Direct Quantification of Complement Biomarkers Using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS).

The classical complement pathway is a critical therapeutic target for numerous diseases, but the lack of robust, translatable biomarkers hinders drug development. Here, we report the development and qualification of a quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) platform to measure key classical pathway proteinsC1q, C 1s (total and active), and the native C1q/C 1s complex. The platform was established across human, cynomolgus monkey, and mouse samples, including serum, plasma, cerebrospinal fluid, and brain homogenates. We employed two complementary workflows: an affinity capture LC-MS/MS method to quantify the intact C1q/C 1s complex and a direct digestion strategy to differentiate total C 1s from its active form. The assays met rigorous qualification standards for accuracy, reproducibility (CV ≤15%) and freeze-thaw stability. Applying this platform, we established that cynomolgus monkeys exhibit complement profiles highly similar to those in healthy humans, validating their use as a translational model. In contrast, mice display markedly higher baseline levels of active C 1s, revealing a critical species-specific difference in complement regulation. This qualified, cross-species platform provides an essential tool for bridging preclinical and clinical research and enabling reliable pharmacodynamic assessment of complement-targeting therapeutics.