Exploring Unconventional SAM Analogues To Build Cell-Potent Bisubstrate Inhibitors for Nicotinamide...

Nicotinamide N-methyltransferase (NNMT) methylates nicotinamide and has been associated with various diseases. Herein, we report the first cell-potent NNMT bisubstrate inhibitor II399, demonstrating a K_i of 5.9 nM in a biochemical assay and a cellular IC₅₀ value of 1.9 μM. The inhibition mechanism and cocrystal structure confirmed II399 engages both the substrate and cofactor binding pockets. Computational modeling and binding data reveal a balancing act between enthalpic and entropic components that lead to II399's low nM binding affinity. Notably, II399 is 1 000-fold more selective for NNMT than closely related methyltransferases. We expect that II399 would serve as a valuable probe to elucidate NNMT biology. Furthermore, this strategy provides the first case of introducing unconventional SAM mimics, which can be adopted to develop cell-potent inhibitors for other SAM-dependent methyltransferases.