Introduction: The Critical Role of Enzymatic Cleavage in Antibody–Drug Conjugates

Antibody–drug conjugates (ADCs) represent one of the most promising classes of targeted cancer therapeutics, combining the high specificity of monoclonal antibodies with the potent cytotoxicity of small-molecule drugs. Since the approval of the first ADC, gemtuzumab ozogamicin, in 2000, the field has undergone a rapid evolution, with nine ADCs now on the market and dozens more in clinical trials. A central design challenge in every ADC is the linker that connects the antibody to the payload. This linker must be stable during circulation to prevent systemic toxicity, yet must release the cytotoxic agent efficiently once inside the target tumor cell. Enzymatic cleavage processes have emerged as a highly controlled and versatile strategy for achieving this balance. Recent advances in enzyme engineering, linker chemistry, and process development have dramatically improved the manufacturability, homogeneity, and therapeutic index of ADCs. This article explores the latest innovations in enzymatic cleavage techniques for ADC production, from the basic mechanisms to cutting-edge developments and future directions.

Understanding Enzymatic Cleavage in the Context of ADC Design

Enzymatic cleavage in ADCs relies on the use of specific enzymes to hydrolyze peptide bonds or other linker structures under predetermined conditions. The most common approach involves incorporating a cleavable linker between the antibody and the drug. This linker is designed to be stable in the bloodstream (pH 7.4, low protease activity) but becomes a substrate for intracellular enzymes once the ADC is internalized by the target cell. Typically, the cleavage occurs inside lysosomes or endosomes, where enzymes such as cathepsins, glucuronidases, or matrix metalloproteinases are active. The released drug then diffuses to its intracellular target, often microtubules or DNA, inducing cell death.

The choice of enzyme and linker dictates the release profile, which in turn influences the ADC’s efficacy, toxicity, and pharmacokinetics. Two main types of enzymatic cleavage are used in current ADCs: protease-sensitive linkers (e.g., valine-citrulline, cleaved by cathepsin B) and glycosidase-sensitive linkers (e.g., β-glucuronide, cleaved by β-glucuronidase). More recently, researchers have explored linkers that respond to other tumor-associated enzymes, such as legumain, urokinase plasminogen activator, or fibroblast activation protein, to achieve even greater selectivity.

Key Enzyme Families Used in ADC Cleavage

  • Cysteine cathepsins (e.g., cathepsin B): The workhorses of ADC release. These lysosomal proteases recognize dipeptide sequences such as Val-Cit (valine-citrulline) or Phe-Lys. The Val-Cit linker is used in approved ADCs like brentuximab vedotin (Adcetris) and enfortumab vedotin (Padcev). Despite its widespread use, cathepsin B cleavage can be inefficient in certain tumor types with low cathepsin B expression, motivating the search for alternatives.
  • Glucuronidases: The β-glucuronide linker is cleaved by human β-glucuronidase, a lysosomal glycosidase. This enzyme is abundant in the tumor microenvironment and inside cancer cells, making it a robust cleavage trigger. Trastuzumab deruxtecan (Enhertu) utilizes a tetrapeptide-based linker (GGFG) that is cleaved by lysosomal cathepsins, but newer glucuronide-based linkers have shown promise in preclinical models.
  • Legumain: An asparaginyl endopeptidase overexpressed in many solid tumors. Legumain-sensitive linkers (e.g., Ala-Ala-Asn) provide an alternative cleavage mechanism, particularly for tumors with low cathepsin B activity.
  • Matrix metalloproteinases (MMPs): These extracellular enzymes are upregulated in the tumor stroma. Linkers cleaved by MMP-2 and MMP-9 can be used to release drugs in the tumor microenvironment before internalization, enabling “bystander” killing effects.

Advances in Enzyme Engineering for Site-Specific Conjugation

One of the most transformative developments in ADC manufacturing has been the move from stochastic conjugation (where drugs are attached via native lysine or cysteine residues) to site-specific, enzyme-mediated conjugation. Site-specific conjugation produces homogeneous ADCs with a defined drug-to-antibody ratio (DAR), leading to more predictable pharmacology and fewer batch-to-batch variations. Enzymatic methods enable precise functionalization of the antibody at engineered tags or motifs, avoiding the complex mixtures that result from chemical conjugation.

Transglutaminase-Mediated Conjugation

Microbial transglutaminase (mTG) has become a powerful tool for ADC production. This enzyme catalyzes the formation of an amide bond between the side chain of a glutamine residue and a primary amine. By engineering a specific glutamine tag (e.g., the LLQG motif) into the antibody backbone, mTG can attach a drug-linker payload exclusively at that site. The technology, often called TGase conjugation, has been commercialized and applied to multiple ADC candidates. mTG offers several advantages: high conjugation efficiency, mild reaction conditions (no harsh reagents), and compatibility with a wide range of linker chemistries. Recent research has focused on engineering mTG variants with improved activity and broader substrate tolerance, enabling conjugation at additional sites and with non-natural linkers.

Sortase A Ligation

Sortase A (SrtA), an enzyme from Staphylococcus aureus, recognizes an LPXTG motif and catalyzes transpeptidation with a polyglycine peptide. This “sortagging” approach has been adapted for ADC production, allowing the attachment of drug-linker constructs to the antibody’s C-terminus or at specific internal sites. Sortase-mediated ligation is highly specific and can be performed under physiological conditions. However, the reaction is reversible, requiring excess substrates or engineered SrtA variants (e.g., mutants with improved catalytic efficiency and reduced product inhibition) to drive the reaction forward. Newer sortase variants with enhanced activity and stability are enabling more scalable processes.

Engineered Proteases for Cleavable Linkers

Instead of using native enzymes for conjugate cleavage, researchers are engineering proteases that are specific for non-native linker sequences. This “designer” approach decouples the cleavage trigger from the endogenous enzyme activity, allowing for controlled release even in cells or microenvironments where native enzymes are absent. For example, an engineered variant of human cathepsin B with altered substrate specificity can be used to cleave a synthetic linker that is not recognized by the wild-type enzyme. Such systems could enable “triggered” release by an exogenous enzyme delivered to the tumor site, or by an enzyme-activated prodrug strategy. While still in early research, these advances point toward a future where ADC release is fully programmable.

Innovations in Linker Chemistry for Enzymatic Cleavage

The linker is the heart of an ADC, and its design dictates the stability, release kinetics, and therapeutic index. Recent advances have produced a variety of enzymatically cleavable linkers that address limitations of earlier molecules.

Peptide Linkers Beyond Val-Cit

The Val-Cit dipeptide linker has been the gold standard for many years, but it has drawbacks: it requires relatively high levels of cathepsin B for efficient release, and the PABC (para-aminobenzylcarbamate) self-immolative spacer used with it can be slow. Newer peptide linkers incorporate sequences optimized for rapid cleavage by multiple lysosomal proteases, reducing the dependence on a single enzyme. For instance, the GGFG (glycine-glycine-phenylalanine-glycine) tetrapeptide used in trastuzumab deruxtecan is cleaved by both cathepsin B and other cathepsins, resulting in fast and efficient payload release. Other groups have developed linkers containing unnatural amino acids that resist premature cleavage in circulation while maintaining susceptibility to lysosomal enzymes.

Glucuronide Linkers: An Emerging Platform

β-Glucuronide linkers have gained attention as alternatives to peptide linkers. They are highly hydrophilic, improving ADC solubility and reducing aggregation. Cleavage by β-glucuronidase, an enzyme found in lysosomes and sometimes in the tumor microenvironment, produces a free drug that can diffuse out of the cell, contributing to bystander killing. Glucuronide linkers have been paired with highly potent payloads such as monomethyl auristatin E (MMAE) and duocarmycin. A notable example is the ADC enfortumab vedotin, which uses a glucuronide-based linker (though its primary cleavage is via cathepsin B on the peptide portion; some variants now use glucuronide-only release). Research has shown that glucuronide linkers can achieve >90% drug release within minutes in lysosomal conditions, outperforming some dipeptide linkers.

Self-Immolative and Prodrug-Based Designs

The self-immolative spacer (often a p-aminobenzyl alcohol derivative) amplifies drug release by fragmenting after enzymatic cleavage. Recent innovations include “triple-cleavable” linkers that require two enzymatic steps and one spontaneous fragmentation, providing an extra safety layer against premature release. Another emerging concept is the “enzyme-responsive polymeric linker,” where multiple drug molecules are attached via a single polymeric chain that degrades only in the presence of a specific tumor-associated enzyme. This approach increases the DAR without causing aggregation, a significant challenge for traditional ADCs.

Comparison with Chemical Conjugation: Why Enzymatic Methods Are Gaining Ground

Traditional chemical conjugation relies on the reaction of maleimides with cysteines or activated esters with lysines. While simple and well-established, these methods produce heterogeneous mixtures with variable DARs ranging from 0 to 8. This heterogeneity can lead to suboptimal therapeutic activity, as ADCs with high DAR are cleared faster, while those with low DAR have reduced efficacy. Enzymatic conjugation overcomes these issues by ensuring that each antibody molecule carries a predetermined number of drugs at defined positions. The benefits are clear:

  • Homogeneous DAR: Typically DAR 2 or 4, achieved with >95% consistency.
  • Reduced batch variability: Critical for reproducible manufacturing and regulatory approval.
  • Preserved antibody function: Enzymatic modification can be directed to regions that do not interfere with antigen binding or Fc receptor interactions.
  • Softer reaction conditions: Enzymatic conjugation often occurs at near-neutral pH, room temperature, and in aqueous buffers, minimizing aggregation and denaturation.

However, enzymatic methods are not without challenges. They require engineering of the antibody to incorporate the appropriate tag, which can complicate development and regulatory filings. The enzymes themselves must be produced under GMP conditions, adding cost. Additionally, the efficiency of some enzymatic reactions is lower than that of maleimide chemistry, necessitating process optimization. Despite these hurdles, the advantages in product quality have driven the pharmaceutical industry to invest heavily in enzymatic conjugation platforms.

Manufacturing Scalability and Process Control

The translation of enzymatic cleavage and conjugation from bench to commercial scale demands robust processes. Key considerations include:

  • Enzyme supply: Enzymes like mTG are available in recombinant form at kilogram scale, but for others (e.g., sortase, legumain variants), yield and purity need improvement. Engineered enzymes with higher turnover numbers and thermal stability are being developed to reduce manufacturing costs.
  • Reaction monitoring: Real-time analytical methods (e.g., LC-MS, UPLC, and native mass spectrometry) are used to track conjugation progress and linker cleavage kinetics. Process analytical technology (PAT) tools allow for feedback control, ensuring consistent DAR and minimal byproducts.
  • Purification: After enzymatic cleavage or conjugation, the ADC is typically purified by protein A chromatography, hydrophobic interaction chromatography, or size-exclusion chromatography. Efficient capture of the enzyme (if not immobilized) is necessary to avoid contamination of the final product.
  • Stability: The pH, temperature, and buffer composition during enzymatic processing must be optimized to maintain antibody integrity and linker stability. For example, prolonged exposure to low pH can hydrolyze acid-sensitive linkers.

Several contract development and manufacturing organizations (CDMOs) now offer fully integrated enzymatic ADC manufacturing services, from cell line development to drug product filling, reflecting the growing industrial acceptance of these methods.

Clinical Impact: Case Examples of Enzymatic Cleavage in Approved ADCs

While many approved ADCs still use chemical conjugation (e.g., trastuzumab emtansine uses lysine conjugation), enzymatic cleavage of the linker is a key feature of several marketed products. For instance, brentuximab vedotin (Adcetris) has a protease-cleavable Val-Cit linker attached via a maleimide to a cysteine. Although the conjugation is chemical, the release relies entirely on the enzymatic cleavage of the peptide bond by cathepsin B. The success of this ADC validated the enzymatic cleavage strategy and paved the way for later products. Trastuzumab deruxtecan (Enhertu) uses a tetrapeptide linker (GGFG) that is cleaved by lysosomal cathepsins (not exclusively cathepsin B), leading to fast release of the topoisomerase I inhibitor DXd. The high DAR (about 8) achieved through chemical conjugation, combined with efficient enzymatic release, contributes to its remarkable clinical activity, including in patients with low HER2 expression. Enfortumab vedotin (Padcev) similarly employs a protease-cleavable linker, while polatuzumab vedotin (Polivy) uses a protease-cleavable linker with MMAE. These examples underscore the critical role of enzymatic cleavage in current ADC therapy.

Emerging ADCs with Fully Enzymatic Conjugation

Several ADCs in clinical development use site-specific enzymatic conjugation. For example, ARX788 (an anti-HER2 ADC) uses an unnatural amino acid incorporated via amber suppression technology, enabling site-specific attachment of a cleavable linker. While not strictly enzymatic conjugation, the linker is cleaved by lysosomal enzymes. Other programs, such as those from Ambrx and Sutro Biopharma, leverage engineered enzymes like formylglycine-generating enzyme (FGE) or mTG for precise payload placement. These next-generation ADCs are demonstrating improved pharmacokinetics, lower toxicity, and the ability to combine multiple payloads on a single antibody.

Future Directions: Personalized ADCs and Dual-Payload Systems

The field is moving toward ADCs that can be tailored to individual patient tumors or that can release multiple drugs with different mechanisms. Enzymatic cleavage processes are central to these visions. One exciting direction is the development of dual-payload ADCs that carry two different cytotoxic drugs, each attached to the antibody via a different cleavable linker that responds to distinct enzymatic triggers. For instance, a linker cleavable by cathepsin B would release one drug, while a glucuronide linker would release a second drug upon β-glucuronidase activity. This approach could combat multidrug resistance and provide synergistic killing. Proof-of-concept studies have already demonstrated the feasibility of such constructs.

Another frontier is personalized ADCs in which the linker and enzyme pair are matched to the patient’s tumor enzyme expression profile. Using diagnostics that measure cathepsin B, legumain, or glucuronidase levels in biopsy samples, clinicians could select the optimal ADC for each patient. Enzyme engineering could also produce “activatable” ADCs that are inert until they encounter a tumor-specific enzyme, reducing off-target toxicity. For example, a pro-ADC with a masking peptide that is detached by a tumor-associated protease could become active only at the disease site.

Finally, advances in cell-free protein synthesis and directed evolution will accelerate the discovery of novel enzymes with tailored specificities and stabilities. The integration of artificial intelligence for predicting linker cleavage kinetics and enzyme-substrate interactions promises to further refine the design of enzymatic cleavage processes, making ADC development faster and more precise.

Conclusion

Enzymatic cleavage processes have evolved from a niche concept to a cornerstone of modern ADC design and manufacturing. The ability to control the timing and location of drug release through specific enzymes, combined with site-specific conjugation techniques, has produced ADCs with superior homogeneity, stability, and clinical performance. Advances in enzyme engineering, linker chemistry, and process control continue to push the boundaries of what is possible, enabling the development of more complex constructs with improved therapeutic indices. As the field moves toward personalized and dual-payload ADCs, enzymatic methods will remain at the forefront, offering the precision and flexibility required to realize the full potential of antibody-targeted therapy. For researchers and manufacturers alike, investing in these enzymatic technologies is essential for staying competitive in the rapidly evolving landscape of ADCs.