Demonstrating the potency of an Advanced Therapy

A batch of cell therapy product may meet identity, purity and viability specifications while still leaving one question unanswered: do the cells perform the biological activity for which they were produced? Similarly, a satisfactory potency assay result provides information about the sample tested, but does not, on its own, explain why the manufacturing process can consistently deliver that level of performance. These are two distinct questions, and a potency assurance strategy must address both.

On 2 November 2026, the FDA published its final guidance, Potency Assurance for Cellular and Gene Therapy Products, superseding the 2011 guidance on potency testing. The guidance recommends establishing a science- and risk-based strategy, extending from the early stages of development through to post-marketing monitoring. As final US guidance, it provides a reference for European companies developing products intended for the US market, both in preparing regulatory submissions and in their interactions with the agency. The European Union’s regulatory framework remains distinct.

FDA Guide

What a Potency Assay Demonstrates

Potency refers to a product’s ability to exert its intended biological activity. Measuring it requires first identifying the relevant function and determining how accurately the selected method can assess it. For a cell therapy designed to recognise and eliminate target cells, for example, measuring receptor expression may help characterise the product. A functional cytotoxicity assay addresses a different question: under defined conditions, are the cells capable of exerting the expected activity against their target?

Even functional assays have limitations that must be clearly acknowledged. In vitro conditions cannot fully reproduce what occurs in the patient, and results must be interpreted in light of variability associated with the method, the sample and the reagents. The key consideration, therefore, is how informative the assay is for the decision it is intended to support. A method used for product characterisation during development may involve timelines and complexity that are manageable in a laboratory setting but incompatible with the time available for batch release. A faster method may be suitable for routine testing if its relationship with the relevant biological activity is supported by data.

For some products, combining complementary measurements may be beneficial. In such cases, it is important to explain what each measurement contributes and how potentially discordant results should be interpreted. If receptor expression falls within the expected range but the functional assay indicates reduced activity, the two measurements cannot be considered to offset one another. Instead, the discrepancy warrants investigation: does the observed signal originate from the product itself, sample preparation or assay conditions?

What the Manufacturing Process Must Demonstrate

The laboratory measures the biological activity of the sample it receives. The manufacturing team must be able to establish which conditions have contributed to preserving that activity and which variations could compromise it. This is where the transition from potency testing to potency assurance becomes evident: the final test result becomes more meaningful when interpreted alongside the batch’s manufacturing history.

In a cell therapy manufacturing process, for example, variability in starting materials, culture duration, and freezing and thawing conditions are factors that must be assessed in relation to the specific product. An expansion step may achieve a satisfactory yield while leaving questions about potential changes in functional activity unanswered. Determining whether that step requires specific controls involves linking process data with analytical results. A process parameter warrants a defined limit or targeted monitoring when the available scientific knowledge supports its relevance to the control strategy.

Another consideration is sample representativeness. If testing is performed before freezing but the product is administered after thawing, it is necessary to justify how the test result relates to the product under its intended conditions of use. Addressing this question may require characterisation and stability studies alongside routine controls. The regulatory dossier must clearly establish the links between these different sources of evidence.

Batch Release Puts the Strategy to the Test

For a CMC team, the most informative situations often arise when the data do not align.

Consider a batch that meets the potency assay acceptance criterion, but with a result close to the acceptance limit and a documented deviation during a potentially critical manufacturing step. The test result must be interpreted alongside the deviation investigation, assay performance and the manufacturing history of the batch. The reverse situation requires equal rigour: a manufacturing history with no recorded deviations does not justify disregarding an out-of-specification result.

To make this assessment feasible, analytical development, manufacturing and quality teams must agree in advance on which data to collect, how to interpret them and who is responsible for their evaluation. Establishing a strategy early in development also helps identify gradual changes. If several batches remain within specification but their results progressively approach an acceptance limit, the trend may warrant investigation before an out-of-specification result occurs. The initial assessment should examine both the manufacturing process and the analytical measurement system: changes in reagents, reference materials or laboratory operating conditions may affect the results.

Comparability and Transfer to a New Manufacturing Site

A change of manufacturing site puts both the process and the ability to measure its performance to the test. It is useful to distinguish between two questions: does the analytical method perform comparably in the receiving laboratory? And does the product manufactured at the new site retain its relevant characteristics, including biological activity? The first question concerns analytical method transfer; the second concerns product comparability following the change. Successfully manufacturing the first batch represents an operational milestone. Assessing the transfer requires data to be interpreted against predefined criteria.

Consider a scenario in which the first batches manufactured at the receiving site show lower potency values than historical batches, while remaining within specification. Before attributing the difference to the new manufacturing process, it is important to establish whether the two laboratories use the same reference materials, prepare samples in the same way and obtain comparable results when testing common samples. Once the extent to which the observed difference may be attributable to analytical variability has been established, the modified manufacturing steps can be examined more precisely. The same reasoning applies to changes in manufacturing scale, equipment or starting materials.

Potency Assurance as a Team Effort

If a potency assay yields lower results following a manufacturing site transfer, each function has a role in understanding the discrepancy. Analytical development assesses whether results from the two laboratories are comparable; manufacturing identifies what has changed in the process; quality evaluates how to manage the observed trend; and regulatory affairs determines what evidence is needed to document the change. The decision is informed by bringing these findings together.

References:

1. FDA. Potency Assurance for Cellular and Gene Therapy Products. Guida definitiva, novembre 2026.
2. FDA. FDA Provides Clarity to Support Cell and Gene Therapy Development and Quality. Comunicazione del 2 novembre 2026.
3. EMA. Guideline on potency testing of cell based immunotherapy medicinal products for the treatment of cancer.
4. ICH. Q2(R2) – Validation of Analytical Procedures. Versione finale, 2023.
5. EMA. Comparability considerations for advanced therapy medicinal products (ATMPs). Reflection paper.