We have previously demonstrated that other MEK inhibitors (PD098059, U0126, PD184161)

reduce ERK phosphorylation (MEK activity) and growth in human HCC cells.27, 29 PD0325901 is much more potent than these MEK inhibitors in HCC cells in vitro based on its median inhibitory concentration, which lies in the nanomolar range. In a recent study, Raf-1 small interfering RNA selleck chemicals llc (100 nM) caused a 50% decrease in phosphorylated ERK levels that was associated with a 50% decrease in HCC growth in vitro30 (and unpublished results). Similar results were obtained with ERK1,2 olignonucleotide anti-sense (300 nM) that decreased total ERK levels with a corresponding decrease in HCC cell growth.27 The effective dose and inhibitory effects of the small interfering RNA and anti-sense are comparable to that of

PD0325901 in HepG2 and Hep3B cells in vitro. Taken together, these results suggest that PD0325901 is a MEK inhibitor with absolute specificity. To investigate the efficacy of MEK inhibition in AZD4547 purchase a more clinically relevant model, TGF-α transgenic mice from which the TAMH line was derived were employed. These animals have a human TGF-α transgene that is specifically up-regulated in HCC tumors within the liver.31 The TGF-α transgenic mice are known to develop well-differentiated HCC in 70% of animals by 15 months of age.32 Indeed, studies of rat HCC show that preneoplastic regions in the liver grow at a threefold faster rate with up-regulation of TGF-α.33 Furthermore, because most human HCC tumors have an increased MCE amount of TGF-α present, the TGF-α transgenic mouse is believed to be a valid model of HCC for the current study.34, 35 Because TGF-α is a potent activator of the MEK-ERK pathway, these animals are ideal for treatment with a MEK inhibitor.26 In TGF-α transgenic mice at 30 weeks of age, we previously demonstrated an eightfold increase in P-ERK expression within HCC hepatocytes compared with normal hepatocytes. In addition, the ability of PD0325901 to decrease P-ERK within normal

hepatocytes in the treatment arm correlated with its ability to prevent HCC formation in this model.36 In the current randomized study, the incidence of carcinoma in the diethylnitrosamine accelerated transgenic model was determined. A 3.5-fold decrease in tumor incidence was seen in animals given the MEK inhibitor. We wanted to further examine the mechanism of PD0325901 and determine whether it was preventing formation, halting progression, or causing regression of established tumors in this developmental model. To achieve this, MRI confirmation of the presence of tumors was performed, and then treatment was initiated. After serial examinations, a dramatic regression in tumor volume was observed (Fig. 4). Taken together, the animals that were examined by MRI showed a difference in tumor volume approaching a threefold decrease in the MEK-inhibitor treated mice compared with the vehicle-treated mice (Fig. 5).