Sulfonylurea Bioisosteres of Sulfonic Acid in JFD 00458-Based ST6GAL1 Inhibitors Enhance Membrane Permeability

Sialyltransferases catalyze the transfer of sialic acid to glycoconjugates within the Golgi apparatus, generating cell-surface sialoglycans that regulate cell-cell and immune communication. This process can be modulated by inhibiting α-2,6-sialyltransferase 1 (ST6GAL1), one of the most important sialyltransferases. Most β-galactoside ST6GAL1 inhibitors are polar CMP-Neu5Ac derivatives with limited membrane permeability, a limitation shared by the noncarbohydrate inhibitor JFD 00458. Here, we designed JFD 00458 analogs to improve permeability while maintaining ST6GAL1 inhibition and direct target engagement. Incorporation of sulfonic-acid bioisosteres, particularly sulfonylureas, improved passive permeability, as measured by parallel artificial membrane permeability assay, with the best analog reaching log(P_app [cm/s]) = -4.3. Potency was also improved in an expanded series of sulfonylureas, with the best inhibitor displaying an IC₅₀ of 1.9 µM. Microscale thermophoresis confirmed direct ST6GAL1 binding, with similar apparent K_d values of approximately 22-25 µM for JFD 00458 and the optimized sulfonylureas. Saturation transfer difference NMR identified the substituted phenoxy phenyl core as a common ST6GAL1-contacting epitope, while closer contacts from the sulfonylurea-linked aromatic substituents in 6d and 6e were associated with the highest inhibitory potency across the series. These results establish sulfonylureas as membrane-permeable sulfonic-acid bioisosteres and provide a structure-activity framework for further ST6GAL1 inhibitor optimization.