AlphaLISA Assay: An in-vitro AlphaLISA assay (Perkin Elmer) was developed in order to quantitate the level of PCSK9 secreted into the cell culture media following compound treatment. To detect and measure PCSK9 protein an in-house generated anti-PCSK9 antibody was coupled to AlphaLISA acceptor beads by an external vendor (PerkinElmer) and a second internally developed anti-PCSK9 antibody with an epiptope distinct from that of the acceptor beads was biotinylated in house. Streptavidin coated-donor beads (Perkin Elmer) are also included in the assay mixture which then binds the biotinylated anti-PCSK9 antibody and in the presence of PCSK9 this donor complex and acceptor beads are brought into close proximity. Upon excitation of the donor beads at 680 nm singlet oxygen molecules are released that trigger an energy transfer cascade within the acceptor beads resolving as a single peak of light emitted at 615 nm. The ability of compound to modulate PCSK9 protein levels in conditioned media by AlphaLISA was assessed in the human hepatocellular carcinoma cell line Huh7, stably over-expressing human PCSK9.
Cell Free Assay: In order to eliminate the permeability barrier inherent to the WT7 and SCHH cell-based assays a cell-free system was also established to access compound activity. A sequence containing the full length human PCSK9 (NCBI reference identifier, NM-174936.3, where coding sequence start annotated at position 363) along with 84 additional 3⿲ nucleotides, comprising a V5 tag and polylinkinker followed by an in frame modified firefly luciferase reporter (corresponding to nucleotide positions 283-1929 of pGL3, Gen Bank reference identifier JN542721.1) was cloned into the pT7CFE1 expression vector (ThermoScientific). The construct was then in-vitro transcribed using the MEGAscript T7 Kit (Life Technologies) and RNA subsequently purified incorporating the MEGAclear Kit (Life Technologies) according to manufacturer's protocols. HeLa cell lysates were prepared following the protocols described by Mikami (reference is Cell-Free Protein Synthesis Systems with Extracts from Cultured Human Cells, S. Mikami, T. Kobayashi and H. Imataka; from Methods in Molecular Biology, vol. 607, pages 43-52, Y. Endo et al. (eds.), Humana Press, 2010) with the following modifications.
Inhibition of recombinant PCSK9 (unknown origin) expressed in human HepG2 cells assessed as increase in LDL uptake at 10 uM after 16 hrs by fluorescence assay
Inhibition of recombinant PCSK9 (unknown origin) expressed in human HepG2 cells assessed as increase in LDL uptake at 1.25 uM after 16 hrs by fluorescence assay
Inhibition of recombinant PCSK9 (unknown origin) expressed in human HepG2 cells assessed as increase in LDL uptake at 0.31 uM after 16 hrs by fluorescence assay