Influence of <i>N</i> ‐Functionalization of Amiridine on the Biological Activity of Its Conjugates as Multitarget Agents for Potential Treatment of Alzheimer's Disease

The goal of this mechanistic study was to elucidate the influence of N ‐functionalization of amiridine on the biological properties of its conjugates. We synthesized conjugates of amiridine and salicylimine/amine 9, 10 with linkers containing an N ‐acyl group at the amiridine fragment as analogs of our previously obtained hybrids 1, 2 containing an N ‐aminohexamethylene spacer. A comparative study of selected biological activities of N ‐acylalkylene conjugates 9, 10 with conjugates 1, 2 showed substantial decreases in anti‐butyrylcholinesterase (BChE) activity (35‐fold for the imine and 8‐fold for the amine), loss of anti‐acetylcholinesterase (AChE) activity, and disappearance of the ability to block amyloid beta (1–42) (Aβ 42 ) self‐aggregation. Similar effects were observed for the model compound N ‐hexylamiridine 3 and its N ‐acyl derivative 11 . However, N ‐acyl functionalization at the amiridine pharmacophore did not reduce the ability of the conjugates to block AChE‐induced β‐amyloid aggregation and only slightly decreased their antiradical activity, maintaining the higher activity of amines compared to imines. These effects were consistent with results from quantum mechanical calculations and molecular docking, showing that N ‐acylation of amiridine decreased the proton affinity of its endocyclic N‐atom. Consequently, the amiridine pharmacophore was essentially unprotonated, thereby diminishing the anticholinesterase activity of conjugates 9 and 10 and their ability to block Aβ 42 self‐aggregation.