Higher-order structure comparison of the reference and biosimilar products using spectral distance calculation on the circular dichroism spectra.

A comparative analytical study of the quality attributes of biosimilars and their reference products is an important step in developing biosimilar products. Circular dichroism (CD) spectroscopy is commonly used to demonstrate similarity of higher-order structures. However, in most cases, the similarity of the CD spectra between the reference and biosimilar products is visually determined. Only in a few cases, the fractions of secondary structures (e.g., α-helix and β-strand) estimated from the far-UV/CD spectrum are also compared between the reference and biosimilar products. Therefore, the number of cases in which the CD spectra of the reference product are objectively and numerically compared with those of the biosimilar product is limited. To quantitatively assess the similarity of CD spectra, in this study, we converted the spectra into univariate data, i.e., spectral distances, and applied both the quality range method and the equivalence test to them. These statistical methods were selected based on the recommendation in the U.S. Food and Drug Administration guidance for quantitative quality attributes of high and moderate risk. Application of these method showed that almost all biosimilar products approved in Japan exhibited similarity in CD spectra to each reference product with less than 15 % of the exceptions. Our data indicate that converting the spectra into univariate data allows an objective similarity assessment of higher-order structures between the reference and biosimilar products.