Linker Module

Antibody-drug conjugates (ADCs) combine the cytotoxic potency of a drug with the target specificity of an antibody. In the development of ADCs, the design of suitable linkers is crucial for the stable and efficient delivery of the cytotoxic drug to the target cells. Thus, choosing the right antibody-drug linkage often plays a key role in optimizing the therapeutic efficacy and tolerability of ADCs. An ideal linker should possess exceeding stability in systemic circulation, and readily releases the cytotoxic payload in its active form upon reaching the target site.

Linker design for ADCs is highly dependent on the biolology of the target cells, the characteristics of the antibody, and the structure of the drug molecule. Selection of the optimal linker is achieved through efficacy and toxicity assessments for each individual ADC construct. With years of experience, Creative Biolabs provides professional, state-of-the-art ADC linker design and drug-linker complex synthesis services for our customers. Scientists at Creative Biolabs are experts at organic full synthesis and generating the most suitable linker for your ADCs.

Linkers dictate the mechnasims for drug release in an ADC and they are generally categorized as cleavable and non-cleavalbe linkers. Cleavable linkers respond to physiological stimuli inside the cell, such as acidic or reductive environment, as well as certain lysosomal enzyms. Noncleavable linkers rely on lysosomal proteolytic degradation of the antibody to release the drug molecule.

Cleavable linkers

Cleavable linkers rely on the inherent properties of a cell’s cytoplasmic compartments for selective release of the cytotoxic drug. Such linkers mainly include chemically cleavable linkers that respond to low pH (acid-labile linkers) or reducing environment (disulfide linkers), and enzymatically cleavable linkers that are susceptible to the action of certain lysosomal enzymes (peptide linkers or β-glucuronide linkers).

• pH-sensitive (acid-labile) linkers

Stable under neutral pH conditions during systemic circulation, acid-labile linkers are designed to facilitate release of the drug through hydrolysis in acidic environments once trafficked into the cell’s endosomes (pH 5.0–6.5) or lysosomes (pH 4.5–5.0).

• Disulfide linkers

Breaking of disulfide bonds occurs via reduction. High concentrations of glutathione in the cytosol provides the reducing environment that promotes cleavage of disulfide linkers. Stable at physiological pH in circulation, where concentration of free sulfhydryls is significatnly lower compared to that in the cytosol, disulfide linkers allow selective drug release upon internalization of the ADC.

Cleavable linkers, such as disulfides and hydrazones, were used in the early generations of ADCs. However, they showed limited serum stability and short half-lives in systemic circulation. To overcome these issues, innovative next-generation linkers such as enzymatic cleavable linkers are developed and adopted for the application in ADC to achieve enhavced serum stability and reduced off-target toxicity.

• Protease-sensitive (peptide) linkers

Small peptide sequences such as Val-Cit and Phe-Lys have been developed as linkers for ADCs. These bi-peptide linkers show good stability in serum, yet can be recognized and rapidly hydrolyzed by certain lysosomal proteases, such as cathepsin B, following internalization.

• β-glucuronide linkers

β-glucuronide linkers can be readily cleaved by the abundant lysosomal enzyme β-glucuronidase, facilitating facile and selective release of the active drug.

Non-cleavable linkers

Complete lysosomal proteolysis of the antibody following internalization of the ADC is required for suscessful release of the cytotoxic drug using non-cleavable linkers, which retain the linker and the amino acid that served as the attachment site on the antibody. Compared to cleavable linkers, noncleavable linkers have superior stability within circulation, yet they are also more dependent on efficient internalization and intracellular degradation of the antibody for effective drug activation. As a result, ADCs with noncleavable linkers are restricted to the antigen-positive tumor cells.

With our well-established “DrugLnk” organic synthesis platform, the experienced scientists here at Creative Biolabs offer world-leading custom linker design and drug-linker complex synthesis services to meet your specific demands for ADC developments. Our customarily tailored services and high quality products will contribute to your projects. In the meantime, we also provide other services for the benefit of ADC development. Moreover, Creative Biolabs also provides a variety of ADC linker products and Drug-linker complex products. Please feel free to contact us for more information and a detailed quote.

References：

1. Nolting, B.; et al. Linker technologies for antibody-drug conjugates. Methods. Mol. Biol. 2013, 1045: 71-100.
2. Jain, N.; et al. Current ADC linker chemistry. Pharm. Res. 2015, 32: 3526-3540.