Levoleucovorin Calcium (CL307782), a calcium salt of the folinic acid, is used in cancer chemotherapy as an adjuvant.

Chk1 Assay: For the Chk1 assay, the kinase domain is expressed in Sf9 insect cells, and a biotinylated cdc25c peptide containing the consensus Chk1/Chk2 phosphorylation site (*)(biotin-[AHX]SGSGS*GLYRSPSMP-ENLNRPR[CONH2]) is used as the substrate. A dilution series of CHIR-124 is mixed with a kinase reaction buffer containing a final concentration of 30 mM Tris-HCl (pH 7.5), 10 mM MgCl2, 2 mM DTT, 4 mM EDTA, 25 mMβ-glycerophosphate, 5 mM MnCl2, 0.01% bovine serum albumin, 1.35 nM CHK1 kinase domain, 0.5 μM peptide substrate, and 1 AM unlabeled ATP, plus 5 nM 33Pγ-labeled ATP (specific activity = 2,000 Ci/mmol). Reactions and detection of the phosphate transfer are carried out by a radioactive method. Reactions are incubated at room temperature for 1 to 4 hours and the phosphorylated peptide captured on streptavidin-coated microtiter plates containing stop reaction buffer (25 mM EDTA [ethylenediaminetetraacetic acid], 50 mMHEPES, pH 7.5). Phosphorylated peptide is measured with the DELFIA TRF system using a Europium-labeled anti-phosphotyrosine antibody PT66. The concentration of CHIR-124 for IC50 is calculated using nonlinear regression with XL-Fit data analysis software.