Looking for fingerprints
A key feature of the FDA draft guidance is the idea of a fingerprint-like identification of the protein, a set of quality attributes measured by orthogonal methods that in combination could be used to identify a protein and demonstrate similarity between a biosimilar and a comparator. This concept arose, in part, from experience in the approval of enoxaparin, a generic low-molecular weight heparin approved in July 2010. Enoxaparin was approved without a requirement for clinical trials, based on five criteria that the agency deemed sufficient to demonstrate that enoxaparin had the same active ingredient as the innovator product, Lovenox (4). These criteria were:
"The fingerprint will depend on the individual molecule and the relevant critical quality attributes," says Fiona M. Greer, global director of BioPharma Services Development at SGS M-Scan. "It may be possible to standardize this for a particular class of molecule but probably not feasible between types of product. For example, glycosylation may have an effect on the efficacy in one type of molecule, but in another, changes in carbohydrate are not significant. Whatever the standard attributes are, they should cover both the physicochemical (primary and higher order structure) and biological properties. Without a doubt, the inclusion of information regarding posttranslational modifications will be required."
The fingerprint analogy can only be taken so far. Although real fingerprints are unique and unchanging, the attributes of a protein are harder to pin down. Protein products demonstrate variation in posttranslational modifications that may or may not be important for the biological activity of the protein. This type of heterogeneity is an expected feature of proteins produced in cultured cells, and the approval process for any biologic product typically includes a discussion with the regulatory agency where the range of heterogeneity is defined and limits agreed upon within which the potency and safety of the product remain unaffected.
For biosimilars manufacturers, determining the extent of variability in the comparator protein is an essential part of the characterization process. Typically, this step involves obtaining multiple batches of the comparator product manufactured at different times and evaluated at different points in their viable shelf life, and determining the range of quality attributes that the product displays.
"I wouldn't recommend picking a single batch and trying to produce a protein that's 99% identical to that batch," says Jin Xu, director of protein sciences at the Massachusetts Biomanufacturing Center. The comparator will exhibit a range of physicochemical properties that are acceptable, and the biosimilar has to fall within that range, explains Xu.