Development and validation of a method for quantitation of CTx-1 for the assessment of biosimilarity of FKS518, a denosumab biosimilar

Fresenius Kabi has developed FKS518, a fully human monoclonal antibody biosimilar to denosumab. The clinical development program included two randomized comparative trials: a PK study in healthy volunteers and a safety and efficacy study in osteoporosis patients. The demonstration of similarity and equivalence between FKS518 and reference denosumab included the quantitation of the serum biomarker C-terminal cross-linking telopeptide of Type 1 collagen (CTx-1), a well-established marker of bone resorption. This paper details the development, validation, and analytical performance of the method for CTx-1 quantitation, emphasizing how the Context of Use (CoU) shaped the validation requirements. Application of the method in samples from the pharmacokinetic (PK) equivalence study allowed demonstration of pharmacodynamic (PD) similarity between FKS518 and Prolia. This publication also addresses the use of endogenous serum control (ESC) samples in monitoring assay performance. A retrospective analysis indicated that applying more flexible ranges and/or omitting ESC-based criteria for run acceptance would not have substantially changed the CTx-1 results. While recognizing the value of ESCs for stability and trending analysis and the importance of rigorous biomarker method development and validation in the assessment of biosimilarity, this paper fosters the discussion whether run acceptance based on tight ESC acceptance limits is always necessary.