Phase 1Raw material preparation
01Preparation of S30 cell-free extract (E. coli lysate) (S30 extract preparation / lysate preparation)
Extract activity and specifications are the starting material that rate-limits antibody yield and folding
- Standard method
- Engineered E. coli grown by high-density fermentation is disrupted by high-pressure homogenization, and the supernatant from centrifugation at 30,000 × g is taken
Why it is the standard. Sutro's Open Cell-Free Synthesis (originating from J. Swartz) is the representative configuration in the industry. Because the quality of the extract (activity, reproducibility) rate-limits the yield and folding of the final antibody, specifying the raw material is the starting point. Sources: PMC10045668, sutrobio.com technical materials
With Conventional method (cysteine-maleimide stochastic conjugation)
- Conventional cysteine conjugation does not use a non-natural amino acid and takes the existing interchain cysteines as the attachment points, so the nnAA incorporation machinery itself is not needed
- Method: Step omitted (no pAMF / amber suppressor system is used)
- Check also: The antibody keeps its native sequence. The attachment points are cysteines derived from the interchain disulfides, and no additional genetic modification is needed
About this step and the alternatives
This step prepares the "workshop" in which the antibody is assembled. E. coli are grown to high density and crushed under high pressure to release their contents.
The released liquid (the extract) contains the ribosomes and enzymes that synthesize protein, all together. It is the source of the materials for translation.
The key is the strain modification. To form the correct disulfide bonds of the antibody, the cells are deliberately engineered to have an oxidizing intracellular environment (ΔtrxB Δgor type).
Synthesis of the antibody scaffold takes place inside this bacterial extract. The extract supplies all the ribosomes and enzymes needed for transcription and translation. For correct antibody disulfide bonds, the key is to engineer the strain used to make the extract so that it has an oxidizing intracellular environment (ΔtrxB Δgor type).
Alternatives in practice
- Commercial E. coli cell-free kits · Thermo Fisher Expressway / 1-Step Human Coupled IVT, Promega S30 T7 High-Yield, NEB PURExpress (reconstituted system) — For research and small-scale screening; no in-house extract needed
- Eukaryotic extracts · Promega Wheat Germ Extract (TNT), Promega Rabbit Reticulocyte Lysate, Takara/Cambridge Isotope wheat germ systems — For difficult-to-express proteins involving disulfides or glycans
- Lyophilized extract · Arbor Biosciences myTXTL / in-house lyophilized (Lyo) S30 — Stabilized for storage and transport; reconstituted on site
Supply of non-natural amino acid (pAMF) and orthogonal aaRS / suppressor tRNA (preparation of the non-natural amino acid and orthogonal translation system)
Amber reassignment places an azide scaffold at a defined position and secures a uniform DAR
- Standard method
- Engineered pAMF-tRNA synthetase derived from M. jannaschii and amber suppressor tRNA are prepared separately in E.…
Why it is the standard. pAMF is a non-natural amino acid optimized by Sutro and is the core of achieving a uniform DAR and a defined conjugation position. The fidelity of the orthogonal aaRS/tRNA guarantees incorporation at the defined position. Sources: US9938516B2 (patent on aaRS for pAMF), PMC10045668, Bioconjug. Chem. 2014 (establishment of the pAMF system)
Check also. The efficiency of amber codon reassignment depends on the sequence context and does not reach 100%. A certain proportion of by-products can arise: the non-natural amino acid does not go in at the target position and the original translation stops (truncation), or a different natural amino acid is incorporated by mistake. The incorporation rate is therefore always confirmed in later steps by mass spectrometry and peptide mapping.
With Conventional method (cysteine-maleimide stochastic conjugation)
- Because pAMF is not introduced at an amber codon, standard cell-free translation without added engineered tRNA synthetase / pAMF-tRNA (or, as is usual, mammalian cell expression such as CHO)
- Check also: Because it does not depend on an nnAA, the conventional route generally prepares the antibody itself by expression in CHO cells
About this step and the alternatives
This step supplies the dedicated components for setting up, at the intended position, the "scaffold" to which the drug is attached. The non-natural amino acid pAMF, which is not among the 20 natural ones, serves as that scaffold.
The mechanism is a rewrite of the translation code. A pair of a dedicated synthetase (aaRS) and a suppressor tRNA makes the amber codon (TAG), normally a stop signal, translate as pAMF.
The azide group on pAMF becomes the "hook" that catches the drug in the later click reaction. The starting point of a uniform ADC is set up here.
At the position that serves as the "scaffold" for attaching the drug, an amino acid that is not one of the 20 natural ones, para-azidomethyl-L-phenylalanine (pAMF), is placed. This is achieved with a dedicated pair of a synthetase (aaRS) and a suppressor tRNA that reassigns the amber codon (TAG). The azide group becomes the hook for the later click reaction.
Alternatives in practice
- Ketone-oxime ligation · p-Acetylphenylalanine (pAcF) (Iris Biotech / SiChem) + aminooxy-drug — Oxime linkage; proceeds at pH 4–5; no copper needed
- Alternative route for azide introduction · p-Azidophenylalanine (pAzF) / p-azidonorleucine (SiChem / Iris Biotech) — For SPAAC or the Staudinger reaction; an alternative azide source to pAMF
- Orthogonal chemistry based on natural residues · Selenocysteine-incorporating strains; leaving-group cysteine (Michael addition) reagents — A different principle that does not use an ncAA synthesis system
Phase 4Purification (downstream)
05Harvest and clarification of the reaction mixture → removal of S30-derived cell debris, ribosomes, aggregates and lipids by centrifugation and depth filtration to obtain a clarified lysate that can be loaded onto Protein A (harvest and clarification / preparation of Protein A load)
The S30 reaction mixture is turbid with cell fragments and particles and cannot be loaded onto Protein A without clarification
- Standard method
- Rough clarification by continuous centrifugation, followed by polishing clarification with depth filters to prepare the Protein A feed
- Reagents and materials
- — (clarification; diluted with Protein A equilibration buffer as needed (e.g., 20 mM sodium phosphate, 150 mM NaCl, pH 7.4)). HC Pro is a grade with reduced β-glucan, which interferes with endotoxin LAL testing
Why it is the standard. A depth filter series with a track record in clarifying high-solids cell lysates; it removes cell debris, particles and turbidity in stages.
Check also. After depth filtration, a 0.2 µm (or 0.45/0.2 µm) bioburden-reduction and particle-removal filtration is customarily inserted as a finishing step before Protein A loading.
About this step and the alternatives
This step removes the solid turbidity from the reaction mixture and turns it into a clear liquid that can go onto the purification line. Cell-free reaction mixtures are more turbid than you might expect.
The extract from high-density E. coli leaves large amounts of cell debris, ribosomes and precipitates. They are separated roughly by continuous centrifugation and finished with depth filters.
If the contaminants are not removed here, the next Protein A capture column gets clogged and fouled. Solid-liquid separation is a prerequisite for capture.
The cell-free reaction mixture is turbid because it contains large amounts of S30 extract from high-density E. coli, cell debris, ribosomes and precipitates. To prevent clogging and fouling of the capture chromatography column, solid-liquid separation by centrifugation + depth filtration is essential. Centrifugation is standard at research scale, and centrifugation + depth/microporous filtration at manufacturing scale.
Alternatives in practice
- Depth filter clarification alone (centrifuge-free) · Merck Millistak+ HC Pro single stage / Sartorius Sartoclear P, 3M Harvest RC — Small to medium scale; no continuous centrifuge equipment needed
- TFF microfiltration clarification · Repligen KrosFlo hollow fiber (0.2 µm ME) / Cytiva ULTA Hollow Fiber, Sartorius Sartocon hollow fiber — Continuous clarification with single-use tangential flow
Protein A affinity capture (Protein A capture)
Captures only the antibody through Fc, removes extract contaminants and secures most of the purity
- Standard method
- Protein A affinity chromatography
Why it is the standard. Protein A capture is the de facto standard capture step for mAb/IgG downstream. The Sutro paper also uses the same procedure for cell-free ADCs. The alkali-tolerant SuRe ligand holds up well under CIP. Sources: PMC10045668, Cytiva product literature
Check also. Protein A grabs the Fc of the antibody, so it cannot be used with fragment-type molecules that lack an Fc. Also, because acidic elution is assumed, the elution conditions need to be reviewed for acid-sensitive molecules.
About this step and the alternatives
The central step that selects and grabs only the target antibody from the clear reaction mixture. It is the first gate of the purification line.
Protein A binds specifically to the "tail" region (Fc) of the antibody. This is used to wash away extract-derived proteins and other contaminants in one pass.
Its strength is that most of the purity is gained in this one step. As a platform capture step, it is the foundation of downstream.
Only the target antibody is grabbed from the reaction mixture by its Fc region, and extract-derived proteins and other contaminants are removed in one pass. This is the first gate of downstream, and most of the purity is gained here.
Alternatives in practice
- Protein A resins from other suppliers · Thermo POROS MabCapture A Select、Tosoh TOYOPEARL AF-rProtein A HC-650F、Purolite Praesto Jetted A50 — Selected by alkali stability, dynamic binding capacity and pressure
- Protein G/L as alternative affinity ligands · Cytiva Protein G Sepharose / Capto L, Thermo CaptureSelect FcXL — When Fc/light-chain-based specificity is needed
- Affinity-free purification · Combination of the Merck Viresolve series + depth filtration + low-pH viral inactivation — A platform that does not depend on Protein A
Low-pH hold and neutralization → the Protein A eluate (glycine, pH about 3.2–3.5) is held for a defined time and then neutralized with Tris, to inactivate contaminants and adjust to a pH suitable for the next step (low-pH viral inactivation / neutralization)
The acidic Protein A eluate can go to the later steps only after a low-pH hold and neutralization, which makes this a standard step
- Standard method
- The eluate is held at the defined pH and time in a stirred vessel, then neutralized by pH adjustment with 1–2 M Tris base
Why it is the standard. A standard low-pH hold + Tris neutralization that makes use of the low pH of the Protein A elution; it keeps the additional pH-adjustment burden to a minimum and can be controlled as a discrete unit operation.
Check also. The low-pH hold is a general-purpose step intended for viral inactivation, but cell-free E. coli systems inherently carry little risk of viral contamination, which differs from mammalian cell culture. Even so, control of hold time and pH is indispensable to suppress aggregate formation, and the approach needs to be justified according to the risk from raw materials.
About this step and the alternatives
The eluate that strips the antibody off Protein A is strongly acidic, at around pH 3.2 with glycine. This step bridges to the next one by conditioning it first.
It is held at the defined pH and time in a stirred vessel to inactivate viruses and the like. Then 1–2 M Tris base is added to neutralize it quickly.
If neutralization is neglected, the antibody aggregates from the low-pH exposure, or the binding conditions for the next chromatography no longer fit. It is an unglamorous but indispensable stand-alone operation.
Protein A elution with MabSelect SuRe and similar resins is acidic, at around pH 3.2 with glycine, so the standard is a discrete unit operation as a platform step: a low-pH hold for a defined time (inactivation) followed by prompt neutralization. Without neutralization, low-pH exposure causes aggregation and the binding conditions for the downstream chromatography no longer match.
Alternatives in practice
- Continuous neutralization (inline) · Cytiva ÄKTA process inline pH control / Sartorius Resolute BioSC static mixer — Contact time is tightly controlled; suited to continuous processes
- Acetic acid / citric acid hold + NaOH dilution · Acetic acid / citrate buffer (Merck) + NaOH dilution line — Gentle pH recovery for aggregation-prone molecules
Polishing (aggregate and contaminant removal) and buffer exchange (polishing step, buffer exchange)
Polishes away residual aggregates and contaminants, preparing a clean, mainly monomeric antibody for conjugation
- Standard method
- Preparative size exclusion chromatography
Why it is the standard. The Sutro paper concentrates the ProA pool and then polishes it by prep-SEC into PBS. At commercial scale, SEC is capacity-limited, so it is generally replaced with AEX/CEX/MM chromatography. Source: PMC10045668
About this step and the alternatives
This finishing purification polishes away the trace contaminants that capture could not fully remove. It trims off remaining aggregates (HMW) and trace contaminants.
Preparative size exclusion chromatography separates molecules by size and removes aggregates. At the same time, the antibody is exchanged into a buffer suitable for conjugation.
To make a uniform ADC, the starting antibody must be mainly monomer and clean. The key is to fix the quality of the next step here.
Aggregates (HMW) and trace contaminants that remain after capture are trimmed away in the polishing step, and the antibody is conditioned into a buffer suitable for conjugation. To make a uniform ADC, the starting antibody must be mainly monomer and clean.
Alternatives in practice
- AEX flow-through polishing · Cytiva Capto Q / Q Sepharose FF, Sartobind Q membrane (flow-through mode) — Removes aggregates, DNA and HCP; high throughput
- CEX polishing · Cytiva Capto SP ImpRes / Tosoh TOYOPEARL GigaCap S-650 — Bind-elute polishing of charge variants and aggregates
- Mixed-mode polishing (MMC) · Cytiva Capto adhere / Capto MMC, Merck Eshmuno HCX — Selective removal of hard-to-separate impurities
Anion exchange flow-through polishing → endotoxin (LPS), host-cell DNA and acidic HCP are adsorbed and removed in flow-through mode on a Q membrane / Q resin, reducing the impurities of E. coli-derived products (anion exchange polishing, flow-through)
E. coli systems carry a high endotoxin and DNA burden, so AEX flow-through polishing is essential
- Standard method
- A Q membrane adsorber is run in flow-through mode so the IgG passes through while endotoxin / DNA / HCP are adsorbed
Why it is the standard. Membrane adsorbers are suited to high-flow-rate flow-through removal of large impurities (DNA, endotoxin, aggregates) and have a track record in reducing LPS/DNA in E. coli systems.
Check also. Because the material comes from an E. coli extract, the burden of endotoxin and host DNA is higher than in mammalian cell systems, and size exclusion polishing alone may not reach specifications. Even when an anion exchange flow-through polishing step is added, conditions need care for molecules whose antibody isoelectric point (pI) is low.
With Conventional method (cysteine-maleimide stochastic conjugation)
- Attachment is by thiol-Michael addition between the reduced cysteine thiols and the maleimide, not by SPAAC with azide-DBCO
- Method: The interchain disulfides are partially reduced with TCEP (e.g., TCEP:mAb ≈ 2:1), and then a maleimide-type drug-linker (such as vc-MMAE) is added in a DMA/DMSO co-solvent
- Reagents: TCEP, maleimide-vc-MMAE (DBCO-PEG-drug is not used), co-solvents such as dimethylacetamide (DMA)
- Check also: The DAR is a heterogeneous distribution of 0–8 (average about 3.5–4). There is a risk of reverse disulfide / payload loss, and maleimide stabilization (such as self-hydrolyzing linkers) may be required
About this step and the alternatives
A polishing step that targets the impurities specific to E. coli origin. Endotoxin (LPS), host DNA and acidic HCP are present at a higher burden than in CHO systems.
A Q membrane or Q resin is run in flow-through mode. Near neutral pH, the antibody passes through, and only the negatively charged impurities are caught by the Q adsorber.
The difficulty with microbial systems is that size exclusion polishing alone has trouble reaching specifications. This anion exchange polishing is the standard finish for E. coli systems.
Because S30 is derived from E. coli, the burden of endotoxin (LPS), host DNA and nucleic acid / acidic HCP is higher than in CHO systems, and SEC polishing alone makes it difficult to meet specifications. Flow-through AEX polishing, in which IgG passes through near neutral pH and negatively charged impurities are retained on the Q adsorber, is added as the standard step for microbial and E. coli systems.
Alternatives in practice
- Q membrane adsorbers from other suppliers · Cytiva HiTrap Q HP membrane / Pall Mustang Q capsules — Selected to fit existing equipment and scale
- AEX polishing on a Q resin-packed column · Cytiva Capto Q / Q Sepharose FF packed columns (flow-through) — Higher binding capacity than membranes; reusable after regeneration to reduce cost
Phase 5Site-specific conjugation
10Drug-linker conjugation by SPAAC click reaction (click conjugation)
Copper-free azide-DBCO coupling delivers a defined position and uniform DAR; this is the core step
- Standard method
- Strain-promoted azide-alkyne cycloaddition (SPAAC, copper-free click)
Why it is the standard. The SPAAC between the azide of pAMF and DBCO needs no copper catalyst, is gentle on the antibody, and proceeds at the defined position, which gives a uniform DAR. This is the core reaction that achieves "uniform DAR at a defined position". Sources: PMC10045668, Bioconjug. Chem. (pAMF-DBCO system)
Check also. The selling point of site-specific conjugation is a uniform number of drugs (DAR), but that uniformity directly reflects the incorporation rate of the non-natural amino acid. Molecules with incomplete incorporation get no drug, which shows up in the average DAR and the distribution, so the quality of the synthesis stage decides the final uniformity.
With Conventional method (cysteine-maleimide stochastic conjugation)
- Adds a step in which unreacted maleimide drug is quenched with a sacrificial thiol before it is removed
- Method: Excess maleimide is quenched with a sacrificial thiol such as N-acetylcysteine, and then free drug and the quenched species are removed by ultrafiltration/diafiltration
- Reagents: N-acetylcysteine (quencher)
About this step and the alternatives
This is the heart of the modality: the reaction step that connects the antibody and the drug. The azide group of the pAMF built into the antibody becomes the attachment point.
A drug-linker carrying DBCO (a cyclic alkyne) is connected there by a click reaction that uses no copper (SPAAC). It reacts only at the target position.
Because the conjugation position is defined, the drug-to-antibody ratio (DAR) comes out uniform. This is the greatest feature of the site-specific ADC and leads to stable quality.
A drug-linker carrying DBCO (a cyclic alkyne) is attached to the azide group of the pAMF built into the antibody by a copper-free click reaction. Because the position is defined, the number of attached drugs (DAR) comes out uniform, which is the greatest feature of this modality.
Alternatives in practice
- Oxime ligation (pAcF system) · Aminooxy-drug (BroadPharm / MedChemExpress) + aniline catalyst — Ketone-aminooxy, around pH 4.5
- Copper-catalyzed click (CuAAC) · Alkyne-drug + CuSO4/THPTA/ascorbic acid (Sigma / Click Chemistry Tools) — Fast, but copper removal is required
- Tetrazine-TCO (IEDDA) · TCO-drug + tetrazine-modified antibody (Click Chemistry Tools / Jena Bioscience) — Inverse electron-demand; extremely fast and bioorthogonal
Removal and purification of unreacted drug-linker (free drug removal, purification)
Reliably removes free drug-linker, a CQA directly tied to safety
- Standard method
- Free drug removal and buffer exchange by desalting and ultrafiltration
- Reagents and materials
- Desalting resin (Zeba High-Performance Resin), PBS, activated carbon (Merck) / hydrophobic resin (Purolite/Merck) as needed. Removes free drug and organic solvent
Why it is the standard. Free drug is a CQA directly tied to safety, and its removal is essential. The Sutro paper removes unreacted linker by spin desalting. At scale, TFF + adsorption is standard. Sources: PMC10045668, ScienceDirect (control of free-drug-related species)
Check also. Free drug-linker is a critical quality attribute directly tied to toxicity. Highly hydrophobic drugs can adsorb non-specifically to the antibody or container, escape complete removal and be released again later, so the removal rate is confirmed separately by free drug analysis.
About this step and the alternatives
This purification step reliably removes the "leftovers" after the reaction. The target is free drug-linker that did not get attached.
Desalting and ultrafiltration separate the ADC from the small free drug, and at the same time the buffer is exchanged into one suited to formulation. The removal uses the difference in size.
Free drug carries a toxicity risk and is strictly controlled as a critical quality attribute (CQA). This is an indispensable step for securing safety.
Free drug-linker that remains after the reaction is a toxicity risk, so it is removed reliably. Free drug is strictly controlled as a critical quality attribute (CQA).
Alternatives in practice
- TFF diafiltration · Merck Pellicon 3 Ultracel 30 kDa / Repligen KrosFlo, Sartorius Sartocon — Scalable; buffer exchange and concentration are performed at the same time
- Activated carbon / hydrophobic resin adsorption · Merck activated carbon depth / Purolite hydrophobic resin column — Selective capture of residual free drug
- Protein A re-capture · Cytiva MabSelect PrismA / SuRe reload — The ADC is re-bound and eluted, sweeping away non-binding material
Phase 7Quality attributes and analytics
14Quantification of the drug-to-antibody ratio (DAR) and drug distribution (DAR and drug distribution measurement)
DAR and its distribution determine potency and safety; uniformity is confirmed directly by MS
- Standard method
- Intact DAR measurement by mass spectrometry
- Reagents and materials
- HPLC mobile phase solvents
Why it is the standard. Because site-specific ADCs have a uniform DAR, the DAR can be determined directly by mass spectrometry, and HIC-UV separates the species distribution. Methods that achieve both separation and MS identification, such as native HIC-MS, are also advancing. Sources: PMC10045668, PMID28458283, tandfonline (2024) native HIC-MS
Check also. Intact mass spectrometry quickly shows the average DAR and distribution, but it does not tell the position where the drug is attached. The reliability of the conjugation site must be confirmed separately by peptide mapping (next step).
With Conventional method (cysteine-maleimide stochastic conjugation)
- Identifying the nnAA incorporation site is no longer needed; what is confirmed becomes the occupancy and positional isomers of the conjugated cysteines
- Method: Occupancy and positional isomers of the conjugated cysteines are confirmed by reduced/digested LC-MS/MS after conjugation (the pAMF site is not identified)
- Check also: The evaluation centers on occupancy patterns for up to 8 conjugation sites from the 4 interchain disulfides
About this step and the alternatives
A quality evaluation that measures how many drugs are attached per 1 antibody molecule on average (DAR) and how much they vary (distribution). It is directly tied to potency and safety.
The intact molecule is weighed by mass spectrometry, and the DAR and distribution are worked out from it. If there is too much or too little drug, the properties of the product change.
A uniform DAR (typically 2) is the selling point of site-specific ADCs. This step confirms that uniformity and shows that the product was made as designed.
The average number of drugs attached per 1 antibody molecule (DAR) and its distribution determine potency and safety. A uniform DAR (typically 2) is the selling point of site-specific ADCs, so uniformity is confirmed here.
Alternatives in practice
- Reversed-phase LC reduced subunit MS · Waters ACQUITY UPLC + BioResolve RP mAb Polyphenyl / Agilent 1290 Infinity II Bio LC + 6545XT QTOF — Obtains the DAR distribution at high resolution from the light and heavy chains after reduction
- UV/Vis absorbance ratio method · Agilent Cary / Thermo NanoDrop (ratio of absorbance at 280 nm to the drug absorption wavelength) — Simple DAR calculation for drugs that have a chromophore
- Native SEC-MS / CE-MS · Waters BioAccord + ACQUITY BEH SEC / SCIEX PA800 Plus CESI-MS — Observes drug distribution and minor heterogeneity under non-denaturing conditions
Identification of non-natural amino acid incorporation and conjugation sites (peptide mapping)
Sequence-level confirmation of pAMF incorporation position and occupancy; a CQA specific to this modality
- Standard method
- Tryptic digestion + LC-MS/MS peptide mapping
- Reagents and materials
- Peptide mapping protease, Disulfide reduction alkylation reagents
Why it is the standard. Confirming the position and occupancy of the non-natural amino acid and the conjugation site is a CQA specific to this modality. Incorporation efficiency above 98% has been reported with the pAzF system, and peptide mapping is the standard confirmation method. Sources: US9738724 (site-specific nnAA antibody), Sigma-Aldrich peptide mapping protocol
Check also. Depending on how the enzymatic digestion is done and how unstable the drug is, a conjugate that was originally present can come off during analysis and change the apparent incorporation rate. It is safest to verify the digestion conditions for each molecule and to cross-check with multiple methods.
About this step and the alternatives
This step confirms at the sequence level that pAMF is correctly placed at the intended position and that the drug is attached at that defined position.
The antibody is fragmented with trypsin, and the peptides are read out by LC-MS/MS. What is attached at which position can be tracked precisely.
The key is monitoring incorporation efficiency and codon misreading (misincorporation). It is a checkpoint specific to this modality, which uses a non-natural amino acid.
Confirms at the sequence level that pAMF is correctly placed at the intended position and that the drug is attached at that defined position. Monitoring incorporation efficiency and misincorporation (misreading) is the key point specific to this modality.
Alternatives in practice
- Middle-down subunit MS · IdeS (Genovis FabRICATOR) digestion + Thermo Orbitrap / Waters QTof — Confirms the ncAA incorporation site at the middle-molecule level
- Disulfide/glycan mapping · Thermo Orbitrap + non-reduced digestion, Waters GlycoWorks RapiFluor-MS — Comprehensive identification of bonding patterns
- Incorporation confirmation by intact/reduced mass · Waters BioAccord / Xevo G3 QTof (intact and reduced) — Rapid screening before peptide mapping
Evaluation of purity and aggregates (size variants) (size variant and aggregate evaluation)
Aggregates and fragments are linked to reduced potency and immunogenicity, so monomer purity is monitored
- Standard method
- Quantification of % HMW/monomer/% LMW by analytical SEC-HPLC (280 nm integration)
- Reagents and materials
- HPLC mobile phase solvents
Why it is the standard. SEC-HPLC is the standard for aggregate evaluation of ADCs/mAbs, and CE-SDS is a quantitative purity method that replaces SDS-PAGE and is consistent with ICH Q6B. The Sutro paper also judges quality by analytical SEC. Sources: PMC10045668, ICH Q6B
About this step and the alternatives
A purity evaluation that checks whether the contents of the product are uniform "in terms of size". It confirms that no aggregates or fragments are mixed in.
Analytical SEC-HPLC separates molecules by size, and the 280 nm absorbance is integrated to quantify % HMW (aggregates), monomer and % LMW (fragments).
Aggregates and fragments are linked to reduced potency and immunogenicity. ADCs carrying hydrophobic drugs aggregate especially easily, so monomer purity is monitored strictly.
Aggregates (HMW) and fragments (LMW) are linked to reduced potency and immunogenicity. ADCs carrying hydrophobic drugs aggregate easily, so monomer purity is monitored strictly.
Alternatives in practice
- SEC-MALS · Wyatt (Waters) DAWN / miniDAWN MALS + Optilab dRI — Quantifies aggregate size by absolute molecular weight
- Analytical ultracentrifugation (AUC) · Beckman Coulter Optima AUC (sedimentation velocity) — Matrix-independent orthogonal verification of aggregates
- CE-SDS purity (separate instrument) · SCIEX PA800 Plus / Agilent ProteoAnalyzer — Confirms purity and fragments under reducing and non-reducing conditions
Evaluation of charge variants (charge variant evaluation)
Captures charge skew from deamidation and conjugation and shows batch-to-batch consistency
- Standard method
- Imaged capillary isoelectric focusing (icIEF) separates and quantifies acidic/main/basic peaks based on isoelectric point (pI)
- Reagents and materials
- Carrier ampholytes pI markers
Why it is the standard. icIEF is a standard method widely used for charge variant analysis of biologics and is complementary to CE-SDS. It reflects deamidation, pyroglutamate, fragments and so on. Sources: BioProcess International (icIEF), PMID35634791
About this step and the alternatives
An evaluation that checks whether the product is uniform "in terms of electrical properties". The skew in surface charge is separated and measured.
Imaged capillary isoelectric focusing (icIEF) separates and quantifies acidic, main and basic peaks based on differences in isoelectric point (pI).
Changes in charge that accompany deamidation, C-terminal lysine and conjugation can be captured. It serves as an index showing batch-to-batch consistency (comparability).
The charge profile captures deamidation, C-terminal lysine, and the charge skew that accompanies conjugation. It serves as an index showing batch-to-batch consistency (comparability).
Alternatives in practice
- Ion exchange chromatography (IEX/CEX) · Thermo MAbPac SCX-10 / Tosoh TSKgel CM-STAT + Agilent Bio LC (pH/salt gradient) — Preparative fractionation and quantification of charge variants
- cIEF-MS / CE-MS · SCIEX PA800 Plus CESI-MS / Intabio ZT (icIEF-MS) — Mass identification of the peaks confirms their assignment
- Conventional IEF/cIEF gels · Cytiva PhastSystem / Thermo Novex IEF gels — Low-cost qualitative confirmation
Target binding, affinity / FcRn binding (binding kinetics, affinity measurement)
Binding, affinity and FcRn binding are evaluated to assess the impact on potency and pharmacokinetics
- Standard method
- Binding kinetics (ka/kd) and affinity (KD) measured by surface plasmon resonance (SPR) or biolayer interferometry (BLI)
- Equipment
- SPR surface plasmon resonance, BLI
- Reagents and materials
- SPR / BLI sensor chips
Why it is the standard. SPR/BLI measure binding kinetics label-free in real time and are the standard methods for evaluating the effect of conjugation on binding. The Sutro paper measures FcRn binding on an Octet. Sources: PMC10045668, Cytiva/Sartorius literature
About this step and the alternatives
A functional evaluation that confirms the antibody can still bind its target properly after the drug is attached. If binding strength drops, it directly affects potency.
Surface plasmon resonance (SPR) or biolayer interferometry (BLI) measures how fast binding forms and breaks (ka/kd) and how strong it is (KD).
FcRn binding, which involves the antibody's "tail", is also evaluated. It is an index of half-life in the body, that is, how long the drug lasts.
Confirms that the antibody still binds its target properly after the drug is attached. If binding drops, it directly affects potency. FcRn binding is an index of pharmacokinetics (half-life).
Alternatives in practice
- ELISA binding assay · Tecan/BMG microplate reader + antigen-coated plates — Semi-quantitative binding, high throughput
- Flow cytometry cell-surface binding · BD FACSymphony / Beckman CytoFLEX + antigen-positive cells — Evaluates binding to the target in its native conformation
- KinExA (solution equilibrium) · Sapidyne KinExA 4000 — Accurately measures high-affinity Kd in solution
Potency (cytotoxicity) bioassay (cytotoxicity bioassay / potency test)
The most important potency index, reflecting the mechanism of action from internalization to cell death
- Standard method
- Cytotoxicity assay using a target antigen-positive cancer cell line
- Reagents and materials
- Reporter gene bioassay ADCC CDC
Why it is the standard. A cell-based potency test is the standard potency assay that reflects the mechanism of action of ADCs, and is used for lot-to-lot consistency and release. The Sutro paper also evaluates cell killing with CellTiter-Glo. Sources: PMC10045668, ICH Q6B (biological activity)
Check also. Potency tests that use cells vary widely, and results shift with the state and passage of the cell line. Evaluating relative to a reference standard, and fixing the acceptance criteria and the robustness of the test method in advance, are prerequisites.
About this step and the alternatives
The most important potency evaluation, which confirms in cells whether it can ultimately "kill cancer cells". It directly supports the efficacy of the product.
The ADC is applied to a cancer cell line that carries the target antigen, and the killing activity is measured in a cytotoxicity assay. It is observed under conditions close to the actual action.
Its strength is that the whole chain of the mechanism of action is reflected: internalization into the target → drug release → cell death. It serves as the overall judgment of whether it works.
Ultimately confirms in cells whether it can kill cancer cells. It is the most important potency index, reflecting the mechanism of action from target internalization through drug release to cell death.
Alternatives in practice
- Internalization assay · pH-sensitive dye pHrodo (Thermo) / IncuCyte (Sartorius) live-cell imaging — Visualizes ADC uptake and lysosomal trafficking
- Caspase/apoptosis assays · Promega Caspase-Glo 3/7 / Annexin V flow cytometry — Directly evaluates the mechanism of action (apoptosis)
- Reporter gene bioassay · Promega luciferase reporter cells (GloResponse) — Stable relative potency measurement linked to MoA
Control of process-related impurities (HCP, residual DNA, endotoxin) (process-related impurity tests / HCP, residual DNA, endotoxin)
HCP, DNA and endotoxin from the E. coli extract are controlled below the safety limits
- Standard method
- HCP by sandwich ELISA, residual DNA by qPCR, endotoxin by LAL
Why it is the standard. HCP ELISA, resDNA qPCR and LAL are the standard for controlling process impurities in injectable biopharmaceuticals (ICH Q6B, national pharmacopoeias). For E. coli cell-free systems, controlling E. coli-derived HCP/DNA is the key point. Sources: ICH Q6B, USP/EP (bacterial endotoxins, residual DNA)
Check also. A host cell protein (HCP) ELISA can only measure the contaminants that the antibody (antibody reagent) recognizes, and may miss proteins specific to E. coli. Coverage is complemented by methods such as mass spectrometry, and it is confirmed that the antibody reagent used broadly covers the impurities of the in-house process.
About this step and the alternatives
A control step that confirms the "residues" that get mixed in during manufacturing are held down to a safe level.
Even in a cell-free system, host cell protein (HCP), residual DNA and endotoxin from the E. coli extract can remain. This is a point to watch because the raw material is E. coli.
HCP is measured by sandwich ELISA, DNA by qPCR and endotoxin by LAL. They are directly tied to safety, so they are controlled strictly below the specified limits.
Even in a cell-free system, host cell proteins (HCP), DNA and endotoxin derived from the E. coli extract can be present. As process residues directly tied to safety, they are controlled to below the specified limits.
Alternatives in practice
- HCP mass spectrometry profiling · LC-MS/MS with Thermo Orbitrap / Waters QTof (HCP identification and quantification) — Covers HCPs not covered by ELISA; identifies individual HCPs
- Recombinant factor C (rFC) assay · Lonza PyroGene rFC assay / bioMérieux ENDONEXT — No animal-derived LAL used; high specificity and environmentally friendly
- Fluorescent dye method (residual DNA) · Thermo Quant-iT PicoGreen dsDNA Assay + fluorescence reader — Rapid quantification of total dsDNA (not species-specific)
The reference (“gold standard”) method is the one broadly recognized in published practice and guidelines; your process, product and regulator may justify another. Makers and products are examples, not endorsements. This map is translated from the Japanese edition, where it is updated first (Japanese version).