In vitro activity: PF299804 is a specific inhibitor of the ERBB family of kinases. PF299804 inhibits EGFR signaling and induces apoptosis in the EGFR T790M-containing H3255 GR cell line. PF299804 is effective in gefitinib-sensitive and gefitinibresistant NSCLC cell lines. PF299804 inhibits the growth of H3255 and HCC827 cells engineered to express EGFR T790M. PF299804 inhibits EGFR phosphorylation in the presence of the T790M mutation. PF-299804 is believed to irreversibly inhibit ERBB tyrosine kinase activity through binding at the ATP site and covalent modification of nucleophilic cysteine residues in the catalytic domains of ERBB family members. PF299804 shows significant growth-inhibitory effects in HER2-amplified gastric cancer cells (SNU216, N87), and it has lower 50% inhibitory concentration values compared with other EGFR tyrosine kinase inhibitors, including gefitinib, lapatinib, BIBW-2992, and CI-1033. PF299804 induces apoptosis and G1 arrest and inhibits phosphorylation of receptors in the HER family and downstream signaling pathways including STAT3, AKT, and extracellular signal-regulated kinases (ERK) in HER2-amplified gastric cancer cells. PF299804 also blocks EGFR/HER2, HER2/HER3, and HER3/HER4 heterodimer formation as well as the association of HER3 with p85α in SNU216 cells. A recent research uses forty-seven human breast cancer and immortalized breast epithelial lines to evaluate the inhibition effects of PF299804, the results indicate PF299804 preferentially inhibits growth of HER-2-amplified breast cancer cell lines than nonamplified lines (RR = 3.39, p < 0.0001). PF299804 reduces the phosphorylation of HER2, EGFR, HER4, AKT, and ERK in the majority of sensitive lines. PF299804 exerts its anti-proliferative effect through a combined G0/G1 arrest and an induction of apoptosis.

Kinase Assay: The ERBB1, ERBB 2, and ERBB4 cytoplasmic fusion proteins are made by cloning the ERBB1 sequence (Met-668 to Ala-1211), ERBB2 (Ile-675 to Val-1256), and ERBB4 sequence (Gly-259 to Gly-690) into the baculoviral vector pFastBac using PCR. Proteins are expressed in baculovirusinfected Sf9 insect cells as GST fusion proteins. The proteins are purified by affinity chromatography using glutathione sepharose beads. Inhibition of ERBB tyrosine kinase activity is assessed using an ELISA-based receptor tyrosine kinase assay. Kinase reactions (50 mM HEPES, pH 7.4, 125 mM NaCl, 10 mM MgCl2, 100 μM sodium orthovanadate, 2 mM dithiothreitol, 20 μM ATP, PF299804 or vehicle control, and 1-5 nM GST-erbB per 50 μL of reaction mixture) are run in 96-well plates coated with 0.25 mg/mL poly-Glu-Tyr. The reactions are incubated for 6 minutes at room temperature while being shaken. Kinase reactions are stopped by removal of the reaction mixture, and then the wells are washed with wash buffer (0.1% Tween 20 in PBS). Phosphorylated tyrosine residues are detected by adding 0.2 μg/mL antiphosphotyrosine antibody (Oncogene Ab-4; 50 μL/well) coupled to horseradish peroxidase (HRP) diluted in PBS containing 3% BSA and 0.05% Tween 20 for 25 minutes while being shaken at room temperature. The antibody is removed, and plates are washed in wash buffer. HRP substrate (SureBlue3,3,5,5-tetramethyl benzidine or TMB) is added (50 μL per well) and incubated for 10-20 minutes while it is shaken at room temperature. The TMB reaction is stopped with the addition of 50 μL of stop solution (0.09 N H2SO4). The signal is quantified by measuring absorbance at 450 nm. IC50 values are determined for PF299804 using the median effect method.

Cell Assay: Growth and inhibition of growth is assessed by 5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. This assay, a colorimetric method fordetermining the number of viable cells, is based on the bioreduction of MTS by cells to a formazan product that is soluble in cell culture medium, can be detected spectrophotometrically. The cells are exposed totreatment for 72 hours, and the number of cells used per experiment is determined empirically. All experimental points are set up in 6 to 12 wells, and all experiments are repeated at least thrice. The data are graphically displayed using GraphPad Prism version 3.00 for Windows (GraphPad Software). The curves are fitted using a nonlinear regression model with a sigmoidal dose response.