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Sterile filtration is often required for biologics but presents degradation and compatibility challenges.
Selection of a sterilization strategy for a drug product that requires aseptic manufacturing is generally determined by the stability of the drug substance. For stable products, terminal sterilization, including heat sterilization or exposure to radiation or chemicals such as ethylene oxide or vaporous hydrogen peroxide, is the preferred strategy. These methods are advantageous because the processes can be monitored and validated and they tend to be less prone to error.
Most biologic drug substances, however, are unstable when exposed to heat, radiation, or chemicals and generally require aseptic manufacturing using sterile filtration. Successful sterile filtration requires a drug product formulation with an appropriate viscosity and compatibility with the contact surfaces and shear stresses involved in pumping the fluid. Single-use technology is widely used in sterile fill/finish operations today, reducing the turnover time and cross-contamination risk.
The main factors that must be considered from the outset when developing formulations for biologic drug substances that require sterile filtration include, according to Yunsong (Frank) Li, director of process development at Catalent Biologics, the product’s chemical and physical stability under various stress conditions (e.g., thermal, mechanical, photostability) during product manufacturing, storage, shipping, and administration; compatibility with the materials used during sterile fill/finish processing; and compatibility with the final container closure system and administration device.
Factors related specifically to sterile filtration that can impact the final formulation include the contact surfaces, shear forces involved, and the potential to induce aggregation or particle formation, notes Margaret Faul, vice-president of drug product for Amgen. The method used to sterilize the fill/finish equipment or primary packaging must also be considered because any residual chemicals may interact with the biologic. For instance, antibodies containing methionine residues in the related binding regions of an antibody can become ineffective (lose efficacy) if (accidentally) oxidized (e.g., by vaporous hydrogen peroxide during manufacture), according to Hanns-Christian Mahler, head of drug product services at Lonza Pharma & Biotech.
In addition, Faul observes that the concentrations of excipients or the biologic drug substance may be reduced by the filter. “The material of construction may affect material loss, while the filter system design may affect the flow properties, and the size may affect the overall process throughput,” she explains.
To address these issues, certain excipients may be added to minimize process loss to the filter, negative interactions between the biologic and the filter, or residual sterilization chemicals that might lead to particle formation, aggregation, or decreased rates of filtration, according to Faul.
To ensure that sterile filtration will be effective and that contaminants are excluded, it is therefore necessary to test the process for each batch, adds Andrew Bulpin, head of process solutions at MilliporeSigma. “The adsorption of components of the final formulations and the biologic drug substance to the sterile filter must be analyzed to ensure that the formulation and final concentrations remain within the desired values,” he says.
In addition, Bulpin notes that because biologic drug substances may be sensitive to shear stress, the mixing and pumping during formulation must be gentle to prevent aggregation and/or conformational changes of the biologic. Fill/finish processes must also be performed under controlled temperature conditions to avoid thermal degradation. Many biologics are in fact lyophilized during final fill due to their thermal instability.
“In general,” concludes Mahler, product development should carefully consider the intended method of manufacturing. Less obvious and less regulated, but equally important, is evaluation of whether each critical process unit operation (process step) of the manufacturing process within and beyond normal operating parameters would impact critical quality attributes.
As an example, Mahler points to the type of filling pump chosen, which is not only relevant from a good manufacturing practice and fill precision perspective but can also significantly and adversely impact product quality by causing aggregation and/or precipitation when operating within or beyond its parameters. He also observes that the choice of disposable tubing, filters, and primary packaging components, as well as the capping and 100% visual inspection processes, also play a significant role in impacting product quality and yield.
There are significant advantages with single-use technology in both drug substance and drug product manufacturing. “Its use eliminates the cleaning step, which saves time with batch turn-over, saves costs in facility construction, and improves the process flexibility,” states Li. Using sterile, disposable materials also mitigates the risk of microbial ingress during processing, according to Christy Eatmon, SME for sterile drug products at Thermo Fisher Scientific. “Using disposable technologies makes it possible to complete sterile connections in a non-sterile compounding area. The product can be almost completely processed using closed systems for product flow from a manufacturing area to the aseptic filling core,” she comments.
Li stresses, though, that with disposable systems, it is also necessary to understand and ensure compatibility between the single-use materials employed and the final drug product as part of the formulation development efforts. “It is necessary to ensure that there are no adverse impacts, such as aggregation, denaturation, or adsorption to the product by the materials used in the process. The extractable and leachable profile also needs to be fully understood to ensure that it is compatible with the product formulation,” he explains.
Other issues to consider with the application of single-use Âtechnologies during sterile fill/finish operations include particle shedding, permeation of water or oxygen out of or into the product (possibly leading to degradation), or loss of critical excipients such as polysorbate or preservatives, according to Mahler. “The latter components have been shown to possibly be adsorbed or be able to permeate across disposables, with the potential to lead to significant quality issues in the final product,” he remarks.
Therefore, while there are time savings during production, Faul observes that the advent of single-use technology for sterile fill/finish has impacted the formulation development process by increasing the need to assess and prevent degradations due to interactions with the contact surfaces of the disposable systems.
Generally, many biologic drugs require high doses for injection via the subcutaneous route of administration, with the volume of injection typically less than two milliliters. “This delivery method requires the formulation to be at a very high concentration, and it becomes highly viscous, posing difficulties in both manufacturing and administration,” notes Li. Due to the need of high protein concentrations in many biologic formulations, high viscosities still pose difficulties during sterile fill/finish, adds Bulpin.
Different approaches are under development to address this issue. Viscosity reduction in high-concentration solutions can be achieved through the modification of the formulation, Bulpin notes. One potential solution, according to Faul, is the use of excipients to reduce viscosity. The excipients can bind to certain regions of a protein to reduce viscosity and enable delivery of high-concentration formulations. “The method of interaction can be determined using various analytical methods to support understanding of the binding interactions in formulation development,” she comments. However, according to Li, currently there is no general strategy for using these excipients, and their application must be explored on a case-by-case basis.
Conducting fill/finish processes at higher temperatures is also being investigated, according to Faul. This approach is, of course, only relevant for more thermally stable biologic drug products.
An alternative strategy, according to Bulpin, involves enablement of lower protein concentrations using advanced biologic administration technologies that allow for higher volumes during administration.
One of the biggest challenges faced when formulating biologic drug substances is trying to improve the stability of the molecules to achieve the target shelf-life of the product. “Formulation studies can only stabilize the target molecule to a limited extent. While there are many new excipients and approaches being attempted today, the molecule’s structure itself still plays a critical role in determining the molecular stability,” says Li.
The best way to address this issue is to conduct developability/manufacturability studies as early as the molecular screening step in the drug discovery stage, he notes. “By taking this approach, the most stable molecule will be selected from many molecules having the same bioactivities, and this selected molecule will have the best success rate in the subsequent development and manufacturing stages,” Li adds.
Compatibility of biologic drug substances with various surfaces is another challenge, according to Malgorzata Tracka, senior staff scientist at Thermo Fisher Scientific. “Compatibility studies should be conducted to ensure suitability of the formulation against product contact surfaces, including the primary packaging (IV bags, auto injectors, Âpre-filled syringes, etc.) and manufacturing design (tubing, connectors, filters, etc.),” she observes.
Formulations can be optimized with surfactants using a design-of-experiment approach, and contact surfaces can be selected carefully to minimize their negative impact. “Very often formulations advanced to the sterile fill/finish stage require optimization of surfactants to protect the biomolecules from effects that can lead to aggregation and subvisible particle or visible particle formation,” notes Tracka.
Controlling the fluid viscosity, product sensitivity to shear stress, and thermal stability are particularly important for biologics subjected to sterile fill/finish processing, says Bulpin. “Optimal formulation might reduce product viscosity with the addition of salts and other stabilizers or through pH adjustments, while temperature control during sterile fill/finish will prevent thermal degradation and adequate filter sizing, and process design will reduce pumping requirements and, therefore, shear stress,” he comments.
The key component of successful formulation strategy for biologics that will be subjected to sterile fill/finish processing is early identification of potential degradation factors. It is important to determine if the active will be damaged by the mechanical forces involved during processing or fill/finish activities, according to Eatmon. “The formulation development process involves identifying degradation pathways for the biologic, assessing which may be impacted by sterile fill/finish processing, identifying excipients that minimize potential degradation of the biologic and maintain potency, and empirical testing/verification of assumptions at appropriate milestones,” adds Faul. The potential for excipients to foam during processing or adhere to filter membrane during aseptic filtration must also be evaluated, Eatmon notes.
That is where process, flush, shear-stress, and compatibility studies come in, to ensure the correct mechanical parameters, determine the quantity of material required to saturate the product contact surfaces, identify the appropriate fill speed and pressure, and assure the suitability of the materials in contact with the drug substance.
The final filtration process is often developed on a small scale in parallel to formulation development to determine a chemically compatible filter membrane, according to Bulpin. Extractables in the final formulation, protein adsorption, and the adequate filter size are then factored in.
Other questions must also be addressed, Bulpin adds, such as whether the bulk drug substance will be frozen before final fill and whether the drug product will be lyophilized, because these processing steps require excipients such as sugars to stabilize protein conformation. In addition, based on the hydrophobicity of the biologic, an optimal solvent must be identified (mostly aqueous), the optimal pH conditions under final concentration must be determined, and the need for the addition of antioxidants evaluated. “In essence,” Bulpin remarks, “the entire formulation needs to be tweaked to ensure maximum stability of the product.”
A new form of sterile fill/finish being explored through the development of several new technologies involves the production of sterile biologics in a powdered state, which are then used to produce sterile suspensions for injection. “All of these technologies are relatively new and in various stages of clinical studies,” Li observes.
The sterile filling of powders is also relatively new to the industry and poses additional challenges. Li adds, however, that these technologies are attractive because they can achieve an extremely high “concentration” of biologics even beyond the protein’s solubility.
Pharmaceutical Technology Vol. 43, No. 7
When referring to this article, please cite it as C. Challener, “Considerations for Sterile Filtration of Biologic Drugs," Pharmaceutical Technology 43 (7) 2019.