Solid-Phase Synthesis and Insights into Structure−Activity Relationships of Safinamide Analogues as Potent and Selective Inhibitors of Type B Monoamine Oxidase

Safinamide, (S)-N²-{4-[(3-fluorobenzyl)oxy]benzyl}alaninamide methanesulfonate, which is in phase III clinical trials as an anti-Parkinson drug, and a library of alkanamidic analogues were prepared through an expeditious solid-phase synthesis and evaluated for their monoamine oxidase B (MAO-B) and monoamine oxidase A (MAO-A) inhibitory activity and selectivity. (S)-3-Chlorobenzyloxyalaninamide (8) and (S)-3-chlorobenzyloxyserinamide (13) derivatives proved to be more potent MAO-B inhibitors than safinamide (IC₅₀ = 33 and 43 nM, respectively, vs 98 nM) but with a lower MAO-B selectivity (SI = 3455 and 1967, respectively, vs 5918). The highest MAO-B inhibitory potency (IC₅₀ = 17 nM) and a good selectivity (SI = 2941) were displayed by (R)-21, a tetrahydroisoquinoline analogue of safinamide. Structure−affinity relationships and docking simulations pointed out strong negative steric effects of α-aminoamide side chains and para substituents of the benzyloxy groups and favorable hydrophobic interactions of meta substituents. The significantly diverse MAO-B affinities of a number of R and S α-aminoamide enantiomers, including the two rigid analogues (21) of safinamide, indicated likely enantioselective interactions at the enzymatic binding sites.