Abstract. Biomarkers are used to study drug effects, exposure-response relationships, and facilitate early decision making during development. Denosumab, a fully human monoclonal antibody against receptor activator of nuclear factor-κB ligand, profoundly inhibits bone resorption. C-terminal telopeptides of type I collagen (CTx), a bone resorption biomarker, provides early indications of denosumab effectiveness and informs protracted clinical outcomes (e.g., bone mineral density). Because of the dynamic relationship between denosumab and CTx, a precise and robust assay was desired. Thus, we adopted a fit-for-purpose approach to modify and validate a commercial CTx diagnostic kit to meet the intended applications of a quantitative pharmacodynamic biomarker for denosumab development. Seven standards were prepared to replace five calibrators provided in the kit. Three quality controls (QC) and two sample controls were used to characterize and monitor assay performance. Robotic workstations were used for standard and QC preparation and assay execution. Method validation experiments were conducted with rigor and procedures similar to those used for drug bioanalysis. The method demonstrated a linear range of 0.0490-2.34 ng/mL with four-parameter logistic regression. Inter-assay total error of validation samples in serum was ≤26.7%. Extensive tests were conducted on selectivity in sera from target populations, specificity, stability, parallelism, and dilutional linearity. Applications to samples from numerous clinical studies confirmed that the CTx method was reliable, robust, and fit for use as an early indicator of denosumab effectiveness. Refinement supported the confidence for use in pharmacokinetic/pharmacodynamic modeling, dose selections, correlation to clinical effects, and formulation bioequivalence work.KEY WORDS: bone resorption marker; commercial immunoassay kit; method validation; pharmacodynamic (PD) biomarker; serum C-terminal telopeptides of type 1 (CTx).

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Dried blood spots (DBS) technology has been introduced as a microsampling alternative to traditional plasma or serum sampling for pharmacokinetics or toxicokinetics evaluation. The application of DBS has been established for many small molecule drugs at discovery, nonclinical, and clinical stages. However, the application of DBS for large molecule therapeutics development is not yet well-established. This article describes the method validation of a ligand binding assay (LBA) for DBS sampling of a therapeutic monoclonal antibody-AMG 162 (Denosumab). The original serum LBA was modified for the DBS method. A fit-for-purpose method validation was performed to evaluate accuracy and precision, selectivity, dilutional linearity, and stability. In addition, the parameters relevant to DBS, such as spot volume, extraction recovery, whole blood stability, and hematocrit effects, were evaluated. The validation results demonstrated assay robustness with inter-assay precision of ≤ 19%, inter-assay accuracy of ≤ 9%, and total error of ≤ 24%. Selectivity, extraction recovery, dilutional linearity, and stability were demonstrated. The validation results revealed some limitations of the possible effect of blood hematocrit on therapeutic concentration measurements and the caution required using whole blood for standards and quality controls preparation. This is the first article to describe a thorough method validation of an LBA using DBS for a therapeutic monoclonal antibody. The lessons learned can serve as a model process for future method validation of other LBAs for large molecule therapeutics or biomarkers using the DBS sampling method.

The current ISR acceptance criteria for macromolecules was scientifically and statistically meaningful for methods with a total error of 25% or less.

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