Phase 1Cell culture and production (upstream)
01Selection of host and vector system and construction of cell banks (MCB/WCB) (cell bank construction)
Strain uniformity governs lot-to-lot variation in expression level and impurity profile.
- Standard method
- Cell banking with a BL21(DE3)-type strain + T7/pET expression system (two-tier MCB/WCB)
- Reagents and materials
- Mammalian expression vectors and selection systems, E. coli fermentation media, Selection antibiotics, Expression induction reagents (IPTG, lactose autoinduction)
Why it is the standard. BL21(DE3) is lon/ompT protease-deficient, so there is little degradation of the recombinant product, and the T7-pET system is an industry standard that can be strongly induced with a 1-point IPTG addition. A two-tier bank in line with ICH Q5D (derivation and characterization of cell substrates) is a premise.
Check also. BL21(DE3) + T7/pET is the de facto standard for recombinant protein expression, but for toxic proteins, membrane proteins and proteins with many disulfide bonds, this strain may not give a yield, so early consideration is given to switching to Origami/SHuffle (for oxidative folding in the cytoplasm), protease-deficient strains, or, if glycans are needed, a host other than E. coli.
About this step and the alternatives
First, the seed for "starting from the same cells" is prepared. This is the step that selects the E. coli used for production (BL21(DE3) strain and the T7/pET expression system) and grows it uniformly.
The grown cells are divided into aliquots and frozen, and stored as a two-tier cell bank: the parent MCB and the WCB for mass production made from it. Each time, work starts by thawing from here.
If the seed varies, expression level and impurity profile fluctuate from lot to lot. As the starting point of all manufacturing, building in uniformity is the key.
The "seed" for starting from the same strain every time is cryopreserved. If the strain is not uniform, expression level and impurity profile vary from lot to lot.
Alternatives in practice
- Protease-deficient base strain (for toxic/degradation-prone proteins) · Merck Novagen HMS174(DE3) (K-12 derived, 69453) — Higher recombinant stability than B-strain lines, advantageous for plasmid maintenance
- Periplasmic secretion expression · Merck Novagen pET-22b(+) (pelB signal, 69744)/OmpA signal sequence vectors — Transports disulfide-forming proteins into the periplasm and avoids inclusion body formation
- General-purpose K-12 hosts (pharmaceutical-grade derived strains) · ATCC W3110 (27325)/RV308 (31608) — Low acetate accumulation; suited to high-density culture and secretion systems
Seed culture and inoculum scale-up (seed culture) (revival from a frozen WCB vial → shake flask → stepwise expansion culture in the seed fermenter)
Standard front-end expansion that supplies enough active cells from the frozen cell bank to the production tank
- Standard method
- WCB vial revival → shake flask preculture → logarithmic-phase inoculation in the seed fermenter (about 5–10% inoculation)
Why it is the standard. Multistage expansion with shake flasks + seed fermenters is an established standard procedure in microbial fermentation, supplying active cells in logarithmic growth phase at a fixed OD and fixed inoculation rate to hold down variation in production fermentation.
About this step and the alternatives
This is the "run-up" step that revives a frozen seed vial and grows it stepwise to an amount that can be planted in the production fermenter. It is not put straight into a large tank.
Cell mass is expanded, in steps of roughly 10-fold, from preculture in shake flasks to the seed fermenter. The aim is to hand the healthiest cells, in logarithmic growth phase, on to the next stage.
The quality of the seed grown here directly governs the reproducibility and productivity of the production fermentation. It is an unglamorous preparation step that cannot be skipped.
A frozen WCB vial is not inoculated directly into the production tank; cell mass is increased stepwise (typically in steps of about 10-fold) in shake flasks and seed fermenters before inoculating the production fermenter. The quality of the seed culture is an essential step governing the reproducibility and productivity of the production fermentation.
Alternatives in practice
- Single-use seed expansion · Cytiva WAVE 25 rocking bioreactor + Cellbag/Sartorius Biostat RM — No cleaning validation needed, lower cross-contamination; simplifies the seed stage
- Single-stage inoculation from a high-density vial (HDCB) · Thermo Fisher Scientific HyClone HDCB (high cell density cryovial) — Skips shake flask preculture to shorten the seed stage and the start-up time
Preparation and sterilization of medium and feed (SIP/autoclave sterilization of batch medium, sterile filtration of high-concentration feeds such as glucose, aseptic addition of trace metals and antibiotics) (medium preparation and sterilization)
Standard preparatory step that supplies sterile medium and feed before high-density culture to prevent contamination
- Standard method
- SIP of batch medium (equivalent to 121 °C, 30 min) + 0.2 µm sterile filtration and aseptic transfer of high-concentration glucose feed
- Reagents and materials
- E. coli fermentation media, Trace metal mixes for defined media, Fermentation carbon sources inducers, Antifoam defoamer
Why it is the standard. The industry standard is to prepare separately the heat-labile components that can be filter sterilized (glucose, antibiotics, inducer) and the bulk medium that can be SIP sterilized. Sartopore 2 has a broad track record in sterile filtration validation.
About this step and the alternatives
A preparation step that readies the medium and nutrient feeds used in fermentation and sterilizes them so as not to bring in contaminants. In cooking terms, it is the stage of preparing the ingredients.
The base medium is steam sterilized in the tank (SIP) at the equivalent of 121 °C, and high-concentration glucose feed, trace metals, inducer and antibiotics are sterile filtered through 0.2 µm and poured in while remaining sterile.
Without this preparation, the subsequent high-density fed-batch culture itself cannot take place. It is the essential step that lays the foundation for fermentation.
To make high-density fed-batch culture work, batch medium must be sterilized in situ (SIP) or at high temperature, and separately prepared high-concentration glucose feed, trace metals and inducer must be sterile filtered and supplied aseptically. Without this, the fermentation itself cannot take place; it is an essential preparation step.
Alternatives in practice
- Autoclave sterilization (small scale, glass vessels) · Tuttnauer/Systec autoclaves (121 °C, 20–30 min) — At laboratory scale, the whole medium is moist-heat sterilized in its vessel, with glucose sterilized separately to avoid browning
- Pre-gamma-sterilized single-use vessels + sterile filtration of all feeds · Thermo Fisher HyPerforma Single-Use Bioreactor (S.U.B.) + Sartopore 2 0.2 µm filters — No SIP needed; disposable vessels shorten turnaround
Cell growth by high-density fed-batch culture (main culture)
Raises cell density while suppressing acetate accumulation to maximize the total amount of product.
- Standard method
- Glucose-limited fed-batch culture (exponential feeding + DO-stat control)
- Equipment
- Microbial fermentation bioreactor, Inline pH and DO sensors and probes, pH meter, Capacitance biomass probe
- Reagents and materials
- E. coli fermentation media, Trace metal mixes for defined media, Antifoam defoamer, Fermentation oxygen and process gas supply (gas mixing, MFC)
Why it is the standard. Glucose-limited fed-batch is the standard for obtaining high cell density (tens to over 100 in OD600) while suppressing acetate accumulation. Continuous monitoring of OD600 and DO detects deviations early (Eppendorf Application Note 462 and others).
With Soluble/periplasmic expression
- Induction is weakened and the temperature lowered so that the protein folds correctly without aggregating
- Method: Suppress the expression rate with low-temperature induction (18–25 °C) + low-concentration IPTG or lactose autoinduction to maximize the soluble fraction (in the periplasmic route, adjust induction strength to the capacity of Sec-dependent transport)
- Check also: Contrary to the IB route, overexpression causes aggregation, so the setting is shifted toward suppression. Temperature and inducer concentration must be optimized for each target
About this step and the alternatives
The core culture step that grows E. coli as densely as possible in the fermenter to gain the total amount of target protein. It makes the fullest use of limited tank volume.
Adding nutrients all at once makes the cells run away, so feeding is done by an exponentially increasing little-by-little method, or by DO-stat control that supplies according to the cells' oxygen consumption, suppressing overflow metabolism.
The final yield changes with how the feed is delivered. The measure of "growing slowly and in large amounts" is where skill shows.
Grow the cells as densely as possible in limited tank volume to gain the total amount of target protein. The knack is to feed nutrients little by little rather than all at once.
Alternatives in practice
- pH-stat fed-batch culture · Sartorius BIOSTAT B (pH-triggered pump control) — Detects the pH rise that accompanies substrate depletion and adds feed, suppressing acetate accumulation
- Batch culture · Eppendorf BioFlo 320 (batch mode) — Simple process, suited to screening in early development
- Rocking-motion culture · Cytiva WAVE 25/Sartorius Biostat RM 200 — Gentle shear and single-use; for low to medium density culture
Induction of expression (induction) (IPTG induction)
The timing and amount of induction are the branch point that determines toxicity and yield.
- Standard method
- T7/lac promoter induction with IPTG (temperature and concentration optimized)
- Reagents and materials
- Expression induction reagents (IPTG, lactose autoinduction)
Why it is the standard. The standard induction method for the T7-pET system. IPTG is not metabolized and is highly reproducible. Examples of higher expression at low temperature, low concentration and in rich medium have been reported (PMC4687329 and others).
Check also. IPTG induction is powerful, but from the standpoint of cost and specification control of endotoxin and residual IPTG, commercial processes sometimes choose lactose autoinduction media or arabinose induction systems. Lowering the induction temperature can suppress inclusion body formation and increase the soluble fraction, but total expression falls, so conditions are decided together with the soluble versus inclusion body strategy.
With Soluble/periplasmic expression
- Instead of strong disruption to release IBs, the step switches to selective periplasmic release
- Method: For periplasmic expression, selectively rupture only the outer membrane to release the target by osmotic shock (sucrose/EDTA/Tris buffer) and similar methods (for cytoplasmic soluble expression, gentle disruption is sufficient)
- Equipment: Complete disruption with a high-pressure homogenizer is not essential (osmotic shock is mainly buffer exchange and centrifugation)
- Reagents: Sucrose (about 18–20%), EDTA, Tris (around pH 8)
- Check also: Osmotic shock carries over little HCP, host DNA and endotoxin, giving high initial purity, but yield varies because it depends on the target's titer and retention
About this step and the alternatives
A step that flips the switch to make the target protein when the cells have grown enough. The inducer called IPTG is that switch.
The faucet of expression called the T7/lac promoter is opened and the cell factory is run at full capacity. But forcing production weakens the cells, so temperature and IPTG amount are optimized.
This timing and amount are the branch point separating yield and quality. Too early, too late or too much, and the result falls apart.
Once the cells have grown enough, the switch "make the target protein" is flipped. If timing and amount are wrong, toxicity weakens the cells and yield falls.
Alternatives in practice
- Lactose induction/autoinduction · Merck Novagen Overnight Express Autoinduction System (71300) — No IPTG needed; automatic induction as culture proceeds gives simple operation and good scale reproducibility
- Arabinose induction (araBAD/pBAD system) · Thermo pBAD expression vector system + L-arabinose (Sigma A3256) — Expression level can be fine-tuned in a dose-dependent manner; little leaky expression
Phase 2Harvest and clarification
06Cell harvest and cell disruption (lysis) (harvest and disruption)
Whether the intracellular target can be taken out without over-disrupting the inclusion bodies is the key to downstream steps.
- Standard method
- Cell harvest by centrifugation/microfiltration, then disruption with a high-pressure homogenizer
Why it is the standard. High-pressure homogenization gives uniform, highly reproducible disruption of E. coli and suits high-density culture. At 12,000–20,000 psi, multiple passes give >99.99% disruption (PubMed 36656521 and others). For inclusion body recovery, conditions are kept milder to preserve particle size.
Check also. A high-pressure homogenizer is standard, but increasing the number of passes makes cell debris finer, so downstream clarification and filtration clog more easily. The key is not to over-disrupt when recovering inclusion bodies and, conversely, to raise the disruption rate for soluble protein, so note that conditions point in opposite directions depending on the purpose.
With Soluble/periplasmic expression
- The step itself is unnecessary because inclusion bodies are not formed
- Method: This step is not performed (no IB pelleting, no detergent/urea wash)
- Check also: Not applicable to the soluble/secretion route
About this step and the alternatives
Because the target protein has accumulated inside the cells, this recovery step first collects the cells, breaks them and takes out the contents. It is the bridge from culture to purification.
Cells are gathered by centrifugation or microfiltration and burst all at once with a high-pressure homogenizer. The image is of physically breaking the wall to release the contents.
However, over-disruption is to be avoided. If the inclusion bodies inside are also crushed, downstream steps are disturbed, so adjusting the strength of disruption is the key.
The target protein is inside the cells, so the cells are first collected and broken to take out the contents. Not over-disrupting the inclusion bodies is the key to downstream steps.
Alternatives in practice
- Microfluidizer (multiple recirculations) · Microfluidics Microfluidizer LM20/M-110P — Uniform shear in a fixed-geometry interaction chamber; excellent for fine particle size and reproducibility
- Ultrasonic disruption (laboratory scale) · Sonics Vibra-Cell VCX 750/Qsonica Q700 — For small scale; ice cooling is essential to manage heat
- Chemical/enzymatic lysis · Merck BugBuster Protein Extraction Reagent (70584) + Benzonase — No mechanical disruption equipment needed; gentle, for laboratory screening
Isolation and washing of inclusion bodies (IB) (IB washing)
Inclusion body purity before solubilization determines the success of the later refolding.
- Standard method
- IB pelleting by centrifugation + washing with detergent/low-concentration urea
Why it is the standard. The established method is to centrifuge the lysate at around 13,000 g and recover IB as the insoluble fraction. Washing with detergent and low-concentration urea removes membrane proteins, endotoxin and nucleic acids and raises IB purity before solubilization (Microbial Cell Factories 2015 and others).
With Soluble/periplasmic expression
- Unnecessary because there are no inclusion bodies to solubilize
- Method: This step is not performed (no solubilization with chaotropes or reducing agents)
- Check also: Not applicable to the soluble/secretion route
About this step and the alternatives
In the lysate, the target protein that has clumped as an insoluble mass is called an "inclusion body (IB)." This step sorts them out and washes them.
Inclusion bodies are heavy, so they are sedimented to the bottom by centrifugation and collected, then washed with detergent or dilute urea. Dirt such as membrane fragments, DNA and other bacterial proteins is removed.
How cleanly they can be washed here determines the success of the next refolding. It is a step where the precision of the preparation counts.
The "inclusion bodies," in which the target protein has clumped, are collected by centrifugation, and membranes, DNA and other proteins are washed off. The purity here determines the success of the later refolding.
Alternatives in practice
- IB recovery by depth filter/microfiltration · Merck Millistak+ depth filter/Sartorius Sartoclear — Continuous recovery of IB at large scale where centrifugation is difficult; lower shear
- Optimization of number of washes and detergent type · Stirred tank + Sigma sodium deoxycholate (D6750), etc. — Wash conditions are optimized according to residual HCP/endotoxin
Solubilization of inclusion bodies (solubilization)
The balance of how far it is unfolded determines solubility and refolding yield.
- Standard method
- Solubilization with high-concentration chaotrope (urea/guanidine hydrochloride) + reducing agent
- Reagents and materials
- Chaotropes and denaturants (urea, GdnHCl), Inclusion body reagents, Disulfide reduction alkylation reagents
Why it is the standard. 8 M urea/6 M GdnHCl is the representative method that completely unfolds disulfides and hydrophobic cores to dissolve the protein. On the other hand, a strategy of raising refolding yield with "mild solubilization," which preserves native-like structure, is also established (Microbial Cell Factories 2015, PubMed 32729411). The optimum is subtype dependent.
Check also. Solubilization conditions with urea or guanidine hydrochloride are two sides of the same coin with downstream refolding efficiency. Unfolding all the way to full denaturation makes aggregation likely during refolding, and too mild leaves undissolved material, so solubilization conditions are optimized together with the refolding step and not decided alone.
With Soluble/periplasmic expression
- Refolding is unnecessary because the protein is already folded to its native structure at expression
- Method: This step is not performed (no denaturant removal, no re-formation with a redox pair). In the periplasm, the endogenous disulfide bond formation system (DsbA/DsbC) forms native disulfides
- Check also: Not applicable to the soluble/secretion route. The rate limit set by refolding yield is lifted, while secretion capacity and solubilization capacity become the new rate limits
About this step and the alternatives
A step that "unravels" and dissolves the clumped, insolubilized inclusion bodies once. It is like taking apart a tangled ball of yarn before knitting it again, the stage before that.
The protein structure is unfolded using denaturants such as high-concentration urea or guanidine hydrochloride, and a reducing agent. Setting the strength is the crux.
Unfolding too strongly makes refolding difficult later, and too weakly leaves undissolved material. As the premise for succeeding in the next refolding, the right degree is sought.
The clumped protein is unfolded once and dissolved. Unfolding it too strongly makes it hard to refold later, and too weakly it does not dissolve. Balance is important.
Alternatives in practice
- Mild solubilization (low-concentration chaotrope + pH) · Stirred tank + low-concentration urea 2–4 M/non-denaturing detergent — Preserves native-like structure and improves downstream refolding yield
- Alkaline pH/N-lauroylsarcosine solubilization · Stirred tank + Sigma sodium N-lauroylsarcosine (L9150, pH 9–12) — Avoids strong chaotropes; easy to remove later by dialysis
- High pressure solubilization · High-pressure reactor (up to the ~2000 bar class) — Dissociates aggregates with low-concentration denaturant, reducing waste liquid and denaturant amount
Phase 3Purification (downstream)
Rate-limiting step that prevents aggregation, forms correct disulfides and recovers activity.
- Standard method
- Stepwise denaturant removal by rapid dilution/diafiltration + disulfide re-formation with a redox pair
Why it is the standard. The standard is to remove the denaturant gradually by dilution, dialysis or diafiltration and to promote correct S-S bonds with a redox pair. Arginine suppresses aggregation. In recent years refolding has been monitored and controlled with PAT + kinetic models (PMC7954745, PMC7551518). Optimal conditions vary greatly from product to product.
Check also. Refolding is the step in the E. coli inclusion body route whose success is hardest to predict, and it is rate-limiting for yield. The optimal redox pair ratio, dilution factor and protein concentration must be screened experimentally for each molecule, and there are no established general-purpose conditions. At scale-up, the large volumes that come with rapid dilution translate directly into load on downstream steps.
With Soluble/periplasmic expression
- Instead of stopping a refolding reaction, the extract/lysate is clarified directly
- Method: Stopping refolding by pH adjustment is unnecessary. The periplasmic extract or cytoplasmic soluble lysate is clarified by centrifugation + depth + 0.2 µm filtration and concentrated by TFF
- Check also: The aggregate removal load is smaller than in the IB route, but with soluble expression, more residual HCP/DNA remains, which changes the burden on downstream chromatography
About this step and the alternatives
A step at the heart of the purification part that refolds the unfolded, dissolved protein to its native three-dimensional structure and recovers its activity. It is also called "rewinding."
The denaturant is removed little by little by rapid dilution or diafiltration. Diluting all at once avoids aggregation, in which molecules stick to each other, and a redox combination makes the correct disulfide bonds (intramolecular clasps) form.
Only here is a molecule in a "working form" born. If it folds the wrong way, no activity appears; this is the central step that creates the added value.
The unfolded protein is returned to its native three-dimensional structure to recover activity. The points are to dilute it all at once to prevent aggregation and to have correct disulfide bonds formed.
Alternatives in practice
- On-column refolding (SEC/IEX/immobilized) · Cytiva ÄKTA process + Superdex 200/HiTrap IEX columns — Removes the denaturant gradient in the flow path to suppress aggregation and reduce dilution volume
- Denaturant removal by diafiltration · Merck Pellicon 3 Ultracel TFF cassettes/Repligen KrosFlo hollow fiber — Removes the denaturant gently by stepwise buffer exchange
- High pressure refolding · High-pressure reactor (up to 2000 bar) — Lowers the dilution factor; applied to hard-to-refold targets
Clarification and concentration of the refolding solution (reaction stop by pH adjustment, aggregate removal by depth/0.2 µm filtration, concentration by ultrafiltration before column loading) (clarification and concentration)
Removes aggregates from, and concentrates, the large volume of diluted refolding solution for capture chromatography
- Standard method
- Reaction stop by pH adjustment → aggregate removal by depth + 0.2 µm filtration → concentration by TFF ultrafiltration
Why it is the standard. Clarification + UF to remove precipitate from, and concentrate, the dilute large-volume refolding solution is an established standard sequence, and Millistak+ and Pellicon 3 have a broad track record in bioprocessing.
About this step and the alternatives
The solution refolded by rapid dilution is dilute and large in volume, and also contains aggregates and precipitate formed during re-formation. This intermediate step brings it to a state that can be handed to purification.
First, pH adjustment stops the refolding reaction, and depth filtration and 0.2 µm filtration remove particles and turbidity. Then ultrafiltration by tangential flow filtration (TFF) removes water and concentrates it.
Unless particles are removed and volume reduced, it cannot be loaded onto the next capture chromatography. At industrial scale it is a groundwork step that cannot be skipped.
Rapid dilution refolding yields a very dilute, large-volume solution that contains aggregates and precipitate formed during re-formation. It cannot be loaded onto capture chromatography until the reaction is stopped by pH adjustment, particles are removed by filtration, and the solution is concentrated by ultrafiltration. It is an intermediate step that is indispensable at industrial scale.
Alternatives in practice
- Aggregate removal by centrifugation + concentration by ultrafiltration · Beckman Coulter Avanti JXN-26 + Pellicon 3 TFF — Removes aggregates by sedimentation in systems with poor compatibility with depth filters
- Expanded bed adsorption (EBA) integrates clarification and capture · Cytiva STREAMLINE/expanded bed adsorption columns — Captures directly from unclarified feed, reducing the number of steps
Capture chromatography (capture)
First concentration and purification step that cuts large amounts of impurities at once.
- Standard method
- Capture by ion exchange chromatography (IEX) or affinity (His-tag, etc.)
Why it is the standard. IEX gives high resolution and high binding capacity under mild conditions and is the recommended standard method for capturing recombinant proteins. If tagged, IMAC gives highly selective capture (Antibody Therapeutics 2024 review and others). Choose by presence of a tag and CQAs.
Check also. His-tag affinity makes capture simple, but the tag can affect product quality, immunogenicity and activity, so the final drug substance often requires tag removal (protease cleavage) and re-purification after cleavage; whether this is possible and the cleavage efficiency are confirmed early. When no tag is used, capture by ion exchange is the reference.
About this step and the alternatives
The first capture step of purification, which grabs only the target protein in bulk from the dilute refolded solution. The image is of scooping it up all at once with a net.
It is adsorbed with ion exchange (IEX), which uses differences in charge, or an affinity resin that binds the His-tag (a tag attached as a marker), and eluted afterwards.
In this one move large amounts of impurities are sieved out and, at the same time, the dilute solution is concentrated. Once capture is settled, downstream steps become much easier.
The target protein is grabbed in bulk from the refolded solution, cutting large amounts of impurities at once. This is the first concentration and purification step.
Alternatives in practice
- Immobilized metal affinity (IMAC, His-tag) · Cytiva Ni Sepharose excel (17-3712)/HisTrap excel, Thermo HisPur Ni-NTA — Highly selective capture of His-tagged targets; greatly removes contaminating HCP
- Capture with mixed-mode resins · Cytiva Capto MMC (multimodal cation exchange, 17-5317)/Capto adhere — Can bind even with high-conductivity feed; capture possible without salt concentration adjustment
Intermediate and finishing (polishing) purification and endotoxin removal (polishing step)
Shaves off aggregates, fragments and endotoxin to determine final purity and safety.
- Standard method
- Polishing with a different type of chromatography, hydrophobic interaction (HIC)/anion exchange (AEX), + endotoxin removal
Why it is the standard. HIC is a separate means of removing aggregates, endotoxin and nucleic acids by surface hydrophobicity. AEX (flow-through) is effective for removing negatively charged endotoxin and host DNA (endotoxin removal by anion exchange, Antibody Therapeutics 2024). The point is to layer principles different from capture.
Check also. Endotoxin is premised on a multistage strategy managed not only in this step but also in the dedicated depyrogenation step downstream (#12), so it is safer not to expect it to be removed completely here. Polishing shaves off aggregates and fragments, but there is a tradeoff with recovery.
About this step and the alternatives
A finishing purification step that shaves off the "relatives of the target" (related substances) such as aggregates and fragments remaining after capture, and bacterial endotoxin.
Principles different from the earlier chromatography are combined, such as hydrophobic interaction (HIC), which separates by differences in hydrophobicity, and anion exchange (AEX) of a different type, to target the remaining dirt.
Final purity and safety are decided here. It is the step that decides quality and is directly tied to whether specifications are met.
Shaves off residual related substances (aggregates, fragments) and endotoxin derived from gram-negative bacteria. This step determines final purity and safety.
Alternatives in practice
- Size exclusion (SEC) polishing · Cytiva Superdex 200 prep grade/Superdex 75 (ÄKTA process) — Separates aggregates and fragments by molecular size, with buffer exchange performed at the same time
- Ceramic hydroxyapatite (CHT) · Bio-Rad CHT Ceramic Hydroxyapatite Type I (40 µm, 157-0040) — Separates aggregates and closely related impurities by multimodal selectivity
- Ligands dedicated to endotoxin removal · Polymyxin B immobilized resin (e.g., Thermo Pierce High Capacity Endotoxin Removal Resin, 88274) — Reduces residual LPS by specific adsorption
Endotoxin (LPS) removal and depyrogenation polishing (anion exchange flow-through, or selective removal of LPS by detergent two-phase partitioning) (endotoxin removal)
Removal step essential for E. coli products, reducing E. coli-derived LPS to the specification value
- Standard method
- LPS removal in flow-through mode with Q strong anion exchange resin (product passes through, LPS adsorbs to the resin)
Why it is the standard. Because the phosphorylated lipid A of LPS adsorbs strongly to quaternary ammonium groups, LPS can be selectively captured while the product is recovered in the flow-through. The membrane format is widely used for depyrogenation polishing because of high flow rate and disposability.
Check also. Endotoxin removal by anion exchange flow-through is not universal: if the product itself is an acidic protein, the product also adsorbs to the resin and recovery falls. Depending on the product's isoelectric point, a method with a different principle such as Triton X-114 two-phase partitioning must be used together or switched to.
About this step and the alternatives
If it is made in E. coli, contamination with endotoxin (LPS) cannot be avoided. This is the depyrogenation polishing step that removes it specifically and brings the product within injectable specifications.
Part of LPS (lipid A) sticks strongly to anion exchange resin. Using this property, separation is by the "flow-through" mode, in which the product passes straight through and only LPS is captured on the resin.
As an alternative, there is also the option of selectively pulling out LPS by temperature-dependent two-phase partitioning with a detergent, Triton X-114. It is the last build-in of safety.
With an E. coli host, contamination with endotoxin (LPS) is unavoidable, and a dedicated depyrogenation step is essential to meet injectable specifications. The standard is flow-through purification that exploits the strong adsorption of the lipid A region of LPS to anion exchange resin, or selective removal of LPS by temperature-induced two-phase partitioning with Triton X-114.
Alternatives in practice
- Triton X-114 temperature-induced two-phase partitioning · Stirred tank + Sigma-Aldrich Triton X-114 (X114) — Uses the cloud point to partition and separate LPS into the detergent phase; no resin needed
- Adsorptive removal with LPS affinity resin (polymyxin B) · Thermo Pierce High Capacity Endotoxin Removal Resin (88274)/Seikagaku Corporation Detoxi-Gel — Effective for LPS removal from acidic proteins that are hard to remove by anion exchange
Phase 4Formulation and fill-finish
14Concentration and buffer exchange by ultrafiltration/diafiltration (UF/DF) (UF/DF and buffer exchange)
Final finishing that concentrates the target to the required concentration and exchanges it into a storage buffer.
- Standard method
- Tangential flow ultrafiltration/diafiltration (TFF UF/DF)
- Equipment
- TFF system, TFF cassette, TFF hollow fiber
Why it is the standard. A standard concentration and buffer exchange step used before the final formulation of nearly all biologics. It retains the target by molecular weight cutoff and gives higher yield and longer membrane life than dead-end filtration (Repligen/Sartorius technical documents).
About this step and the alternatives
A finishing step that concentrates the purified target protein to the concentration the product needs and replaces it into a buffer suited to storage.
Ultrafiltration by the tangential flow method (flowing the liquid along the membrane surface) removes water to concentrate, and the following diafiltration washes out salts and unneeded small molecules while replacing the liquid with the formulation solution.
The concentration and liquid properties decided here connect directly to the composition of the final formulation. It is an important finishing step that prepares the foundation of the product.
The target protein is concentrated to the required concentration and replaced into a buffer suited to storage. An important finishing step directly tied to the final formulation.
Alternatives in practice
- Single-pass TFF (SPTFF) · Repligen KrosFlo TFF-SPTFF/Pall Cadence Inline Concentrator — Continuous concentration without recirculation; shorter residence time and smaller footprint
- Dialysis · Thermo Slide-A-Lyzer dialysis cassettes (66810, etc.)/regenerated cellulose dialysis membranes — Convenient for buffer exchange of small-scale, small-volume samples
Sterile filtration and aseptic filling (fill-finish)
Last safety gate that removes microorganisms and finishes the product as a sterile injectable.
- Standard method
- Aseptic filling (vial/syringe) after 0.22 µm sterile filtration
Why it is the standard. The standard final step for injectables. Sterility is assured by 0.22 µm sterile filtration + filter integrity testing (bubble point, etc.). Consistent with the framework of USP/Ph.Eur./ICH Q8-Q11.
About this step and the alternatives
A step that removes microorganisms to make sterile contents and puts them into containers. It is the last gate that secures the safety to be put into the body as an injectable.
Bacteria are filtered out with a 0.22 µm filter to make a sterile bulk, which is filled into vials or syringes in an aseptic environment. No contamination is allowed up to the moment of filling.
No matter how perfect the purification, if bacteria get in here it is all for nothing. It is the step that calls for the most nerve as the close of manufacturing.
Microorganisms are removed to make a sterile final product, which is filled into containers. It is the last gate that secures safety as an injectable.
Alternatives in practice
- Freeze-drying (lyophilization) · SP Scientific LyoStar/GEA/IMA freeze dryers — Secures long-term storability of products that are unstable as liquids
- 0.45/0.22 µm two-stage filtration · Sartorius Sartopore 2 (0.45+0.2 µm double layer) — Improves the reliability of sterile filtration with measures against clogging and redundant sterility assurance
Bioburden reduction filtration (control of bioburden and particulates by 0.45/0.22 µm pre-filtration of the intermediate hold solution before filling) (bioburden reduction filtration)
Lowers bioburden and particulates before sterile filtration to secure the reliability of aseptic filling
- Standard method
- 0.45/0.22 µm pre-filtration (bioburden reduction) of the bulk before filling
- Reagents and materials
- Formulation buffer (bulk before filling)
Why it is the standard. A double-layer (0.45+0.2 µm) configuration captures bioburden and particulates in stages and is an established method to extend the life of, and raise the assurance of, the final sterilizing filter.
About this step and the alternatives
A preliminary step, just before final sterile filtration and aseptic filling, that reduces beforehand the bacteria and particulates that may increase while waiting (hold).
The liquid is filtered once through a 0.45/0.22 µm pre-filter to lighten the load on the sterilizing filter. It keeps the main filter from clogging and also prevents breakthrough of bacteria.
It is, so to speak, a "forerunner clearing the way" placed before the main filter. A standard precaution for running aseptic filling stably.
Just before final 0.22 µm sterile filtration and aseptic filling, it is standard to reduce, by pre-filtration, the bioburden and particulates that may increase during the hold step, which lowers the load on the sterilizing filter and the risk of breakthrough.
Alternatives in practice
- Bioburden sampling before and after intermediate hold + single-layer 0.2 µm · Sartorius Sartopore 2 single-layer 0.2 µm + Sartorius Microsart bioburden testing — Omits the pre-filter to cut filter cost, managed by monitoring
- Suppress growth with a cold hold and simplify pre-filtration · GMP-compliant 2–8 °C cold room/jacketed tank — Suppresses bacterial growth during the hold time and lowers the bioburden load
Phase 5Quality attributes and analytics
17Product potency and content (protein quantitation)
Quantifies the amount of target as basic data for process control and lot specifications.
- Standard method
- Quantitation by reversed-phase/SEC-HPLC or UV (A280) + Bradford/BCA in process
- Reagents and materials
- HPLC mobile phase solvents
Why it is the standard. For process monitoring, Bradford/BCA and A280 are simple and standard. For specification control, HPLC/UV quantitation is used for accuracy. Confirming identity and approximate purity by SDS-PAGE and Western blot is the established practice (standard configuration in many production papers).
Check also. BCA/Bradford and A280 are simple and suited to in-process control, but they are affected by coexisting impurity proteins, nucleic acids and buffer components. For lot specifications, cross-check with quantitation that involves separation, such as SEC/reversed-phase HPLC, or with methods that use a reference standard for calibration.
About this step and the alternatives
The most basic quantitative test, which pins down in numbers how much target protein was produced and how much is in the product.
For the final product, it is measured precisely by reversed-phase or size exclusion HPLC or by UV absorbance (A280), and during the process it is quickly checked by simple methods such as the Bradford method.
This number is foundational data used for both process control and lot pass/fail decisions. It becomes the reference point for all quality evaluation.
The amount of target protein produced and contained is expressed numerically. Basic data used for both process control and lot specifications.
Alternatives in practice
- Bradford/BCA assays · Thermo Pierce BCA Protein Assay Kit (23225) + microplate reader — Simple and high-throughput for in-process monitoring
- A280 (extinction coefficient conversion) · Thermo NanoDrop One/Implen NanoPhotometer — No dilution needed, rapid with small volumes; applies to high-purity samples
- SDS-PAGE + Western blot · Bio-Rad Mini-PROTEAN/Cytiva Amersham WB system — Less quantitative, but used together for identity and molecular weight confirmation
Purity and related substances (size-based: aggregates, fragments) (purity, aggregates/fragments)
Evaluates by a separate method the amount of aggregates and fragments, which bear directly on immunogenicity.
- Standard method
- Purity evaluation by SEC-HPLC (size exclusion) + CE-SDS (reducing/non-reducing)
- Reagents and materials
- HPLC mobile phase solvents
Why it is the standard. SEC is the representative method for separating aggregate/monomer/fragment, and CE-SDS is the standard method for purity and size heterogeneity as a quantitative version of SDS-PAGE. Evaluating size-based related substances with both, by different methods, is the norm for biologics (purity and impurity evaluation in ICH Q6B).
About this step and the alternatives
A purity test that measures how many "relatives" of different sizes are mixed in with the target protein. The aim is two directions: aggregates that are too large and fragments that are too small.
Evaluation combines size exclusion HPLC (SEC), which separates by molecular size, and CE-SDS, which does electrophoresis in a thin capillary (under both reducing and non-reducing conditions).
Aggregates in particular lead directly to the cause of the body treating them as foreign and mounting an immune response (immunogenicity). That is why their content is strictly controlled.
Measures how much aggregate and fragment is mixed in with the target protein. Aggregates bear directly on immunogenicity, so they are strictly controlled.
Alternatives in practice
- SEC-MALS (multi-angle light scattering) · Wyatt Technology (Waters) DAWN MALS + Optilab dRI — Absolute molecular weight and absolute quantitation of aggregates are possible
- Analytical ultracentrifugation (AUC) · Beckman Coulter Optima AUC — Matrix-independent, orthogonal evaluation of aggregates
- SDS-PAGE (reducing/non-reducing) · Bio-Rad Mini-PROTEAN TGX/Thermo NuPAGE — Simple, low-cost screening; orthogonal confirmation of CE-SDS
Purity and charge heterogeneity (charge variants)
Captures charge changes from deamidation and cleavage to manage manufacturing variation.
- Standard method
- Imaged capillary isoelectric focusing (icIEF)/IEX-HPLC
- Reagents and materials
- Carrier ampholytes pI markers
Why it is the standard. icIEF is the standard method for separating and quantifying charge variants at high resolution, and IEX-HPLC is also widely used together. It captures charge changes such as deamidation and C-terminal cleavage and addresses the heterogeneity evaluation of ICH Q6B.
About this step and the alternatives
A purity test that distinguishes molecules whose charge has changed through deamidation or partial cleavage (charge variants). It captures "subtle variants" of the same molecule.
Separation and quantitation use imaged capillary isoelectric focusing (icIEF), which separates by differences in charge, and ion exchange HPLC (IEX-HPLC).
An indicator that captures lot-to-lot variation and degradation progressing during storage early. It is the eye that watches whether quality is consistent.
Distinguishes molecules whose charge has changed through deamidation, cleavage and so on. Serves as an indicator that captures manufacturing variation and degradation.
Alternatives in practice
- IEX-HPLC (cation/anion exchange) · Thermo MAbPac SCX-10/ProPac WCX-10 + Agilent 1260 Bio — High-resolution separation and quantitation of charge variants with a salt/pH gradient
- Conventional cIEF/IEF gel · SCIEX PA 800 Plus (cIEF kit, A80976)/Cytiva PhastSystem IEF gel — Alternative when no imaging instrument is available
Primary structure and identity (molecular weight, sequence) (identity and molecular weight)
Precisely confirms molecular weight, sequence and modifications to assure that it is the target.
- Standard method
- Intact mass spectrometry
- Reagents and materials
- Peptide mapping protease, Disulfide reduction alkylation reagents, HPLC mobile phase solvents
Why it is the standard. Intact mass confirms overall molecular weight, and peptide mapping confirms sequence coverage and PTMs (oxidation, deamidation, etc.); this is the reference method for identity. The multi-attribute method (MAM) with high-resolution MS can evaluate multiple CQAs at once (Agilent primer, ICH Q6B).
Check also. Intact mass analysis is the core of identity confirmation, but on its own it may not fully distinguish residual N-terminal methionine, disulfide bonds or differences in partial modification. Combining it with confirmation by sequence coverage from peptide mapping (LC-MS/MS) is standard.
About this step and the alternatives
An "identity check" test that confirms whether what was made is really the target protein and whether sequence and modifications are as designed. It plays the core role in identity.
Intact mass spectrometry, which weighs the molecule whole without breaking it apart, precisely measures the overall mass. From small deviations in mass, sequence differences and unexpected modifications are brought out.
Core data that backs that the product is "certainly that." It is the final proof that does not miss mix-ups or failed production.
Precisely confirms whether what was made is really the target protein and whether sequence and modifications are as expected. Core data for identity confirmation.
Alternatives in practice
- N-terminal sequencing (Edman degradation) · Shimadzu PPSQ-50A protein sequencer — Directly confirms N-terminal identity and processing
- Disulfide bond mapping (non-reduced peptide map + MS) · Thermo Orbitrap Exploris + Waters ACQUITY UPLC — Verifies correct disulfide bridging
- Amino acid composition analysis · Hitachi L-8900 amino acid analyzer/Agilent AdvanceBio AAA — Orthogonal confirmation of composition and content; also used for quantitation
Higher-order structure and conformation (higher-order structure analysis)
Whether the protein is correctly folded and its higher-order structure maintained governs efficacy.
- Standard method
- Structure confirmation by circular dichroism (CD) spectroscopy + fluorescence spectroscopy
- Reagents and materials
- Dilution buffer (low-absorbance phosphate/fluoride systems for CD, avoiding Cl-), reference standard
Why it is the standard. The representative methods: far-UV CD (190–250 nm) for secondary structure and near-UV CD/tryptophan fluorescence for tertiary structure. They can confirm the correctness of higher-order structure and thermal stability (Tm) of refolded product (refolding monitoring literature).
About this step and the alternatives
A higher-order structure test that confirms whether the molecule is correctly folded. Even if the sequence is right, the drug will not work if the shape is broken.
Circular dichroism (CD) spectroscopy looks at the rough backbone (secondary structure), and fluorescence spectroscopy at the three-dimensional arrangement (tertiary structure). The soundness of folding is evaluated from two viewpoints.
Even if apparent purity and sequence are in order, it is meaningless if the shape is not genuine. This step assures that the "working form" is maintained.
Confirms whether the protein is correctly folded (secondary and tertiary structure). Even with the same sequence, it will not work if the structure is broken.
Alternatives in practice
- Differential scanning fluorimetry (DSF/nanoDSF) · NanoTemper Prometheus NT.48 (nanoDSF)/Thermo QuantStudio (SYPRO Orange DSF) — Evaluates thermal stability (Tm) quickly with a small amount; for formulation screening
- FTIR spectroscopy · Bruker Tensor II/Thermo Nicolet iS50 (ATR-FTIR) — Effective for secondary structure evaluation of β-sheet content and aggregation
- HDX-MS (hydrogen-deuterium exchange) · Waters HDX system + SYNAPT/Trajan LEAP HDX + Orbitrap — Analyzes higher-order structure and conformational changes at the residue level
Biological activity and potency (potency testing)
Measures the power to work by function, deciding whether the product holds up.
- Standard method
- Product-dependent functional assay (cell-based bioassay/enzyme activity/binding assay)
- Reagents and materials
- Reporter gene bioassay ADCC CDC
Why it is the standard. The principle is that potency is measured with a functional assay that reflects the product's mechanism of action, and ICH Q6B calls for setting potency. Binding kinetics are quantified by SPR/BLI and activity by cell/enzyme assays. Methods differ by product, so there is no uniform reference method.
About this step and the alternatives
A biological activity and potency test that directly measures how much "power to work" the product actually has. Even if purity and shape are good, it is not a product if it does not work.
Depending on the contents, a suitable functional test is chosen, such as a cell-based bioassay in which the product acts on cells and the response is observed, enzyme activity, or a binding assay to the target.
Whereas the other tests look at "shape and purity," this looks at the "function" itself. It is the decisive test that directly backs efficacy.
Measures how much "power to work" there actually is. Even with good structure and purity, there is no product without activity.
Alternatives in practice
- Enzyme activity assay (specific activity) · Molecular Devices SpectraMax plate reader (kinetic measurement) — Directly quantifies the specific activity of enzyme drugs
- ELISA-based binding assay · Tecan/BioTek plate washer + plate reader — Evaluates target binding activity at high throughput
- Reporter gene cell assay · Promega luciferase reporter system + luminometer — Measures cell-based potency with high relevance to the mechanism
Process-related impurities: host cell protein (HCP) (HCP measurement)
E. coli-derived contaminating proteins affect immunogenicity and stability even in trace amounts.
- Standard method
- E. coli HCP quantitation by sandwich ELISA
Why it is the standard. Sandwich ELISA with anti-E. coli antibody is the industry standard for HCP quantitation and can detect down to the ppm level (Cygnus Technologies E. coli HCP kit). Addresses process-related impurity control in ICH Q6B. Confirming the validity of antibody coverage is a premise.
Check also. A host cell protein ELISA can measure only HCP that the antibodies recognize, so certain hard-to-detect HCP may be missed. To cover HCP specific to the product and process comprehensively, coverage is supplemented by mass spectrometry (LC-MS/MS), and the validity of a product-specific assay is confirmed as needed.
About this step and the alternatives
A test for process-related impurities that measures how much contaminating protein (HCP) derived from the host E. coli remains in the product.
Trace HCP is quantified by sandwich ELISA, a highly sensitive method that detects by sandwiching between antibodies. It is confirmed that the set upper limit is not exceeded.
Even tiny amounts can trigger an immune response or lower product stability. That is why control to keep levels within a strict upper limit is indispensable.
Measures how much E. coli-derived contaminating protein remains. Even trace amounts affect immunogenicity and stability, so it is strictly controlled.
Alternatives in practice
- HCP identification and quantitation by LC-MS/MS · Thermo Orbitrap Exploris/SCIEX TripleTOF + nanoLC — Identifies and absolutely quantifies individual HCP species; complements HCP not detected by ELISA
- 2D-DIGE/2D electrophoresis · Cytiva Ettan IPGphor/2D Electrophoresis system — Overview of the HCP profile; used for antibody coverage evaluation
Process-related impurities: residual host DNA (residual DNA measurement)
Residual host DNA is an essential safety item with an upper limit set by regulation.
- Standard method
- E. coli residual DNA quantitation by real-time qPCR (TaqMan)
- Makers
- Bio-Rad, Thermo Fisher, Applied Biosystems
Why it is the standard. resDNASEQ, based on TaqMan qPCR, is the standard for residual host DNA quantitation, with high sensitivity and specificity. WHO and national agencies specify upper limits for residual DNA, and it addresses their control (Thermo Fisher resDNASEQ).
About this step and the alternatives
A test for process-related impurities that measures how much residual DNA derived from the host E. coli remains in the product.
TaqMan-format real-time qPCR, which follows a specific sequence by fluorescence, accurately quantifies even very small amounts of DNA.
A clear upper limit is set for residual DNA by regulation. It is an essential item that must always be confirmed in securing safety.
Measures the amount of residual host-derived DNA. An upper limit is set by regulation, making this an essential item for securing safety.
Alternatives in practice
- Fluorescent dye (intercalator) method · Thermo Quant-iT PicoGreen dsDNA Assay (P7589) + fluorescence plate reader — Rapid quantitation of total dsDNA; no species specificity, suited to screening
- Digital PCR (dPCR) · Bio-Rad QX600 Droplet Digital PCR/Thermo QuantStudio Absolute Q — Absolute quantitation without a standard curve; excellent for low copy number and tolerance to inhibitors
Safety testing: bacterial endotoxin (endotoxin test)
E. coli-derived endotoxin causes fever and shock, so being within limits is always confirmed.
- Standard method
- Bacterial endotoxins test (BET) by the LAL method
- Reagents and materials
- Charles River Endosafe LAL cartridges (horseshoe crab-derived amebocyte lysate, chromogenic substrate built in), control standard endotoxin (CSE)
Why it is the standard. BET with LAL (horseshoe crab-derived) is the standard for detecting gram-negative bacterial endotoxin and is listed in USP <85>/Ph.Eur. 2.6.14/ICH Q4B. Quantitation is by kinetic chromogenic/turbidimetric methods. Endosafe is cartridge-based and rapid (USP <85>).
Check also. The LAL (horseshoe crab reagent) method is standard, but some products and excipients interfere by inhibiting gelation or color development, so the presence of interference is confirmed by a recovery test. Where animal-derived reagents are to be avoided, the recombinant Factor C (rFC) method is spreading as an alternative.
About this step and the alternatives
E. coli is a gram-negative bacterium, and its endotoxin causes fever and shock. This is a safety test that confirms whether the amount of this endotoxin is within limits.
The amount of endotoxin present is judged by the bacterial endotoxins test (BET) using the LAL method, which uses a component derived from horseshoe crab. It is a test that cannot be avoided for injectables.
A specification item that must always be performed before shipment. It becomes one of the last bastions protecting the safety of a product administered directly to patients.
Endotoxin from E. coli, a gram-negative bacterium, causes fever and shock, so for injectables it must always be confirmed to be within limits.
Alternatives in practice
- Recombinant factor C (rFC) method · Lonza PyroGene Recombinant Factor C Assay (50-658U) + fluorescence plate reader (Ex/Em 380/440 nm) — No animal-derived materials, sustainable; no glucan cross-reactivity
- Gel-clot LAL · Charles River/Lonza gel-clot LAL reagents + temperature-controlled bath — Minimal equipment; suited to limit tests, with simple qualitative judgment
- Monocyte activation test (MAT) · Human monocytes/PBMC + IL-6 ELISA (plate reader) — Also detects non-endotoxin pyrogens; alternative to the rabbit pyrogen test
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).