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Release date:2019/12/19 16:30:42
Antibody-drug conjugates (ADCs) are now established as the main therapies for clinical cancer treatment. The principle of ADCs is based on monoclonal antibodies, which specifically direct toxic agents to diseased tissues while minimizing peripheral damage to healthy tissues.

The
development of ADC involves a critical understanding of target antigen selection, conjugate internalization by tumor cells, drug potency, and stability of the linker between drug and antibody.

One of the
main challenges in developing safe and effective antibody-drug conjugates is the generation of suitable chemical linkers between cytotoxic drugs and monoclonal antibodies. A stable ADC linker ensures that less cytotoxic payloads fall off before reaching tumor cells, increasing safety and limiting dose.

In antibody-drug conjugates currently approved by the US FDA, two common ways of linking cytotoxic anticancer agent payloads include cleavable and non-cleavable linkers.
cleavable-and-non-cleavable-linkers
Image resource: royalsocietypublishing.org

Cleavable linkers play a key role in the success of antibody-drug conjugates. They use the inherent properties of tumor cells to selectively release cytotoxins from ADCs. They are stable in the blood circulation for a long period of time. There are three commonly used mechanisms: 1) protease sensitivity, 2) pH sensitivity, and 3) glutathione sensitivity.

The protease sensitivity strategy utilizes predominant proteases found in lysosomes of tumor cells to recognize and cleave specific peptide sequences in the linker. Dubowchik and Firestone et al. pioneered the discovery of the valine-citrulline (VC) dipeptide as an intracellular cleavage mechanism by cathepsin B.

The acid-sensitive strategy is to use a lower pH of the endosome (pH = 5-6) and lysosome (pH = 4.8) compartments compared to the cytoplasm (pH = 7.4) to trigger the hydrolysis of an acid-labile group within the linker, such as a hydrazone.

The third release strategy utilizes higher intracellular glutathione concentrations than in plasma. Therefore, the disulfide-containing linker releases cytotoxins after reduction by glutathione.

Non-cleavable linker has no obvious drug release mechanism, and the ADC prepared by this strategy relies on the complete lysosomal proteolytic degradation of the antibody that releases the antibody-drug after internalization. Through this degradation, the non-cleavable linker carrying the drug will also be coupled to the conjugated amino acid of the antibody. Therefore, ADCs with non-cleavable linkers are more dependent on the biology of the target cells than cleavable linkers.

One of the advantages of non-cleavable linkers over cleavable linkers is their increased plasma stability, which can improve the therapeutic index. Studies have shown that non-cleavable linked ADCs generally perform better than their cleavable counterparts in vivo. 

 
Linker Strategy Advantages Disadvantages
Cleavable Linker Disulfide Intracellular release of payload Potential premature cleavage during circulation
Pyrophosphate diester Stability during circulation
Hydrophilicity
Traceless release of payload
Unknown mechanism of lysosomal cleavage
Non-cleavable Linker Stable linker without cleavage mechanism Stability during circulation An amino acid residue attached on the released payload
Cleavable vs non-cleavable linkers
 
Moreover, due to the fact that payload derivatives from non-cleavable ADCs can kill target cells, non-cleavable linkers can potentially provide a larger therapeutic window compared to cleavable linkers. Finally, it is expected to reduce off-target toxicity compared to cleavable linker conjugates because non-cleavable ADCs can provide greater stability and tolerance.

Biochempeg provides GMP standard PEG derivatives and bulk orders via custom synthesis, offering the opportunity to match customers’ special quality requirements.  ADC linkers with molecular weights, branching, and functional groups not listed in our online catalog may be available by custom synthesis.

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