ADC Linkers

With the success of the marketed ADC drugs, ADCETRIS and Kadcyla, the list of ADCs that are undergoing clinical trials has begun to grow. There are currently 40 ADCs in different phases of the clinical trial. Of the 40 ADCs being tested, there are 34 provided structures; 24 of these ADCs are linked to thiol groups of cysteine residue on an mAb while the remaining 10 structures are linked to a lysine. A single mAb may contain multiple cysteine or lysine residue which may cause varied conjugations.

To prevent the production of heterogeneous ADCs, there is more research being done to improve the site specificity of the conjugation. This includes altering the mAb to introduce discrete, available cysteine or non-natural amino-acid groups with orthogonally-reactive functional groups such as aldehyde, ketone, azido or alkynyl. Site-specificity improves the homogeneity of ADCs and allows for reactions other than thiol or amine conjugations to be used. This broadens the diversity of linkers that can be used to enhance linker design in future generations of ADCs.

There are two main categories of ADC linkers, cleavable and non-cleavable. The cleavable linkers use inherent properties of tumor cells for selective release of cytotoxins from the ADCs.
There are three commonly used mechanisms which include protease-sensitivity, pH-sensitivity and glutathione-sensitivity. Protease-sensitive cleavable linkers use the dominant proteases found in tumor cell lysosomes for recognition and cleavage of a specific peptide sequence in the linker. A commonly used ADC linker makes use of a valine-citrulline (vc) dipeptide, first discovered by Dubowchik et al., as an intracellular cleavage mechanism by cathepsin B. Acid-sensitive linkers use a lower pH in the endosomal (pH = 5-6) and lysosomal (pH = 4.8) compartments, in contrast to cytosol (pH = 7.4) to trigger hydrolysis of acid labile groups within a linker such as hydrazone. Glutathione-sensitive linkers use higher concentrations of intracellular glutathione than in the plasma. This causes disulfide bridges to release the cytotoxin upon reduction by glutathione. Contrarily, non-cleavable linkers degrade only when the anti-body degrades. Once the antibody degrades, the linker carries amino acids from the antibody.

One of the common ADCs being used is VC-MMAE, also known as MC-VC-PABC-MMAE. This ADC makes up 13 of the 18 disclosed auristatin structures as seen below in Table 1.

Table 1: Clinical ADCs Using VC-MMAE  
Common nameOriginatorSpecificity target nameTargeted disease
CDX-011Seattle Geneticsanti-GPNMBbreast cancer
Seattle Geneticsanti-PSMAprostate
RG-7596Seattle Geneticsanti-CD79bNHL
RG-7593Seattle Geneticsanti-CD22NHL
RG-7599Seattle Geneticsanti-NaPi2bovarian cancer
SGN-LIV1ASeattle Geneticsanti-Liv1LIV1+ breast cancer
ASG-22MESeattle GeneticsNectin-4solid tumors
ASG-15MESeattle GeneticsSLTRK6bladder cancer
RG-7450Seattle Geneticsanti-STEAP1prostate
RG-7458Seattle Geneticsanti-MUC16ovarian
AGS67EAgensysanti-CD37NHL, CLL, AML
HuMax-TFSeattle Geneticsanti-TFsolid tumors
MLN-0264Seattle Geneticsanti-GCCgastrointestinal cancer


The linker payload, MC-VC-PABS-MMAE, is conjugated to cysteine of the antibody, producing an ADC with a DAR of 4. The structure of the MC-VC-PABC-MMAE was optimized for the use of a treatment for Hodgkin lymphoma in drugs such as ADCETRIS. The structure of the MC-VC-PABS-MMAE ADC is displayed in Figure 1a-c. The mc spacer provides room so that the vc group can be recognized by Cathepsin B and proceed with the cleavage process.


BroadPharm offers a wide variety of ADC linkers to further empower the advancement of ADC drug development. These products include our high purity PEG linkers, Val-Cit dipeptides, and various payloads.

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