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Previous studies have demonstrated that cognitive function can be restored in mouse models of Alzheimer¿s disease (AD) following administration of sildenafil, a specific PDE5 inhibitor. Another very potent PDE5 inhibitor with a longer half-life and safe in chronic treatments, tadalafil, may represent a better alternative candidate for AD therapy. In this project we first measured the blood and brain levels of tadalafil to prove for the first time that the compound crosses BBB and that chronic treatment leads to accumulation in the brain of the J20 transgenic mouse model of AD. We demonstrated the presence of PDE5 mRNA in the brain of the mice and also in the human brain. After a 10 week treatment with either of these PDE5 inhibitors, the cognition of the J20 mice improved when compared with the transgenic mice that received vehicle. Biochemical analysis revealed that neither sildenafil nor tadalafil altered the amyloid burden, although both compounds reduced Tau phosphorylation in the mouse hippocampus. This study provides evidence of the potential benefits of a chronic tadalafil treatment in AD therapy. We demonstrate that tadalafil could be considered as an optimal candidate for drug re-positioning and as a good candidate to enhance cognition too. The cyclic nucleotides cAMP and cGMP are important second messengers that are required for the proper development and function of the brain. Modulation of intracellular cAMP/cGMP concentrations happens by activation or inhibition of AC/GC respectively and to inhibit signalling, both are degraded by different PDE. It has even been suggested that an imbalance in the cyclic nucleotides signaling caused by an inadequate control by these enzymes may contribute to the synaptic deficits occurring in neurodegenerative disorders that curse with dementia. Nevertheless, levels of cGMP or cAMP and the expression of PDEs in AD brains are not well characterized. In our study we used CSF samples from patients diagnosed with clinical and prodromal stages of AD to assess the levels of cAMP and cGMP and characterize the implication of these cyclic nucleotides in the clinical and pathological progression of AD. We have developed and implemented a high throughput LC-MS/MS method that offers a very sensitive, robust and highly selective. Further, the expression level of different types of PDEs was also studied in the temporal cortex of AD patients. Our results support the hypothesis that cGMP-specific PDEs inhibitors may affect memory-related processes by promoting increases of cGMP. Considering the numerous complex and different pathological mechanisms involved in AD progression, treatments targeting a single cause may lead to limited benefits. The goal of the next part of this study was the identification of a novel mode of action for this unmet need. Pharmacological tool compounds were utilized simultaneously for in-vitro and in-vivo proof of concept. A synergistic effect was observed in the amelioration of AD signs using the combination therapy in Tg2576 mice. Finally, a therapeutic agent, CM-414, inhibiting simultaneously two targets from tool compounds, was generated and tested in Tg2576 mice. CM-414 reversed cognitive impairment, reduced amyloid and tau pathology, and rescued dendritic spine density loss in the hippocampus in AD mice. Importantly, the effect obtained was present after a 4-weeks wash-out period.