MK-8776 (SCH 900776)

SCH 900776 is a less potent inhibitor of Chk2 and CDK2 with IC50 of 1.5 μM and 0.16 μM, respectively. SCH 900776 shows no significant inhibition of cytochrome P450 human liver microsomal isoforms 1A2, 2C9, 2C19, 2D6, and 3A4. SCH 900776 induces a dose-dependent loss of DNA replication capability 24 hours after hydroxyurea exposure. SCH 900776 enhances the γ-H2AX response of hydroxyurea, 5-fluoruracil, and cytarabine. In combination with an antimetabolite, SCH 900776 induces accumulation of γ-H2AX within 2 hours, indicative of replication fork collapse and double stranded DNA breaks. Additionally, SCH 900776 suppresses accumulation of the Chk1 pS296 autophosphorylation in a dose-dependent manner. Exposure of proliferating WS1 cells to SCH 900776 is associated with rapid, dose-dependent accumulation of Chk1 pS345, indicating that cycling populations of normal cells induce Chk1 pS345 following exposure to SCH 900776 as part of a futile cycle, perhaps driven by AT-family kinases and DNA-PK.^[1]

Administered 30 minutes after gemcitabine, 4 mg/kg SCH 900776 is sufficient to induce the γ-H2AX biomarker while 8 mg/kg leads to enhanced tumor pharmacodynamic and regression responses relative to gemcitabine or SCH 900776 alone. Dose escalation of SCH 900776 (16 mg/kg and 32 mg/kg) induces incremental improvements in tumor response. Importantly, doses of SCH 900776 associate with robust biomarker activation and improved tumor response are not associated with enhanced toxicity of gemcitabine on hematological parameters in BALB/c mice. ^[1]

• Chk1 SPA assay

  An in vitro assay utilizing recombinant His-Chk1 expressed in the baculovirus expression system as an enzyme source and biotinylated peptide based upon CDC25C as substrate. His-Chk1 is diluted to 32 nM in kinase buffer containing 50 mM Tris pH 8.0, 10 mM MgCl₂, and 1 mM DTT. CDC25C (CDC25 Ser216 C-term biotinylated peptide) peptide is diluted to 1.93 μM in kinase buffer. For each kinase reaction, 20 μL of 32 nM Chk1 enzyme solution and 20 μL of 1.926 μM CDC25C are mixed and combined with 10 μL of SCH 900776 diluted in 10% DMSO, making final reaction concentrations of 6.2 nM Chk1, 385 nM CDC25C and 1% DMSO after addition of start solution. The reaction is started by addition of 50 μL of start solution consisting of 2 μM ATP and 0.2 μCi of ³³P-ATP, making a final reaction concentration of 1 μM ATP, with 0.2 μCi of ³³P-ATP per reaction. Kinase reactions run for 2 hours at room temperature and are stopped by the addition of 100 μL of stop solution consisting of 2 M NaCl, 1% H₃PO₄, and 5 mg/mL Streptavidin-coated SPA beads. SPA beads are captured using a 96-well GF/B filter plate and a Filtermate universal harvester. Beads are washed twice with 2 M NaCl and twice with 2 M NaCl with 1% phosphoric acid. Signal is then assayed using a TopCount 96-well liquid scintillation counter. Dose-response curves are generated from duplicate 8 point serial dilutions of SCH 900776. IC50 values are derived by nonlinear regression analysis.

• Cell lines

  mESCs

• Concentrations

  10 μM

• Incubation Time

  1 h

• Method

  Cells were treated with inhibitors or vehicle for 1 hour

• Animal Models

  Female nude mice injected subcutaneously with A2780 or MiaPaCa2 cells

• Dosages

  ~50 mg/kg

• Administration

  Administered intraperitoneally