Discovery of Novel Pyrazolopyrimidinone Derivatives as Phosphodiesterase 9A Inhibitors Capable of Inhibiting Butyrylcholinesterase for Treatment of Alzheimer’s Disease

Discovery of multitarget-directed ligands (MTDLs), targeting different factors simultaneously to control the complicated pathogenesis of Alzheimer’s disease (AD), has become an important research area in recent years. Both phosphodiesterase 9A (PDE9A) and butyrylcholinesterase (BuChE) inhibitors could participate in different processes of AD to attenuate neuronal injuries and improve cognitive impairments. However, research on MTDLs combining the inhibition of PDE9A and BuChE simultaneously has not been reported yet. In this study, a series of novel pyrazolopyrimidinone–rivastigmine hybrids were designed, synthesized, and evaluated <i>in vitro</i>. Most compounds exhibited remarkable inhibitory activities against both PDE9A and BuChE. Compounds <b>6c</b> and <b>6f</b> showed the best IC₅₀ values against PDE9A (<b>6c</b>, 14 nM; <b>6f</b>, 17 nM) together with the considerable inhibition against BuChE (IC₅₀, <b>6c</b>, 3.3 μM; <b>6f</b>, 0.97 μM). Their inhibitory potencies against BuChE were even higher than the anti-AD drug rivastigmine. It is worthy mentioning that both showed moderate selectivity for BuChE over acetylcholinesterase (AChE). Molecular docking studies revealed their binding patterns and explained the influence of configuration and substitutions on the inhibition of PDE9A and BuChE. Furthermore, compounds <b>6c</b> and <b>6f</b> exhibited negligible toxicity, which made them suitable for the further study of AD <i>in vivo</i>