Phase 1Cell culture and production (upstream)
01Cell line and seed train / N-1 high-density perfusion (seed train)
N-1 perfusion raises the seeding density to 10–20 × 10^6, shortening the time to steady state and reducing the number of seed train stages.
- Standard method
- N-1 perfusion (high-density perfusion seed culture) + single-use bioreactor
Why it is the standard. Raising the seeding density of the production culture to 10–20 × 10^6 cells/mL by N-1 perfusion is a widely adopted configuration to shorten the time to steady state and reduce the number of seed train stages. CHO is the de facto standard host for antibody production (with cell substrate control under ICH Q5A/Q5D as a premise).
Check also. Building a high-density seed by perfusion from N-1 allows the production culture to start up quickly, but media consumption is large, and it presupposes maintaining sterility for several weeks from the start-up stage and managing hollow-fiber clogging. Because the density and state of the seed cells determine whether the production culture reaches steady state, the seeding conditions should be worked out at actual scale and then fixed.
About this step and the alternatives
In continuous manufacturing, where the production culture runs for months without stopping, there is no room to stumble at the start. N-1 perfusion is that run-up: the step one stage before the production culture in which the seed cells are grown to high density in advance.
It works by feeding fresh medium into the bioreactor while continuously drawing off spent medium. Only the cells are kept in the vessel by a hollow-fiber filter, so nutrients are constantly replaced and the cells grow to a density orders of magnitude higher than usual.
The key is speed of start-up. If a dense, ample seed is prepared, the production culture reaches steady state at once after transfer. The cell density established here determines how well long-term operation gets off the mark.
In continuous manufacturing the production culture runs for weeks to months without stopping. To secure enough cells at start-up, high-density seed cells are produced by perfusion from the step before the production culture (N-1), so that the production culture reaches steady state quickly.
Alternatives in practice
- Stepwise-expansion seed train derived from conventional fed-batch · Sequential scale-up with Cytiva Xcellerex XDR-50/200 / Sartorius BIOSTAT STR 50/200 (Gibco Dynamis / EfficientFeed feeds) — Low risk and well proven, but start-up takes longer and the seeding cell density is lower
- High-seed intensified N-1 (direct seeding from bolus frozen vials) · Cytiva WAVE 25/200 rocking bag + XCell ATF 4/6, large-volume frozen vials (Cytiva/Sartorius cryobags) — Shortens the seed train and allows the production culture to start at high density, but requires frozen stock preparation and logistics
Production culture: perfusion culture (ATF/TFF cell retention)
Retains cells in the vessel and keeps producing antibody at high density and steady state; the heart of continuous manufacturing.
- Standard method
- Perfusion culture + ATF (alternating tangential flow) hollow-fiber cell retention
Why it is the standard. ATF moves the culture fluid back and forth through the hollow fiber with a diaphragm pump, suppresses filter clogging through the backflush effect, and can maintain a high density of 100–130 × 10^6 cells/mL for a long period, so it is a representative perfusion configuration. Together with TFF it is one of the two main cell retention methods (PMID 29509101, etc.).
Check also. Perfusion presupposes continuous operation for weeks to months, so long-term sterility of the medium, clogging of the hollow-fiber filter and a decline in product passage (sieving) tend to be rate-limiting. Even at steady state, glycans and charge variants can change slightly in the later part of a run, so consistency of quality is monitored continuously in-process.
About this step and the alternatives
This is the heart of continuous manufacturing. Fresh medium is poured into the bioreactor without pause, spent medium and the antibody that has been made flow out, and only the cells are kept inside the vessel. That cycle is kept running for weeks without stopping.
The part that keeps the cells in is a hollow fiber called ATF (alternating tangential flow). It moves the medium back and forth to prevent clogging while sieving out only the cells. This allows a high cell density to be maintained while the vessel stays small.
The aim is steady-state operation in which the product is "the same quality whenever you take it." Unlike a batch, it does not create peaks and valleys but keeps a constant state, so a small facility can keep producing uniform antibody over a long period.
Fresh medium is fed continuously, and spent medium and the antibody produced are withdrawn continuously, while the cells are retained in the vessel to maintain high density and steady state. This is the heart of continuous manufacturing, which keeps producing at consistent quality with smaller equipment.
Alternatives in practice
- TFF (tangential flow filtration) cell retention · Repligen KrosFlo TFF hollow-fiber module + peristaltic / magnetically levitated pump — Simpler equipment than ATF, but membrane fouling and reduced product sieving occur more readily
- Acoustic wave cell retention (acoustic settler) · FloDesign Sonics/Pall Cadence Acoustic Cell Separation (formerly ekko) — Membrane-free and free of clogging, but limited in throughput and equipment cost at large scale
- Inclined-plate / centrifugal settler · Settler with built-in inclined sedimentation plates, or continuous centrifugation (Carr UniFuge / CARR Centritech type) — Avoids membrane fouling, but maintenance, shear and scale-up are issues
Process monitoring and control (PAT)
Real-time Raman measurement and automatic control are the key to keeping quality constant over weeks of perfusion.
- Standard method
- Real-time measurement by inline Raman spectroscopy + PLS multivariate model
- Reagents and materials
- No reagents are needed because the measurement is optical. YSI 2900/2950 glucose and lactate analyzers and Roche Cedex Bio HT (metabolites) serve as offline references for calibration and model building.
Why it is the standard. Raman can estimate glucose, lactate, glutamine, VCD, titer and more simultaneously from 1 scan, and automatic glucose control has been reported to raise titer and reduce glycation, making it a representative perfusion PAT tool. Consistent with the FDA PAT framework (ICH Q8–Q11, ASTM E2363).
Check also. The real-time values of nutrients, metabolism and titer from Raman are estimates from a multivariate (PLS) model built for each product, not direct measurements. The model drifts when the medium lot or the cell state changes, so periodic verification and recalibration against offline reference measurements is a prerequisite.
About this step and the alternatives
It is not realistic to watch a perfusion culture that runs nonstop for weeks by sampling it by hand every time. What is needed is a system that keeps measuring the contents of the vessel instead of going out to measure them. That is this PAT (process analytical technology).
The core is inline Raman spectroscopy. Light is shone into the culture fluid, and the concentrations of nutrients, metabolites and antibody are read in real time from how the light is scattered back. A multivariate model translates that signal and returns the moment-by-moment state as numbers.
The values obtained are fed back automatically and immediately to media feeding and so on. This autopilot, which spots a deviation and corrects it at once, is what keeps quality constant over long operation. Continuous manufacturing works only when PAT works.
In perfusion that runs for weeks, nutrients, metabolism and titer must be grasped in real time without waiting for sampling, and quality must be kept constant by automatic feedback. Process control by PAT is the key to continuous manufacturing.
Alternatives in practice
- Viable cell density measurement by capacitance (dielectric) spectroscopy · Hamilton Incyte Arc capacitance probe, Aber Instruments Futura/Solo dielectric spectroscopy — Detects only viable cells and is independent of medium changes, but is specialized for VCD and cannot measure metabolites
- Near-infrared (NIR) spectroscopy · Bruker MATRIX-F NIR, NIR probes from Sartorius and other vendors — May suffer less fluorescence interference than Raman, but is often inferior in information content and selectivity
- Inline pH, DO and automated cell counter · Hamilton OneFerm/VisiFerm DO and pH probes, Beckman Coulter Vi-CELL BLU automated cell counting — Measurement of established basic parameters. Direct measurement rather than multivariate prediction
Phase 2Purification (downstream)
04Clarification and harvest (continuous clarification) (harvest and clarification)
Continuously removes cells and debris from the ATF permeate and supplies a clarified load to downstream chromatography without interruption.
- Standard method
- Continuous depth filtration + 0.2 µm bioburden reduction filtration (direct filtration of ATF permeate)
Why it is the standard. ATF perfusion keeps the cells in the vessel, so the permeate has a low cell density to begin with, and the combination of depth + 0.2 µm bioburden reduction filtration is the standard configuration for continuous clarification. It can eliminate the need for flocculation and large centrifuges.
With Column-free continuous precipitation capture (PEG / metal salt, tubular reactor)
- Because the antibody is captured by selective precipitation with a soluble precipitant instead of Protein A affinity resin, the equipment and reagents change entirely
- Method: Continuous precipitation capture: ZnCl2 (or CaCl2) is added to the perfusion permeate through a static mixer to nucleate, and PEG is added in the following reactor to precipitate the antibody. The precipitate is concentrated and washed (dewatered) by single-pass TFF, and impurities are removed by countercurrent multistage diafiltration
- Equipment: Tubular flow reactor or oscillatory flow reactor + static mixer, hollow-fiber single-pass TFF (for dewatering and washing). Multicolumn chromatography skids (BioSMB / Cadence, etc.) are not used
- Reagents: Cooperative precipitation system of PEG (e.g., 6 w/v%) + ZnCl2 (e.g., 8 mM), or CaCl2 / caprylic acid. Protein A resin (MabSelect, etc.) and affinity elution acid buffer are not required
- Check also: The precipitants (PEG, Zn) remain in the product, so reliable removal downstream is essential. Implementation examples in continuous GMP operation are mostly at the research / pilot stage, and the track record as a fully approved commercial platform is more limited than for Protein A
About this step and the alternatives
The culture fluid flowing out of perfusion (ATF permeate) still carries bits of cells and the seeds of turbidity. This clarification and harvest step filters them out continuously and hands clean feed to the purification that follows.
Filtration is done in two stages. First, depth filtration traps debris and colloids, and then 0.2 µm bioburden reduction filtration finishes the job. Instead of collecting the fluid and processing it all at once as in a batch, it keeps filtering continuously as the fluid flows.
The aim is not to place unnecessary load on the next chromatography. If fluid is passed with turbidity still in it, the resin deteriorates sooner. Supplying a clear load solution without interruption is an unglamorous but essential pretreatment.
Residual cells, debris and colloids are removed continuously from the perfusion harvest fluid (ATF permeate), supplying a clear load without interruption so that the downstream chromatography is not burdened.
Alternatives in practice
- Continuous centrifugation + depth filtration · Continuous disc-stack centrifuge (Alfa Laval CultureOne / GEA kytero) + Merck Millistak+ depth filter — Advantageous for fed-batch harvests with high solids. Tends to be excessive for the clear permeate of perfusion
- Removal of cells and debris by acoustic settling · FloDesign Sonics/Pall Cadence Acoustic Separator — Reduces consumables, but depth filtration is needed in combination for final clearance of fine debris particles
Continuous capture (Protein A, multicolumn) (capture)
Captures antibody continuously by switching columns, greatly raising resin utilization and productivity; the primary purification step.
- Standard method
- Multicolumn continuous capture (PCC / periodic counter-current)
Why it is the standard. The periodic counter-current method, in which 2 or more columns are fed in series and antibody breaking through the 1st column is captured by the 2nd, greatly improves productivity and resin utilization over batch (with CaptureSMB, productivity +35% and resin utilization +45%), and is the representative configuration for continuous capture. Alkali-tolerant MabSelect PrismA is the de facto standard resin.
Check also. Multicolumn continuous capture is a method that uses the resin up to just before breakthrough and greatly raises productivity, but resin lifetime, the decline in binding capacity and leakage of the Protein A ligand over a long campaign must be tracked continuously. In continuous operation, decide in advance what span of operation counts as 1 lot (lot definition).
With Column-free continuous precipitation capture (PEG / metal salt, tubular reactor)
- Rather than a separate low-pH hold step, acidic redissolution of the precipitate also serves as viral inactivation
- Method: The precipitate is redissolved at low pH (e.g., phosphoric acid pH 2.5 to acetic acid pH 3.5), and that low-pH hold (about 60 min) is used for viral inactivation. This differs from a configuration in which the Protein A eluate is separately adjusted to low pH and held
- Reagents: Redissolution and inactivation buffer: 50 mM phosphate pH 2.5 or acetate pH 3.5 (glycine-based systems have also been examined in some cases)
- Check also: If the redissolution pH is too low there is a risk of acid denaturation and aggregation of the antibody, and if it is too high the inactivation LRV is insufficient, so the redissolution pH must be verified to meet the viral inactivation requirement as well
About this step and the alternatives
This is the primary purification that picks out only the antibody from a mixed culture fluid. A "magnet specific to antibodies" called Protein A is built into the resin, the antibody alone sticks to it, and it is released later and recovered. Its selectivity is orders of magnitude higher than other steps.
The trick that makes it continuous is to use several columns in rotation (PCC, periodic counter-current). When 1 column is about to fill with antibody, flow is handed to the next, and the full column goes to elution and washing. Like a relay, the flow continues without a break.
The advantage is that each column can be used right up to its limit. The resin is not left idle, and buffer consumption is kept down. The design takes in, without waste, the antibody that arrives continuously from perfusion.
The primary purification that captures only the antibody with high selectivity is made continuous by switching between multiple columns. The resin is used up to just before breakthrough, capturing antibody supplied without interruption from perfusion with high resin utilization and low buffer consumption.
Alternatives in practice
- One-column continuous chromatography (OCC) / 3–4-column PCC (high-breakthrough utilization) · Cytiva ÄKTA pcc 75 / Pall Cadence BioSMB (configurations with fewer columns) — Simplifies equipment and can run with few columns, but resin utilization and productivity are lower than with multicolumn SMB
- Affinity capture alternatives to Protein A / non-affinity capture · Thermo POROS CaptureSelect-type affinity resins, or capture by cation exchange (Cytiva Capto S ImpAct) — Advantageous for avoiding Protein A resin cost and leaching, but selectivity and platform suitability decrease
Viral inactivation (continuous, low pH) (viral inactivation)
A safety step that holds the eluate at low pH and secures residence time to inactivate enveloped viruses.
- Standard method
- Continuous low-pH viral inactivation
- Equipment
- Continuous viral inactivation reactor, Inline pH and DO sensors and probes
Why it is the standard. The CFIR creates secondary flow with bend inversions and gives a narrow residence time distribution close to plug flow, so the required hold time can be assured for all particles. It has achieved an LRV equivalent to batch (around 4.1 for XMuLV) in continuous mode, with a reported reactor specific productivity about 15-fold higher (doi:10.1002/bit.26872). Low-pH inactivation is one element of the dual virus safety approach consistent with ICH Q5A.
Check also. The key to making viral inactivation work in continuous flow is controlling the residence time distribution (variation in how the fluid flows), which assures a minimum hold time for every molecule. The reported viral clearance (LRV) values apply to specific flow rates and reactor conditions, so it should be confirmed at actual scale that the shortest hold time is secured even when the flow rate fluctuates. Low-pH inactivation is for enveloped viruses; small non-enveloped viruses require a separate virus filtration step.
With Column-free continuous precipitation capture (PEG / metal salt, tubular reactor)
- Instead of neutralizing a capture eluate, conditioning becomes solubilization of the acidic redissolved solution, followed by neutralization and desalting
- Method: The acidic redissolved solution is neutralized inline (alkaline stock + WFI through a static mixer), and residual PEG, metal salts and salt concentration are adjusted by single-pass TFF / diafiltration to the load conditions of the downstream polishing step
- Reagents: In addition to the alkaline stock solution and WFI for neutralization, diafiltration buffer is used to dilute and remove residual precipitants
About this step and the alternatives
This is a safety gate that renders any virus that may have slipped in harmless. Holding the Protein A eluate acidic (low pH) breaks enveloped viruses, which are wrapped in a membrane, and they lose their infectivity.
To work, it needs a certain "soak time." In a batch, time is gained by holding the solution in a hold tank, but in continuous manufacturing the solution keeps flowing. So the solution is passed through a long, thin continuous reactor, and the required residence time is built into the flow.
The aim is to ensure safety without stopping. The beauty of this continuous low-pH approach is that the specified treatment time is secured without a holding tank.
A safety step in which the Protein A eluate is held at low pH to inactivate enveloped viruses. Instead of a batch hold tank, it is carried out in a continuous reactor that secures a defined residence time while the solution keeps flowing.
Alternatives in practice
- Securing residence time with continuous stirred-tank reactors in series (CSTRs in series) · 2–3 single-use stirred tanks in series + inline pH control — Residence time distribution is easy to secure, but equipment footprint and dead volume increase
- Plug-flow hold in a tubular (serpentine) reactor · Serpentine / zigzag tube reactor (approximates plug flow) — Compact plug flow, but the residence time distribution tends to broaden when the flow rate fluctuates
- Continuous detergent (solvent/detergent) inactivation · Triton X-100 replacements (e.g., low-toxicity detergents from Merck and other vendors) in a continuous mixing skid — Effective against enveloped viruses, but low pH is required for small non-enveloped viruses
Inline neutralization and conditioning (adjusting the low-pH inactivation solution to the load conditions of the next step) / The acidic pool is continuously mixed with alkali and dilution buffer in a static mixer to return pH and conductivity to the polishing load conditions (neutralization and conditioning)
Because the low-pH inactivated solution cannot be passed to the next step unless it is returned to the AEX load pH and conductivity
- Standard method
- Concentrated acid/alkali stock solutions and WFI are pumped to a static mixer, and inline neutralization and conditioning are performed under pH and conductivity feedback control
Why it is the standard. Cytiva inline conditioning is a system that continuously generates buffer at target pH and conductivity from concentrated stocks and controls it by pH/conductivity feedback. It is standardly adopted for neutralization and load adjustment after low-pH inactivation in continuous trains.
With Column-free continuous precipitation capture (PEG / metal salt, tubular reactor)
- Residual PEG, Zn, HCP and DNA must be removed together, so a flow-through anion exchange membrane adsorber becomes the reference method
- Method: Flow-through anion exchange (combined with mixed-mode in some cases). Precipitant residues and HCP/DNA are removed together in flow-through mode (an example with more than 2 LRV for HCP and DNA)
- Equipment: Anion exchange membrane adsorber (Sartobind Q / Mustang Q, etc.). Operated as continuous flow-through
- Check also: Because the precipitant load is high, membrane capacity and cleaning must be designed with margin; otherwise residual PEG/Zn is carried over into the 2nd polishing step and beyond
About this step and the alternatives
The solution after low-pH inactivation is strongly acidic, around pH 3.5. It cannot be passed to the next chromatography as it is. This neutralization and conditioning step returns the pH and salt concentration to conditions the next step can accept.
Alkali and dilution buffer are mixed continuously into the acidic fluid. A static mixer (fixed blades inside a tube) mixes it quickly and uniformly, and the pH is brought to near neutral and the conductivity to the polishing load conditions while the fluid keeps flowing.
Speed and uniformity are everything here. If neutralization is slow or mixing is uneven, the antibody clumps and forms aggregates. It is an essential bridge before polishing, controlled precisely while watching pH and conductivity.
Continuous low-pH viral inactivation is performed with a hold at about pH 3.5, and that solution cannot be passed directly onto polishing such as anion exchange. A step of inline neutralization that continuously adjusts the pH to near neutral and the salt concentration to the load conditions is invariably included as standard. The neutralization rate and poor mixing can cause aggregate formation, so control is essential.
Alternatives in practice
- Semi-continuous conditioning with neutralization in a surge tank + inline dilution · Single-use surge bag + peristaltic pump + static mixer — Cushions variation in buffer preparation and is easy to control, but is inferior in full continuity and footprint
- Inline buffer blending built into the chromatography system · Cytiva ÄKTA process BufferPro / chromatography skids with built-in inline conditioning — Prepares buffer directly from concentrated stocks and can reduce buffer preparation tanks, but the calibration and validation burden is heavy
Polishing (impurity removal, continuous chromatography) (polishing step)
Intermediate to final purification that removes HCP, DNA, aggregates and charge species to bring CQAs within specification.
- Standard method
- Anion exchange
Why it is the standard. The 2-step approach, in which AEX flow-through removes DNA/HCP/virus while CEX or multimode separates aggregates and charge species, is a widely adopted mAb polishing configuration. In continuous mode, MCSGP (multicolumn gradient counter-current), which balances yield and purity, is a promising option.
About this step and the alternatives
This is a polishing step that further refines the antibody roughly picked up at capture. It removes residual host cell protein and DNA, leached Protein A, aggregates and molecules with skewed charge, bringing quality within specification.
Anion exchange chromatography is used, mainly in "flow-through" mode, in which negatively charged impurities are adsorbed onto the resin while the antibody passes straight through. Only the targeted contaminants are held back, and the antibody flows out.
To make it continuous, 2 columns are switched or the fluid is passed continuously in flow-through mode, so the flow is never stopped. It is the finishing stage, moving from intermediate to final purification, that brings the critical quality attributes (CQAs) within specification.
Intermediate to final purification that removes host cell proteins (HCP), DNA, leached Protein A, aggregates (HMWS) and charge variants and brings CQAs within specification. For continuous operation, 2-column and flow-through modes are used.
Alternatives in practice
- Mixed-mode chromatography (hydrophobic + ion exchange) · Cytiva Capto adhere (multimode AEX) / Capto MMC (multimode CEX) — Effective for hard-to-separate impurities, but condition optimization is complex
- Hydrophobic interaction chromatography (HIC) · Cytiva Capto Phenyl ImpRes / Tosoh TOYOPEARL PPG-600M — Strong at removing aggregates, but high-salt conditions require dilution and downstream adjustment
- Flow-through coupling (integrated AEX + virus filtration) · Sartobind Q + Sartorius Virosart HF connected in series — Step integration reduces the footprint. Load management of the virus filter is key
2nd polishing: cation exchange (CEX) bind-and-elute / The high-pI antibody is bound and eluted to remove charge variants, aggregates, residual HCP and leached Protein A (polishing step)
The standard mAb platform has 2 polishing steps, and AEX alone is insufficient for removing charge variants and aggregates
- Standard method
- Multicolumn continuous cation exchange chromatography (bind-and-elute)
Why it is the standard. BioSMB is a commercial system that supports multicolumn continuous bind-and-elute operation and is standardly adopted for the CEX stage of continuous mAb polishing. Capto SP ImpRes/Capto S ImpAct are commercially available resins widely used in antibody CEX polishing for removing charge variants and aggregates.
About this step and the alternatives
The standard in antibody purification is to stack two polishing stages after Protein A capture. The second stage is this cation exchange (CEX). It finishes off, by a different yardstick, the contaminants that the preceding anion exchange could not fully remove.
This time the antibody itself is bound to the resin and later eluted, in "bind-and-elute" mode. The antibody has a high isoelectric point and tends to be positively charged, so it is held firmly by the negatively charged resin. When it is released by changing the conditions, it separates from molecules with skewed charge.
What can be removed are charge variants, aggregates, residual host proteins and leached Protein A. In continuous operation, several columns are arranged to increase the robustness with which charge heterogeneity is suppressed. It is the stage that firms up safety and uniformity at the end.
In the standard antibody purification platform, Protein A capture is followed by 2 polishing steps (flow-through AEX + bind-and-elute CEX). CEX binds and elutes the high-pI antibody and is invariably included as a standard polishing step that, with high clearance capacity, reduces charge variants, aggregates, residual HCP and leached Protein A that AEX cannot fully remove. In continuous operation, multicolumn CEX increases the robustness of charge variant reduction.
Alternatives in practice
- Continuous counter-current multicolumn CEX (PCC format) · Pall Cadence BioSMB / Cytiva ÄKTA pcc 75 — High resin utilization and productivity, but control and validation are complex
- CEX polishing with membrane adsorbers (flow-through / bind-and-elute) · Sartorius Sartobind S、Pall Mustang S — High flow rate and single-use with no resin CIP needed, but binding capacity is lower than that of resins
- Polishing with multimode (mixed-mode) resins · Cytiva Capto MMC ImpRes、Tosoh TOYOPEARL MX-Trp-650M — High salt tolerance and selectivity, but condition optimization takes effort
Virus filtration (continuous, small-pore nanofiltration) (virus filtration)
Physically removes parvovirus-class viruses by size exclusion, doubling viral safety with a principle different from low pH.
- Standard method
- Parvovirus-removal nanofilter (long-duration operation in continuous / constant-pressure mode)
Why it is the standard. Parvovirus-class size exclusion filters (removal of ≥20 nm) provide an LRV of ≥4 log and, because their mechanism is a different principle from low-pH inactivation, meet the dual viral safety approach of ICH Q5A as the standard. For continuous operation, grades that withstand constant-pressure, long-duration loading (BioEX, etc.) are selected.
Check also. In long-duration constant-pressure continuous operation, flux decline from membrane fouling and how to assure filter integrity during operation become issues. It is required by regulation as a removal mechanism of a different principle from low-pH inactivation, and in continuous manufacturing the linkage of 1 filter to 1 lot and the control of load limits are built into the operating design.
About this step and the alternatives
This is another gate for viral safety. Where low-pH inactivation "destroys and disables" the virus, this step "physically filters it out." Two stages with different principles secure safety twice over.
A nanofilter with very small pores is used. It sorts by molecular size, lets the antibody pass, and holds back even the far smaller parvovirus-class small viruses. Size exclusion works like a fine-meshed sieve.
It is also indispensable from a regulatory standpoint, and its value lies in being a different principle from inactivation. Continuous manufacturing supports a mode of running for a long time at constant pressure, building up safety without stopping.
A second layer of viral safety that physically removes even small viruses such as parvovirus by size exclusion. Required by regulation as a removal mechanism based on a principle different from low-pH inactivation.
Alternatives in practice
- Higher-flux virus filters for single-pass use · Asahi Kasei Planova BioEX (high-flux next-generation membrane), Merck Viresolve Pro — Throughput increases and area can be reduced, but prefilter design and pressure management are important
- Combined use of viral clearance credit from AEX · Sartobind Q / Cytiva Capto Q flow-through — Reduces the nanofilter load and secures the total LRV. Complements rather than replaces membrane filtration
Phase 3Formulation and fill-finish
11Continuous concentration and buffer exchange (SPTFF UF/DF) (UF/DF)
Exchanges into formulation buffer and concentrates to the 100–200 mg/mL class in a single pass without a holding tank.
- Standard method
- Continuous UF + diafiltration by single-pass TFF (SPTFF)
Why it is the standard. SPTFF concentrates in a single pass without a circulation reservoir, so it excels in low shear, compact size and compatibility with continuous flow, and Cadence / Pellicon SPTFF are the representative modules. Implementation examples of PAT for continuous UF/DF have also been reported, and it is the standard choice for final concentration in integrated continuous manufacturing.
About this step and the alternatives
This step brings the purified antibody to the "concentration" and the "liquid it is in" that a drug needs. The antibody is exchanged into a formulation buffer suited to storage and administration and, if necessary, concentrated in one go to the 100–200 mg/mL class for high-concentration formulations.
Single-pass TFF (a one-pass version of tangential flow filtration) is used. Water is removed while the liquid flows along the membrane, so the antibody is concentrated and the buffer is exchanged by diafiltration. It has no holding tank and finishes in a single pass.
The reason to skip the holding tank and use a single pass is to fit naturally into continuous flow. It is the entry point to formulation, performing concentration and buffer exchange at the same time without stopping the flow.
The purified antibody is exchanged into the formulation buffer and concentrated to the target concentration (100–200 mg/mL class for high-concentration formulations). To suit continuous flow, this is done by a single-pass method without a holding tank.
Alternatives in practice
- Conventional batch TFF (reservoir recirculation) · Merck Pellicon 3 cassettes + retentate tank recirculation skid, Cytiva ÄKTA flux — Well proven, but reservoir and recirculation increase shear and processing time, making it unsuited to continuous operation
- Multistage SPTFF in series (pre-concentration + final concentration) · Pall Cadence SPTFF arranged in series (pre-concentration → DF → final concentration) — Allows high concentration factors to be shared across stages and pressure across the membrane to be evened out, but piping and control become complex
Final filtration and aseptic filling (fill-finish)
An essential final step: excipients are finally adjusted, the product is filtered through a 0.2 µm sterilizing-grade filter, and DS/DP is filled aseptically.
- Standard method
- 0.2 µm sterilizing-grade filtration + aseptic filling (after addition of polysorbate / excipients)
Why it is the standard. Sterilizing-grade 0.2 µm filtration (with a validated filter) and aseptic filling are the established final steps of sterile drug manufacturing, consistent with ICH Q5A and aseptic GMP (Annex 1). The key points are pool/surge management of the continuous DS and the final filter integrity test.
Check also. In continuous manufacturing the drug substance (DS) flows without interruption, so what counts as 1 lot (batch) and at what interval the integrity test of the final filter is performed must be defined in advance in line with aseptic GMP (Annex 1). Final bioburden control and assurance of sterility at filling cannot be omitted even in continuous operation.
About this step and the alternatives
At last, this is the final step of putting the drug into containers. First, excipients such as polysorbate are added to make the final adjustment of the formulation, the solution is filtered through a 0.2 µm sterilizing-grade filter, and the drug substance (DS) or final drug product (DP) is filled into containers while remaining sterile.
The main concerns here are sterility and bioburden (the amount of microorganisms) control. Sterilizing filtration removes microorganisms, and a closed environment is maintained to keep contamination out until the moment of filling.
No matter how cleanly continuous manufacturing has run for weeks, microbes entering at the final fill would ruin it all. This is the non-negotiable closing stage for bringing a safe drug to the world.
Excipients are finally adjusted, and the drug substance (DS) and final drug product (DP) are filtered through a 0.2 µm sterilizing-grade filter and filled aseptically into containers. Even in continuous manufacturing, final bioburden control and aseptic filling are essential.
Alternatives in practice
- Freeze-drying (when frozen-storage stability is required) · GEA/IMA/Optima freeze dryers + vial filling line — Advantageous for long-term stability, but process cost and time increase substantially
- Prefilled syringe / autoinjector filling · Bausch+Ströbel/Groninger PFS lines, BD/Gerresheimer syringes — High administration convenience, but silicone and aggregation risks and compatibility testing are required
Phase 4Quality attributes and analytics
13Purity and aggregates (size heterogeneity) (SEC)
Continuously quantifies aggregates (HMWS), which are directly tied to immunogenicity, to assure monomer purity during long-term operation.
- Standard method
- Quantification of monomer, HMWS and LMWS by size exclusion HPLC (SEC-HPLC/UPLC)
- Reagents and materials
- HPLC mobile phase solvents
Why it is the standard. SEC-HPLC is an established pharmacopoeial method (USP/Ph. Eur.) for the separation and quantification of aggregates and fragments. It is common to use SEC-MALS or AUC alongside as confirmation by a different method, and it is used to monitor aggregate trends between lots.
About this step and the alternatives
A purity test that checks whether the antibody is uniformly intact as a single molecule or whether clumps have formed by sticking together. Size exclusion HPLC lines molecules up by size and sieves them.
What is of interest is the native monomer, high-molecular-weight aggregates in which two or more molecules have stuck together, and fragments that have broken into smaller pieces. Larger species elute first and smaller ones later, so the amount of each can be measured at once.
Aggregates in particular are a critical quality attribute directly tied to the risk of triggering an immune reaction in the body. In continuous operation that keeps running for a long time, purity is monitored continuously to confirm that it stays constant.
Dimers and high-molecular-weight aggregates (HMWS) are critical CQAs directly tied to immunogenicity risk. In continuous manufacturing, monomer purity is checked continuously to assure consistency of quality during long-term operation.
Alternatives in practice
- SEC-MALS (multi-angle light scattering detection) · Wyatt/Waters DAWN MALS + Optilab dRI, connected upstream of the SEC column — Measures absolute molecular weight independent of the column, but the instrumentation and analysis are sophisticated
- Analytical ultracentrifugation (AUC, sedimentation velocity) · Beckman Coulter Optima AUC — An orthogonal method independent of dilution/matrix, but throughput is low
- Field-flow fractionation (AF4) · Wyatt Eclipse AF4 + MALS — Can separate even large aggregates, but method optimization is difficult
Purity and size (CE-SDS)
Evaluates molecular size, fragments and non-reduced species by electrophoresis; a critical CQA showing that heavy/light chain integrity is constant.
- Standard method
- Capillary electrophoresis SDS (CE-SDS, reduced/non-reduced)
- Reagents and materials
- Disulfide reduction alkylation reagents
Why it is the standard. CE-SDS is a quantitative, automated version of SDS-PAGE and a pharmacopoeia-aligned method widely adopted for purity and fragment specification testing (USP <129>). It is the standard for evaluating molecular integrity under both reduced and non-reduced conditions.
About this step and the alternatives
An antibody is a Y-shaped molecule built from heavy and light chains. This is a purity test, from the standpoint of size, to confirm that this assembly has not come apart or fragmented. Molecules are separated by making them migrate under an electric field.
Capillary electrophoresis SDS is used. Molecules migrate in order of size inside a thin tube, separating antibody of the native size from small broken fragments and measuring the amount of each. Under both reduced and non-reduced conditions, how the chains are linked is also confirmed.
This is confirmation of a critical quality attribute: that the association of heavy and light chains and the degree of fragmentation do not drift over long-term continuous operation. It serves as evidence that the product keeps the same form.
Purity based on molecular size, and fragments and non-reduced species, are evaluated by electrophoresis. A critical CQA showing that heavy/light chain association and fragmentation remain constant throughout continuous operation.
Alternatives in practice
- SDS-PAGE (Coomassie / silver staining) · Bio-Rad Criterion/Mini-PROTEAN + ChemiDoc imager — Simple and low-cost, but inferior to CE-SDS in quantitation and resolution
- Microchip CE · Agilent 2100 Bioanalyzer / Bioanalyzer Protein 230, PerkinElmer LabChip GXII — Fast and low-volume, but limited in resolution and validation scope
Charge heterogeneity (charge variants) (icIEF)
A critical CQA: keeps the acidic/basic ratio of charge variants, which can affect activity and stability, constant.
- Standard method
- Imaged capillary isoelectric focusing (icIEF)
- Reagents and materials
- Carrier ampholytes pI markers
Why it is the standard. icIEF quantifies acidic, main and basic species with high resolution based on pI, and is the representative method widely used for monitoring charge heterogeneity from development through QC release. Cation exchange HPLC (CEX) is used alongside for confirmation by a different method.
About this step and the alternatives
Even for the same antibody, slight chemical changes give individuality to how it carries electric charge. This test tells those differences apart. It captures subtle differences that arise in manufacturing and storage, such as deamidation, C-terminal lysine and glycation.
Imaged capillary isoelectric focusing (icIEF) is used. Each molecule's "position where it comes to rest electrically" is lined up in a row, and the species that lean acidic and those that lean basic are separated and made visible.
Charge skew is a critical quality attribute that can affect activity and stability. As continuous operation goes on, the ratio of acidic to basic species is checked to stay constant, assuring the uniformity of the product.
Charge variants from deamidation, C-terminal lysine, glycation and so on are critical CQAs that can affect activity and stability. The ratio of acidic/basic species is confirmed to be constant throughout continuous operation.
Alternatives in practice
- Cation exchange HPLC (CEX, salt/pH gradient) · Thermo MAbPac SCX-10 / Waters/Agilent LC + strong CEX columns — Highly reproducible and amenable to fractionation, but separation is based on retention rather than pI
- Native MS / CEX-native MS coupling · Thermo Q Exactive/Exploris + native ESI, online CEX-MS — Allows structural assignment of charge variants, but advanced instrumentation and expertise are required
N-glycan profile (glycosylation)
A critical CQA: monitors Fc glycans, which govern effector functions such as ADCC, for the effect of long-term culture.
- Standard method
- Fluorescently labeled HILIC-FLR of released N-glycans (with MS added as needed)
- Reagents and materials
- N-glycan release and labeling (RapiFluor-MS / 2-AB), Analytical antibody enzymes
Why it is the standard. PNGase F release → labeling with 2-AB etc. → HILIC-FLR separation is regarded as the reference method for released N-glycan analysis, and with MS added it can even identify structures. Because glycans are directly linked to effector function, they are a priority monitoring item in continuous culture.
About this step and the alternatives
Sugar chains are attached to the tail region (Fc) of the antibody, and how they are attached influences how the drug works. Reading this glycan pattern is the glycosylation profile analysis.
The method is to cut only the glycans off the antibody, attach a fluorescent tag, and line them up by type with a separation method called HILIC (adding mass spectrometry if necessary). The proportions of galactose, fucose, high mannose and so on are read off.
These glycans are a critical quality attribute that affects functions such as ADCC, which calls in immune cells, and how readily the antibody disappears from the body. How culture conditions change the glycans over long-term perfusion is monitored carefully.
N-glycans on the Fc region (galactosylation, fucosylation, high mannose) are critical CQAs that affect effector functions such as ADCC and clearance. In long-term perfusion operation, the effect of culture conditions on glycans is monitored.
Alternatives in practice
- Capillary electrophoresis with laser-induced fluorescence (CE-LIF) · SCIEX PA 800 Plus + Fast Glycan / APTS labeling kit — High resolution and high sensitivity, but separate labeling chemistry and capillary management are required
- Middle-up HILIC-HRMS / intact MS · Waters/Thermo UPLC + high-resolution MS (Orbitrap Exploris, Xevo G3 QTof) — Can assign glycoforms at the molecular level, but a separate method is more reliable for profile quantitation
Identification of primary structure and post-translational modifications (peptide mapping)
Identifies the amino acid sequence and PTM sites such as oxidation and deamidation, providing the basis for identity and modification CQAs; a foundational analysis.
- Standard method
- Tryptic peptide mapping by LC-MS/MS (high-resolution MS)
- Reagents and materials
- Peptide mapping protease, Disulfide reduction alkylation reagents, HPLC mobile phase solvents
Why it is the standard. Peptide mapping by enzymatic digestion + LC-MS/MS is an established method for sequence confirmation and quantitation of PTM sites, and is central to characterization and comparability assessment (ICH Q6B). Use as a multi-attribute method (MAM) to monitor multiple CQAs at once has also become widespread.
About this step and the alternatives
A foundational analysis that checks whether the antibody is "as designed," down to the finest unit. It pinpoints, site by site, whether the amino acid sequence is correct and where chemical modifications have occurred.
The procedure is to first cut the antibody into peptide fragments with an enzyme called trypsin and then run them through LC-MS/MS (high-resolution mass spectrometry). The sequence is read from the mass of each fragment, and the whole molecule is confirmed like a map.
Its strength is that post-translational modifications such as oxidation, deamidation and C-terminal processing can be located by position. It is the underlying analysis that supports confirmation of product identity and the basis for quality attributes related to modification.
Confirms the amino acid sequence and identifies, site by site, post-translational modifications such as oxidation, deamidation and C-terminal processing. A foundational analysis that provides the basis for identity confirmation and modification CQAs.
Alternatives in practice
- Intact/subunit mass spectrometry · Thermo Orbitrap (intact / IdeS-digested subunits), Waters BioAccord — Quickly confirms the overall mass and major modifications, but site specificity is low
- Multi-attribute method (MAM) · Thermo Orbitrap Exploris + Chromeleon/BioPharma Finder (MAM workflow) — Can monitor multiple CQAs with 1 method, but method development and validation are heavy
Potency (bioassay) and biological properties (potency, bioassay)
Shows target binding and functions such as neutralization and ADCC; an essential potency test that assures efficacy.
- Standard method
- Binding ELISA / SPR (binding) + cell-based or reporter gene bioassay (functional potency)
- Reagents and materials
- Reporter gene bioassay ADCC CDC
Why it is the standard. The combination of binding (ELISA/SPR/BLI) and functional potency (cell/reporter assays) is the standard for potency assessment, managed as relative potency against a reference standard (ICH Q6B). In continuous manufacturing, confirming consistency across intervals is key to quality assurance.
Check also. Potency (bioassay) is an essential test that assures product efficacy, but its variability is large and it is not suited to immediate process control. In continuous operation, decide in advance in which intervals and how often to sample to confirm consistency of function (how lots / intervals are defined).
About this step and the alternatives
Even if purity looks uniform, whether the drug "works" is a separate matter. This potency (bioassay) test directly verifies whether the antibody properly binds the target and exerts the intended function.
It is measured from two angles. First, ELISA or SPR shows how strongly the antibody binds to the target. Next, an assay using cells or a reporter gene confirms whether functions such as neutralization and ADCC actually appear.
This is an indispensable test that assures the efficacy of the product itself. Even in continuous manufacturing, it confirms that function does not drift between lots or operating intervals, assuring consistency of effect.
Shows that the antibody binds the target and exerts the intended function (neutralization, ADCC, etc.). An essential potency test that assures product efficacy. Even in continuous manufacturing, consistency of function is confirmed for each lot / interval.
Alternatives in practice
- Binding kinetics by biolayer interferometry (BLI) · Sartorius Octet R8/RH16 + Anti-hIgG Fc / Protein A biosensors — High-throughput and label-free, but sensitivity can be inferior to SPR for small molecules and low-affinity interactions
- FcγRIIIa binding / ADCC reporter assay · Cytiva Biacore (immobilized FcγRIIIa) / Promega ADCC Reporter Bioassay — Reflects effector function, but has limits as a substitute for cell-based full-length ADCC
Process-related impurities and safety (HCP, DNA, leached Protein A, endotoxin)
Continuously monitors HCP, residual DNA, leached Protein A and endotoxin to assure safety within specification.
- Standard method
- HCP-ELISA、qPCR
Why it is the standard. HCP ELISA, qPCR for residual DNA, Protein A ELISA and LAL are established pharmacopoeia-aligned tests for process-related impurities / safety (ICH Q6B, USP/Ph. Eur.). In continuous manufacturing, monitoring impurity trends over long-term operation is important for release criteria.
Check also. HCP ELISA can only pick up the host cell proteins that the antiserum used can recognize, so coverage is limited. At important process changes or when in doubt, coverage is confirmed separately by mass spectrometry or similar. Trend monitoring that shows continuous compliance with specification during continuous operation is a prerequisite.
About this step and the alternatives
A safety test to check that no "impurities" left behind during manufacturing are mixed into the drug. Targets are host cell proteins, residual DNA, leached Protein A and endotoxin.
The measurement method differs for each impurity. Host proteins are measured by HCP ELISA, which captures them with a dedicated antibody, and residual DNA is quantified down to trace levels by qPCR. Each is checked to be within the upper limit set by the specification.
All of these are critical quality attributes directly tied to patient safety. Throughout continuous operation, which runs for a long time without stopping, it is assured that they remain continuously within specification.
Host cell proteins (HCP), residual DNA, leached Protein A and endotoxin are critical CQAs directly tied to safety. They are assured to remain within specification continuously, including during continuous operation.
Alternatives in practice
- HCP identification and quantitation by LC-MS/MS · Thermo Orbitrap Exploris / SCIEX + nanoLC (HCP spike standard) — Can identify and quantify individual HCPs, but is more laborious than ELISA and standardization is a challenge
- Recombinant factor C (rFC) endotoxin test · Lonza PyroGene rFC assay + fluorescence plate reader — Eliminates the need for horseshoe crab-derived LAL, which is advantageous for animal welfare and supply, but regulatory acceptance must be confirmed
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).