mAbs Present Twin Challenge
At 155 K or more Daltons, mAbs present a complicated challenge for biosimilar developers. Just correctly identifying the complete structure of the reference product can be daunting. Originators will not share information such as “the exact sequence and all of the post-translational modifications,” says Fiona Greer, Ph.D., global director, biopharma services development for SGS M-Scan, a provider of analytical services with biologics experience.
“We’re talking about a different scale of protein to conquer with the antibody,” explains Dr. Greer. “Smaller proteins like growth hormones and even erythropoietin—which is a bigger molecule but that’s because 30 percent of its mass is carbohydrate—are relatively straightforward in terms of physicochemical characterization.”
The second big challenge is proving that your product is sufficiently comparable to the reference product. To some extent the same analytical tools are used for each task (mass spec, peptide mapping, various chromatographic approaches, etc.) to analyze both primary and higher-order structure. De novo analysis approaches are used in determining the reference compound’s structure, keeping in mind there are always small variations among some critical attributes that regulators have deemed acceptable.
Demonstrating structural similarity is a bit different. “Actually we interrogate the two molecules (reference and biosimilar) to see if there are any differences, so we keep drilling down. On the first pass, the molecular weights might be the same. Then we might do the peptide map to spot any differences. If there aren’t any, we would we go to the next level, each time going deeper into the molecules,” said Dr. Greer.
“Physicochemical comparison is only the first step,” she stresses. “The second step would be to see if those differences impact the biological activity or safety profile. We have learned in Europe that EMA expects the primary amino acid structure to be the same to be a biosimilar—this is now clearly stated in the revised Quality Guideline. However, there can be slight differences, for example, in the carbohydrate structure or C-terminal lysine heterogeneity if this does not affect the efficacy or safety of the molecule—but this is always treated on a case-by-case basis.”