The development of antibody drugs relies on a well-structured Quality by Design (QbD) approach for upstream production. By leveraging the advantages of Design of Experiments (DoE) and high-throughput parallel bioreactor technology, researchers can efficiently complete laboratory-scale R&D processes. A critical step in this process is the early identification of the relationship between critical quality attributes (CQAs) and critical process parameters (CPPs), which helps define the design space during development.
Once small-scale R&D processes are successfully established, the next challenge is scaling up to pilot and production levels. Ideally, an efficient scale-up process can transition from R&D to full production within 2–4 months. However, achieving seamless process transfer and scale-up requires deep expertise and experience from the entire process technology team.
Key Considerations in Process Scale-Up
The primary objective of process scale-up is to maintain a stable cellular environment for consistent product expression as culture volumes increase. The key metrics for assessing process scale-up include:
- Cell density and growth rate
- Viability and productivity
- Product expression rate
- Glycosylation profiles
During scale-up, key process control parameters can be categorized as:
- Volume-independent factors, such as temperature, dissolved oxygen (DO), and pH.
- Volume-dependent factors, such as agitation speed and aeration rate, which are influenced by vessel size and geometry.
Since R&D employs bioreactors from various suppliers, differences in materials (disposable vs. glass), aspect ratios, impeller designs, and tank geometries present challenges in maintaining consistent stirring and aeration conditions post-scale-up. To ensure a uniform cultivation environment, developers rely on the following scaling strategies:
Common Scale-Up Strategies
1. Constant Blade Tip Velocity
Agitation shear forces significantly affect cell viability, with different cell lines exhibiting varying tolerances. Early CHO cell lines were highly sensitive to shear stress, but modern engineering cell lines demonstrate greater resilience.
Blade tip velocity, determined by impeller diameter and rotational speed, is a key factor in maintaining consistent shear forces. Since larger bioreactors require larger impellers, their rotational speed is reduced accordingly to preserve similar shear conditions. This method is particularly effective for small-to-medium scale-ups.
2. Constant Mixing Time
Mixing time, a critical parameter in chemical engineering, ensures uniform nutrient and gas distribution. While small-scale bioreactors (e.g., 2L) achieve rapid mixing, larger volumes require increased impeller speeds, which can elevate shear stress and impact cell viability. As a result, this strategy is more applicable to chemical processes than to cell culture.
3. Constant Oxygen Transfer Coefficient (kLa)
The volumetric oxygen transfer coefficient (kLa) determines oxygen availability, which is crucial for cell metabolism. Maintaining a constant kLa ensures a uniform oxygen transfer environment across scales. However, kLa is influenced by multiple factors, including agitation and aeration rates, requiring extensive optimization. As reactor volume increases, CO2 accumulation becomes a challenge, further complicating kLa maintenance.
4. Constant Power per Unit Volume (P/V)
P/V, a function of impeller power number (Np), vessel geometry, and liquid properties, influences mixing and mass transfer efficiency. It is widely regarded as the most effective and commonly used scale-up criterion. Given cell line-specific shear tolerances, the recommended P/V range typically falls between 10–40 W/m³.
Additional Considerations in Large-Scale Cultivation
Beyond the primary scale-up criteria, larger-scale bioreactors must account for the effects of CO2 accumulation on cell growth and protein expression. Unlike small-scale systems where gas exchange naturally removes metabolic CO2, large-scale cultures experience reduced CO2 stripping efficiency due to lower kLa CO2 values. Factors such as gas saturation and bubble dynamics must be managed through enhanced aeration strategies or integrated CO2 removal systems.
Adapting to Evolving Process Challenges
As antibody technology advances, the challenges of process scale-up continue to evolve. Process developers must remain flexible, tailoring scale-up strategies based on cell line characteristics and product-specific requirements. A holistic approach—balancing P/V, blade tip velocity, CO2 removal, and kLa—ensures optimal process performance while maintaining CQAs.
By integrating advanced process engineering principles, biopharmaceutical companies can achieve reliable and efficient scale-up, facilitating the seamless transition from R&D to large-scale production.
