Development of the Complement C5 Assay by LC-MS/MS in Monkey Serum and Comparison with Enzyme-Linked Immunosorbent Assay.

Complement C5 (C5) is the key component for the complement activation pathway, which is important for innate immunity, and inhibition of C5 is considered to be effective in antibody-mediated rejection in organ transplantation. Thus determination of C5 levels in systemic circulation is a simple way to understand efficacy of drugs that aim to inhibit C5 production. We have developed a simple liquid chromatography with tandem mass spectrometry (LC-MS/MS) assay for C5 in cynomolgus monkey serum. C5 in monkey serum was subjected to tryptic digestion, and two signature peptides, DSSVPNTGTAR and LQGTLPVEAR, were assayed by LC-MS/MS with electrospray ionization in the positive ion mode. Assay reproducibility in serum samples was evaluated, and the assay was applied to the C5 assay in monkey serum after administration of C5 siRNA encapsulated in lipid nanoparticles to monkeys. The time profiles of C5 after administration of C5 siRNA were comparable between the two signature peptides by LC-MS/MS and were also similar to those by an enzyme-linked immunosorbent assay using an assay kit. These findings suggest that the established LC-MS/MS assay of C5 is reliable to determine C5 levels in monkey serum.