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Regulatory requirements for therapeutic proteins: the relationship between the conformation and biological activity of filgrastim.

Higher order structure, including conformation, is considered a critical quality parameter of therapeutic proteins, and is mandatory information in development of first use and bio-similar therapeutic protein drugs, the assumption being that the biological activity of a protein is directly dependent on its adoption of a 'correct' conformation. Studies on the relationship between conformation and activity depend on the ability to induce conformational changes in proteins, and conventional approaches such as thermal or chemical denaturation are incompatible with bioactivity measurements. To explore the relationship between bio-activity and conformational studies, we have studied variants of the therapeutic protein filgrastim (rec met huGCSF) which have been mutated by the replacement of helical alanine residues with glycine, to destabilise the conformation of the molecule. In the GCSF A-G mutant series studied, single conformation-destabilising amino-acid substitutions significantly reduced the biological activity. These effects were not, however correlated with changes in secondary structure measurable by far-UV Circular Dichroism (CD) spectroscopy. Only the more extensively mutated double and triple substitutions showed measurable reductions in alpha-helical structure by CD. We conclude that in this system, GCSF does not readily adopt a reduced-activity altered conformational state which can be detected by low resolution techniques such as CD. In contrast, reductions in biological activity do reflect reductions in conformational stability, possibly caused by time-dependent degradation of the protein in the cell-proliferation bioassay. Although not a formal model of biosimilarity, we suggest that our results could inform the regulatory process in determining appropriate experimental approaches to meeting regulatory requirements for higher order structural analysis of therapeutic proteins.