Description

The incorporation of sulfur fluoride exchange (SuFEx) functional groups into protein building blocks is revolutionising protein engineering and therapeutic design by enabling covalent binding of a drug to its target for prolonged activity. However, conventional approaches are challenging, due to limited reactivity and poor site accessibility.

SuFEx-reactive unnatural amino acids—such as fluorosulfate-L-tyrosine (FSY), meta-fluorosulfate-L-tyrosine (mFSY), and fluorosulfonyloxybenzoyl-L-lysine (FSK)—can be site-specifically incorporated into proteins as latent bioreactive building blocks. These aryl fluorosulfate-modified residues remain inert until positioned in close proximity to target nucleophiles (Lys, His, or Tyr) on interacting proteins, enabling precise SuFEx-mediated covalent linkages and increasing tumour retention for drugs.

Citation: Gao et al., 2025

Most biologics bind their targets reversibly. The drug eventually dissociates, requiring repeated dosing. Covalent binding, where the drug forms a permanent chemical bond with its target, can extend duration of action substantially. The challenge is controlling where and when covalent bond formation occurs.

SuFEx-reactive non-canonical amino acids (fluorosulfate-L-tyrosine, meta-fluorosulfate-L-tyrosine, fluorosulfonyloxybenzoyl-L-lysine) solve this through proximity-driven reactivity. These ncAAs remain inert in circulation but form covalent bonds specifically when positioned close to nucleophilic residues (lysine, histidine, tyrosine) on the target protein. In a HER2/EGFR bivalent construct, SuFEx-mediated covalent binding increased tumour retention compared to reversible binding (Gao et al., 2025).

Prolonged target occupancy can in principle reduce dosing frequency and improve efficacy, particularly for oncology targets where sustained receptor blockade matters. This covalent biologic approach combines the selectivity of protein therapeutics with the irreversible pharmacology traditionally associated with small-molecule drugs.

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