Structure–Activity Relationships of Neplanocin A Analogues as S‑Adenosylhomocysteine Hydrolase Inhibitors and Their Antiviral and Antitumor Activities

On the basis of the potent inhibitory activity of neplanocin A (1) against S-adenosylhomocysteine (AdoHcy) hydrolase, we analyzed the comprehensive structure–activity relationships by modifying the adenine and carbasugar moiety of 1 to find the pharmacophore in the active site of the enzyme. The introduction of 7-deazaadenine instead of adenine eliminated the inhibitory activity against the AdoHcy hydrolase, while 3-deazaadenine maintained the inhibitory activity of the enzyme, indicating that N-7 is essential for its role as a hydrogen bonding acceptor. The substitution of hydrogen at the 6′-position with fluorine increased the inhibitory activity of the enzyme. The one-carbon homologation at the 5′-position generally decreased the inhibitory activity of the enzyme, indicating that steric repulsion exists. A molecular docking study also supported these experimental data. In this study, 6′-fluoroneplanocin A (2) was the most potent inhibitor of AdoHcy hydrolase (IC₅₀ = 0.24 μM). It showed a potent anti-VSV activity (EC₅₀ = 0.43 μM) and potent anticancer activity in all the human tumor cell lines tested.