July 16, 2020

Eliminating Residual Impurities Starts with a Strategic Plan

Eliminating Residual Impurities Starts with a Strategic Plan

Identifying the source, assessing the risk, and removing residual impurities requires a strategic approach.

angellodeco/Stock.Adobe.comResidual impurity testing is essential to ensure the quality and safety of pharmaceutical products, but it can be particularly challenging with biologics because there are so many potential sources of impurities, from the media and host cells to misfolded or aggregated product to downstream purification reagents and process equipment. A risk-based approach to identifying potential residual impurities is important for understanding any risks to the product quality and potentially the patient. It also allows the development of an optimal process with a suitable control strategy and test methods in a timely fashion.

The implementation of a risk-based strategy for the testing of residual impurities, meanwhile, offers the potential to significantly reduce the amount of testing needed while still meeting regulatory requirements and ensuring patient safety. It does require, however, a thorough understanding of the process combined with appropriate control strategies to minimize the impurities that may be present at the end of the production process (drug substance and drug product). While building a knowledge base on residual impurities requires investment, that investment pays off in the long term because it enables risk-based residual impurity control and minimizes the need for residual impurity testing.

Numerous risk factors

Residual impurities in biopharmaceutical processes can be traced back to raw materials or can form during upstream or downstream processing steps and upon storage. They can be derived from the host cell or the product itself, media components, surfactants, virus inactivation agents, inorganic or organic contaminants from permanent and single-use process equipment, the pharmaceutical formulation, and/or a breach of sterility. “A strategy for the control of residual impurities has to consider risk factors that are specific to each of these process- or product-related impurities,” asserts Michael Jahn, group head of forensic chemistry at Lonza.

This quantity and heterogeneity of possible impurities is perhaps the single biggest factor influencing the development of risk-based strategies for the testing of residual impurities, adds Ashleigh Wake, UK chemicals and pharmaceuticals business director for Intertek.

“Some residual impurities, such as antibiotics and stabilizers, can be relatively easily defined and a strategy put in place that may include a limit specification in a release test or validating the removal during product purification. Others are more complex to define, such as those that originate from the cellular system including residual DNA or host-cell proteins (HCPs),” Wake observes. “In the latter case, for instance, consideration must be made for total HCP levels per regulatory guidance, but also for any single HCP of particular concern, for which a more specific monitoring approach would need to be developed.”

For some residual impurities such as HCPs, safety thresholds (at approximately 1–100 ppm) are well established, whereas quality thresholds regarding, for example, the presence of cell-derived lipases that potentially can degrade the excipient polysorbate with detrimental consequences—visual particle formation or loss of protective activity against interfacial stress on the active protein—are not established yet, according to Jahn. Leachables, meanwhile, are typically controlled in a risk-based fashion with high-risk materials assessed in laboratory studies, he notes. “A common basis for the establishment of risk factors is an in-depth understanding of the underlying processes,” Jahn summarizes.

Other factors relate to the testing process itself, according to Lisa Sapp, biopharma market manager with Agilent Technologies. The selected isolation method—precipitation, filtration, centrifugation, extraction, etc.—should not affect the impurities with respect to concentration or composition. If derivatization is necessary to improve the sensitivity and detectability of the analyte in question, any loss in recovery must be accounted for. Filtration of samples must not cause any interference either. The chosen separation and detection technologies must also be suitable to the analysis without degradation of the impurities occurring.

Overall, potential process-related and other residual impurities need to be evaluated with regard to their criticality and their potential to pose any risk to patient safety, asserts Stefan Braun, regulatory affairs manager with Rentschler Biopharma. “The risk assessment should aim at identifying and assessing those molecules where an element of risk may remain even at very low concentration, with potential risk evaluated in a multi-stage approach,” he notes.

The best solution to managing risk factors is to use quality by design (QbD) to classify them, applying tools such as Ishikawa fishbone diagrams or failure mode and effects analysis (FMEA), agrees Luc-Alain Savoy, global head of biologics at SGS.

“Critical quality attributes will be identified during this risk-assessment step and their criticality addressed from a safety perspective. Efficient purification processes will be designed and validated to remove the residual impurities. Monitoring the efficiency of the residual impurities will be guaranteed through the development of robust analytical methods,” he explains. Savoy also recommends that methods be developed following an analytical QbD approach.

Specific factors to assess for possible impurities include requirements established in regulatory guidelines, acute and chronic toxicity, risk of co-purification, and natural occurrence in humans, according to Braun. If one or more of these factors is deemed critical, the potential impurity should be further evaluated in the next step of the risk assessment.

For process-related impurities, Braun notes that as long as the theoretical amount of the substance in the drug substance is below the toxicologically relevant limit (acceptable level), the substance is rated as uncritical and analytical testing in the drug substance is not recommended. If the theoretical amount in drug substance exceeds the acceptable level, the molecule is recommended for testing on drug substance level. Depletion of such molecules in the final process design should, however, be confirmed during process characterization and process validation.


Many regulations

There are several relevant regulations that address the identification and evaluation of residual impurities. The International Council for Harmonization (ICH) Q6B guideline (1) provides general information on setting and justifying release and in-process specifications for both process and product-related impurities in drug substances and how the suitability of any method should be addressed within the guidances outlined in ICH Q2 (R1) (2). European Pharmacopoeia monograph <0784> (3) and United States Pharmacopeia General Chapter <1132> (4) provide specific guidance on the control of host-cell-derived process-related impurities using appropriate risk-management strategies. The ICH Q9 guideline (5) discusses a risk-based approach for determining the testing requirements for monitoring the removal of reagents from processes; ICH Q3C (R6) (6) addresses residual solvents; and ICH Q3D (7) covers elemental impurities.

At this time, one area that is not currently regulated in detail by the health authorities is extractables and leachables. “Biopharmaceutical manufacturers have to establish ‘in-house’ risk-based strategies, with the disadvantage of insecurity with regard to health authority expectations, but with the advantage of the possibility to apply process knowledge and extractables/leachables subject-matter expertise beyond a generic regulatory framework,” Jahn explains.

Challenges galore

Risked-based strategies work well when the process is well-defined, that is, specified impurities are analyzed and meet the specifications set forth. Issues arise, says Tilak Chandrasekaran, biopharma marketing manager for Agilent Technologies, when an unspecified impurity appears and is present at a low level. “This situation often happens if the starting material or reagents are from a different supplier and not fully characterized, and tracking the root cause for these impurities can become a time-consuming and costly process,” he explains.

Such situations can also arise if documented knowledge from early development stages is not maintained as projects progress through the development cycle, according to Chandrasekaran. “Although full CMC [chemistry, manufacturing, and controls] information is not required until the market application dossier is submitted, a useful strategy is to prepare a comprehensive report that details the origin, genetic construct, cloning, and cell substrate preparation. Such a document should be reviewed regularly and serve as a repository for information as the project moves from early- to late-stage development,” he states.

Finding suitable evaluation criteria that take into account the large number of different types of process-related impurities and defining a mathematically and scientifically sound way for determining toxicologically relevant limits for critical impurities can also be a challenge, according to Aline Denzel, quality control manager at Rentschler Biopharma. Furthermore, from a contract development and manufacturing organization’s (CDMO’s) perspective, she notes that the developed concepts should be applicable to many different customer projects.

Therefore, Denzel says all raw materials used at a manufacturing site should be evaluated initially to define potential critical process-related impurities. A toxicological assessment with a relevant permitted daily exposure (PDE) value should be compiled by a registered toxicologist if required. She also notes that most PDE values are calculated based on the body weight of an adult, so application of the concept for pediatric products must be considered separately, as must different routes of administration.

One difficulty in evaluating media and feeds, however, is the tendency of suppliers of these materials to not fully share the compositions of their proprietary products with their customers. “Other than getting suppliers to be more transparent, the best approach is to have the supplier provide a positive list of all potential raw materials and media components for the customer to use in worst-case models. The manufacturer must then agree that the toxicological limits established by the customer will not be exceeded and the media/feed does not contain any other critical substances,” Braun says.

From a physical perspective, one of the greatest challenges to residual impurity testing relates to the difficulties in accurately measuring compounds of interest within sample matrices, particularly at the low levels observed for most residual impurities. Chandrasekaran notes that most conventional detectors lack the sensitivity to accurately measure residual impurities in process fluids. “Developing a non-denaturing method is critical to avoid inaccurate quantification.” Often the methodologies required to achieve the specification limits are more challenging to develop and validate and require the application of a wider range of technologies that are not traditionally employed for batch release, Wake adds.

Accurately detecting impurities when performing cleaning verification is also difficult because the levels are very low, says Chandrasekaran. “The method must be sufficiently sensitive or cross-contamination can occur, which can be a safety issue, particularly with HCPs, which are known to have an immunogenic response at low levels,” he comments.

On a company level, there may be reluctance within regulatory and quality departments to change from well-established testing strategies to a more-risk-based approach, according to Jahn. The availability of and reference to industry white papers might be convincing, he says. As an example, he points to the BioPhorum Operations Group (BPOG) and its guide for the assessment of leachables from process materials, which presents an outline for a material risk evaluation. “The numerical scoring and weighing can be used as a template for the risk categorization of the many materials that are applied in a modern single-use biopharmaceutical production suite, which cannot be assessed by individual laboratory testing,” he says.

The most important element for the establishment of a successful risk-based strategy for residual impurity testing is the knowledge space, Jahn states. For example, he notes that if a platform drug substance downstream process has shown for different platform molecules that a supplement added during fermentation can be depleted to a satisfactory level, it might be decided that the depletion is not explicitly tested for the next platform molecule. “In the absence of the knowledge space, however, neither risk factors, nor regulatory requirements and challenges, can be appropriately addressed without extensive testing,” observes Jahn.

Indeed, the fundamental risk with any impurity is determining if it is present and if it is, what the specification for that impurity should be, Wake says. “With process-related impurities, such as surfactants and antibodies, this is a relatively definable process and regulatory guidance is hugely relevant. The lack of safety profiles for product-related species makes the process more difficult, and hence there is a greater risk to define them and effectively specifications become more risk loaded,” she observes.

This risk must be evaluated and defined for these impurities on a case-by-case basis. “The real challenge becomes, therefore, the ability to define ‘safe limits’ and then develop and validate a suite of methods that covers all of these species,” Wake concludes.

Knowing all of the production and raw materials that can contribute to residual impurities becomes very important, therefore, and a key element of effective risk-based strategies, according to Braun. Sapp adds that understanding the impact of raw or starting materials is key, and selecting only those materials or batches that are fit for purpose will minimize the impact of impurities formed during production.

“In addition, potential process-related impurities can be quickly identified and evaluated and, if testing becomes necessary, an appropriate analytical method can be developed,” Braun observes. In this context, he also notes that close contact with a registered toxicologist is an advantage for quickly determining PDE values for critical process-related impurities.

This knowledge also allows for weighting of the risk by the probability of occurrence for those residual impurities qualified as critical, according to Savoy. As an example, he points to an antibiotic that if present as a residual impurity at certain levels would constitute a potential risk for the patient’s safety, but that if added during upstream processing and prior to all downstream purification steps would be expected to have a negligible residual quantity that would likely be below the detection limits of applicable analytical methods.


Avoid shortcuts and inflexibility

Residual impurity testing for biopharmaceuticals is complicated and time-consuming, even when risk-based strategies are employed. Short cuts should be avoided and will typically only lead to complications and delays, according to Jahn. As an example, he points to the importance of pro-actively justifying why certain testing can be omitted in a risk-based extractables and leachables program without impacting patient safety. “Realizing only at or after a biologics license application submission that the data package is not sufficient might endanger commercialization with a much higher financial impact compared to the costs of a testing regimen,” he says.

Savoy adds that while previous experience and process understanding should be leveraged when developing risk-based testing strategies, it is equally important to not rely too heavily on previous experience to the point where control or verification steps are skipped.

It is also essential to avoid taking a “tick-box” approach, according to Wake. “Any program should be bespoke and based on a true understanding of the molecule and what parameters are pertinent to truly assessing the quality, and inherently, the safety of the product. Science should be the base of everything!” she asserts.

A rigid and inflexible approach to developing a risk-based strategy for residual impurity testing is also to be avoided, says Denzel. “The risk-based strategy should be developed in such a way that it allows the requirements of different clinical phases and projects to be addressed flexibly. The requirements for Phase I and Phase III products are different, and thus the risk assessment is a living document that should be adapted based on the status of a project, even if the process stays the same,” she observes.

“A lifecycle management system for the risk assessment of process-related impurities should be in place that includes and evaluates relevant changes in regulatory requirements (e.g., changes in guideline PDE values), in the manufacturing process (e.g., changes to raw materials), and in clinical requirements (e.g., changes in the maximum daily dose). The impact of the changes on the analytical approaches must constantly be evaluated,” Denzel continues.

Limit vs. quantitation tests

One final consideration of note, according to Wake, is when to use a limit test or full quantitation. “Limit tests are best suited when there is a really clear specification and do work well in that situation. Full quantitation methods are often more challenging, but there are specific situations in which they are needed,” she says. Analytical limit tests, according to Denzel, are a cost-effective tool to show the absence of process-related impurities in drug substances, while quantitative tests can provide useful information during process characterization and may be employed to develop data-based models reflecting purification factors for well-known process-related impurities.


1. ICH, Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products, Step 4 version (1999).
2. ICH, Q2(R1), Validation of Analytical Procedures: Text and Methodology, Step 4 version (1994).
3. EDQM, EurPh, Products of Recombinant DNA Technology (0784) (Strasborg, France).
4. USP, USP General Chapter <1132>, “Residual Host Cell Protein Measurement in Biopharmaceuticals,” USP 39-NF 34 (Rockville, MD, 2016).
5. ICH, Q9, Quality Risk Management, Step 4 version, (2005).
6. ICH, Q3C (R6), Maintenance of the Guideline for Residual Solvents, Final version (2016).
7. ICH, Q3D (R1), Guideline for Elemental Impurities, Final version (2019).

Tags: Residual impurity, residual impurity testing, elemental impurities