Two Main Chemical Approaches
Linker-payloads are conjugated onto antibodies through two broad strategies: random (conventional) conjugation, which targets abundant native lysine or cysteine residues, and site-specific conjugation, which uses engineered handles to attach payload at precisely defined positions. Both rely on solvent-accessible reactive amino acids as the anchor points.
Lysine Conjugation
An antibody has roughly 40–90 solvent-accessible lysine residues, and their nucleophilic primary amines readily react with electrophilic linker chemistries. The standard chemistry uses an NHS (N-hydroxysuccinimide) ester on the linker, which forms a stable amide bond directly with the lysine amine. This can be done as a "one-step" reaction (linker-payload already bears the NHS ester) or a "two-step" protocol, where lysines are first activated with an NHS-ester-bearing crosslinker and then reacted with the payload separately. Because so many lysines are available and reactive, this method produces highly heterogeneous drug-to-antibody ratios (DAR 0–8) — Kadcyla and the original Mylotarg were both made this way.
Cysteine Conjugation
The other classic route exploits cysteine thiols, most commonly generated by partially reducing the antibody's four interchain disulfide bonds (IgG1 has 8 such cysteines), exposing free thiols for conjugation. These thiols react with maleimide- or haloacetamide-functionalized linkers via thiol-Michael addition or alkylation, forming a stable thioether bond. Brentuximab vedotin (Adcetris) is produced this way, typically yielding DAR 4 or 8 depending on reduction extent. Cysteine conjugation gives more homogeneous products than lysine chemistry but still isn't fully site-specific since multiple thiols are available.
Site-Specific Conjugation Technologies
To get a single, defined DAR and eliminate batch-to-batch variability, several engineered approaches have been developed:
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Engineered cysteine (THIOMAB): a cysteine is genetically inserted at a defined antibody position; native disulfides and the new cysteine are reduced, then reoxidized (often with CuSO4) to restore structural disulfides while leaving the engineered thiol free for maleimide conjugation, giving precise DAR2 or DAR4 products.
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Disulfide re-bridging: interchain disulfides are selectively reduced (e.g., with TCEP or DTT), then a bifunctional bridging reagent (dibromomaleimide, divinylpyrimidine) reconnects the two cysteines while simultaneously installing the payload, preserving native sequence and reducing the max attachable payloads from 8 to 4.
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Non-natural amino acids (ncAA): an amber stop codon is inserted at a chosen site, and the cell's translation machinery incorporates an unnatural amino acid (e.g., p-acetylphenylalanine, pAcF) bearing a bioorthogonal handle (ketone, azide, alkyne) that reacts selectively with a complementary linker group (e.g., alkoxyamine forms an oxime bond).
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Enzymatic conjugation: enzymes such as transglutaminase, Sortase A, or glycotransferases recognize a specific peptide tag or glycan motif and covalently attach the linker-payload at that exact site; glycan remodeling at Asn-297 is a common Fc-based version that avoids sequence mutation.
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Selective/buried lysine and affinity-peptide methods: newer chemistries (e.g., β-lactam or phenyl-oxadiazole methylsulfone reagents) target a single unusually reactive or buried lysine without any antibody mutation, enabling one-step, mutation-free site-specific conjugation at neutral pH.
Typical Conjugation Workflow
A representative cysteine-based conjugation process (e.g., THIOMAB-style) runs as follows: partial/selective reduction of target cysteines with a mild reductant such as TCEP or TSPP, removal of excess reductant, addition of excess (often ~10 equivalents) linker-payload bearing a maleimide group, incubation at room temperature for 1–4 hours, then purification (e.g., by desalting or chromatography) to remove unconjugated drug and verify DAR by mass spectrometry or HIC. Lysine-based one-step conjugation is similarly simple — mixing antibody and NHS-ester (or β-lactam) linker-payload in PBS at pH 7.4 for a few hours at room temperature — but requires tighter control of equivalents to manage the resulting DAR distribution.
Choosing an Approach
Site-specific chemistries (THIOMAB-style cysteine engineering, disulfide re-bridging, and enzymatic tags) are the areas seeing the most active pharma R&D investment right now, since DAR homogeneity is directly tied to improved therapeutic index and is a major differentiator in next-generation ADC programs.
Reviews:
(1) Dumontet, C., Reichert, J. M., Senter, P. D., Lambert, J. M., & Beck, A. (2023). Antibody–drug conjugates come of age in oncology. In Nature Reviews Drug Discovery (Vol. 22, Issue 8, pp. 641–661). Nature Research. https://doi.org/10.1038/s41573-023-00709-2