As defined in the US Code of Federal Regulations, potency is the specific ability or capacity of a product to affect a given result. Potency is a critical quality attribute of biological products that has been historically determined using some form of bioassay. A biological system is generally used to report on the potency of the product. The system may be animal, organ, tissue, or cell culture based. The concept of potency denotes an important feature of complex biologics: the biological activity produced as a direct result of the tertiary/quaternary structure of a molecule. Although a product may be well characterized by physicochemical (e.g., HPLC) assays, such tests are typically unable to confirm its higher-order structure. This molecular structure results in the mechanism of action (MoA) of a drug, which is the link between clinical response and activity measured in a bioassay. Because a bioassay or bioactivity assay can reflect the MoA, it is an important component of the complete analytical profile accumulated before a product is released for commercial use.



PRODUCT FOCUS: ALL BIOLOGICS

PROCESS FOCUS: MANUFACTURING

WHO SHOULD READ: PRODUCT AND PROCESS DEVELOPMENT, ANALYTICAL, QUALITY, AND REGULATORY AFFAIRS

KEYWORDS: STABILITY, LOT RELEASE, SURROGATE ASSAYS, BINDING ASSAYS, POTENCY ASSAYS

LEVEL: INTERMEDIATE

As a result of using a living system, bioassays present some challenges. First, maintenance of animals or cell lines may be difficult and expensive. Second, bioassays can be costly to perform, in part because they often require specialized equipment and lengthy training. Third, bioassays frequently lack the precision and robustness of other analytical methodologies, such as high-performance liquid chromatography (HPLC).

For all those reasons, technology transfers of bioassays can be especially difficult. In fact, bioassays are generally the most challenging analytical technique to transfer to another laboratory, be it in-house or to a contract laboratory. Given the challenges, it is natural that alternative strategies to potency testing would be investigated. The potential use of binding and physicochemical assays in addition to (or in lieu of) cell-based bioassays is of growing interest. Unfortunately, the regulatory and scientific pathways under which such methods may be appropriately used are not clearly defined at present.

Strategy Forums



A Chemistry and Manufacturing Controls (CMC) Strategy Forum on the roles of bioactivity assays in lot release and stability testing was held in January 2007 in Washington, DC. Its purpose was to promote an understanding of the design and utility of bioassays throughout product development — and to delineate the conditions under which noncell-based surrogate assays could be used to determine product potency. Topics discussed included appropriate assay selection at each stage of product development, the potential use of a binding assay for potency testing, and the conditions under which multiple surrogate assays may be needed. A second CMC forum was held in Paris in April 2008 to follow up on the success of that first one and to expand the discussion on many of the same topics.

Case studies were presented both by biopharmaceutical companies and regulatory agencies on the role of the potency assay in correlating product biological activity to structure and MoA, the development and use of surrogate assays for monoclonal antibody (MAb) testing, and the replacement of bioassays with binding or physicochemical assays for complex biological products. Perspectives were provided on the necessary characteristics of a reliable biological assay and the required level of correlation between the bioassay and surrogate assays. Open forums were held to discuss and gain consensus on the following topics:

• How is a potency assay defined and developed throughout the course of a product's life cycle?

• What characterization studies should be done to show that a surrogate to biological activity is valid?

• Under what circumstances might more than one surrogate assay be needed for potency testing?

• What role might physicochemical assays have in lot release and stability testing, given that they are generally less variable and more sensitive to change?

Those and other relevant questions were discussed at both forums. The Washington, DC forum consisted of two sessions; the Paris forum was a single session. Each session included three presentations followed by an interactive, moderated discussion with questions and comments from the audience. Although this review includes topics and opinions presented at both forums, the format follows that of the first meeting because the two meetings featured the same speakers and similar panel discussions.

Design and Utility of Bioassays