Antibodies · Process map

Bispecific antibodies

This reference process map organizes the manufacture and analysis of bispecific antibodies (BsAb) in the order of expression, purification, formulation and characterization of quality attributes. The best method depends on the molecular format (IgG-like or fragment-based) and on control of chain association, so alternative and complementary methods are listed for each step as well.

15 steps in 6 phases · the reference method for each step, the equipment and makers behind it, and the alternatives used in practice

Editor's note. The difficulty with bispecific antibodies lies not in capture but in the "chain selection" that follows. Knobs-into-Holes promotes correct pairing but is not a cure-all, and half-antibodies and light chain mispairing remain. That is exactly why the core of purification is not Protein A but polishing, that is, the design skill to pare away mispaired species and homodimers by HIC, relying on small differences in hydrophobicity. Where the field stumbles is the idea of buying purity downstream while leaving chain-ratio optimization in culture for later. It is fair to say that it is overwhelmingly cheaper not to "let mispairing be made" than to "remove it," and that both yield and recovery are decided by the chain-balance design upstream. On the quality control side too, SEC and CE-SDS alone cannot tell correct pairing from mispairing, so one must not overlook that the CQA is closed only when masses are matched by subunit LC-MS.

Used to makeAntibody that serves as the foundation of an antibody-drug conjugate (ADC) (bispecific ADC)

This map assumes IgG-like, Fc-retaining (Knobs-into-Holes). Steps marked ◇ change with Fragment-based, Fc-free (tandem scFv / BiTE type). This switches to a design based on a single-chain tandem scFv without Fc (BiTE type, e.g., blinatumomab). Without Fc, Protein A cannot be used for capture and it changes to IMAC (His-tag/Ni-NTA) or CH1/κ chain affinity (CaptureSelect CH1-XL / KappaSelect, etc.); being a single chain, removal of chain mispairing, half-antibodies and homodimers and LC-MS evaluation of chain pairing become unnecessary; with no Fc, N-glycan evaluation is also out of scope; and because of the short half-life (about 2 hours) the product becomes a lyophilized, continuous infusion formulation, so the reference methods for multiple steps branch in two.

At a glance

Design and cell line construction
1. 01 Molecular design that assures format selection and correct chain pairing (cell line construction and molecular design) ◇
Cell culture and production (upstream)
1. 02 Cell culture that expresses the target heterodimer at high purity and titer (production culture / upstream)
Harvest and clarification
1. 03 Removal of cells and cell debris and pretreatment for the capture step (harvest and clarification) ◇
Purification (downstream)
Formulation and fill-finish
1. 10 Stable liquid / lyophilized formulation and aseptic filling (fill-finish) ◇
Quality attributes and analytics

Phase 1Design and cell line construction

Molecular design that assures format selection and correct chain pairing (cell line construction and molecular design)

Suppresses heavy chain homodimers and light chain mispairing at the design stage; the starting point that even dictates the downstream purification strategy.

Standard method: Heavy chain heterodimerization (Knobs-into-Holes)
Equipment: Mammalian expression vectors and selection systems, Single cell cloning colony picker
Makers: Cytiva, Sartorius, ATCC, Lonza, Dassault Systèmes BIOVIA
Reagents and materials: Selection antibiotics

Why it is the standard. Knobs-into-Holes is the representative foundational technology for bispecific antibody heterodimerization and achieves preferential heterodimerization of the heavy chains (PubMed 29193902). Light chain mispairing must be solved separately by CrossMab / common light chain / DuoBody, etc., and the format determines the type of mispairing and the purification strategy.

Check also. Knobs-into-Holes (a design that makes the heavy chains fit together with a protrusion and a cavity), which pairs the heavy chains asymmetrically, helps heavy-heavy pairing but is not a cure-all, and light chain mispairing must be suppressed separately by measures such as CrossMab or a common light chain. The optimal design changes with the format (IgG-like or fragment-based), and there is no single standard.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Small fragments without Fc can also be produced in E. coli. There is no concept of chain-ratio optimization, and periplasmic secretion or inclusion bodies + refolding become options. Mammalian cells (CHO/HEK) and Pichia are also used (blinatumomab itself is made in CHO).
Method: E. coli periplasmic secretion expression (or inclusion body expression + refolding). Mammalian cell and Pichia expression are also options. Chain-ratio optimization is unnecessary
Equipment: Microbial fermenter (for E. coli) + refolding tank if needed
Check also: In E. coli systems, solubilization and refolding steps are additionally required when inclusion bodies form. Glycosylation, PTMs, titer and immunogenicity vary by host

About this step and the alternatives

A bispecific antibody houses 2 heavy chains of different character and up to 2 light chains in 1 molecule. If nothing is done, identical chains pair with each other or light chains get swapped, and large amounts of unintended combinations form.

So a "knob" and a "hole" are carved into the interface between the heavy chains so that each fits only its correct partner (Knobs-into-Holes). Like a key and a keyhole, it is a mechanism by which only the intended combination binds stably.

Building correct pairing in at the molecular design stage is the starting point of making a bispecific antibody. What is engineered here determines the purity and yield of every later step.

A bispecific antibody mixes 2 different heavy chains and up to 2 different light chains. Left alone, large amounts of homodimers and light chain mispairing form. So building heavy chain heterodimerization and correct light chain pairing in at the design stage is the starting point of manufacturing.

Alternatives in practice

DuoBody (controlled Fab-arm exchange, cFAE) · Genmab DuoBody platform (F405L / K409R mutations in CH3). In-house, reductive arm exchange with 2-MEA (2-mercaptoethylamine, Sigma-Aldrich M9768) — 2 finished antibodies are mixed and exchanged by reduction. Few half-antibody by-products and robust at scale
Common light chain · Common light chain platforms such as Regeneron Veloci-Bi / Merus Biclonics (in-house expression systems) — Fixing the light chain to 1 type eliminates mispairing itself. Difficulty of expression cell line construction rises
DVD-Ig (dual-variable-domain) · AbbVie DVD-Ig format expression plasmids (in-house design) — Purification and symmetric structure same as ordinary IgG, free of mispairing; large size
Electrostatic steering (charge-pair) mutations · In-house plasmids carrying Chugai/Genentech-type charge-pair mutations (e.g., D399K / K409D in CH3) — Promotes heterodimerization by electrostatic repulsion. Can be combined with KiH

Phase 2Cell culture and production (upstream)

Cell culture that expresses the target heterodimer at high purity and titer (production culture / upstream)

If the expression ratio of the 3–4 chains is upset, half-antibodies and homodimers increase, affecting correct-pairing purity and yield.

Standard method: Fed-batch culture with a stable CHO cell line (including chain-ratio optimization and codon adjustment)
Equipment: Single-use bioreactors, Cell retention device, Inline pH and DO sensors and probes, Culture control system
Makers: Cytiva, Sartorius, Repligen, Thermo Fisher, Sigma-Aldrich
Reagents and materials: CHO media and feeds, Antifoam defoamer, Cell culture supplements

Why it is the standard. A titer of 6 g/L has been reported in single-clone fed-batch (PMC5098433), and heterodimer assembly can be optimized by chain-expression-ratio control through non-optimized codons (PMC5297534). The chain ratio decides the correct-pairing purity (from ≪3% to over 80%) (ScienceDirect S1369703X20304113).

About this step and the alternatives

A step in which 3 to 4 kinds of polypeptides, such as the designed heavy and light chains, are made simultaneously by a stable mammalian cell line, the CHO cell. In fed-batch mode, in which nutrients are added during culture, the target antibody is accumulated in the culture fluid.

With more kinds of chains, control is harder than for an ordinary antibody. If the ratio at which each chain is made is upset, pairs of identical chains and half-sized antibodies (half-antibodies) increase.

So the chain expression ratio and the way the genes are read are adjusted to even out the ratio. It is the key adjustment, like tuning the proportions of a recipe, aimed at balancing high yield with correct pairing.

A bispecific antibody requires simultaneous expression of 3 to 4 kinds of polypeptides and is harder to control than an ordinary mAb. If the expression ratio of each chain is upset, homodimers and half-antibodies increase. Chain-ratio tuning and fed-batch operation are therefore used to balance yield and correct-pairing rate.

Alternatives in practice

Co-culture of 2 cell lines (single-reactor co-culture) · The above XDR / Biostat STR run in co-culture mode (each chain expressed by a separate line) — Chain ratio controlled by the ratio of lines. Process stabilization requires development effort
Perfusion culture · Repligen XCell ATF 6/10 or Cytiva KrosFlo KPS hollow fiber — Higher titer through high density and continuous production. Equipment becomes more complex
High-intensity fed-batch · The above STR with concentrated feed and temperature shift — Raises titer with existing equipment. Management of metabolic waste is the key
Transient expression · Thermo Fisher ExpiCHO-S or Expi293F (ExpiFectamine), MaxCyte electroporation for large-scale transfection — Supplies small amounts quickly without cell line construction. Unsuited to GMP production

Phase 3Harvest and clarification

Removal of cells and cell debris and pretreatment for the capture step (harvest and clarification)

Pretreatment that physically removes cells, particulates and HCP/DNA to prevent clogging and load on downstream columns.

Standard method: Continuous centrifugation + depth filtration + 0.2 µm sterile filtration
Equipment: Harvest centrifuges, Depth filter, Sterile filter (0.2 µm)
Makers: Cytiva, Merck, Sartorius, Pall, Alfa Laval
Reagents and materials: Membrane filter

Why it is the standard. Platform clarification common to mAbs and bispecifics. The 2 stages of depth filter + membrane are the industry standard and reduce the HCP/DNA load before the capture step.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Protein A cannot be used because there is no Fc. For His-tagged constructs capture is by IMAC, or by CH1/κ chain affinity that targets the constant region of the fragment.
Method: Primary capture by IMAC (immobilized metal affinity, using His-tag) or CH1/κ chain affinity chromatography
Equipment: Affinity chromatography column
Reagents: Ni-NTA / Ni-Sepharose (IMAC), or affinity resins such as CaptureSelect CH1-XL and KappaSelect (Protein A is unsuitable)

About this step and the alternatives

The fluid after culture contains not only the target antibody but also the cells themselves, cell fragments, and host cell-derived protein and DNA. This is the step that first sorts these out physically.

Centrifugation, which sinks heavy material by spinning, drops cells and debris (fragments), and fine-pored depth filtration and 0.2 µm filtration remove particulates. A clear supernatant (clarified fluid = HCCF) is taken out.

If the turbidity is not removed completely here, the downstream purification columns clog and performance drops. It is the groundwork step of making a clear fluid before handing it to purification.

The culture fluid contains cells, particulates, host cell protein (HCP) and DNA. Unless these are first removed physically, downstream chromatography columns clog and performance drops. So a clear clarified fluid (HCCF) is made by centrifugation and filtration before proceeding to purification.

Alternatives in practice

Depth filtration alone (centrifuge-free) · Merck Millistak+ HC Pod + prefilter (Clarisolve multilayer) — Single-use, no centrifuge required. High-density harvest increases the required area
With flocculant / flocculation · The above depth filters + flocculant (Merck pDADMAC, caprylic acid/acetic acid) — Pre-flocculates particulates, DNA and HCP to lower the load. Condition optimization required
Acoustic cell separation (acoustic wave separation) · FloDesign Sonics (now Cytiva) / Pall Cadence Acoustic Separator — Low shear and well suited to clarification in continuous perfusion. Throughput is limited

Phase 4Purification (downstream)

Primary capture and concentration by Protein A affinity chromatography (capture)

The gate that catches dilute antibody at once by specific Fc binding, concentrating it and removing most contaminant protein in one step.

Standard method: Protein A affinity chromatography
Equipment: Chromatography system
Makers: Cytiva, Purolite, Sartorius, Thermo Fisher, Tosoh
Reagents and materials: Protein A resin, Elution buffer, Neutralization buffer

Why it is the standard. Standard capture of Fc-containing bispecific antibodies. A typical configuration is Protein A capture followed by 2 polishing steps (Antibody Therapeutics, academic.oup.com/abt/4/2/73). However, it is not applicable to formats without Fc or with Fc on only one arm, and is replaced by light chain-based resins (KappaSelect/LambdaFabSelect) or mixed mode.

Check also. Protein A capture presupposes binding to Fc, so it cannot be used as is for fragment-based formats without Fc, and one switches to another capture method such as CH1 or κ chain affinity or IMAC using a His-tag. In asymmetric molecules only one arm may carry Fc, so the binding behavior should be measured before adoption.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Low-pH viral inactivation is tied to Protein A elution and to clearance of adventitious viruses of mammalian origin. With an E. coli host and no Protein A, the premise of mammalian virus risk changes, and this step must be omitted or separately justified.
Method: With an E. coli host and no Protein A, low-pH viral inactivation can be omitted (because the premise of mammalian-derived virus risk does not hold). Performed as before only for mammalian cell expression
Check also: Whether it can be omitted depends on viral safety assessment and risk assessment. For expression in mammalian cells, the conventional low-pH hold is maintained

About this step and the alternatives

The target antibody dissolved in the clarified fluid is still very dilute. Protein A capture is the first gate of purification, which catches it at once, concentrates it and drops most of the impurities.

Protein A has the property of sticking only to the Fc region, the "tail" of the antibody. It is packed in a column, the antibody is adsorbed, and then the conditions are changed to release it.

Because it targets Fc, most contaminant proteins are shaken off in this one step. It can be called the leading step of downstream processing, doing capture and concentration at the same time.

The target antibody is dissolved very dilutely in the clarified fluid. Protein A capture is the first gate that catches it at once, concentrates it and drops most of the impurities. Because it binds specifically to Fc, contaminant proteins can be removed to a large extent in a single step.

Alternatives in practice

Light chain type-specific affinity capture · Thermo Fisher CaptureSelect KappaXL / LambdaFabXL affinity resins, Cytiva KappaSelect / LambdaFabSelect — Captures κ/λ chains on a separate axis to separate the correctly paired species. Effective for κ+λ type bsAb
Differentiated Protein A (one-arm / FcRn selection) · Thermo Fisher CaptureSelect FcXL / FcRn affinity resin — Capture design that separates half-antibodies and homodimers by differences in Fc binding properties
Mixed-mode capture (Protein A-free) · Cytiva Capto MMC ImpRes、Tosoh TOYOPEARL MX-Trp-650M — For capture when acid elution is to be avoided or for Fc-deficient formats. Selectivity requires optimization

Low-pH viral inactivation (hold + neutralization): the Protein A elution pool is held at low pH for a fixed time to inactivate enveloped viruses, then neutralized, and precipitate is removed by depth/0.2 µm filtration (low-pH viral inactivation)

Because the Protein A eluate is at low pH, enveloped viruses can be reliably inactivated immediately afterwards; an essential safety step

Standard method: Protein A elution pool adjusted to pH 3.4–3.6 with 1 M acetic acid etc. and held for 60 min, then neutralized to around pH 5 with Tris/sodium hydroxide, depth fil…
Equipment: Inline pH adjustment, Depth filter, Sterile filter (0.2 µm)
Makers: Cytiva, Merck, Sartorius, 3M, Merck Millipore
Reagents and materials: Neutralization buffer

Why it is the standard. The low-pH hold is the first stage of viral clearance by a separate principle in line with ICH Q5A; it can use the Protein A elution conditions as they are, with minimal acid addition. For acid-labile formats it is excluded from consideration and an alternative method is adopted.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

For single-chain tandem scFv, the impurity concepts of half-antibodies, chain mispairing and homodimers do not exist. The removal targets change mainly to aggregates, fragments and misfolded species.
Method: The objective of removing chain mispairing/homodimers disappears. Redefined as removal of aggregates, fragments and misfolded species by SEC or ion exchange
Check also: HIC remains useful for separating aggregates and hydrophobic variants, but the main purpose of removing half-antibodies/homodimers does not apply

About this step and the alternatives

The liquid released from Protein A is acidic (low pH) to begin with. Using this property as it is, adjusting to around pH 3.5 with acetic acid and leaving it for about 60 minutes makes viruses wrapped in a membrane lose their strength.

When the hold is over, the pH is brought back to around pH 5 with alkali for neutralization. Precipitate and aggregates form at this point, so they are removed by depth filtration and 0.2 µm filtration before handing the material on.

Since the product is made in mammalian cells, viral safety is protected in two tiers, inactivation and removal. The low-pH hold carries one wheel of that, and at the same time lowers the load on the following columns.

The eluate of Protein A affinity capture is at low pH, and by using this and holding at around pH 3.5 for 30 to 60 minutes, enveloped viruses are inactivated. Precipitates and aggregates formed on neutralization are removed by depth filtration + 0.2 µm filtration, lowering the load on the subsequent chromatography steps. A standard step that forms one wheel of the two-tier defense (inactivation + removal) for ensuring viral safety.

Alternatives in practice

Viral inactivation with detergent · Polysorbate 80 + tri-n-butyl phosphate (TnBP) from Merck/Croda, or environmentally friendly alternatives of the Nonidet/Simulsol type (Triton X-100 replacements) — For enveloped viruses. Useful for molecules not tolerant of low pH; a removal step is required
Continuous inline low-pH inactivation (coil residence type) · Cytiva ÄKTA readyflux + residence coil (incubation chamber), Sartorius Resolute BioSC Inactivation — Residence time distribution is designed and the operation made continuous. No batch hold tank required

Removal of homodimers, half-antibodies, mispaired species and aggregates (the core of this modality) (polishing step / HIC)

The difficulty unique to this modality: separating homodimers and mispaired species with the same pI as the target by differences in hydrophobicity.

Standard method: Hydrophobic interaction chromatography
Equipment: Chromatography system
Makers: Cytiva, Bio-Rad, Sartorius, Tosoh
Reagents and materials: HIC resin, Mixed mode resin

Why it is the standard. Homodimers have almost the same pI as the target and are hard to separate by IEX, so HIC, which uses differences in surface hydrophobicity, is a promising solution of a separate principle (MDPI Membranes 15/10/299, PMC12566258). Capto MMC ImpRes mixed mode can remove half-antibodies, hole-hole homodimers and aggregates simultaneously (ScienceDirect S1046592819304851). Homodimer removal by AEX weak partitioning has also been reported (ResearchGate 371184890).

Check also. This is the core of the modality, but homodimers and mispaired species have properties close to the target, and there is no guarantee that a single hydrophobic interaction chromatography (HIC) step can separate them completely. Conditions are worked out for each product, and ion exchange or mixed mode is layered on as a separate principle when necessary. The final judgment on whether separation was achieved is backed up by mass spectrometry (LC-MS).

About this step and the alternatives

This is the biggest difficulty unique to bispecific antibodies. It is the core step that sorts out of the target the homodimers in which identical chains have paired, the half-antibodies that are only half, the mispaired species with the light chains swapped, and the aggregates.

The troublesome part is that these have almost the same molecular weight and charge as the target. They are hard to tell apart by ion exchange, which separates by differences in charge. So attention goes to small differences in how poorly the molecular surface mixes with water (hydrophobicity).

Hydrophobic interaction chromatography scoops out only the target, relying on that subtle difference. The optimal solution varies by product, so separate separation principles are combined to polish the material.

This is the biggest difficulty unique to bispecific antibodies. Homodimers and mispaired species have nearly the same molecular weight and charge as the target and are hard to separate by ordinary ion exchange. A method that uses small differences in hydrophobicity is therefore the key. The optimal step varies by product, so multiple separation principles are combined, each of a different principle.

Alternatives in practice

Mixed-mode chromatography · Cytiva Capto MMC ImpRes / Capto adhere ImpRes, Tosoh TOYOPEARL MX-Trp-650M — Removes homodimers, half-antibodies and aggregates together. No high salt required; yield needs development
AEX weak partitioning · Cytiva Capto Q ImpRes / Capto adhere, Sartorius Sartobind Q (membrane) — Weakly retains impurities under flow-through conditions. Reduces HCP/DNA/aggregates
CEX bind-and-elute polishing · Cytiva Capto SP ImpRes、Tosoh TOYOPEARL GigaCap S-650M、Bio-Rad Nuvia HR-S — Separates mispaired species and aggregates by charge differences. Salt gradient elution
Preparative SEC · Cytiva Superdex 200 prep grade / HiScale columns, Tosoh TOYOPEARL HW-55 — Separates aggregates/fragments by size. Low throughput, limited at scale

Anion exchange chromatography (flow-through polishing): the 2nd polishing step that removes HCP, host DNA, leached Protein A, endotoxin and residual virus by a separate principle (2nd polishing / AEX flow-through)

The standard finishing polish that removes trace HCP/DNA/leached Protein A and residual virus. HIC alone is insufficient

Standard method: The antibody is passed through (flow-through) at neutral pH and low to medium conductivity, and negatively charged impurities are adsorbed and removed on the AEX membrane/resin
Equipment: Chromatography system, Membrane chromatography
Makers: Cytiva, Bio-Rad, Merck, Sartorius, Pall
Reagents and materials: AEX resin

Why it is the standard. AEX flow-through is the industry-standard polish for removing HCP, DNA, leached Protein A and virus. Capto adhere is multimodal and also effective for impurities with a close isoelectric point. Converting to membrane adsorbers raises productivity.

About this step and the alternatives

A second polishing step that polishes further by a different yardstick after aggregates and mispaired species have been removed by differences in hydrophobicity. It is the finishing side of the standard antibody purification configuration of capture plus 2 polishing steps.

At neutral pH and a low salt concentration, the target antibody passes straight through (flow-through) and only the negatively charged impurities stick to the column. The targets are DNA, acidic host protein, leached Protein A, endotoxin and so on.

Because the principle differs from the preceding step, they can complement each other for what was left behind. It also works on removing residual virus and brings the purity closer to the product specification.

Capture plus 2 polishing steps (HIC etc. + ion exchange) is the standard configuration of an antibody purification platform. After aggregates and mispaired species are removed by HIC, AEX flow-through adsorbs and removes negatively charged impurities (DNA, acidic HCP, leached Protein A, endotoxin) and also contributes to viral clearance. HIC alone often does not reach the purity and viral safety needed to meet specification.

Alternatives in practice

Cation exchange (bind-and-elute polishing) · Cytiva Capto SP ImpRes、Tosoh TOYOPEARL GigaCap S-650M — Binds on the basic side and elutes with a salt gradient. Separates charge variants/aggregates
Ceramic hydroxyapatite (mixed-mode) polishing · Bio-Rad CHT Ceramic Hydroxyapatite (Type I/II) / CFT — Removes aggregates and leached Protein A through phosphate/calcium multimodal retention

Ensuring viral safety, buffer exchange and concentration (virus filtration + UF/DF)

Because of mammalian cell origin, viral safety is secured by 2 means of different systems, and the final buffer exchange and concentration are completed.

Standard method: Nanofiltration (virus filtration, about 20 nm) + ultrafiltration/diafiltration (UF/DF)
Equipment: Virus removal filter, TFF system
Makers: Cytiva, Merck, Sartorius, Pall, Asahi Kasei
Reagents and materials: TFF cassette, Prefilter

Why it is the standard. The 2 means of different systems, low-pH inactivation (Protein A step) + nanofiltration, are the viral safety platform for mAbs/bispecifics (ICH Q5A). Performing final buffer exchange and concentration by UF/DF is the industry standard.

About this step and the alternatives

Since the product is made in mammalian cells, preparation against any viral contamination is layered. The fluid is passed through an extremely fine mesh of about 20 nm (nanofiltration), which lets the antibody through and physically sieves out viruses.

Next, ultrafiltration/diafiltration (UF/DF) exchanges the contents of the fluid into the buffer for the final formulation and concentrates it to the target concentration.

The final touch of viral safety and adjustment of the solution properties for formulation. Both safety and formulation suitability are set at this stage at the same time.

Since the product is made in mammalian cells, preparation against any viral contamination must be layered two and three deep. The material is then exchanged into the final formulation buffer and concentrated to the target concentration. Both safety and suitability for formulation are completed here.

Alternatives in practice

Solvent/detergent (S/D) treatment · TnBP + polysorbate 80 / Triton X-100 replacement detergents, removed downstream by chromatography/TFF — Complements enveloped virus inactivation. A removal step is required separately
Single-pass TFF (SPTFF) · Merck Pellicon 3 Single-Pass / Cadence, Cytiva ÄKTA flux SPTFF configuration — Continuous concentration without recycle. Process linking and reduced residence
Viral clearance at the chromatography step (AEX flow-through) · Sartorius Sartobind Q、Cytiva Capto Q ImpRes — Adds to the LRV by chromatography in addition to filtration. Robust with membranes

UF/DF and final filtration: concentration to the target concentration by ultrafiltration, replacement with formulation buffer by diafiltration, and 0.2 µm bioburden reduction filtration to give the drug substance (bulk) (drug substance preparation and final filtration)

Final buffer exchange and concentration, and finishing the drug substance by 0.2 µm bioburden reduction filtration; an essential final DSP step before formulation

Standard method: Ultrafiltration membrane
Equipment: TFF system, Sterile filter (0.2 µm)
Makers: Cytiva, Merck, Sartorius, Repligen
Reagents and materials: TFF cassette, Excipients and stabilizers

Why it is the standard. UF/DF is the standard final DSP operation that performs concentration and buffer exchange of high-concentration mAb drug substance together. Regenerated cellulose membranes have low adsorption and suppress aggregation. Sartopore 2 has a track record for bioburden protection with its redundant layer configuration.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Without Fc, blood half-life is extremely short (about 2 hours for BiTE). The formulation and dosing design change from a stable liquid with ordinary subcutaneous/intravenous administration to a lyophilized formulation + continuous intravenous infusion (several weeks) (e.g., blinatumomab = Blincyto).
Method: Formulation design premised on lyophilized formulation and continuous intravenous infusion after reconstitution and dilution. Stabilizer screening is also optimized for continuous administration and long-term stability at low concentration
Check also: Because of the short half-life, continuous infusion by pump is essential, and additional evaluation of stability in the infusion bag (IV solution stabilizer) is required

About this step and the alternatives

Nanofiltration only sieves out viruses and neither concentrates nor exchanges buffer. So UF/DF using tangential flow filtration concentrates the material to the target protein concentration and replaces the liquid with the formulation buffer.

Since the method draws off only water and small molecules while the liquid flows along the membrane surface, the antibody itself is not damaged and can be made concentrated and to the target composition.

Finally, 0.2 µm filtration reduces microorganisms (bioburden), giving bulk drug substance of specified concentration and formulation. It is the final unit operation of downstream processing, which hands the material to filling or frozen storage.

Nanofiltration (virus removal filtration) performs neither buffer exchange nor concentration, so before formulation the material must be concentrated to the target protein concentration by UF/DF using tangential flow filtration and replaced with the formulation buffer. Finally, 0.2 µm bioburden reduction filtration is performed, and the bulk drug substance of specified concentration and formulation goes on to filling and frozen storage. This is the final unit operation of the DSP.

Alternatives in practice

Continuous concentration by single-pass TFF (SPTFF) · Merck Pellicon 3 Single-Pass / Cadence Inline Concentrator, Repligen TangenX — High-concentration formulations at low shear and continuously. Small equipment footprint
UF/DF by hollow fiber TFF · Cytiva ÄKTA flux + hollow fiber cartridges, Repligen KrosFlo TFF — Suited to shear-sensitive molecules. Optimized by membrane area/flow path design

Phase 5Formulation and fill-finish

Stable liquid / lyophilized formulation and aseptic filling (fill-finish)

Protects the aggregation-prone molecule with stabilizers and surfactant, and secures shelf life by aseptic filling.

Standard method: Liquid formulation based on excipient screening (buffer + sugar/amino acid + polysorbate) + sterile filtration and filling
Equipment: Single-use mixers, Sterile filter (0.2 µm), Vial filling machine, Lyophilizer
Makers: Cytiva, Merck, Sartorius, GEA, Syntegon
Reagents and materials: Excipients and stabilizers, Surfactants

Why it is the standard. Measures for high concentration, stability, viscosity and interfacial stress follow the established principles of mAb formulation (ICH Q8). Some bispecifics are highly unstable as molecules, and the selection of polysorbate and stabilizing sugars/amino acids is the key to controlling aggregation and particles.

Check also. How readily bispecifics aggregate and how vulnerable they are to interfaces and freeze-thaw differ greatly by format, and the formulation of an ordinary monoclonal antibody often cannot be reused as is. Excipients are screened again for each molecule and decided by measuring the behavior of particles and aggregation.

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Because it is a single-chain molecule, there is no heavy chain heterodimer correct pairing rate, mispairing or homodimer to evaluate. LC-MS focuses on intact mass, sequence confirmation and evaluation of misfolded/fragmented species.
Method: Confirmation of mass and single-chain identity by intact LC-MS. Quantitation of correct chain pairing rate/mispairing and homodimers is out of scope
Check also: If a two-chain format (e.g., Fab×scFv) is adopted, some chain pairing evaluation remains

About this step and the alternatives

Depending on its shape (format), a bispecific antibody can aggregate more readily than an ordinary antibody and may also be vulnerable to contact with liquid surfaces and to freeze-thaw. A step that firms up its protection through the formulation is therefore needed.

The buffer, sugars and amino acids, and polysorbate selected by excipient screening are blended, and the antibody is protected by stabilizers and surfactant. The product is then filled into containers while kept sterile.

Whether particles and aggregation can be suppressed is directly tied to the product's shelf life. Protective formulation and aseptic technique secure quality until it reaches the patient.

Depending on the format, a bispecific antibody can aggregate more readily than an ordinary mAb and can also be vulnerable to interfaces and freeze-thaw. It is therefore protected with stabilizers and surfactant, filled aseptically, and given shelf life. Suppressing particles and aggregation is directly tied to quality.

Alternatives in practice

Lyophilized formulation · SP Scientific LyoStar / LyoConstellation, GEA / IMA Life freeze dryers — Advantageous for long-term stability and room-temperature transport. Increases development and cost; a reconstitution step is required
High-concentration subcutaneous (SC) formulation · High-concentration UF/DF (Pellicon 3) + viscosity evaluation (m-VROC rheometer), PFS/autoinjector filling — Home administration at over 100 mg/mL. Viscosity and aggregation control are challenges
Self-buffering formulation (no adjuvant/external buffer needed) · Uses the buffering capacity of the antibody itself; the above preparation tanks and aseptic filling — Minimizes excipients. Verification of pH stability is essential

Phase 6Quality attributes and analytics

Identification and quantitation of the correct chain pairing rate and of mispaired/homodimer impurities (CQA) (intact/subunit mass spectrometry)

Correct pairing of the 4 chains is the premise of binding 2 antigens, and mispairing is the center of quality, directly tied to potency and safety.

Standard method: Intact/subunit LC-MS
Equipment: LC-MS (liquid chromatography mass spectrometry), HIC-HPLC for DAR
Makers: Agilent, Waters, Bruker, Thermo Fisher, New England Biolabs
Reagents and materials: Analytical antibody enzymes, Disulfide reduction alkylation reagents

Why it is the standard. For mispairing in asymmetric bispecifics, a representative approach is to systematize it with 3 approaches based on different principles: rapid intact LC-MS for early development, a QC-friendly HIC method for release testing, and peak identification by 2D-LC/MS (mAbs 2018.1511198, PMC6284573). Hole-hole homodimers can be monitored by analytical HIC (PubMed 31344474). Highly sensitive quantitation of homodimers by mixed-mode SEC × native MS has also been reported (Anal. Chem. 2019, acs.analchem.9b02793).

Check also. Intact/subunit LC-MS (mass spectrometry) is central to confirming correct pairing, but when the mass difference between the mispaired species and the target is very small, as with a common light chain, intact mass alone may not tell them apart. The material is cut down to subunits or peptides and confirmed from a different angle.

About this step and the alternatives

To bind 2 kinds of antigen correctly, it is a major premise that the 4 chains are assembled as intended. This step confirms that correct pairing rate by LC-MS, which measures mass precisely.

Impurities such as mispaired species and homodimers are identified and quantified from both the mass of the whole (intact) molecule and the mass of the parts it has been broken into (subunits).

Mispaired species not only lose potency but can also bear on safety. Continuing to measure mispairing from process development through release testing is at the center of quality control for this modality.

To bind 2 antigens correctly, it is a major premise that the 4 chains are assembled correctly. Mispaired species and homodimers not only lose potency but also bear on safety. So quantifying mispairing continuously, from process development through release testing, is at the center of quality.

Alternatives in practice

Native MS · Thermo Fisher Q Exactive UHMR / Orbitrap Ascend, Waters BioAccord (mixed-mode SEC or direct injection) — Directly identifies the masses of correctly paired/mispaired species under non-denaturing conditions. Instrument/conditions are specialized
Reversed-phase LC-MS (RP-UPLC-MS) · Waters ACQUITY BioResolve RP mAb Polyphenyl column + BioAccord / Xevo — High separation of subunit masses. Good resolution of glycans/PTMs
2D-LC-MS (HIC/IEX × RP-MS) · Agilent 1290 Infinity II 2D-LC + 6545XT QTOF, Waters 2D configuration — Species separation in the 1st dimension, then MS. Comprehensive analysis of complex mispaired species
Disulfide bond mapping · Non-reduced peptide map + Orbitrap Exploris / Eclipse — Confirms interchain disulfide connectivity. Detects half-antibodies/scrambling

Quantitation of size variants such as aggregates, fragments and half-antibodies (CQA) (SEC + CE-SDS)

Aggregates are tied to immunogenicity, and fragments and half-antibodies directly to reduced potency, so size purity is cross-checked by multiple principles.

Standard method: SEC (quantitation of aggregates/fragments) + CE-SDS (size purity under reduced/non-reduced conditions)
Equipment: SEC-HPLC, CE-SDS
Makers: Agilent, Waters, SCIEX, ProteinSimple, Bio-Techne
Reagents and materials: Disulfide reduction alkylation reagents

Why it is the standard. SEC-MALS is the industry standard used for molecular weight identification of monomer, dimer, oligomers, aggregates and hinge fragments. CE-SDS is a separate method for evaluating size purity under denaturing conditions: non-reduced for intact purity, and reduced for monitoring heavy chain / light chain / non-glycosylated heavy chain (ICH Q6B).

Check also. Size exclusion chromatography (SEC) can underestimate aggregates because of dilution and interaction with the column, and the half-antibody can be confused with the normal molecule by size alone. It is combined with CE-SDS (separation by electrophoresis), and the results are cross-checked by multiple principles.

About this step and the alternatives

Aggregates are directly tied to immunogenicity (the risk that the body regards them as foreign), and fragments and half-antibodies lead to reduced potency. This step quantifies such species of different size and assures size purity.

SEC, which separates by size, measures the amount of aggregates and fragments, and CE-SDS confirms the size purity under reduced and non-reduced conditions respectively. The half-antibody unique to bispecifics is also a detection target.

Multiple methods of different principles are used, and the results are cross-checked against each other. Making up for the blind spot of one measurement with another method makes size purity solid.

Aggregates are directly tied to immunogenicity risk, and fragments and half-antibodies lead to reduced potency. Bispecifics also produce a size species unique to them, the half-antibody. So measuring size purity by multiple principles and cross-checking each against the others is indispensable.

Alternatives in practice

SEC-MALS (multi-angle light scattering) · Wyatt/Waters DAWN + Optilab dRI + ACQUITY/Agilent LC — Determines the true order of aggregates from absolute molecular weight. Corrects for column interaction
Analytical ultracentrifugation (AUC, sedimentation velocity) · Beckman Coulter Optima AUC (successor to ProteomeLab XL-I) — Matrix-free aggregate quantitation. Orthogonal confirmation; low throughput
MFI / subvisible particle measurement · ProteinSimple (Bio-Techne) MFI 5000, HORIBA / light obscuration HIAC (Beckman Coulter) — Counts subvisible particles of 2–100 µm. Formulation stability evaluation

Domain-by-domain evaluation of charge variants and PTMs such as deamidation/oxidation (CQA) (charge heterogeneity / icIEF)

Deamidation and oxidation change potency and stability, so which arm is modified is evaluated separately for each domain.

Standard method: icIEF
Equipment: icIEF, LC-MS (liquid chromatography mass spectrometry), CEX-HPLC
Makers: Bio-Techne, SCIEX, Thermo Fisher, Cytiva, Agilent, Waters, Promega
Reagents and materials: Carrier ampholytes pI markers, Peptide mapping protease, Disulfide reduction alkylation reagents

Why it is the standard. icIEF is the industry standard for charge variant analysis for its high resolution, short run time and QC suitability (chromatographyonline). For bispecifics, domain-specific charge variants and PTMs can be detected by cutting the molecule into F(ab)2/Fc etc. with IdeS and then performing reduced, denaturing icIEF (ScienceDirect S0731708524001602). In anti-CD3 bispecifics, PTM susceptibility differs between the arms (PMC11926628).

With Fragment-based, Fc-free (tandem scFv / BiTE type)

Fragments without Fc (especially E. coli expression) have no N-glycans (aglycosylated), so N-glycan profiling does not apply.
Method: Because they are aglycosylated, N-glycan HILIC analysis is not applicable (out of scope)
Check also: Glycan evaluation is needed only for designs expressed in mammalian cells that have glycosylation sites

About this step and the alternatives

Small chemical modifications of a protein (PTMs), such as deamidation, oxidation and C-terminal lysine, change the charge of the molecule and can affect potency and stability. This step captures those changes in charge.

icIEF, which separates components by differences in charge, is used to separate charge variants. In a bispecific, which of the 2 arms a modification occurred on determines potency.

So the whole is not viewed all at once; it is broken down domain by domain to grasp what kind of modification has occurred where. Site-specific control supports quality.

Modifications such as deamidation, oxidation and C-terminal lysine change the charge and can affect potency and stability. In bispecifics, which arm a modification occurs on matters for potency. So charge variants and PTMs are evaluated separately for each domain.

Alternatives in practice

CEX charge variant analysis · Thermo Fisher MAbPac SCX-10 / ProPac WCX, Agilent Bio MAb / 1290 Bio LC — Separates and quantifies acidic/basic species by chromatography. MS coupling is also possible
Native CE-MS (microfluidic) · SCIEX/908 Devices ZipChip + Orbitrap, SCIEX CESI 8000 Plus + MS — Separates charge variants while identifying mass. Trace amounts, high separation
Middle-up subunit LC-MS · Genovis IdeS treatment + Waters BioAccord / Xevo — Evaluates deamidation/oxidation sites separately for Fc and Fab

Quantitation of the N-glycan (Fc glycan) profile (CQA) (N-glycan profile / HILIC)

Fc glycans determine effector function and PK through FcγR/FcRn binding, so composition is managed lot by lot.

Standard method: Fluorescently labeled HILIC of released N-glycans
Equipment: HILIC, LC-MS (liquid chromatography mass spectrometry)
Makers: Agilent, Waters, SCIEX, Bio-Techne, NEB
Reagents and materials: Analytical antibody enzymes, N-glycan release and labeling (RapiFluor-MS / 2-AB)

Why it is the standard. The reference method for identification and quantitation of N-glycans is "release, dye labeling (2-AB/RFMS), then HILIC-FLD or HILIC-MS" (PMC8617915). Middle-up HILIC-MS can complement the released-glycan method as a MAM. Fc glycans are a CQA directly tied to effector function and PK (ICH Q6B).

About this step and the alternatives

Sugar chains (glycans) hang from the Fc region of the antibody. Through binding to receptors such as FcγR and FcRn, they determine effector function and blood half-life.

The glycans are cut off from the main body, tagged with a fluorescent label, and the N-glycan profile is quantified by a method called HILIC. The presence or absence of fucose and the proportions of galactose and high mannose are measured.

These proportions affect potency and clearance (how the antibody disappears from the body). So it is required to grasp the glycan composition numerically and keep it constant even when the lot changes.

Fc glycans influence effector function and half-life through FcγR and FcRn binding. The proportions of afucosylation, galactosylation and high mannose affect potency and clearance. So glycan composition is quantified and controlled between lots.

Alternatives in practice

Middle-up glycan profile · Genovis IdeS + reduction, then HILIC/mass analysis of Fc/2 subunits on Waters BioAccord — Profiles Fc glycans in their molecular context without release. Simplified pretreatment
Glycan CE-LIF · SCIEX PA800 Plus + Fast Glycan (APTS labeling) kit, Bio-Techne Maurice — High separation and high sensitivity with APTS labeling. Strong for sialic acid isomers
Site-specific glycan analysis by peptide map · Trypsin digestion + Thermo Fisher Orbitrap Exploris / Eclipse — Identifies occupancy and minor heterogeneity at each glycosylation site. Greatest information content

Evaluation of binding to 2 antigens and of biological activity reflecting the mechanism of action (CQA) (binding activity and biological activity / SPR, bioassay)

Confirms simultaneous binding of 2 antigens, the reason bispecifics exist, and potency that reflects the mechanism of action, to assure efficacy.

Standard method: Confirmation of sequential, simultaneous binding to 2 antigens by SPR (surface plasmon resonance) + cell-based bioassay (e.g.
Equipment: SPR surface plasmon resonance, BLI, Plate reader, Flow cytometry
Makers: Cytiva, Sartorius, Agilent, Tecan, Gator Bio
Reagents and materials: SPR / BLI sensor chips, Reporter gene bioassay ADCC CDC

Why it is the standard. The "simultaneous binding of 2 antigens" that is the essence of a bispecific can be demonstrated by saturating one antigen on SPR and then injecting the other and confirming the second-stage signal (the same applies with the injection order reversed). Binding to both targets is typically evaluated with ELISA/SPR/cell-based dual-function bridging systems (PMC9548515). For release potency, a cell-based assay reflecting the mechanism of action is the reference (ICH Q6B).

Check also. Simultaneous binding of 2 antigens by surface plasmon resonance (SPR) physically backs up the function of the molecule, but is by itself a separate concept from regulatory potency. The potency for release decisions is set by a cell-based bioassay that reflects the mechanism of action, such as cell cross-linking.

About this step and the alternatives

The reason for a bispecific antibody to exist is "binding to 2 targets simultaneously." It is meaningless if it works on only one. This step therefore verifies binding to the 2 antigens and the mechanism of action from the functional side.

SPR, which captures binding in real time through the reflection of light, confirms sequential and simultaneous binding to the 2 antigens. It checks whether the antibody can grasp the other while still holding one.

In addition, a bioassay using cells measures potency that reflects the actual action, such as cross-linking cells. It is the finishing step that assures, through potency, that the antibody functions as designed.

The reason for a bispecific antibody to exist is "binding to 2 targets simultaneously." It is meaningless if only one side works. So binding to both antigens, especially simultaneous binding, is confirmed, and potency is assured with a potency measure that reflects the mechanism of action, such as cross-linking cells.

Alternatives in practice

Dual-target bridging ELISA · Tecan / Agilent BioTek automated ELISA workstations, standard microplate readers — Capture with antigen 1, detect with antigen 2 to confirm bivalent binding. Simple, high throughput
BLI · Sartorius Octet R8 / RH96, Gator Bio Gator Prime — Sequential binding of 2 antigens, label-free and in parallel. Also robust with crude samples
Reporter gene bioassay · Promega reporter bioassay systems (NFAT-luc, etc.) + GloMax / PHERAstar — Quantifies MOA-linked function. GMP-compatible, low variability
Cytotoxicity assay (TDCC, etc.) / flow cytometry · Cytek Aurora / BD FACSymphony, Promega CellTox/CytoTox detection + plate reader — Directly evaluates T-cell redirected killing. High biological relevance

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).