– S-Adenosylhomocysteine (SAH) hydrolase is responsible for the reversible hydrolysis of SAH into adenosine and homocysteine. Inhibition of this enzyme leads to the accumulation of SAH within cells, thereby increasing the SAH to S-adenosylmethionine (SAM) ratio and subsequently inhibiting SAM-dependent methyltransferases. (−)-Neplanocin A, a potent and irreversible inhibitor of SAH hydrolase (Ki= 8.39 nM), exhibits significant antitumor activity against mouse leukemia L1210 cells and holds broad-spectrum antiviral properties. Its efficacy notably surpasses that of the reversible inhibitor 3-deazaneplanocin, especially in combating vesicular stomatitis, evidencing a higher potency with ID50 values of 0.07 μg/ml for Neplanocin A versus 0.3 μg/ml for 3-deazaneplanocin.