Biopharmaceuticals are large, complex and heterogeneous proteins with variable molecular weights, typically ranging from 18,000 to 45,000 Da. The active substance of a biopharmaceutical is a collection of large protein isoforms and not a single molecular entity. Therefore it is highly unlikely that the active substances between two products are identical. Currently there are no analytical techniques to establish biopharmaceutical equivalence.

Compared to the manufacture of small-molecule pharmaceuticals (molecular weight between 100 and 1,000 Da), the manufacture of biopharmaceuticals requires a greater number of batch records (>250 vs. <10), more product quality tests (>2,000 vs. <100), more critical process steps (>5,000 vs. <100) and more process data entries (>60,000 vs. <4,000) than of chemical drugs.

The molecular size and complexity of biopharmaceuticals, and their production in living cells, make the final product very sensitive to changes in production conditions. Changes may occur to the expression systems used for production, culture conditions, e.g. temperature and nutrients, purification and processing, formulation, storage and packaging.

Small changes in, or differences between, manufacturing processes may have a significant impact on the quality, purity, biological characteristics and clinical activity of the final product. Even when biosimilars are produced from the same genetic construct, using the same technique, formulation and packaging as the innovator product, there is no guarantee that they will be comparable with the reference product.

Structural differences between proteins may arise for a number of reasons, including oligomerisation, modification of the protein primary sequence, glycosylation patterns or the conformational state. Even if molecular characteristics and bioavailability are similar between products, it cannot be assumed that their clinical activity will be the same. The only way to ascertain the safety and efficacy of a biosimilar will be to conduct pre-clinical tests and clinical trials, and implement tailored pharmacovigilance plans.