A properly selected linker helps the antibody to selectively deliver the cytotoxic drug to tumor cells and accurately releases the cytotoxic drug at tumor sites. In addition to conjugation, the linkers play critical roles in ADCs’ stability during the preparation, storage and ADC systemic circulation. The design of linkers for ADCs is challenging as it has to meet stringent criteria of extracellular stability and intracellular release. Intracellular acid and reducing environment, cathepsin enzymatic hydrolysis environment can catalyze or activate the cleavage and release of the payload.
Nowadays, ADC linkers developed rapidly, it's covers DBCO, tetrazine, TCO, BCN and cycloprpene etc., biotin linkers, PEG linkers, peptide linkers, glucuronide linkers, photocleavable linkers, dye probe linkers, etc.
Being non-toxic and non-immunogenic, PEGs are very suitable for biological applications, they can be added to media and attached to surfaces and conjugated to molecules without interfering with cellular functions or target immunogenicities.
The useful characteristics of PEG Linkers have been increasingly recognized in bioconjugation, biolabeling and ADC research and development. This is, in part, due to their water solubility, lack of toxicity and well-defined chain lengths.
PEG-linkers are particularly attractive as a linker for conjugation. Water solubility, lack of toxicity; low immunogenicity and well-defined chain lengths and molecular weights are specific characteristics of PEG moieties relevant to pharmaceutical applications.
In an antibody-drug conjugate, the link between the antibody and the cytotoxic agent plays a critical role in the development of the ADC. Further, the nature of the linker can significantly affect the potency, selectivity, and the pharmacokinetics of an antibody-drug conjugate. A common feature of numerous linkers used to prepare antibody-drug conjugates is their hydrophobicity – or their inability to dissolve in water. In most cases, the average drug-to-antibody ratio or the number of drug molecules per antibody does not exceed 3-4 drug molecules per antibody molecule. Attempts to increase the drug-to-antibody ratio have often failed. This is especially the case when both the cytotoxic agent and the linker are hydrophobic. Other reasons may be caused due to aggregation of the antibody-drug conjugates, the loss of affinity for the target antigen, and/or rapid clearance from circulation.
Many of the problems seen with hydrophobic linkers can be overcome by sulfonate- or PEG-containing hydrophilic (aqueous-soluble) linkers which are characterized by high solubility in both organic and aqueous solutions. The attachment of PEG-based linkers to proteins and other biomolecules decreases aggregation and increases solubility.
Furthermore, branched or multi-arm PEG linkers enable conjugation of hydrophobic organic molecule drugs, such as a maytansinoid, at a higher ration of drug-to-antibody ratio than hydrophobic SPDB and SMCC linkers used earlier without triggering aggregation or loss of affinity of the resulting conjugate. This, in turn, results in the ability to deliver a higher concentration of cytotoxic drugs into the target cells per antibody binding event.
Related: Common Structures and Applications of PEGs
-N-BIS(PEG3-AMINE).gif)