New chalcogen-optimized 1,3,5-triazines as dual 5-HT6R/FAAH modulators: A versatile approach to neurodegenerative disorders.

The clinical failure of selective serotonin 5-HT6 receptor (5-HT6R) antagonists in Alzheimer's disease (AD) highlights the need for multitarget therapeutic strategies addressing the multifactorial nature of neurodegeneration. Building upon our pioneering discovery of the first-in-class dual 5-HT6R/FAAH (fatty acid amide hydrolase) modulators among O-ether triazine compounds, we here report a comprehensive lead-optimization campaign centered on the triazine-based compound MR3b, identified as a promising lead in the search for AD treatment. Structural modifications based on scaffold contraction and chalcogen bioisosterism generated a focused library of 1,3,5-triazine derivatives with diversified GPCR and FAAH profiles. Several compounds displayed nanomolar affinities for 5-HT6R, 5-HT2AR, and D2R, alongside improved FAAH inhibition and antioxidant properties. Sulfur and selenium substitutions markedly enhanced receptor affinity and reduced cytotoxicity compared to the oxygen-containing lead. Selected compounds demonstrated significant neuroprotective effects in cellular models of AD-related pathology, including mitochondrial dysfunction, amyloid-β, and glutamate-induced toxicity. Furthermore, the thio-analogue 4c effectively reversed memory deficits in vivo, showing superior CNS penetration (Kp,brain = 0.78), an expanded therapeutic window (NOR test), and improved safety relative to MR3b. This study identified compound 4c as a second-generation lead and underscores the potential of multitarget triazine-based ligands combining serotonergic modulation and FAAH inhibition as potential disease-modifying candidates for AD and related neurodegenerative disorders.