How to Choose the Right ADC Linker: Cleavability, Conjugation Chemistry, and PEG Design

Update: October 2026

Antibody-drug conjugates (ADCs) are targeted therapeutic agents composed of monoclonal antibodies, linkers, and cytotoxic payloads. The antibody serves as a targeting vehicle, helping deliver the cytotoxic drug to antigen-expressing tumor cells, where the payload can exert its antitumor activity.

The linker is a critical component of ADC design, connecting the antibody to the cytotoxic payload and influencing plasma stability, payload release, bystander effects, and the therapeutic window. Here in this article, we will brief illustrate how to choose the right ADC linker, including choose the ADC linker types, its conjugation chemistry and the use of hydrophilic PEG linkers in ADC linker.

What Is An Ideal ADC Linker?

An ideal ADC linker should help the antibody deliver its payload to target cells while supporting appropriate payload release or intracellular processing. Key design considerations include:

● Balance between Plasma Stability & Payload Release: The linker should remain sufficiently stable in circulation to limit premature payload release while enabling effective payload release or processing at the intended site of action.
● Synthetic & Conjugation Feasibility: The linker should have a well-defined chemical structure that is readily synthesizable and compatible with selected conjugation strategy,, such as conventional (Lys/Cys) or site-specific conjugation strategies.
● Controlled Catabolite Properties: The physicochemical properties and biological activity of the resulting payload-containing catabolites can influence intracellular retention, membrane permeability, and the potential for a bystander effect.
● Impurity and Degradation Control: Free payload, linker-related impurities, and degradation products should be controlled during conjugation, purification, and storage to support product quality and Chemistry, Manufacturing, and Controls (CMC) requirements.

ADC Linker Types: Cleavable vs. Non-Cleavable Linkers

ADC linkers are broadly categorized into two major types: cleavable and non-cleavable. The choice of linker profoundly dictates both the site and mechanism of payload activation, directly impacting efficacy and safety profiles.

Cleavable ADC Linkers: Triggered Payload Release

Cleavable linkers are designed to respond to specific biological or chemical conditions, enabling payload release through bond cleavage. Depending on the linker design, cleavage may occur in intracellular compartments, such as endosomes or lysosomes, or under other relevant biological conditions. Three commonly studied mechanisms are:

● Protease-sensitive linkers: Peptide sequences, such as Val-Cit, can be cleaved by susceptible proteases, often in lysosomal compartments.
● Acid-sensitive linkers: Acid-labile bonds, such as hydrazones, can undergo hydrolysis under acidic conditions.
● Reduction-sensitive linkers: Disulfide bonds can be reduced under suitable intracellular redox conditions, including those associated with glutathione.

Non-Cleavable ADC Linkers: Release Through Intracellular Antibody Degradation

Non-cleavable linkers lack a specifically designed cleavage trigger. Instead, these ADCs rely on complete lysosomal antibody degradation following endocytosis to release the payload, accompanied by the detached linker fragment.

For example, ado-trastuzumab emtansine (Kadcyla, T-DM1) uses an SMCC-derived linker that forms a stable thioether linkage. Following intracellular processing, a major active catabolite is lysine–SMCC–DM1, in which the payload remains attached to the linker and an amino acid residue derived from the antibody.

Common non-cleavable chemistries primarily include stable thioether-based linkers, most notably maleimidocaproyl (MC) and SMCC-derived linkers. Non-cleavable linkers are exceptionally well-suited for high-antigen-expression tumors where systemic safety and plasma stability are paramount.

Selection factor	Cleavable linkers	Non-cleavable linkers
Payload release	Released through a designed cleavage mechanism, such as enzymatic, acid-sensitive, or reduction-sensitive cleavage	Usually released as a linker–payload catabolite following intracellular antibody degradation
Common types	Protease-sensitive peptide linkers (e.g., Val-Cit); hydrazone; disulfide	Stable linkages, such as the thioether-based linker used in T-DM1
Main advantage	Enable efficient payload release and, depending on payload properties, a bystander effect	Generally high plasma stability and reduced premature payload release
Key limitation	Premature cleavage or insufficient cleavage at the target site may compromise performance	Activity depends on intracellular processing and the potency of the resulting linker–payload catabolite
Key evaluation	Plasma stability, cleavage kinetics, release efficiency, and species-dependent stability	Plasma stability, antibody internalization and degradation, and catabolite activity
Selection consideration	Match the cleavage mechanism to the payload and biological environment	Consider target antigen expression, internalization, and the activity of the resulting catabolite

Table 1. Cleavable vs. Non-Cleavable Linkers

Key takeaway: Neither type is universally superior; selection depends on the antibody, payload, target biology, and desired ADC properties.

Learn More Details About: What Are ADC Linkers: Cleavable vs. Non-Cleavable Linkers

Match Conjugation Chemistry to the Antibody and Payload

Selecting the appropriate conjugation chemistry is essential for achieving consistent ADC quality, reliable drug-to-antibody ratio (DAR), and favorable stability and biological activity. The choice depends on the antibody format, available reactive groups, payload properties, and desired conjugation profile. Different conjugation strategies offer distinct advantages and limitations in reaction selectivity, product heterogeneity, and linker stability.

Lysine/Amine Conjugation via NHS Esters

N-hydroxysuccinimide (NHS) esters are widely used for amine-reactive bioconjugation. They react with primary amines, including those on solvent-accessible lysine residues on the antibody surface, to form stable amide bonds. Because multiple lysine residues may be available for modification, conventional lysine conjugation typically produces a mixture of positional isomers and DAR species.

Cysteine/Thiol Conjugation via Maleimides

Maleimide-based linkers are widely used in ADC development and are featured in several approved ADCs. Maleimides react efficiently with thiol groups to form thioether-linked conjugates under suitable conditions.

In conventional cysteine conjugation, partial reduction of interchain disulfide bonds in an IgG antibody can generate accessible thiol groups for payload attachment. For a typical IgG1 antibody, reduction of its four interchain disulfide bonds can theoretically expose up to eight thiol groups, although the actual number of available thiols depends on the antibody and process conditions.

Bioorthogonal Click Chemistry & Site-Specific Conjugation

Bioorthogonal click chemistry offers an alternative approach to ADC conjugation, particularly when combined with strategies that introduce defined reactive handles at selected antibody sites. For example, azide-functionalized antibodies can react with strained cyclooctyne groups, such as dibenzocyclooctyne (DBCO) or bicyclononyne (BCN), through strain-promoted azide–alkyne cycloaddition (SPAAC).

Reactive handles may be introduced through engineered amino acids, enzymatic modification, or other site-specific labeling approaches.

How PEG Linkers Affect ADC Linker Design?

Hydrophilic polyethylene glycol (PEG) linkers are attractive components in ADC design because they can help mitigate the hydrophobicity of cytotoxic payloads. PEG-containing linker structures have been incorporated into approved ADCs, including Trodelvy® and Zynlonta®, and have been investigated as a strategy to improve aqueous solubility, reduce aggregation, and support favorable pharmacokinetic properties.

Many cytotoxic payloads are highly hydrophobic. When combined with hydrophobic linker components, they can increase the overall hydrophobicity of an ADC, potentially contributing to aggregation, reduced target binding, and accelerated clearance from circulation. These challenges can become more pronounced as the drug-to-antibody ratio (DAR) increases.

Incorporating hydrophilic groups, such as PEG chains or sulfonate-containing moieties, can help offset payload-associated hydrophobicity. Depending on the molecular design, hydrophilic linkers may improve aqueous solubility, reduce aggregation, and enable higher payload loading while maintaining acceptable conjugate stability and biological activity.

Branched or multi-arm PEG linker architectures provide additional design flexibility by allowing multiple payloads to be attached through a defined linker structure. In selected ADC systems, these architectures have supported higher DAR values than conventional linker designs while maintaining favorable physicochemical properties.

Potential Advantages of Hydrophilic PEG Linkers

● Improved aqueous solubility: PEG chains can help counterbalance the hydrophobicity of the conjugates.
● Reduced aggregation: Appropriate hydrophilic modifications may reduce aggregation associated with hydrophobic drug loading.
● Support for higher DAR: In selected designs, improved hydrophilicity can facilitate higher payload loading while maintaining acceptable developability.
● Flexible linker design: PEG molecular weight, chain length, and architecture can be tailored to project-specific requirements.
● Potential pharmacokinetic benefits: Depending on the complete ADC design, hydrophilic linkers may help improve plasma exposure and reduce premature clearance.

Customizable PEG Linker Architectures

Hydrophilic PEG linkers can be designed with different structural and functional features to meet specific ADC research requirements:

● Cleavable or non-cleavable designs: Selected according to the intended payload-release mechanism and intracellular processing requirements.
● Linear, branched, or multi-arm architectures: Designed to accommodate different payload-loading strategies.
● Adjustable PEG chain length and molecular weight: Selected to balance hydrophilicity, steric effects, and overall conjugate properties.
● Multiple payload attachment sites: Branched architectures can accommodate more than one payload per linker, depending on the design.
● Functional-group customization: Reactive groups can be selected to match the intended antibody-conjugation chemistry and payload structure.

Conclusion

ADC design is a coordinated process that requires careful optimization of the target, antibody, linker, and cytotoxic payload. No single linker is ideal for every ADC.

The best linker is not necessarily the newest or most structurally sophisticated. It is the one that best fits the target, therapeutic indication, payload, and development stage while helping maximize the therapeutic window and minimize CMC risks.

PEG Linkers from Biopharma PEG

Biopharma PEG offers a broad selection of PEG linkers for ADC development and bioconjugation research, with functional groups and linker architectures tailored to different conjugation strategies.

● NHS ester PEG linkers for amine conjugation.
● Maleimide PEG linkers for thiol conjugation.
● Azide, DBCO, and BCN PEG linkers for click chemistry applications.
● Linear, branched, and multi-arm PEG linkers, with customizable molecular weights, functional groups, and architectures, depending on product availability and project requirements.

We support both small-quantity orders for early-stage research and larger-quantity supply for scale-up and later-stage development, subject to product availability and project requirements. For product information or custom PEG linker requirements, contact [email protected].

References:
[1] Su Z, Xiao D, Xie F, et al. Antibody–drug conjugates: Recent advances in linker chemistry[J]. Acta Pharmaceutica Sinica B, 2021, 11(12): 3889-3907.
[2] Su D, Zhang D. Linker Design Impacts Antibody-Drug Conjugate Pharmacokinetics and Efficacy via Modulating the Stability and Payload Release Efficiency. Front Pharmacol. 2021 Jun 23;12:687926. doi: 10.3389/fphar.2021.687926. PMID: 34248637; PMCID: PMC8262647.
[3] Linker Design in Antibody-Drug Conjugates: Balancing Stability and Drug Release. Pharmaceutics 2026 Aug 3;18(8):954.
[4] Linker Design Impacts Antibody-DrugConjugate Pharmacokinetics and Efficacy via Modulating the Stability and Payload Release Efficiency. Front Pharmacol. 2021; 12: 687926.
[5] Fu Q, Kong X, Liu Y, Liu M, Huang K, Wang PG, Wu K. The role of hydrophilic linkers in next-generation antibody-drug conjugates. J Control Release. 2026 Jan 6;391:114612. doi: 10.1016/j.jconrel.2026.114612. Epub ahead of print. PMID: 41506379.

Related Articles:
What Are ADC Linkers: Cleavable vs. Non-Cleavable Linkers
Review: Hydrophilic linkers in Antibody-drug Conjugates
What Are PEG Linkers & Their Applications?
How to Choose PEG Linker: Molecular Weight, Structure and Functionality