Quantification of Lipoprotein-Associated PCSK9 in Mouse and Human Plasma

Background: PCSK9 is a major regulator of plasma LDL-C. Monoclonal antibodies to PCSK9 lower LDL-C by 45-65% and Lp(a) by 9-38%. The canonical function of PCSK9 is binding of LDL-receptor (LDLR) via its extracellular EGF-A domain, and subsequently mediating LDLR degradation. However, PCSK9 also weakly associates with plasma lipoproteins, with 20-40% of total plasma PCSK9 found on LDL. However, most LDL particles do not contain PCSK9. Whether PCSK9 also associates with other lipoproteins such as Lp(a) are not well described. Methods: Sensitive and quantitative sandwich-based ELISA assays were developed to measure PCSK9 associated plasma lipoproteins in both mouse and human plasma. For human plasma, commercial rabbit polyclonal antibodies binding to the C-terminal region of PCSK9 (Abgent, ThermoFisher) or REGN727 human monoclonal antibody were bound to microtiter well plates. Plasma was added and monoclonal antibodies MB47 and LPA4, binding to apoB-100 and apo(a) respectively, were used to detect PCSK9-apoB-100 and PCSK9-Lp(a) complexes with a chemiluminescent ELISA. For mouse assays, REGN727 was used as the capture antibody as it detects mouse PCSK9 and monoclonal antibody LF3 was used to detect mouse apoB. Results: PCSK9-apoB and PCSK9-Lp(a) complexes could be detected in both human plasma and in various mouse models expressing apo(a) or Lp(a). The signal to noise ratio was ~20 fold in various clinical samples, including in healthy subjects and in patients with cardiovascular disease. In 536 clinical samples from the Health Professional Follow-Up Study, PCSK9-Lp(a) correlated strongly with Lp(a) (r=0.59, p<0.001, age-adjusted) but not other lipid variables. PCSK9-apoB correlated weakly with PCSK9-Lp(a) (r=0.30, p<0.001, age-adjusted) and LDL-C (r=0.22, p<0.001, age-adjusted). These associations were virtually the same in 526 women in the Nurses’ Health Study. Conclusions: Novel ELISAs were generated to quantitate lipoprotein-associated PCSK9 in transgenic mouse and human plasma, including on apoB and Lp(a). Changes in PCSK9-Lp(a) complexes may provide insights into the Lp(a)-lowering effect of PCSK9 antibodies. Whether these assays will predict CVD outcomes waits to be determined in PCSK9 antibody and epidemiological studies.