In vitro activity: Lomeguatrib inactivates O6-alkylguanine-DNA-alkyltransferase (ATase) with a IC50 10-fold lower than O6-Benzylguanine. Lomeguatrib inhibits the activity of ATase in Raji cells with IC50 of 10 nM. Lomeguatrib effectively inactivates MGMT in MCF-7 cells with IC50 of ~6nM ). Lomeguatrib (10 μM ) substantially increases the growth inhibitory effects of temozolomide in MCF-7 cells (D60= 10 μM with Lomeguatrib vs 400 μM without).

Kinase Assay: Briefly, 200 μg of extracted cellular protein from HeLaS3 cells in 200 μL of 70 mM HEPES buffer (with 1 mM dithiothreitol (DTT), 5 mM EDTA, pH 7.8) is incubated at 37°C with a defined concentration of Lomeguatrib (added as a DMSO solution). After 30 min an excess of [3H]-methylated DNA(120 000 cpm) is added, and the incubation is continued for an additional 90 min. The reaction is stopped by the addition of 400 μL TCA (13%), and the DNA is hydrolyzed by heating the reaction mixture for 30 min at 98°C. The precipitated protein is washed three times with 400-μL portions of 5% TCA, solubilized in 0.1 N NaOH, and analyzed by liquid scintillation counting using the cocktail Rotiszint eco plus and a TRI-CARB. Enzyme activity is expressed as fmol of [3H]methyl transferred to TCA-insoluble protein material per mg of total cellular protein. Percent inhibition is calculated relative to untreated control samples. Each assay is repeated three times, and IC50 values are determined graphically from plots of percent inhibition vs inhibitor concentration

Cell Assay: To determine toxicity, the MTT growth inhibition assay is employed. Cells (1000 per well) are plated into a 96-well plate and following a 24 hattachment period, Lomeguatrib is added to the cells. After 2 h incubation with Lomeguatrib (10 μM) at 37°C, 5% CO2, increasing doses of temozolomide or vehicle are added and the cells are incubated for a further 4-5 days. At the end of the exposure period, 150 μg MTT is added to each well and plates are incubated for 3 h at 37°C, 5% CO2. The media are removed and the formazan crystals formed in the viable cells are solubilised in 200 μL DMSO. The absorbances at 540 and 690 nm are determined using a ELISA plate reader and growth inhibition calculated as a percentage of the A540-A690 of untreated wells.