Phase 1Cell culture and production (upstream)
01Construction and quality control of host strain and cell banks (MCB/WCB)
Fixes the starting point of all production to 1 bank and cuts lot-to-lot variation through strain identity, copy number and passage stability.
- Standard method
- Master/working cell bank system (2-tier bank) + strain identity, purity and genetic stability testing
Why it is the standard. ICH Q5D (derivation and characterization of cell substrates) calls for a 2-tier bank system and identity and purity testing, so this is the standard configuration in industry. Integrating systems (such as the AOX1 locus) are the mainstream in yeast, and confirming the integrated copy number and passage stability is essential.
About this step and the alternatives
All production begins from a single cell of well-established identity. This step cryopreserves that "seed" and secures the foundation.
From the master cell bank (MCB), the working cell bank (WCB) for daily use is set aside in two tiers, and identity, purity, gene copy number and stability after passaging are tested.
If the strain is not uniform, yield and quality vary from lot to lot. Fixing the strain first supports the reproducibility of every downstream step.
Everything in production starts from 1 cell bank of well-established identity. If the strain is not uniform and stable, yield and quality vary from lot to lot. So the first thing to lock down is the strain's identity, purity, copy number and passage stability.
Alternatives in practice
- Strain identification and mutation monitoring by whole-genome sequencing (WGS) · Illumina NextSeq 2000/Oxford Nanopore GridION + Qiagen DNeasy — Comprehensively confirms genetic stability across MCB/WCB generations. Strong at detecting insert copy number and accumulated mutations
- Confirmation of monoclonality and viability by flow cytometry · Sartorius iQue3/BD FACSymphony + Thermo Fisher LIVE/DEAD Fixable dyes — Quantifies clonal purity and the fraction of viable cells. Supports single-cell cloning
- Measurement of plasmid/episome retention (comparison without antibiotic) · Applied Biosystems QuantStudio 5 (Bio-Rad QX200 for ddPCR) — Stability assessment when non-integrating vectors are used. Monitors variability in copy number
Seed culture and scale-up (seed train)
The contents of 1 bank vial cannot go directly into the production tank, so stepwise expansion and sterility checks secure a uniform start for the main fermentation.
- Standard method
- Stepwise seed train
Why it is the standard. In high-density yeast fermentation, a two-phase process that uses glycerol as the carbon source and methanol for induction is the norm, and securing cell mass through glycerol assimilation at the seed stage is the standard configuration.
With Intracellular inclusion bodies + refolding
- IB production builds IB quality through high specific growth, so the induction mode and starting material change
- Method: After high-density fed-batch, induce with IPTG (lac/T7 system) or a temperature shift so that the product accumulates as insoluble inclusion bodies rather than soluble protein
- Reagents: IPTG (inducer)
About this step and the alternatives
A single frozen vial cannot simply be put into a huge production tank. The cells would be too dilute to start growing.
Cell mass is increased stepwise, from flask to small tank to production tank, with growth and sterility checked at each stage.
The goal is to start the main fermentation uniformly. The more closely cell mass and vigor are matched here, the smaller the variation in the production run.
The contents of 1 bank vial cannot be put straight into the production tank. Cell mass is increased stepwise, from flask to small tank to production tank. Growth and sterility are checked at each stage so that the main fermentation can start from a uniform starting point.
Alternatives in practice
- High-density seeding by N-1 perfusion · Sartorius BIOSTAT STR + Repligen XCell ATF perfusion device/Cytiva ÄKTA flux TFF — Seeds at high VCD to shorten the start-up of the main fermentation. Medium exchange is also easy
- Direct inoculation from a frozen seed (cryobag) · Sartorius CryoVessel/Cytiva Cryostore cryobags + Planer Kryo controlled-rate freezer — Reduces the number of seed train stages and lowers contamination. HD-cryo secures the inoculum scale
Main fermentation (fed-batch, high-density culture and methanol induction) (main culture, fed-batch)
Central step that determines yield, controlling expression and secretion through ultra-high-density culture and methanol induction.
- Standard method
- Fed-batch culture
- Equipment
- Microbial fermentation bioreactor, Inline pH and DO sensors and probes, Transfer pump
Why it is the standard. K. phaffii is the most widely used expression host after E. coli, characterized by eukaryotic post-translational modifications (disulfide bonding, glycosylation) and highly efficient secretion. The AOX1 (PAOX1) two-phase process is the most widespread configuration (confirmed in reviews such as FEMS Yeast Res 2025).
Check also. Methanol induction is specific to certain hosts and promoters, such as the AOX1 system of Pichia/Komagataella, and does not apply to Saccharomyces or constitutive expression strains. In large-scale culture, managing methanol's flammability, oxygen demand and heat generation is a challenge, and in recent years more processes choose methanol-independent induction systems.
About this step and the alternatives
This is the heart of the process, where yield is decided. Yeast is grown to ultra-high density while the target protein is secreted into the medium.
Dissolved oxygen, pH, temperature and nutrient supply are tightly controlled, and the run is operated as fed-batch, adding feed little by little. In AOX1 systems, methanol is added to induce expression.
How well this step is built determines both the load on downstream steps and the final yield. Culture quality translates directly into the quantity and quality of the product.
This is the central step that determines yield. Yeast can be grown to ultra-high density and secretes the target protein into the medium. Dissolved oxygen, pH, temperature and substrate feed are tightly controlled, and in AOX1 promoter systems expression is induced with methanol.
Alternatives in practice
- Methanol-free induction (constitutive or alternative promoters such as GAP) · Sartorius BIOSTAT B (glucose fed-batch, Eppendorf BioFlo 320) — Avoids handling flammable methanol and eases explosion-proof requirements. Constitutive expression with PGAP/PDH systems
- S. cerevisiae galactose induction (GAL system) · Eppendorf BioFlo 320 + galactose fed-batch (Sigma-Aldrich D-Galactose) — Induction system when the host is changed to Saccharomyces. GAL1/GAL10 promoters
- Higher productivity by continuous/perfusion fermentation · Sartorius BIOSTAT STR + Repligen XCell ATF/Cytiva ÄKTA flux — High-density continuous production with cell retention. Improves volumetric productivity
In-process control (IPC: monitoring of cell mass, substrates, metabolites and titer) (in-process analysis, IPC)
Measures cell mass, residual substrate and titer as needed, correcting errors in induction timing and feed rate during the run.
- Standard method
- Offline analysis
- Reagents and materials
- HPLC mobile phase solvents, API reference standards and impurity standards (CRS), ELISA
Why it is the standard. Knowing residual substrate and cell mass bears directly on optimizing methanol induction. While inline near-infrared and Raman are advancing within the PAT (process analytical technology, ICH Q8) framework, offline HPLC+OD measurement is still the standard configuration.
With Intracellular inclusion bodies + refolding
- The product accumulates inside the cells instead of being secreted, so the clarified side is the product-discard side and the cells are recovered
- Method: Recover the cell pellet by centrifugation (discard the supernatant). Primary clarification here is not product recovery but a step that concentrates the cells for downstream disruption
- Check also: In the default (secreted) case the product is on the supernatant side, but on the switch side it is on the cell side. The recovery target is reversed
About this step and the alternatives
You cannot see inside the tank during fermentation. That is why you have to keep measuring "what is happening right now."
Samples are taken as needed and analyzed for cell mass, remaining nutrients (glycerol/methanol), metabolites and the titer of the target product.
From these values, the start of methanol induction and the feed rate are decided and fine-tuned. They are the compass for keeping the run on course.
Without seeing "what is happening right now" during fermentation, induction timing and feed rates go wrong. Cell mass, residual glycerol/methanol, metabolites and the titer of the target product are measured as needed, and the run is corrected accordingly.
Alternatives in practice
- Real-time estimation of substrate and cell mass by inline Raman/near-infrared · Endress+Hauser Kaiser Raman Rxn2/Sartorius BioPAT Spectro + Bruker MATRIX-F — Continuously monitors glucose, methanol and VCD without sampling. Directly linked to PAT/feedback control
- Methanol and glucose measurement with capillary biosensors/enzyme electrodes · YSI 2950/2900D Biochemistry Analyzer (Xylem)/Trace Analytics TRACE C2 methanol analyzer — Frequent offline substrate quantitation. Reflected in the fermentation feed
Phase 2Harvest and clarification
05Cell separation and clarification (harvest)
Pretreatment that removes cells and debris to recover the secreted product on the supernatant side and prevents clogging of downstream chromatography.
- Standard method
- Primary clarification by disc stack centrifugation + depth filtration (secreted production = supernatant recovery)
Why it is the standard. For microbial fermentation broth with high cell density, primary clarification by disc stack centrifugation followed by finishing with depth filtration is standard. For secretory yeast production (e.g., secreted insulin), high recovery into the supernatant by centrifugation has been reported.
With Intracellular inclusion bodies + refolding
- In the default secreted route there is no inclusion body step at all; on the switch side it is a core step that starts up for the first time
- Method: Centrifuge the high-pressure homogenization lysate and recover the IB pellet, then wash out membranes and soluble HCP with buffer containing 0.5% Triton X-100, 1–2 M urea and EDTA
- Check also: On the default side this step is skipped (because the product is soluble)
About this step and the alternatives
The fermentation broth is a cloudy soup of cells and cell debris. The target protein is the secreted type, so it is dissolved in the clear supernatant.
Cells are removed by centrifugation (disc stack type) and depth filtration, giving primary clarification to a transparent liquid.
The aim is to prevent clogging of the downstream chromatography resin. The more turbidity is removed here, the more smoothly purification flows.
The fermentation broth is cloudy with cells and cell debris. The secreted protein is on the supernatant side, so the cells are removed first to give a clear liquid. This is pretreatment to avoid clogging the downstream chromatography resin.
Alternatives in practice
- Clarification by TFF/MF (microfiltration) or hollow fiber · Repligen KrosFlo (0.2 µm hollow fiber)/Cytiva ÄKTA flux + Sartorius Sartocon — Continuous clarification without a centrifuge. For shear-sensitive processes and single-use adoption
- Flocculation pretreatment + depth filtration · Merck Millistak+ + flocculant addition (Sigma-Aldrich pDADMAC/chitosan) — Reduces particulates and DNA/HCP upstream to lighten the filter load
Inclusion body (IB) recovery and washing → recover the IB pellet from the lysate by centrifugation and wash with buffer containing low-concentration urea/detergent (Triton X-100) to remove membranes, host proteins and nucleic acids (inclusion body washing)
To wash away host-derived contaminants from the inclusion body surface and secure the purity and reproducibility of solubilization and refolding
- Standard method
- Recover IB by centrifugation, then wash stepwise several times with Tris buffer containing 0.5% Triton X-100, 1–2 M urea and EDTA, centrifuging each time to recover the pellet
Why it is the standard. Stepwise washing with detergent plus a low-concentration chaotrope is an established standard method for inclusion body purification and efficiently reduces membrane, HCP and DNA contamination
Check also. Inclusion body recovery and washing is a step belonging to the E. coli-type route in which the product accumulates insolubly, and it does not normally appear in the standard yeast flow, in which the product is secreted into the supernatant. In a design that expresses the product soluble and secreted, this step is unnecessary altogether.
With Intracellular inclusion bodies + refolding
- Solubilization is complete dissolution with a denaturant, which has the opposite aim to the mechanical disruption and soluble-fraction recovery of the secretion route
- Method: Fully denature and solubilize the washed IB with 6 M guanidine hydrochloride or 8 M urea plus a reducing agent (deliberately unfolding the protein)
- Reagents: Guanidine hydrochloride (6 M) or urea (8 M) + DTT or β-mercaptoethanol
About this step and the alternatives
This is an auxiliary route for when E. coli is made to produce the protein as an insoluble lump called an "inclusion body." It is a separate route from secretory production in yeast.
The lysate is centrifuged to recover the lump of target product (the IB pellet), which is washed stepwise with buffer containing detergent and low-concentration urea.
Washing off the attached cell membranes, host proteins and DNA secures the yield and purity of the later solubilization and refolding.
When the protein is expressed in E. coli as insoluble inclusion bodies, the IB pellet obtained after mechanical disruption carries cell membrane fragments, HCP and DNA. Unless these are removed by stepwise washing with Triton X-100 or low-concentration urea, the yield and purity of the downstream solubilization and refolding drop greatly. This is the standard pretreatment in inclusion body processes and sits between disruption and solubilization.
Alternatives in practice
- IB washing, concentration and buffer exchange by crossflow (TFF) · Repligen KrosFlo KMPi (0.2/0.45 µm hollow fiber)/Sartorius Sartoflow + Hydrosart — Makes the repeated centrifugal recovery continuous. Integrates washing and concentration of the IB slurry
- Low-pressure repeated homogenization + washing to reduce undisrupted cells · GEA Niro Soavi PandaPLUS 2000 (low-pressure multipass) — Obtains high-purity IB. Suppresses residual soluble HCP/membrane fragments
Cell disruption and solubilization for intracellular products (auxiliary route) (homogenization disruption)
Auxiliary route that physically disrupts the tough yeast cell wall in non-secreted or inclusion body types to release the intracellular target.
- Standard method
- Mechanical disruption by high-pressure homogenization + separation of the soluble fraction by centrifugation/filtration
Why it is the standard. Because yeast has a strong cell wall, high-pressure homogenization is the standard disruption method at large scale. Inclusion body recovery is followed by denaturation and refolding steps. In secretory production this route is unnecessary, and the resulting simplification of the process is an advantage of yeast secretion systems.
Check also. Cell disruption is an auxiliary route for when the product is not secreted and accumulates inside the cells, and it is omitted from the standard flow, which is based on supernatant recovery. Because the yeast cell wall is tough, entering this route means allowing for the work of developing the disruption conditions and for a heavier downstream purification load as more host proteins and DNA are released.
With Intracellular inclusion bodies + refolding
- The rate-limiting step that becomes an actual process step only on the switch side; it governs yield and aggregation
- Method: Remove the denaturant stepwise by dilution refolding (6–10-fold dilution) or dialysis, and re-form disulfide bonds with an oxidized/reduced glutathione pair. Add arginine to suppress aggregation
- Reagents: Oxidized glutathione (GSSG)/reduced glutathione (GSH) (e.g., 0.5 mM/1.25 mM), L-arginine
About this step and the alternatives
In types where the target is not secreted but accumulates inside the cells, the cell shell must first be broken to get the contents out.
The yeast cell wall is tough, so physical disruption (homogenization), which smashes the cells under high pressure, is the basic approach. Centrifugation or filtration then separates the soluble fraction.
An auxiliary route to the main route of secretory production. Where the target accumulates changes the entry point of recovery in this way.
In types where the target is not secreted but accumulates inside the cells (or as inclusion bodies), the cell wall must first be broken to get the contents out. Because yeast has a tough cell wall, physical disruption is the basic approach.
Alternatives in practice
- Bead mill disruption · Glen Mills/WAB Dyno-Mill MULTI LAB/Retsch glass bead systems — Effective for high cell density and small to medium volumes. Cooling is essential and shear is strong
- Enzymatic cell wall lysis (zymolyase/lyticase) · Stirred tank + Zymolyase-20T/100T (Nacalai/AMSBIO), Lyticase (Sigma-Aldrich L2524) — Gentle and avoids fine debris. Low-pressure disruption can also be combined after spheroplast formation
Refolding → remove the denaturant stepwise from the solubilized denatured protein by dilution/dialysis and restore the native structure in the presence of a redox pair (refolding)
To restore the activity of the denatured protein from inclusion bodies by refolding it to the native three-dimensional structure with correct disulfide bonds
- Standard method
- Remove the reducing agent from the solubilized solution, then dilute into a refolding buffer containing a redox pair (such as cysteine/cystine)
Why it is the standard. Dilution/dialysis refolding with arginine and a redox pair is a well-established refolding method widely used at industrial scale, and it can restore the native structure and disulfide bonds while suppressing aggregation
Check also. Refolding is needed only when the process goes through inclusion bodies, and it does not occur in the standard route in which the protein is secreted in its native structure. Note that refolding requires dilute conditions to suppress aggregation, so yield and tank volume tend to become the rate-limiting factors.
With Intracellular inclusion bodies + refolding
- Refolded products often carry no affinity tag, so the capture principle changes
- Method: Capture by anion exchange (AEX) is the basic approach, not by affinity such as Protein A (to handle the large volume and low concentration of the diluted refolding solution)
- Reagents: Strong anion exchange resin (e.g., Q Sepharose)
About this step and the alternatives
Immediately after inclusion bodies are dissolved with urea or similar, the protein is in an unfolded (denatured) state and has no activity. It is like an unfolded piece of origami.
While the denaturant is removed little by little by dilution or dialysis, the protein is refolded into its original three-dimensional structure with the help of a redox system that connects cysteines to each other correctly.
Because the protein aggregates easily, the knack is to proceed slowly under dilute conditions so the molecules do not collide. This is the crux that decides the success of the inclusion body route.
Immediately after inclusion bodies are solubilized with urea or guanidine, the protein is denatured and reduced and has no activity. Refolding is therefore essential: the denaturant is removed stepwise by dilution or dialysis while a redox pair such as cysteine/cystine promotes correct disulfide bond formation. It is standard to work under dilute conditions to suppress aggregation, and the step sits between solubilization and capture.
Alternatives in practice
- On-column refolding (refolding the immobilized protein on the column) · Cytiva ÄKTA pure/avant + gradient refolding on HisTrap FF (IMAC) — Reduces dilution volume and lowers aggregation. Refolding with a denaturant gradient after His-tag immobilization
- Gentle denaturant removal by pulse dilution/stepwise dialysis · Repligen KrosFlo (DF)/Spectra/Por dialysis membranes (Repligen) — Suppresses aggregation by avoiding supersaturation. For large-volume processing, TFF diafiltration is advantageous
Phase 3Purification (downstream)
09Capture (capture chromatography)
With no general-purpose affinity ligand, capture is done in one pass by tag purification or ion exchange, a foundation step that removes most impurities.
- Standard method
- Affinity chromatography
Why it is the standard. A general-purpose affinity ligand equivalent to Protein A for antibodies does not exist for recombinant proteins in general, so tag purification (IMAC) or ion exchange using charge differences is the standard choice for capture. Cytiva and Tosoh resins are widely adopted in the industry.
About this step and the alternatives
Even the clear supernatant still contains large amounts of host proteins and pigments. Here the target alone is grabbed in one pass.
The target protein is selectively captured with an affinity resin or similar, sharply narrowing the volume while most impurities are washed away.
Clearing most of the load at this first gate makes the downstream polishing far easier. It is the foundation of the whole purification.
The clarified supernatant still contains large amounts of host proteins and pigments. The first chromatography captures the target protein in one pass, narrowing the volume while removing most impurities. It builds the foundation that makes later steps easier.
Alternatives in practice
- Capture by multimodal (mixed-mode) chromatography · Cytiva Capto MMC ImpRes/Merck Eshmuno HCX + ÄKTA process — Binds even at high conductivity and captures directly from clarified fluid. Reduces HCP/DNA at the same time
- Expanded bed adsorption (EBA) integrates clarification and capture · Cytiva (formerly Streamline/UpFront FastLine) EBA column — Loads unclarified fluid directly and omits centrifugation/filtration. For particle-containing feeds
- Target-specific affinity (antibody, substrate analog, tag) · Thermo Fisher POROS/Cytiva Capto/HiTrap custom ligands, anti-FLAG (Merck/Sigma), Strep-Tactin (IBA) — High selectivity gives high purity in 1 step. Ligand leakage must be managed
Intermediate purification and polishing (removal of aggregates, host impurities and variants)
Polishing step that shaves off aggregates, residual host proteins and molecular variants with IEX and HIC, which work on different principles, to specification purity.
- Standard method
- Ion exchange
Why it is the standard. IEX, which separates by charge, and HIC, which separates by hydrophobicity, are highly orthogonal, making them a well-established combination widely used to remove aggregates and host-derived impurities. Cytiva's Capto Phenyl ImpRes has a confirmed track record for intermediate/polishing use.
About this step and the alternatives
Even after passing through capture, aggregates, remaining host proteins and slight molecular variants persist stubbornly.
Two or so chromatography steps, such as ion exchange, which exploits differences in charge and other properties, are stacked to shave off similar contaminants one by one.
This is the finish that raises purity up to specification. It is a delicate polishing step that separates the last small differences.
Even after capture, aggregates, residual host proteins and slight molecular variants remain. Around 2 chromatography steps with different properties are stacked to shave these off and polish to a purity that meets specifications.
Alternatives in practice
- Final polishing by SEC (gel filtration) · Cytiva Superdex 200 pg/Sephacryl S-200 HR + ÄKTA process — Final separation of aggregates and fragments. Note low throughput and dilution
- Polishing with multimodal AEX/CEX · Cytiva Capto adhere (AEX-MM)/Merck Eshmuno HCX + ÄKTA — Removes HCP/aggregates/DNA in flow-through mode. Selectivity tolerant of salt and pH
- Membrane chromatography (flow-through AEX membrane) · Sartorius Sartobind Q/Merck Millipore Natrix Q/Pall Mustang Q — High flow rate and disposable, removes DNA/virus. A polish that prioritizes flow rate over binding capacity
Virus/impurity clearance and bioburden reduction (virus clearance)
Virus risk is low, but process-derived impurities and bioburden must be reliably reduced and clearance secured.
- Standard method
- Nano/sterile filtration + clearance at each chromatography stage, with built-in endotoxin and DNA removal steps
Why it is the standard. Yeast has a low risk of producing human-type enveloped viruses, but in line with the thinking of ICH Q5A/Q6B, designing reduction of process-derived impurities and bioburden is standard. AEX and membrane adsorbers are effective for removing DNA and endotoxin.
Check also. Because the risk of human pathogenic viruses in yeast systems is inherently lower than in mammalian cells, the dedicated virus clearance steps that are essential for mammalian cells are not necessarily required as they are. However, reducing impurities and microbial load must still be reliably secured as a separate matter, and the final need is decided on the basis of virus safety evaluation and risk assessment.
About this step and the alternatives
Because the product is used as an injectable, process-derived impurities and microbial load must be reliably reduced.
In yeast systems the risk of human pathogenic viruses is lower than in mammalian cells, but clearance by nano/sterile filtration and by each chromatography stage is combined.
Removal of endotoxin and DNA is also secured here. The idea is to build safety up through the whole process.
In yeast systems the risk of human pathogenic viruses is lower than in mammalian cells, but process-derived impurities and microbial load still must be reliably reduced. Clearance is secured by a combination of filtration and chromatography.
Alternatives in practice
- Adding virus inactivation steps such as low pH/detergent treatment · Stirred tank + hold at around pH 3.5/solvent/detergent (S/D) treatment (Triton X-100, Merck) — Inactivates enveloped viruses. Built in before or after chromatography
- Evaluation of clearance by chromatography steps (spiking studies) · Model virus spiking such as MVM/MuLV + qPCR/TCID50 (QuantStudio, Applied Biosystems) — Calculates LRV for each chromatography/filtration step. Essential for validation
Phase 5Quality attributes and analytics
14Confirmation of identity and primary structure (molecular weight, sequence) (identity and primary structure confirmation)
Confirms at the molecular level, by intact mass and peptide mapping, that it is the intended protein, preventing mix-ups and mutations.
- Standard method
- Intact mass measurement by LC-MS + tryptic digest peptide mapping
- Reagents and materials
- Peptide mapping protease, Disulfide reduction alkylation reagents, HPLC mobile phase solvents, Analytical antibody enzymes
Why it is the standard. Intact mass rapidly and quantitatively evaluates molecular weight, mass variants and process-derived modifications, and peptide mapping confirms sequence identity and post-translational modifications (deamidation, oxidation, etc.); this is the standard for characterizing recombinant proteins (ICH Q6B).
Check also. In Pichia-derived proteins, proteolytic cleavage at the N- or C-terminus and addition of O-linked glycans can occur, and the intact mass may show multiple peaks or shifts. Do not judge from 1 molecular weight value alone; read the result by directly confirming the sequence and modification sites by peptide mapping.
About this step and the alternatives
This step confirms at the molecular level that "the intended protein has really been made." It amounts to checking the identity of the product.
Mass spectrometry (LC-MS) measures the overall molecular weight, and a map of peptides cut with an enzyme is used to check the amino acid sequence and modifications.
The linchpin for not missing mix-ups or unexpected mutations. It is the starting point of characterization, assuring identity.
Confirms at the molecular level that "the intended protein has really been made." Intact mass gives the overall molecular weight, and peptide mapping confirms the amino acid sequence and modifications. It is the linchpin for not missing mix-ups or mutations.
Alternatives in practice
- N-terminal amino acid sequencing (Edman degradation) · Shimadzu PPSQ-53A protein sequencer — Direct determination of the N-terminal sequence and confirmation of processing. Low sensitivity; complements MS
- Identification by Western blot/immunoassay · Bio-Rad V3 Western Workflow/Cytiva Amersham ImageQuant + specific antibodies — Rapid and specific identity confirmation. MS handles the details of primary structure
Purity and heterogeneity (size variants, charge variants, aggregates) (purity and heterogeneity)
A test tied directly to safety and efficacy that quantifies size variants and charge variants separately to judge whether the purity profile is within specifications.
- Standard method
- CE-SDS
- Reagents and materials
- Carrier ampholytes pI markers, HPLC mobile phase solvents
Why it is the standard. CE-SDS is a quantitative version of SDS-PAGE and the standard for size purity, and SEC-HPLC is the standard analysis for aggregates/fragments. Evaluating charge heterogeneity by IEX or icIEF is the configuration in line with ICH Q6B. The purity profile is specified by combining methods with different principles.
Check also. CE-SDS (electrophoresis that quantifies SDS-PAGE), the standard for size purity, does not complete the purity picture on its own. The premise is to layer methods with other principles: aggregates by size exclusion chromatography (SEC) or analytical ultracentrifugation, charge variants by ion exchange or isoelectric focusing.
About this step and the alternatives
Variation in purity bears directly on safety and efficacy. Here the heterogeneity of the contents is made visible.
Differences in size (fragments and aggregates) and in charge (variants such as deamidation) are measured separately, each with a dedicated method.
Judges whether the measured values fall within the specification range. It is the gate that assures the product can be made homogeneously.
Purity bears directly on safety and efficacy. Size differences (fragments, aggregates) and charge differences (variants such as deamidation) are measured separately to judge whether they fall within specifications.
Alternatives in practice
- SDS-PAGE (Coomassie/silver staining) · Bio-Rad Criterion/Mini-PROTEAN TGX + Coomassie/SilverQuest (Thermo) — Simple, low-cost purity check. CE-SDS is superior in quantitation and resolution
- Absolute molecular weight and aggregate quantitation by SEC-MALS · Wyatt (Waters) DAWN MALS + Optilab dRI + Agilent 1260 SEC — Absolute molecular weight independent of standards. Determines the stoichiometry of aggregates
- Confirmation of aggregates by analytical ultracentrifugation (AUC) as a separate method · Beckman Coulter Optima AUC — Confirms aggregates/association state without contact with a matrix. Useful as an orthogonal method
Glycan profile (evaluation of high-mannose type and hypermannosylation) (glycan profile)
Hypermannosylation specific to yeast can affect immunogenicity, so released glycans are monitored lot to lot.
- Standard method
- Fluorescent labeling of glycans released with PNGase F
- Reagents and materials
- N-glycan release and labeling (RapiFluor-MS/2-AB), Analytical antibody enzymes, HPLC mobile phase solvents
Why it is the standard. Yeast expression causes hypermannosylation with excess terminal mannose, so glycan control is important. PNGase F release → fluorescent labeling → HILIC-UPLC-FLR/MS is the standard workflow that separates high-mannose structures at high resolution (established in Waters applications and elsewhere).
Check also. Yeast glycans undergo "hypermannosylation," in which terminal mannose is extended excessively, which can affect immunogenicity and in vivo clearance, and it is the most important control item for yeast systems. For strains engineered for human-type glycosylation the picture changes completely and the criteria change as well. Note also that the standard PNGase F removes N-linked glycans, so O-linked glycans require separate evaluation.
With Intracellular inclusion bodies + refolding
- Because the host changes from yeast to E. coli, the HCP ELISA antibody system is entirely different
- Method: Quantify with an E. coli HCP ELISA, not a yeast-specific HCP ELISA. Change the primers/standards for residual DNA qPCR to match the host
- Reagents: Sandwich ELISA kit based on anti-E. coli HCP antibody
About this step and the alternatives
The glycans yeast attaches are of the high-mannose type, rich in mannose. In addition, "excessive mannosylation," in which the outer chains extend long, occurs.
The glycans are cut off with an enzyme (PNGase F), labeled with fluorescence, separated and identified, and the pattern is read.
These glycans can affect immunogenicity and disappearance from the body, so they must be monitored to see whether they are kept consistent between lots.
Yeast glycans are of the high-mannose type, and "hypermannosylation," in which the outer chains extend long, occurs. Because this can affect immunogenicity and clearance, released glycans are separated and identified, and monitored to see whether they are kept consistent between lots.
Alternatives in practice
- Monosaccharide composition analysis (HPAEC-PAD) · Thermo Fisher Dionex ICS-6000 + CarboPac PA20/PA1 — Quantifies monosaccharide ratios for mannose types (yeast high-mannose) and others. No labeling required
- Structural assignment by sequential exoglycosidase digestion · NEB/Agilent exoglycosidases (α-mannosidase, etc.) + Waters UPLC — Determines linkage and branching. Orthogonal confirmation of the mapping
- MS confirmation of glycans from glyco-engineered strains (human-type glycosylation) · Thermo Orbitrap/Waters Xevo G3 QTof + RapiFluor-MS — Confirms glycan homogeneity in humanized strains such as GlycoSwitch. Monitors residual high-mannose
Process-derived impurities (host cell protein HCP, residual DNA)
Core safety test that detects residual yeast HCP and DNA down to trace levels and confirms they have been brought down to or below the acceptable level.
- Standard method
- Yeast-specific HCP ELISA (sandwich format) + qPCR quantitation of residual DNA
Why it is the standard. Cygnus is the industry-standard vendor supplying HCP ELISA kits for various hosts, including yeast. For residual DNA, qPCR is highly sensitive and can quantify down to the femtogram range. ICH Q6B treats process-derived impurities as targets to be controlled by specification, and HCP and residual DNA are representative examples. Specific limits vary by region and product, so there is no fixed numerical value.
Check also. An ELISA that measures host cell protein (HCP) sees only what its antibody panel covers, so it is safer to choose a kit matched to the host and strain and to cross-check for gaps with a method of different principle such as LC-MS/MS. The qPCR for residual DNA also depends on the target sequence, so the validity of the detection range must be confirmed.
About this step and the alternatives
If yeast-derived proteins or DNA remain in the product, they become an immunogenicity and safety risk. Even trace amounts cannot be overlooked.
A yeast-specific HCP assay (sandwich ELISA) and qPCR quantitation of residual DNA detect down to very small amounts.
The core test for securing safety, confirming that levels have been reduced all the way to an acceptable level. Purification performance is graded here.
If yeast-derived proteins or DNA remain in the product, they become an immunogenicity and safety risk. Trace amounts are detected to confirm that they have been reduced all the way to an acceptable level. This is the core test for securing safety.
Alternatives in practice
- Identification and quantitation of individual HCPs by LC-MS/MS (DIA, etc.) · Thermo Orbitrap Exploris/SCIEX ZenoTOF 7600 + Spectronaut/DIA-NN — Identifies and quantifies individual HCPs that ELISA cannot see (risk HCPs). Complements antibody coverage
- Total residual DNA measurement by threshold/fluorescent dye methods · Molecular Devices/BioTek reader + Thermo Fisher Quant-iT PicoGreen dsDNA — Rapid quantitation of total dsDNA. qPCR handles sequence specificity
- Validation of the HCP ELISA by 2D-DIGE/antibody coverage evaluation · Cytiva Amersham WB/2D Ettan DIGE + Typhoon imager — Confirms coverage of the anti-HCP antibody. For validation of the ELISA method
Potency (biological activity, content) and general properties (potency, content)
A specification for efficacy that assures whether the product works and how much it contains, by a bioassay against a reference standard and a content measurement.
- Standard method
- Bioassay against a reference standard
- Reagents and materials
- Reporter gene bioassay ADCC CDC, SPR / BLI sensor chips
Why it is the standard. A280 is a rapid, reagent-free standard method that calculates concentration directly from Trp/Tyr absorption and the extinction coefficient (Beer-Lambert law). For potency, the configuration in line with ICH Q6B is a relative-potency bioassay that reflects the mechanism of action, compared with a reference standard.
Check also. Bioassays use cell or biological responses and so have large variability, and potency is always judged as relative potency against a reference standard. Potency must be designed in a system that reflects each product's mechanism of action and cannot be equated with mere protein content (concentration).
About this step and the alternatives
What ultimately matters is "whether it works (activity)" and "how much is in it (content)." The contents of the product are assured by numbers.
Content is judged by measuring protein concentration, and potency by a biological test (bioassay) that reflects function, in comparison with a reference standard.
This is the specification that secures efficacy itself. It supports every vial that reaches a patient having the same effect.
Ultimately this assures "whether it works (activity)" and "how much is in it (content)." Protein concentration is measured, and potency is judged against a reference standard with a bioassay that reflects function. It is the specification that secures the efficacy of the product.
Alternatives in practice
- Binding-activity-based potency measurement by ELISA/SPR · Cytiva Biacore 8K/Sartorius Octet (BLI) + SpectraMax — Evaluates potency using binding activity as the indicator. Faster and less variable than cell assays
- Protein quantitation by BCA/Bradford colorimetric methods · Thermo Fisher Pierce BCA Protein Assay/Bio-Rad Bradford + plate reader — Simple quantitation of total protein concentration. Serves as the denominator for specific activity
- Absolute content by amino acid analysis · Agilent 1260 AdvanceBio AAA/Hitachi L-8900 amino acid analyzer — Primary-standard-type determination of absolute protein amount. Useful for assigning values to reference standards
Safety testing (endotoxin, bioburden/sterility) (endotoxin and sterility)
Final gate for patient safety, confirming by pharmacopoeial methods that endotoxin and microorganisms are absent from the injectable.
- Standard method
- Bacterial endotoxin test (LAL
Why it is the standard. USP <85> (BET) is a standard harmonized with the European and Japanese pharmacopoeias. In May 2025 USP <86> (recombinant reagents such as rFC) became official, institutionalizing non-animal-derived alternatives. For sterility and bioburden, following pharmacopoeial compendial methods is a mandatory requirement.
Check also. For endotoxin, sterility and bioburden, assurance by pharmacopoeial (compendial) methods is a mandatory requirement. As an alternative to conventional horseshoe crab-derived LAL reagents, the non-animal-derived recombinant Factor C (rFC) method has been institutionalized, and its adoption is spreading from the viewpoint of sustainability and supply stability.
About this step and the alternatives
Since the product enters the body as an injectable, it must be assured to be free of pyrogens and microorganisms. This is the final gate for patient safety.
Confirmed by methods defined in the pharmacopoeia, such as the endotoxin test (LAL) and the bioburden/sterility tests.
Within limits and sterile. Only after passing this gate is the product in a state to be released to the market.
Because the product enters the body as an injectable, it must be assured to be free of pyrogens (endotoxin) and microorganisms. Compliance within limits and sterility are confirmed by compendial (pharmacopoeial) methods. It is the final gate for patient safety.
Alternatives in practice
- Endotoxin measurement by the recombinant Factor C (rFC) method · Lonza PyroGene rFC + FLUOstar (BMG) fluorescence reader/bioMérieux ENDONEXT — Independent of horseshoe crabs; supports animal welfare. Included in USP <86>, so it can serve as an alternative
- Bioburden by rapid microbiological methods (growth-based fluorescence/ATP) · bioMérieux BACT/ALERT, BioFire/Charles River Celsis ATP bioluminescence — Rapid determination of sterility/bioburden. Shortens release
- Pyrogenicity evaluation by the monocyte activation test (MAT) · Merck PyroMAT/Lonza MAT kit + plate reader — Also detects non-endotoxin pyrogens. An 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).