Antibody fragments

Everything starts with deciding "what to make, and how to have it made." You choose the form of the fragment (Fab/Fab'/scFv), a piece cut out of the antibody, and build its blueprint into the host.

The key is choosing the producer. It is a small molecule that does not need sugar decorations, so it can be made cheaply and quickly in E. coli rather than in mammalian cells. But it does not work unless the disulfide bonds that tie the chains together form correctly.

So many processes put secretion into the periplasm (the gap inside the outer membrane of the bacterium), an oxidizing environment, first. It is a move that secures, at the very start, the ease of forming those bonds.

Antibody fragments are small and carry no glycans, so they can be made cheaply and quickly in E. coli rather than in mammalian cells. However, Fab/Fab' do not function unless the intrachain and interchain disulfide bonds are assembled correctly. Many processes therefore make secretion into the oxidizing environment of the periplasm the first choice.

Alternatives in practice

• Cytoplasmic oxidative folding expression · New England Biolabs SHuffle T7 Express (ΔtrxB/Δgor + DsbC) / ATUM CyDisco system (Erv1p + PDI co-expression) — Soluble expression and disulfide formation in the cytoplasm without making inclusion bodies. For high-level expression where secretion is difficult.
• Inclusion body expression + in vitro refolding · High-expression strains (BL21(DE3) / pET system, Merck Novagen) + high-pressure homogenizer (Avestin EmulsiFlex-C3 / GEA PandaPLUS) — For toxic or aggregation-prone sequences at high yield. Refolding is required downstream.
• Secretion expression in methylotrophic yeast · Pichia pastoris (Komagataella phaffii, BioGrammatics/ATUM strains) + Sartorius BIOSTAT STR — Capacity for post-translational modification and disulfide formation. Note glycan addition.
• Cell-free protein synthesis (CFPS) · New England Biolabs PURExpress / Thermo 1-Step Human CFPS / Sutro Biopharma XpressCF system — For rapid screening and incorporation of non-natural amino acids. Limited economies of scale.

This is the central step where the product is "actually made." E. coli is grown in defined medium to as high a density as possible, and at the right moment expression is induced, as if flipping a switch.

The aim is to draw out enough material from the limited volume of the tank. The cells are fattened up with nutrients added in succession, and the timing of induction and the temperature are tuned finely.

The hard part comes after induction. As fragment accumulates, stress is put on the cells, and early lysis and degradation easily lower quality. That is why management of operation that balances yield and quality is the key.

To obtain enough fragment from a limited culture volume, cells are grown to as high a density as possible before expression is induced. After induction, however, cell stress from fragment accumulation, early lysis and protease degradation tend to occur, so management of operating conditions that balance yield and quality is the key.

Alternatives in practice

• Continuous perfusion / high-density inoculation · Sartorius BIOSTAT STR + ATF (Repligen XCell ATF 6/10) or TFF perfusion — Concentrated inoculation from the cell bank shortens start-up and reaches ultra-high density.
• Batch culture · Eppendorf BioFlo 320 / DASGIP parallel bioreactors — Simple process, suited to screening. Low volumetric productivity.
• PAT real-time control · Sartorius BioPAT Spectro (Raman) / Hamilton Incyte Arc (capacitance viable cell density) / Sartorius BioPAT Fundalux — Substrates/metabolites are monitored in real time by Raman and viable biomass by permittivity, to optimize feeding.

The fragment that has been produced is still locked inside the cells. This step is the recovery work of getting it out. First the cells are collected by centrifugation.

Next, the product accumulated in the periplasm is released. Osmotic shock or heat extraction is like "loosening the shell and squeezing out the contents."

How gently it is done matters. If done roughly, host proteins and even DNA leak out together, and the later purification load jumps. The key is to aim only at the target while keeping contaminants down.

The target fragment accumulates in the periplasmic space or inside the cell, so after culture the cells must first be collected and the protein then extracted from them. If the extraction is done wrongly, host protein and DNA contaminate the material in excess and the purification load in later steps jumps.

Alternatives in practice

• Whole-cell high-pressure disruption · Avestin EmulsiFlex-C160 / GEA PandaPLUS (for inclusion body / cytoplasmic expression) — Total solubilization rather than selective periplasmic extraction. Increases HCP and DNA load.
• Clarification and cell removal by TFF · Cytiva ÄKTA readyflux + ULTA hollow fiber (0.2 µm) / Repligen KrosFlo hollow fiber — Continuous clarification without centrifugation. Easy to convert to single-use.
• Inclusion body washing + solubilization · High-pressure homogenizer (Avestin) + centrifuge (Beckman Avanti) — Inclusion bodies are washed with detergent / low-concentration urea and then solubilized with a high-concentration denaturant. Pretreatment for refolding.

The extracted fluid is still turbid, mixed with cell fragments, nucleic acids and the seeds of cloudiness. This step is the "groundwork" that turns it into a crystal-clear load solution.

A disc-stack centrifuge throws off the large particles, and a depth filter with graded density (finished at 0.45/0.2 µm if needed) removes even the fine cloudiness.

Why go to the trouble? Because fluid that is passed without clarifying clogs the next affinity column, raises backpressure and lowers yield. This step builds the foundation for stable chromatography.

Periplasmic extracts from E. coli contain large amounts of cell debris, nucleic acids and colloidal impurities, and loading them as is causes clogging of the affinity column, rising backpressure and lower yield. This is the standard step that clarifies by continuous centrifugation and a graded-density depth filter (finished with 0.45/0.2 µm as needed) to secure a stable chromatography load.

Alternatives in practice

• Depth filters from alternative manufacturers · Sartorius Sartoclear Stax / 3M Zeta Plus Encapsulated / Pall Stax — Spreads supply risk and allows performance comparison. Clarity is optimized by choice of charge/grade.
• Centrifuge-free depth + body feed · Merck Clarisolve (with body feed / flocculation pretreatment) + Millistak+ — Flocculant and body feed directly clarify high-turbidity harvest without centrifugation.
• Tangential flow microfiltration (MF) · Cytiva ULTA / Repligen KrosFlo hollow fiber (0.2/0.45 µm) — Membrane-based continuous, closed-system clarification. Flux management is required.

Real purification starts here. The first move is "grabbing." A dedicated resin that sticks only to the light chain or CH1 domain selects and catches only the target fragment.

While it is held, large amounts of contaminants such as host protein, DNA and endotoxin can be shaken off at once. Purity and concentration rise sharply in one step.

How much contamination can be dropped in this first capture determines how difficult the later purification design is. It is the linchpin of purification, building the foundation of overall recovery and purity.

The clarified fluid contains large amounts of host protein, DNA and endotoxin. In the first capture step, only the target fragment is specifically grabbed and contaminants are dropped all at once, which simplifies the design of subsequent steps and determines the overall recovery and purity.

Alternatives in practice

• Protein L affinity (κ VL binding) · Cytiva Capto L (17547801) / MabSelect VL (prepacked: HiTrap) — Captures Fab/scFv/dAb broadly regardless of light chain type. Unsuitable for λ-only molecules.
• Immobilized metal affinity chromatography (IMAC) · Cytiva Ni Sepharose excel/Capto / Thermo HisPur Ni-NTA / Roche cOmplete His-Tag — For His-tagged constructs. Note tag removal and host-derived metal-binding impurities.
• Non-affinity capture by mixed-mode / ion exchange · Cytiva Capto MMC ImpRes / Capto adhere / Merck Eshmuno CMX — Lower ligand cost and no ligand leakage. Heavy burden of condition optimization.

Fragment extracted from inclusion bodies or the cytoplasm is in a denatured state, unraveled like a string. This step is the work of "folding it again" into the correct three-dimensional structure.

The procedure is to loosen the denaturation in stages while having disulfide bonds form under redox conditions. Unless the bonds form correctly, the fragment has no ability to bind the target in the first place.

The difficulty is the competition with aggregation. Refolding and clumping are a hair's breadth apart, and the degree of dilution, the redox balance and the choice of additives directly determine yield.

Fragment extracted from inclusion bodies or the cytoplasm is denatured, without its disulfide bonds formed. Unless it is refolded into the correct three-dimensional structure, binding activity does not appear. Refolding competes with aggregation, so the design of dilution, redox and additive conditions determines the yield.

Alternatives in practice

• On-column (SEC/IEX) refolding · Cytiva HiLoad Superdex 200 / Capto S ImpRes + ÄKTA — Refolds while passing a denaturant gradient, keeping concentration without a dilution tank.
• Detergent-mediated refolding · Dilution/dialysis system + addition of sodium lauroyl-L-glutamate, etc. — Suppresses aggregation through micelles. Detergent removal is required downstream.
• Switch to periplasmic secretion (no refolding needed) · DsbC co-expression secretion system of step [0] (E. coli W3110 + pelB/STII) — A design change that avoids making denatured material in the first place. Trade-off with expression level.

Even after purity is raised greatly by capture, subtle impurities remain that cannot be removed completely. This step is the final polishing.

What remains are dimers and high-molecular-weight aggregates, host protein, endotoxin and variants generated during elution. These are separated out by cation exchange or the like, which uses differences in charge.

The goal is to drop them to within specification. Only after passing this step is the product finished at pharmaceutical-grade purity usable in injectables. It is the final polishing that decides product quality.

After capture, dimers and high-molecular-weight aggregates, host protein, endotoxin and variants generated during elution remain. 2-stage polishing that exploits differences in charge and hydrophobicity brings these down to within specification and finishes the material at pharmaceutical-grade purity.

Alternatives in practice

• Hydrophobic interaction chromatography (HIC) · Cytiva Capto Phenyl (high sub) / Butyl Sepharose HP, Tosoh TOYOPEARL PPG-600M — Loaded at high salt, separates aggregates and hydrophobic variants. Dilution and salt load are issues.
• Mixed-mode chromatography · Cytiva Capto adhere / Capto MMC ImpRes, Merck Eshmuno HCX — Dual selectivity of charge + hydrophobicity removes HCP/aggregates. Condition screening is needed.
• Endotoxin-removal membrane adsorber (flow-through polishing) · Sartorius Sartobind Q / STIC PA, Pall Mustang Q — High flow rate and single-use polishing of DNA/endotoxin/HCP. Capacity is lower than resin.

Being small, the fragment is excreted quickly by the kidney and cannot stay long in the blood. This step is a modification that compensates for that weakness by "adding a weight."

The method is to attach a large PEG with a maleimide, only at the targeted position, to the free thiol (the reactive sulfur "hand") of the cysteine in the hinge of Fab'.

Because PEG increases the apparent molecular size, the time spent in the body is extended and the number of doses can be reduced. Cimzia (certolizumab pegol) is a representative example.

Fab is small and rapidly cleared by the kidney, so its blood half-life tends to be short. Attaching a large PEG site-specifically to the hinge cysteine of Fab' increases the apparent molecular size, prolongs residence in the body and allows the dosing frequency to be reduced. Cimzia is a representative example.

Alternatives in practice

• Genetic-fusion half-life extension (PAS/XTEN) · PASylation (XL-protein GmbH) / XTEN (Amunix/Sanofi) constructs + the expression system of [0] — No chemical modification needed, and homogeneous. Expression/purification behavior changes with the increase in molecular weight.
• Albumin-binding domain / anti-HSA fusion · ABD fusion (Affibody) / anti-HSA VHH fusion constructs — Binds endogenous albumin and extends half-life via FcRn. No PEG needed.
• Random (amine) PEGylation · NHS-ester PEG (NOF SUNBRIGHT MEGC series / JenKem mPEG-NHS) + reaction tank — Targets Lys side chains. Simple, but non-site-specific, with risks of heterogeneity and reduced activity.

When making a product in E. coli, endotoxin cannot be avoided. It is an outer membrane component of Gram-negative bacteria and causes fever even in minute amounts, so this step brings it down to a safe level.

The procedure is to reduce endotoxin, together with the remaining DNA, to specification with AEX (anion exchange) or membrane adsorbers. In addition, 0.2 µm filtration removes particulates and microorganisms.

In E. coli systems, where the risk of mammalian viruses is low, this is the biggest safety issue. It is the linchpin of safety assurance, finishing the product in a state that can be used with confidence as an injectable.

In E. coli systems the risk of mammalian viruses is low, but endotoxin derived from Gram-negative bacteria is the biggest safety issue. By reducing endotoxin to specification in the final steps and removing particulates and microorganisms by filtration, safety as an injectable is assured.

Alternatives in practice

• Virus removal nanofiltration · Asahi Kasei Planova 20N/BioEX, Merck Viresolve Pro, Sartorius Virosart HF — For assurance when derived from mammalian/yeast. May not be required in E. coli systems.
• Low-pH viral inactivation · Hold tank + pH/inline control (ÄKTA/Sartorius) — Inactivates enveloped viruses. Depends on the acid stability of the fragment.
• Endotoxin-specific affinity/adsorptive removal · Polymyxin B-immobilized resins from Merck and other vendors, Sartobind STIC PA (charged membrane) — Dedicated removal for high endotoxin loads. Note the recovery.

Even in E. coli systems, the possibility that viruses enter from outside through the medium, raw materials or the process environment is not zero. This step is the insurance that physically removes them.

The method is size exclusion with a filter of very small pores. Even small parvoviruses are held back by the "sieve" and not let through. It is customary to do this on a clear, low-viscosity pool.

The point is protection by a "different principle." Non-mammalian cell processes cannot use chemical means such as destroying virus at low pH, so another mechanism, filtration, is layered on.

Even for E. coli-derived products, safety against adventitious viruses and small viruses (parvovirus) originating from medium, raw materials and the process environment is required. In non-mammalian cell processes, where chemical means such as low-pH inactivation cannot be used, nanofiltration by size exclusion is incorporated as standard as a virus removal step of a different principle. It is customary to perform it on the clarified, low-viscosity pool after polishing.

Alternatives in practice

• Parvovirus-retentive nanofilter · Asahi Kasei Planova 20N / Merck Viresolve Pro / Sartorius Virosart CPV — Guarantees retention of the smallest parvovirus (~20 nm). Watch for flux decline and yield.
• Dual-layer PES virus removal filter · Merck Viresolve Pro (PES) / Sartorius Virosart HF — High flux, emphasizing throughput. Compatibility to be examined for the different membrane materials.
• General-purpose virus removal filter (retrovirus retention) · Pall Pegasus SV4 / Asahi Planova 35N — Larger pore size with retrovirus-class retention. Small viruses are not guaranteed.

This step prepares the purified fragment as the "drug contents" that can be stored and administered to patients. First it is exchanged into the formulation buffer by UF/DF (ultrafiltration/diafiltration).

Next it is concentrated to the required concentration, and stabilizers and surfactant are added to make it less prone to degradation. Finally it is filled into vials or syringes in a sterile environment.

Not producing aggregates or fine particles here is the deciding factor of quality. It is the final stage of formulation, which sets storage stability and ease of administration at the same time.

The purified fragment is brought to a composition that can be stored stably and administered to patients. The buffer is exchanged into the formulation buffer, concentrated to the required concentration, stabilizers and surfactant are added, and the product is filled aseptically into vials/syringes. Absence of aggregates and particles here is the deciding factor of quality.

Alternatives in practice

• Freeze-dried (lyophilized) formulation · SP Scientific/ATS LyoStar / GEA / IMA Life freeze dryers — For molecules with insufficient liquid stability. Increases manufacturing time and cost.
• High-concentration formulation (for subcutaneous administration) · Sartorius Sartoflow / Merck Pellicon + high-concentration-compatible cassettes, viscosity measurement (m-VROC) — Makes subcutaneous administration possible at >100 mg/mL. Viscosity and aggregation control are important.
• Single-use aseptic filling / isolator filling · Cytiva SU filling / Vanrx SA25 Aseptic Filling Workcell / Groninger Isolator — Reduces cross-contamination and is flexible for small lots. Capital investment/validation.

This is the last gate, just before the formulated bulk is filled aseptically. The bulk is passed through a sterilizing-grade PES membrane (0.2 µm), and the remaining microorganisms are removed by filtration.

Because the protein cannot be damaged by heat or chemicals, sterility is assured by the gentle means of filtration. An integrity test also confirms that "the membrane has no holes."

This is an essential step common to all biopharmaceuticals, not only fragments. It guards the last line of safety: the sterility of the product.

The formulated bulk that has been buffer-exchanged and concentrated by UF/DF must be passed through a 0.2 µm sterilizing-grade filter just before aseptic filling to remove residual microorganisms. The upstream bioburden reduction filtration lowers the microbial count to protect the final filter, and it is the final sterile filtration that assures the sterility of the product without heat or chemicals. Both are essential steps common to all biopharmaceuticals.

Alternatives in practice

• Sterilizing-grade PES membranes from alternative manufacturers · Merck Millipore Express SHC / Durapore — Supply redundancy and compatibility comparison. Verified by bacterial challenge.
• Sterilizing-grade PES filter in a different configuration · Pall Supor EKV / Fluorodyne EX EDF — Option when high throughput is prioritized or low adsorption is required.

From here on, the analyses check that "the intended molecule has been made correctly." This step is the identity check (identification) of the molecule.

Intact mass spectrometry captures the weight of the whole molecule and modifications such as PEGylation, and peptide mapping confirms the whole amino acid sequence and even small changes such as oxidation and deamidation.

The purpose is to prove that "the same molecule has been made without omission" from lot to lot. It is the starting point of quality assurance, backing product identity and integrity at the structural level.

This step proves at the structural level that "the intended molecule has been made correctly." Intact mass confirms the overall molecular weight and modifications such as PEGylation, and peptide mapping confirms the full sequence and minor modifications such as oxidation and deamidation, assuring lot-to-lot identity and integrity.

Alternatives in practice

• N-terminal sequence / amino acid composition analysis · Shimadzu PPSQ-53A Edman sequencer / Hitachi L-8900 amino acid analyzer — N-terminal confirmation and composition verification. Low throughput but orthogonal confirmation.
• Middle-up/subunit mass spectrometry · Waters BioAccord / Thermo Orbitrap + IdeS (Genovis FabRICATOR) digestion — Analyzes the localization of modifications by subunit mass after reduction and limited digestion.
• Native MS / CE-MS · Thermo Q Exactive UHMR (native) / SCIEX/Intabio ZT CE-MS, Agilent 7100 CE — For retaining higher-order structure and online identification of charge variants.

Fragments are molecules prone to aggregation and degradation. This step is a purity test that measures how much clumping (aggregates) and how many cut-off pieces (fragments) there are.

Size exclusion chromatography (SEC) separates molecules in order of size and precisely quantifies the proportions of native monomer, high-molecular-weight aggregates (HMW) and low-molecular-weight fragments (LMW).

Aggregates are especially important, as they are directly tied to the risk of immunogenicity, in which the body regards them as foreign. That is why it is a critical quality attribute that must always be confirmed to be within specification lot by lot.

Aggregates are a critical quality attribute (CQA) directly tied to immunogenicity risk. Fragments are prone to aggregation and degradation, so monomer content and the proportion of high-molecular-weight aggregates and fragments are measured precisely, and compliance with specification is confirmed lot by lot.

Alternatives in practice

• SEC-MALS (absolute molecular weight/aggregates) · Wyatt (Waters) DAWN/miniDAWN MALS + Optilab dRI, Tosoh UP-SW3000-LS — Determines absolute molecular weight and association number independent of column calibration.
• Analytical ultracentrifugation (AUC, sedimentation velocity) · Beckman Coulter Optima AUC — Orthogonal verification of SEC independent of dilution/matrix. Low throughput.
• AF4/DLS · Wyatt Eclipse AF4 + DAWN, Malvern Zetasizer / Wyatt DynaPro (DLS) — Detection of large associates and particles. AF4 is low-shear and advantageous for fragile associates.

Fab has a structure in which a fragment of the heavy chain and the light chain are fastened together by a disulfide. This step is the test that checks whether the chains are properly assembled.

CE-SDS (capillary gel electrophoresis) separates molecules by size in the presence of SDS. Measuring under both non-reduced and reduced conditions shows purity and chain integrity.

What is to be confirmed is whether free chains or extra fragments are mixed in. It is a purity evaluation that backs up, quantitatively, that the chains are correctly assembled into one.

Fab has a structure in which a heavy chain fragment and a light chain are joined by a disulfide, so one must check that the chains are properly assembled and that there are no free chains or fragments. Molecules are separated by size in the presence of SDS, and purity and chain integrity are evaluated quantitatively.

Alternatives in practice

• SDS-PAGE (Coomassie / silver staining) · Bio-Rad Criterion / Thermo Bolt (Bis-Tris) + SimplyBlue/silver staining — Low cost and good visibility. Inferior to CE-SDS in quantitation and resolution.
• Reduced/non-reduced peptide map comparison · Waters/Thermo LC-MS (step [12]) — An orthogonal method that confirms chain integrity at the primary sequence level.
• Microchip electrophoresis · Agilent 2100 Bioanalyzer (Protein 230) / PerkinElmer LabChip GXII — Small-volume, rapid screening. Moderate quantitative accuracy.

Even for the same molecule, slight modifications can change its electrical properties. This step is the test that tells apart that variation in charge (charge variants).

Modifications such as deamidation and oxidation change the charge of the molecule and can affect activity and stability. icIEF (isoelectric focusing) separates and quantifies acidic and basic peaks from differences in isoelectric point.

What is to be confirmed is whether the charge profile is stable between lots. It backs up, from the charge standpoint, that the product is made uniformly every time.

Modifications such as deamidation and oxidation change the charge of the molecule and can affect activity and stability. Acidic and basic peaks are separated by differences in isoelectric point (charge) and quantified, and the charge profile is confirmed to be stable between lots.

Alternatives in practice

• Cation exchange chromatography (CEX/WCX) · Thermo MAbPac SCX-10 / ProPac WCX-10, Cytiva ÄKTA + analytical CEX — Charge variants can be fractionated, and MS coupling is possible. Resolution is complementary to icIEF.
• icIEF-MS / CE-MS coupling (online identification) · SCIEX/Intabio ZT + X500B, Agilent CE-MS — Assigns the molecular modification (deamidation, etc.) of charge peaks online.
• Capillary zone electrophoresis (CZE) · SCIEX PA800 Plus (CZE kit) / Agilent 7100 CE — Fast, high-efficiency charge separation. Based on mobility rather than pI.

Even if purity and structure are in order, whether it "works" is a separate matter. This step is a functional test that measures whether the fragment properly binds its intended target.

SPR (surface plasmon resonance) measures the speed and strength of binding to the target antigen, and relative potency. It can directly confirm that activity has not been impaired in the course of manufacturing.

Binding activity is itself the proof of being a "molecule that works." It is always measured lot by lot, and it is an indispensable test used to judge whether a lot may be released.

This proves, on the functional side, that the fragment properly binds its intended target, in other words that it is a "molecule that works." Because it confirms lot by lot that activity has not been impaired by manufacturing, binding activity is an essential test for release decisions.

Alternatives in practice

• Binding ELISA (relative potency) · Tecan Spark / Molecular Devices SpectraMax + antigen-coated plates — High throughput and low cost. Kinetics are not obtained.
• Cell-based bioassay · Promega Bioassay (NFAT/reporter), Molecular Devices/Tecan readers — Evaluates neutralization/function as biological activity. High biological relevance.
• Biolayer interferometry (BLI) · Sartorius Octet RH16/R8 + ProA/AHC biosensors — Rapid with disposable sensors and tolerant of crude samples. Resolution is lower than SPR.

Finally, the residues that are directly tied to patient safety are checked. This step is a pre-release check of whether host-derived impurities and microorganisms have been removed to within specification.

E. coli-derived host protein (HCP ELISA), residual DNA and endotoxin are quantified by highly sensitive methods, and the sterility of the final product is confirmed as well.

These cannot be overlooked, because they can cause immune reactions or fever even at trace levels. Only after confirming that all are within specification can the product be released.

Whether E. coli-derived host protein, DNA and endotoxin have been removed to within specification is directly tied to patient safety. These can cause immune reactions or fever even at trace levels, so they are quantified by highly sensitive methods and, together with the sterility of the final product, always confirmed before release.

Alternatives in practice

• HCP identification and quantitation by LC-MS/MS · Thermo Orbitrap (Exploris/Astral) / SCIEX ZenoTOF + nanoLC — Identifies and absolutely quantifies individual HCP species. Complements the antibody coverage of ELISA.
• Recombinant factor C (rFC) endotoxin method · Lonza PyroGene rFC / bioMérieux ENDONEXT + fluorescence reader — Independent of horseshoe crab resources and highly specific. Pharmacopoeial listing is progressing.
• Ligand leakage / residual PEG measurement · ELISA (CaptureSelect / Protein L ligand antibodies), residual PEG by LC-MS/CAD/NMR — Dedicated control of affinity resin leakage and residual PEG. Product-specific.