. Therefore, insulin nasal administration was suggested for the treatment of AD and PD, both in diabetic and non-diabetic patients. However, the adverse effects of chronic insulin prompted the research of alternative strategies, such as the novel antidiabetic drugs based on the incretin hormones glucagon-like protein-1 (GLP-1) and glucose-dependent insulinotropic Peptide (GIP). The rapid inactivation of these incretins by dipeptidyl-peptidase IV (DPP-IV) suggested the development of DPP-IV-resistant GLP-1 receptor agonists (GLP-1Ras), the recent dual GLP-1/GIP receptor agonists and the DPP-IV inhibitors (DPP-IVis). This review will first describe the experimental, pathophysiological and clinical approach for AD and PD treatment with insulin. Afterwards, the main pharmacologic properties of GLP-1Ras and of DPP-IVis will be discussed, detailing their ability to cross the BBB and get access to the brain for GLP-1Ras, and the novel strategies for BBB crossing as regards DPP-IVis. Emphasis will be placed on the main findings obtained from AD and PD experimental models about the neuroprotective effects of these drugs. For AD, the improvement of learning and memory exerted by incretin-based drugs correlated with reduction of chronic inflammation, brain Aβ plaque, tau hyperphosphorylation, protection of mitochondria, enhancement of energy utilisation. For PD, both GLP-1Ras and of DPP-IVis reversed the nigrostriatal dopaminergic cell loss progression, restored dopamine synthesis, exerted anti-inflammatory activity and improved motor functions. Finally, the encouraging results of the first clinical trials on AD and PD patients and the adverse effects of GLP-1Ras and DPP-IVis will be discussed, highlighting how the above-mentioned neuroprotective effects have a great potential to be translated into clinical settings and that the incretin-based approach represents novel promising strategy for the treatment of AD and PD, although more convincing clinical pieces of evidence are required. In perspective, the new approaches that are being developed to allow GLP-1Ras and DPP-IVis rapid entering into the Central Nervous System may substantially contribute to their repurposing for neurodegenerative disease.