In vitro activity: CCT245737 is a potent inhibitor of recombinant human CHK1 with IC50 of 1.4±0.3 nM (mean±SD, n = 3, EZ Reader II assay). There is> 1,000-fold selectivity for CHK1 versus the functionally important kinases CDK1 and CHK2 (IC50=1.26-2.44 and 9.03 μM, respectively), and at least a 90-fold selectivity against cross-reacting kinases such as ERK8, PKD1, RSK1 and 2. CCT245737 potently inhibits cellular CHK1 activity (IC50 30-220nM) and enhances gemcitabine and SN38 cytotoxicity in multiple human tumor cell lines and human tumor xenograft models. It can abrogate an etoposide-induced G2/M arrest. CCT245737 has high cell permeability, as measured by transport across a CaCo2 cell monolayer.

Kinase Assay: CCT245737 shows<50% 10='' 121='' inhibition='' at='' m='' for='' representing='' a='' selectivity='' chk1='' of=''>5000-fold over these enzymes. Commercial in vitro 33P radiometric kinase assays are carried out against 124 human kinases using 10 μM CCT245737 at ATP concentrations corresponding to the kinase Km, ATP. Other kinase IC50 determinations for CHK2 and FLT3 are performed using a commercial assay or in-house with recombinant human CHK1 on a LabChip(R) EZ Reader II or CDK1 in a DELFIA assay.

Cell Assay: Cytotoxicity is determined as the drug concentration that gave 50% inhibition of tumor cell proliferation (GI50) using a 96h (i.e. 4-doublings) Sulforhodamine B (SRB) assay. Inhibition of intracellular CHK1 activity is measured using a cell based ELISA for the abrogation of an etoposide induced G2 checkpoint (mitosis induction assay, MIA).