A PCSK9-binding antibody that structurally mimics the EGF(A) domain of LDL-receptor reduces LDL cholesterol in vivo

Proprotein convertase subtilisin-like/kexin type 9 (PCSK9) regulates LDL cholesterol levels by inhibiting LDL receptor (LDLr)-mediated cellular LDL uptake. We have identified a fragment antigen-binding (Fab) 1D05 which binds PCSK9 with nanomolar affinity. The fully human antibody 1D05-IgG2 completely blocks the inhibitory effects of wild-type PCSK9 and two gain-of-function human PCSK9 mutants, S127R and D374Y. The crystal structure of 1D05-Fab bound to PCSK9 reveals that 1D05-Fab binds to an epitope on the PCSK9 catalytic domain which includes the entire LDLr EGF(A) binding site. Notably, the 1D05-Fab CDR-H3 and CDR-H2 loops structurally mimic the EGF(A) domain of LDLr. In a transgenic mouse model (CETP/LDLr- hemi), in which plasma lipid and PCSK9 profiles are comparable to those of humans, 1D05-IgG2 reduces plasma LDL cholesterol to 40% and raises hepatic LDLr protein levels approximately fivefold. Similarly, in healthy rhesus monkeys, 1D05-IgG2 effectively reduced LDL cholesterol 20%-50% for over 2 weeks, despite its relatively short terminal half-life (t(1/2) = 3.2 days). Importantly, the decrease in circulating LDL cholesterol corresponds closely to the reduction in free PCSK9 levels. Together these results clearly demonstrate that the LDL-lowering effect of the neutralizing anti-PCSK9 1D05-IgG2 antibody is mediated by reducing the amount of PCSK9 that can bind to the LDLr.-Ni, Y. G., S. Di Marco, J. H. Condra, L. B. Peterson, W. Wang, F. Wang, S. Pandit, H. A. Hammond, R. Rosa, R. T. Cummings, D. D. Wood, X. Liu, M. J. Bottomley, X. Shen, R. M. Cubbon, S.-p. Wang, D. G. Johns, C. Volpari, L. Hamuro, J. Chin, L. Huang, J. Z. Zhao, S. Vitelli, P. Haytko, D. Wisniewski, L. J. Mitnaul, C. P. Sparrow, B. Hubbard, A. Carfi, and A. Sitlani. A PCSK9-binding antibody that structurally mimics the EGF(A) domain of LDL-receptor reduces LDL-cholesterol in vivo. J. Lipid Res. 2011. 52: 78-86.