Antibody-drug conjugates (ADCs) are a new class of biotherapeutics used in the treatment of tumors, infectious diseases, and immune disorders. ADCs are formed by linking a monoclonal antibody targeting a specific antigen with a small molecule cytotoxic drug via a linker. They combine the potent cytotoxic effects of traditional small molecule chemotherapy with the tumor-targeting specificity of antibody drugs.

• ▶ Antibody: Targets specific antigens on the surface of cancer cells, which are overexpressed compared to normal cells or absent/low in normal tissues. The antibody should have minimal nonspecific binding to other tissue cells.

• ▶ Linker: Allows covalent attachment of the cytotoxic agent to the antibody. It acts as a bridge in the ADC structure, designed to be either cleavable or non-cleavable. The linker must be carefully designed for stability to prevent premature release in physiological conditions and for release at specific locations such as inside tumor cells.

• ▶ Cytotoxic Drug: A highly potent cytotoxic agent with low immunogenicity. It should also possess an active group for attachment to the linker and have a well-defined mechanism of action.

Figure 1. Structure of ADC [1]

As the bridge linking antibodies and cytotoxic drugs, the linker significantly impacts the stability and payload release profile of ADCs, thereby influencing drug efficacy and safety. Depending on their release mechanisms, linkers can be classified into two types: "cleavable" and "non-cleavable". Cleavable linkers contain chemical triggers that release the payload upon cleavage, offering high flexibility. The released payload can diffuse into surrounding cells, exerting bystander effects. Non-cleavable linkers, lacking chemical triggers, cannot induce bystander effects and are primarily used in blood cancers and tumors with high antigen expression. Additionally, they exhibit reduced off-target toxicity but may contribute to drug resistance.

Ideal linkers are stable in the circulation, which prevents premature payload release and systemic toxicities, but then rapidly cleaved once internalised in malignant cells, promoting efficient cell death. Different types of linkers have their respective advantages and disadvantages. In clinical-stage ADC drugs, the most commonly used linker type is the cleavable linker, with a minority utilizing non-cleavable linkers.

PEG (polyethylene glycol) is one of the most widely used linkers in targeted therapy, offering the following advantages:

• ▶ Modulation of physicochemical properties, such as balancing the hydrophobicity of small molecule toxins, enhancing solubility, increasing Drug-to-Antibody Ratio (DAR), and reducing ADC aggregation.

• ▶ Improvement of pharmacokinetic properties, including influencing the release efficiency of small molecule toxins, thereby affecting therapeutic efficacy.

• ▶ Altering ADC clearance rates, increasing half-life, and modifying biodistribution.

PEG linkers thus play a crucial role in enhancing the efficacy, safety, and pharmacological properties of ADCs in targeted therapy.

Since 2019, ADC (Antibody-drug conjugate) drugs have experienced accelerated approvals, with an increasing use of PEG (polyethylene glycol) derivatives as linkers. Currently, there are 15 ADC drugs approved globally. Among them, Trodelvy® (sacituzumab govitecan) and Zynlonta® (loncastuximab tesirine) utilize PEG derivatives as linkers.

Table 1. Approved ADCs and 2023 sales

Trodelvy has a unique pH-sensitive, hydrolyzable linker (CL2A) and a topoisomerase inhibitor payload that could deliver powerful activity against Trop-2–expressing cell, with a high DAR of 7.6 and a bystander killing effect. Trodelvy® utilizes a hydrolyzable CL2A linker, which conjugates to the antibody through cysteine residues and to the payload (SN-38) via a carbonate bond with a p-aminobenzyl carbonate (PABC) spacer, forming govitecan (US 8420086). The structure is shown in Figure 2, with the linker in blue and the payload in red.

Research indicates that the PEG chain can enhance the solubility of the linker. Introducing a PABC spacer significantly improves the serum stability of the ADC. Additionally, a lysine residue is incorporated at the N-terminus of the PABC spacer to resist enzymatic cleavage by cathepsin B. In acidic environments, the carbonate bond cleaves, releasing the payload SN-38 (the cleavage site is indicated by arrows in the diagram).

Figure 2. Structure of Trodelvy [2]

Zynlonta® uses a valine-alanine (Val-Ala) dipeptide linker, which connects at the C-terminus to a PABC spacer and at the N-terminus to a maleimide-propionyl (MP) group and an eight-unit polyethylene glycol (PEG8) chain. This forms the MP-PEG8-VA-PABC linker, which, when conjugated to the payload (SG3199), creates tesirine (US 9889207) (Figure 3). Under the action of cathepsin B, the amide bond at the C-terminus of the alanine residue is cleaved, and the PABC spacer undergoes self-degradation, releasing the payload (cleavage site indicated by arrows in the diagram).

Figure 3. Structure of Zynlonta [2]

Using various bifunctional PEG linkers allows for the simple and rapid formation of different types of ADC linkers with diverse structural combinations, enabling precise site-specific conjugation. Additionally, multi-arm PEG derivatives can increase drug loading capacity and facilitate the construction of ADC molecules carrying multiple different toxins. This supports the development of next-generation ADC drugs.

Huateng Pharma is dedicated to being your most reliable partner to provide high-quality PEG linkers to promote the progress of your ADC R&D. PEG Linker is one of the most widely used linkers  in ADCs, which can increase the water solubility, improve the DAR value and increase the cycle half-life of ADCs.

References:
[1] A comprehensive review of key factors affecting the efficacy of antibody drug conjugate. Biomed Pharmacother. 2023 May;161:114408.

[2] Chia CSB. A Patent Review on FDA-Approved Antibody-Drug Conjugates, Their Linkers and Drug Payloads. ChemMedChem. 2022 Jun 3;17(11):e202200032. doi: 10.1002/cmdc.202200032. Epub 2022 Apr 5. PMID: 35384350.

Related Articles:
A Comprehensive Guide to ADC Payload Classses  
Summary of ADCs in Clinical Phase 3  
Approved Antibody–Drug Conjugates (ADCs) and In Clinical Trials  
ADC Linker - Development and Challenges