An α-mannose-6-phosphonate (Man6Pn) ligand equipped with an azide-functionalized linker through a hydrophilic triethylene glycol (PEG3) spacer. The terminal azide can be covalently coupled to alkyne partners via copper-catalyzed azide-alkyne cycloaddition (CuAAC), or to dibenzocyclooctyne (DBCO) and related strained alkynes using copper-free click chemistry, forming stable 1,2,3-triazole linkages. This compound enables the construction of lysosome-targeted therapeutics and research tools that exploit the mannose-6-phosphate receptor (M6PR), including applications in enzyme replacement therapy (ERT) for lysosomal storage disorders and in lysosome-targeting chimera (LYTAC) approaches for targeted protein degradation. The phosphonate group serves as a hydrolytically stable, non-cleavable mimic of the natural mannose-6-phosphate (Man6P) ligand, providing resistance to phosphatase-mediated dephosphorylation while preserving M6PR recognition. Mannose-6-phosphonate (M6Pn) ligands are less sensitive to phosphatase degradation than natural Mannose-6-phosphate (M6P) ligands and may offer improved in vivo stability for LYTACs therapeutic applications.