SCH900776 S-isomer (MK-8776 S-isomer) is an effective, specific and orally bioavailable inhibitor of checkpoint kinase Chk1 (IC50: 3 nM). It also inhibitors Chk2 (IC50: 1500 nM) and cyclin-dependent kinase CDK2 (IC50: 160 nM).

CDK2:160 nM, Chk2:1500 nM, Chk1:3 nM

SCH 900776 does not significantly inhibit HLMs CYP450 isoforms 1A2, 2C9, 2C19, 2D6, and 3A4. SCH 900776 induces a dose-dependent loss of DNA replication capability 24 hours after hydroxyurea exposure. 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. SCH 900776 also dose-dependently suppresses accumulation of the Chk1 pS296 autophosphorylation.

After pretreatment 30 minutes with gemcitabine, SCH 900776 (4 mg/kg) 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. SCH 900776 (16/32 mg/kg) dose-dependently induces improvements in tumor response. 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.