SB-525334 (6-[2-tert-butyl-5-(6-methyl-pyridin-2-yl)-1H-imidazol-4-yl]-
quinoxaline) has been characterized as a potent and selective inhibitor of the
transforming growth factor-beta1 (TGF-beta1)
receptor, activin receptor-like
kinase (ALK5). The compound inhibited ALK5
kinase activity with an IC(50) of 14.3 nM and was approximately 4-fold less potent as an inhibitor of ALK4 (IC(50) = 58.5 nM).
SB-525334 was inactive as an inhibitor of ALK2, ALK3, and ALK6 (IC(50) > 10,000 nM). In cell-based assays,
SB-525334 (1 microM) blocked TGF-beta1-induced phosphorylation and nuclear translocation of Smad2/3 in renal proximal tubule cells and inhibited TGF-beta1-induced increases in
plasminogen activator inhibitor-1 (PAI-1) and
procollagen alpha1(I)
mRNA expression in A498 renal epithelial
carcinoma cells. In view of this profile,
SB-525334 was used to investigate the role of
TGF-beta1 in the acute
puromycin aminonucleoside (PAN) rat model of renal disease, a model of
nephritis-induced renal
fibrosis. Orally administered doses of 1, 3, or 10 mg/kg/day
SB-525334 for 11 days produced statistically significant reductions in renal
PAI-1 mRNA. Also, the compound produced dose-dependent decreases in renal
procollagen alpha1(I) and
procollagen alpha1(III)
mRNA, which reached statistical significance at the 10-mg/kg/day dose when compared with vehicle-treated PAN controls. Furthermore, PAN-induced
proteinuria was significantly inhibited at the 10-mg/kg/day dose level. These results provide further evidence for the involvement of
TGF-beta1 in the profibrotic changes that occur in the PAN model and for the first time, demonstrate the ability of a small molecule inhibitor of ALK5 to block several of the markers that are predictive of
fibrosis and renal injury in this model.