In vitro activity: SB505124 is identified as a reversible ATP competitive and selective ALK inhibitor of ALK4 and ALK5. SB505124 shows no toxicity to renal epithelial A498 cells at concentrations up to 100 μM for 48 hours, and blocks TGF-β–induced apoptosis of FaO cells and NRP 154 cells in a concentration-dependent manner. In human umbilical vein endothelial cells (HUVEC), SB505124 (500 nM) blocks the changes of TGF-β1 on F-actin assembly and prevents ROS production induced by TGF-β. By inhibiting TGF-beta1 signaling, SB505124 leads to decreased deferoxamine (DFO)-induced neurogenesis. A recent study shows that SB505124 suppresses the migration and invasion of breast cancer MCF-7-M5 cells.

Kinase Assay: Kinase assays are performed as described by Laping et al., 2002 using the kinase domain of ALK5 and full-length N-terminal fused GST-Smad3. Kinase assays are performed with 65 nM GST-ALK5 and 184 nM GST-Smad3 in 50 mM HEPES, 5 mM MgCl2, 1 mM CaCl2, 1 mM dithiothreitol, and 3 μM ATP. Reactions are incubated with 0.5 μCi of [33P]γATP for 3 hours at 30 °C. Phosphorylated protein is captured on P-81 paper , washed with 0.5% phosphoric acid, and counted by liquid scintillation. Alternatively, Smad3 or Smad1 protein is also coated onto FlashPlate Sterile Basic Microplates. Kinase assays are then performed in FlashPlates with same assay conditions using either the kinase domain of ALK5 with Smad3 as substrate or the kinase domain of ALK6 (BMP receptor) with Smad1 as substrate. Plates are washed three times with phosphate buffer and counted by TopCount.

Cell Assay: Cell viability is measured as described by Laping et al., 2002 or by using the modified tetrazolium salt WST-1. XTT assay: The cells (A498, FaO and NRP 154 cells) are serum-deprived for 24 hours and then treated with SB505124 for 48 hours to assess the cellular toxicity. Cell viability is determined by incubating cells for 4 hours with XTT labeling and electron coupling reagent according to the manufacturer's directions. Live cells with active mitochondria produce an orange-colored product, formazan, which is detected using a plate reader at between A450 nm and A500 nm with a reference wavelength greater than 600 nm. The absorbance values correlate with the number of viable cells. Modified tetrazolium salt WST-1: Approximately 2000 cells are seeded in 96-well dishes in 100 μL of 0.2% FBS phenol red-free media overnight. The cells are treated with 50 μL of SB505124 (to achieve the final concentrations indicated) for 30 minutes before being treated with or without TGF-β1 and TNF-α to a final volume of 200 μL. Cell growth is measured at the indicated time points by incubating each well with 10 μL of WST-1 for 3 hours at 37 °C. Metabolically active cells cleave WST-1 to water-soluble formazan, which is directly quantitated with an enzyme-linked immunosorbent assay plate reader. Each experiment is done at least twice, and treatment for each cell line is done in triplicate.

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