Single-chain antibodies possess compact molecular size, rapid tissue penetration and favourable tumour-targeting capacity, making them promising building blocks for next-generation antibody-drug conjugates. Nevertheless, practical deployment has long been hindered by multiple conjugation-related bottlenecks. Traditional random chemical modification disrupts antigen-binding activity, generates highly heterogeneous drug-antibody ratio (DAR) mixtures, and triggers severe product aggregation. Many enzymatic site-specific strategies suffer from low catalytic turnover, steric hindrance between bulky cytotoxic payloads and compact single-chain antibody frameworks, undesired reverse transpeptidation reactions, and troublesome side-product formation. Triglycine, acting as a core nucleophilic recognition motif and flexible spacer, addresses these persistent obstacles and drives major progress in single-chain antibody-based drug conjugation research.
As the optimal N-terminal acceptor substrate for sortase‑A‑mediated transpeptidation, triglycine contains no bulky amino-acid side‑chains. Its high conformational flexibility effectively relieves steric repulsion between large linker-payload moieties and the compact single-chain antibody tertiary structure. When integrated onto linker-payload constructs, triglycine enables efficient enzyme-substrate docking, markedly elevating ligation efficiency even for small single-chain antibody scaffolds that lack spacious Fc domains. It restricts covalent linkage exclusively to the genetically defined C-terminal site of single-chain antibodies, eliminating off-target modification across variable-region residues that would otherwise impair target-binding affinity. The resulting conjugates exhibit highly homogeneous DAR distribution, greatly simplifying downstream purification and analytical characterisation workflows.
Beyond improved coupling efficiency and positional precision, triglycine‑assisted conjugation minimises harmful side reactions. Devoid of reactive side‑chain functional groups, triglycine avoids non-specific cross-linking, aberrant cyclisation and premature payload cleavage frequently observed with peptide sequences bearing charged or aromatic residues. After enzymatic ligation, the formed peptide-bond linkage demonstrates excellent plasma stability, resisting non-specific proteolytic degradation during systemic circulation. It prevents premature payload shedding which would otherwise produce severe off-target toxic effects. In tumour-cell lysosomes, the triglycine-containing segment can cooperate with lysosomal proteases to trigger controlled payload liberation, balancing circulating stability and intracellular drug-release performance for single-chain antibody conjugates.
Triglycine-based site-specific conjugation brings tangible manufacturing benefits for single-chain antibody ADC development. Compared with non-natural amino-acid incorporation or cysteine-engineering workflows, this approach demands minimal genetic modification to single-chain antibody sequences, lowering recombinant expression burden and reducing immunogenic risks introduced by extra foreign peptide tags. The aqueous-phase enzymatic reaction proceeds under mild physiological conditions, preserving the structural integrity of aggregation-prone single-chain antibody molecules. Reaction by-products are well-defined and easily removed, shortening overall production cycles and improving process reproducibility for pre-clinical and scalable preparation.
Even so, technical constraints remain to be carefully managed. Sortase‑A catalysis is inherently reversible; excess triglycine-linked payload substrate or timely removal of sortase enzyme after reaction is required to suppress reverse cleavage. Over-extended triglycine repeats may introduce unexpected protease susceptibility in plasma. Researchers need to fine-tune triglycine position, spacer length and enzyme-reaction parameters to optimise the balance between conjugation yield, serum stability and payload-release kinetics.
Triglycine resolves multiple historic pain-points of single-chain antibody drug conjugation, including steric hindrance-driven low yield, heterogeneous DAR, antigen‑binding impairment and complicated processing requirements. By serving as both sortase‑recognition element and flexible hydrophilic spacer, it enables high-fidelity site-specific construction of single-chain antibody conjugates with well-defined quality attributes. It provides a versatile, enzymatically-driven technical platform for developing small-format targeted therapeutics for tumour therapy.
