SCH900776 (MK-8776) is an agent targeting cell cycle checkpoint kinase 1 (Chk1) with potential radiosensitization and chemosensitization activities.

Chk1:3 nM, CDK2:0.16 μM

Administering 4 mg/kg of SCH 900776 30 minutes after gemcitabine triggers the γ-H2AX biomarker, while a dose of 8 mg/kg of SCH 900776 boosts tumor pharmacodynamics and degradation response. Incremental increases to 16 mg/kg and 32 mg/kg of SCH 900776 further improve tumor response. Notably, within BALB/c mice, the dose of SCH 900776 correlates with strong biomarker activation, independent of gemcitabine's intensified hematologic toxicity.

After 24 hours of exposure to hydroxyurea, SCH 900776 induces a dose-dependent loss of DNA replication capacity. SCH 900776 enhances the response of γ-H2AX to hydroxyurea, 5-fluorouracil, and cytarabine. By combining with antimetabolites, SCH 900776 triggers the accumulation of γ-H2AX within 2 hours, indicating replication fork collapse and subsequent double-strand DNA breaks. Additionally, SCH 900776 inhibits the accumulation of Chk1 pS296 autophosphorylation in a dose-dependent manner. SCH 900776 is a weak inhibitor of Chk2 and CDK2, with IC50 values of 1.5 μM and 0.16 μM, respectively. SCH 900776 does not significantly inhibit human liver microsomal cytochrome P450 subtypes 1A2, 2C9, 2C19, 2D6, and 3A4.

The Millipore Kinase Profiler service is used to generate general selectivity data for SCH 900776 against a broad range of serine/threonine and tyrosine kinases. Assays are typically run at two concentrations of SCH 900776 (0.5 and 5 μM), at a fixed (10 μM) concentration of ATP. Data are provided as percent activity remaining, relative to uninhibited controls.

For cell growth assays, cells are seeded at low density (500-1000 cells) in 96-well plates and then incubated with drug for 24 h (8 wells per concentration). Following treatment, cells are washed and grown in fresh media for 5-7 days at 37°C. Prior to attaining confluence, cells are washed, lysed, and stained with Hoechst 33258. Fluorescence is read on a microplate spectrofluorometer. Results are expressed as mean and standard error for the concentration of drug that inhibited growth by 50%.