Potency

Potency is the quantitative measure of a product’s ability to elicit its intended biological effect. As a Critical Quality Attribute (CQA), it directly links the drug product to clinical efficacy. The potency assay should, wherever possible, reflect the product’s specific mechanism of action (MOA).

• High Variability: Cell-based bioassays are inherently more variable than standard physicochemical assays.
• Assay-to-Clinic Correlation: Demonstrating that an in vitro assay result is truly representative of the in vivo clinical effect is a significant challenge.
• Reference Standard Lifecycle Management: Potency is a relative measure, making the integrity of the in-house reference standard paramount. Significant effort is required to qualify the primary reference, establish a tiered system of working standards, perform ongoing stability monitoring, and execute technically complex bridging studies whenever a new standard is introduced.

• Early Development: A qualified, "fit-for-purpose" cell-based assay is expected to demonstrate biological activity and consistency. Acceptance criteria are typically broad.
• Mid-Development: The method is optimized for better precision and robustness. Formal validation begins, and criteria start to tighten.
• Late-Stage / BLA Submission: A fully validated, robust bioassay with a statistically justified specification is required. The acceptance range must be supported by clinical and manufacturing data.

• Patient Risk: A batch with sub-potent results can lead to under-dosing and a lack of clinical efficacy. An error in potency measurement could also lead to over-dosing, potentially causing safety issues related to exaggerated pharmacology or toxicity.
• Manufacturing Risk: Potency assays are often highly variable. This can lead to the rejection of valid batches or significant delays in product release due to invalid or out-of-specification results. Furthermore, the long-term risk of assay drift, where subtle, unmonitored changes in critical reagents or cell bank viability cause a gradual shift in assay performance, can lead to erroneous batch data and complex, costly investigations.

AAV gene therapy: Potency provides functional context to physical measurements such as genome titer and Empty/Full ratio. While particle counts define strength, only genome-containing capsids confer biological potency.

Monoclonal antibodies (mAbs): Potency is sensitive to glycosylation and aggregation. Glycan patterns can modulate effector functions (ADCC, CDC), while aggregation reduces activity.

Potency assays are recognized as among the most technically challenging to develop and validate for biologics. Regulators expect early establishment of an assay that meaningfully reflects the product’s mechanism of action. Inadequate potency assays remain a frequent cause of regulatory delay in both development and licensure.

• Reflection of Mechanism of Action (MOA): Global regulators (FDA, EMA) stress that the potency assay must reflect the product's biological MOA wherever possible.
• Validation Expectations: Bioassays must be validated according to the principles of ICH Q2, acknowledging their inherent variability. USP chapters like 〈1032〉 and 〈1033〉 provide detailed guidance.
• Specification Setting (ICH Q6B): A specification for potency is mandatory for all biologics and must be justified by data from clinical trial batches.