Aim: Develop LC-MS/MS-based assays to measure total and free complement C5 in cynomolgus monkey serum as a target engagement biomarker for pharmacokinetic/pharmacodynamic correlation study. Materials & methods/results: The C5-specific signature peptide derived from pellet digestion of serum proteins with and without prior immunodepletion of the drug-bound C5 by protein A beads was quantified to assess free and total C5 levels, respectively. Conditions for immunodepletion by protein A were optimized to ensure complete depletion of IgGs (and drug-bound C5). The effect of sample dilution on drug-target dissociation and thus free C5 measurement was evaluated by applying a mathematical simulation.
Conclusion: The procedure described here allows for the assessment of protein target engagement, aiding in pharmacokinetic/pharmacodynamic correlation analysis and human dose projection.
Keywords: LC–MS/MS; complement component C5; immunodepletion; protein A; quasi-equilibrium; target engagement; total and free target.