Structure-guided design of C3-branched swainsonine as potent and selective human Golgi α-mannosidase (GMII) inhibitor

The human Golgi α-mannosidase, hGMII, removes two mannose residues from GlcNAc-Man₅GlcNAc₂ to produce GlcNAcMan₃GlcNAc₂, the precursor of all complex N-glycans including tumour-associated ones. The natural product GMII inhibitor, swainsonine, blocks processing of cancer-associated N-glycans, but also inhibits the four other human α-mannosidases, rendering it unsuitable for clinical use. Our previous structure-guided screening of iminosugar pyrrolidine and piperidine fragments identified two micromolar hGMII inhibitors occupying the enzyme active pockets in adjacent, partially overlapping sites. Here we demonstrate that fusing these fragments yields swainsonine-configured indolizidines featuring a C3-substituent that act as selective hGMII inhibitors. Our structure-guided GMII-selective inhibitor design complements a recent combinatorial approach that yielded similarly configured and substituted indolizidine GMII inhibitors, and holds promise for the potential future development of anti-cancer agents targeting Golgi N-glycan processing.