Published on 29/11/2025

Assay Transfers During Bridges: Bias and Correction

Assay transfers during bridges play a significant role in ensuring biosimilar products maintain their characteristics and performance consistent with their reference products. This article provides a comprehensive tutorial on the processes involved in assay transfers during bridging, focusing specifically on bias and correction, as well as the parameters that need to be monitored to ensure compliance with regulatory expectations, particularly those articulated by the US FDA, EMA, MHRA, and PIC/S. We will delve into biosimilar analytical comparability, CQA mapping, fingerprint analytics, process drift monitoring, equivalence testing, and bridging justifications.

Understanding Biosimilar Analytical Comparability

Biosimilars are biological products highly similar to an already approved reference product. The comparison process between the biosimilar and the reference product requires rigorous evaluation to demonstrate that no clinically meaningful differences exist in terms of quality, efficacy, and safety. This process is known as biosimilar analytical comparability.

In establishing biosimilar analytical comparability, a number of critical quality attributes (CQAs) must be addressed. CQAs are defined as the physical, chemical, biological, or microbiological properties or characteristics that should be controlled to ensure quality. Any change in the manufacturing process that may affect these attributes must be closely monitored.

Key aspects of biosimilar analytical comparability include:

• Reference Product Selection: Choosing a reference product that aligns with regulatory guidelines and expectations.
• Analytical Methods: Utilizing a variety of analytical techniques to assess requisite CQAs of both the biosimilar and the reference product.
• Comparative Data: Generating comparative data that establish similarity across a comprehensive range of attributes.

Both the FDA and EMA have provided extensive guidance on establishing analytical comparability in biosimilars, highlighting the need for rigorous assessment and validation.

Critical Quality Attribute (CQA) Mapping

CQA mapping is an essential element in ensuring that all relevant attributes of the biosimilar product are quantitatively and qualitatively aligned with the reference product. The purpose of CQA mapping is to create a structured approach for identifying, controlling, and monitoring key performance indicators throughout the development and manufacturing processes.

Steps involved in CQA mapping include:

• Identification: Identify all CQAs relevant to the biosimilar product, taking into account both the composition and manufacturing process.
• Risk Assessment: Evaluate the risks associated with each CQA and prioritize based on impact and likelihood of variation.
• Control Measures: Implement control measures during the manufacturing process that ensure alignment with the defined specifications.

CQA mapping should also be integrated into the process development and validation strategy, especially during the performance qualification (PPQ) and continued process verification (CPV) phases. By establishing clear controls and monitoring practices, companies can better ensure that any variability in the process does not translate into variabilities in product quality.

Fingerprint Analytics in Assay Transfers

Fingerprint analytics is a powerful tool used in the comparative assessment of biosimilars versus reference products. This approach involves systematically evaluating product features—such as purity, potency, and biological activity—against established fingerprints of the reference product. Fingerprint analytics allows for a comprehensive view of the biosimilar product’s attributes and ensures that they are sufficiently aligned with the reference product.

The application of fingerprint analytics involves several steps:

• Criteria Definition: Define the critical parameters to be analyzed and compared against the reference product’s fingerprint.
• Data Collection: Gather analytical data on the defined parameters using validated methods.
• Comparative Analysis: Conduct rigorous statistical analyses to determine the degree of similarity or variance between the products.

Regulatory authorities recommend that fingerprint analytics results be documented thoroughly, with emphasis given to methodologies and statistical treatments employed during the assessments. Such details are crucial for providing comprehensive evidence during regulatory submissions.

Process Drift Monitoring

Process drift refers to variations in processes that can lead to unintentional changes in product attributes over time. Regular monitoring of the manufacturing process is essential to identify any drifts that may affect product quality or yield. The identification of bias in the comparison between a biosimilar and its reference product necessitates a structured process drift monitoring strategy.

The process drift monitoring strategy should encompass:

• Routine Sampling: Implement a routine sampling mechanism to monitor CQAs during the entire manufacturing process.
• Statistical Process Control (SPC): Use SPC techniques to detect variations that may signify process drift. Control charts and trend analysis can be effective tools.
• Root Cause Analysis: Conduct root cause analysis when drift is detected to establish if interventions are necessary to maintain product quality.

Implementing process drift monitoring not only ensures ongoing compliance with analytical comparability but also supports the overall quality assurance processes in manufacturing biosimilars.

Equivalence Testing for Assay Transfers

Equivalence testing is the evaluation of whether two products (the biosimilar and the reference biologic) administer similar levels of efficacy and safety. This form of testing is especially critical in the development of biosimilars, as it helps substantiate claims of similarity or lack of clinically significant differences.

There are several types of equivalence testing that may be employed during assay transfers:

• Statistical Equivalence Testing: Used to evaluate if the differences between treatments or products are statistically insignificant.
• Bioanalytical Validation: Ensures that analytical methods used for testing biosimilars are bioequivalent to those used for reference products.
• Clinical Studies: Conducting bioequivalence studies to gather clinical data supporting the hypothesis of equivalence.

An important aspect of equivalence testing is adhering to predefined boundaries to establish equivalence, known as the equivalence margins. These margins help define acceptable variances and must be clearly outlined in regulatory submissions. Guidance documents from the EMA detail standards for equivalence testing that should be integrated in planning testing strategies.

Bridging Justifications for Assay Transfers

Bridging justifications are necessary when transitioning from one analytical method or platform to another in the context of assay transfers. These justifications provide compelling evidence that the new method produces comparable results to the original method. A well-developed bridging justification is essential for regulatory submission assurance.

Effective bridging justifications include the following components:

• Objective Comparison: Clear objectives that describe the purpose and necessity of the method change.
• Scientific Rationale: A robust scientific rationale supported by data demonstrating that the methods yield comparable results.
• Regulatory Compliance: Addressing how the new method complies with relevant regulations and guidelines, including expectations from the US FDA, EMA, and PIC/S.

By providing a transparent rationale and comprehensive data in support of bridging justifications, sponsors can facilitate smoother regulatory reviews and enhance the likelihood of successful product approval.

Conclusion: Implementing a Comprehensive Validation Strategy

The complexities of assay transfers during bridges necessitate a thorough understanding of biosimilar analytical comparability, CQA mapping, fingerprint analytics, process drift monitoring, equivalence testing, and bridging justifications. Each of these components plays a critical role in the successful validation of biosimilars as well as ensuring compliance with regulatory standards.

Establishing and adhering to a comprehensive validation strategy not only minimizes risks but also builds a robust foundation for continuous quality assurance. By following best practices and remaining informed about the evolving regulatory landscape, pharmaceutical professionals can confidently navigate the biosimilar development process, ensuring both patient safety and product integrity.