Cysteine conjugation exploits the unique nucleophilicity of the thiol (–SH) side chain on cysteine residues. Under mild, controlled conditions, free thiols react selectively with thiol-reactive electrophiles—most commonly maleimides, haloacetamides, and vinyl sulfones—forming stable thioether or thiosuccinimide bonds.

In native IgG antibodies, the four interchain disulfide bonds can be selectively reduced using TCEP or DTT to liberate free cysteines at defined positions, enabling conjugation with a theoretical DAR of up to 8. Partial reduction protocols yield mixed DAR distributions; careful stoichiometry control can produce a mixture resulting in a lower-DAR product.

Cysteine conjugation is compatible with a broad range of cytotoxic payloads and is validated across multiple approved ADC platforms.

Fc glycan remodeling enables site-specific conjugation through the conserved N-linked glycan at Asn297 of IgG antibodies.

Chemoenzymatic approaches using endoglycosidases and engineered glycosynthases can install bioorthogonal handles such as azido sugars for subsequent click-based payload attachment. Compared with traditional lysine conjugation, glycan-mediated approaches typically produce narrower DAR distributions and improved conjugate homogeneity while avoiding direct modification of the antigen-binding region.

In addition, enzymatic remodeling introduces added process and analytical complexity relative to more established cysteine-based site-specific conjugation methods. Despite these challenges, Fc glycan conjugation remains an increasingly attractive platform for highly defined ADCs and next-generation antibody conjugates including AOCs, ISACs, and radioconjugates. Xcellon Biologics has developed the needed process methods to deliver these conjugates on the scales needed to support our clients' programs.

Microbial transglutaminase (mTGase) catalyzes acyl-transfer reactions between glutamine residues and primary amine-containing substrates, forming stable isopeptide linkages under mild conditions. In ADC development, engineered glutamine motifs such as LLQGA or accessible Fc glutamine residues including Q295 can serve as site-selective conjugation handles.

Because Q295 is partially shielded by the conserved Fc glycan, many workflows require prior deglycosylation to enable efficient conjugation. Under optimized conditions, mTGase-mediated conjugation can generate highly homogeneous ADCs with controlled DARs, most commonly DAR 2 or DAR 4. Xcellon Biologics has developed the needed process methods to deliver these conjugates on the scales needed to support our clients' programs.

ThioMab and related engineered cysteine platforms enable site-specific ADC conjugation through the introduction of strategically positioned cysteine residues within the antibody scaffold. These engineered thiols provide defined conjugation handles that reduce the heterogeneity associated with conventional stochastic interchain cysteine conjugation methods.

Beyond our core platform chemistries, Xcellon supports a growing portfolio of emerging and bespoke bioconjugation strategies tailored to unique program requirements.

Bio-orthogonal Click Chemistry: Incorporation of non-canonical amino acids via amber suppression enables CuAAC or strain-promoted copper-free click conjugation. Tetrazine-TCO ligation offers some of the fastest bio-orthogonal reaction rates known.

SpyCatcher / SpyTag: Isopeptide bond-forming protein tag systems allow modular, covalent assembly of antibody fragments with payloads or effector domains—useful for bispecific and multimerization strategies.

NHS Ester (Lysine) Conjugation: For programs where DAR heterogeneity is acceptable and rapid turnaround is prioritized, conventional NHS ester conjugation remains cost-effective with a well-understood regulatory history.

Sortase A & Butelase: Transpeptidase-mediated ligation at C- or N-terminal recognition sequences enables site-specific attachment of peptide-payload cassettes with full sequence control.