The Effects of a Novel MEK Inhibitor PD184161 on MEK–ERK Signaling and Growth in Human Liver Cancer1

The MEK–ERK growth signaling pathway is important in human hepatocellular carcinoma (HCC). To evaluate the targeting of this pathway in HCC, we characterized a novel, orally-active MEK inhibitor, PD184161, using human HCC cells (HepG2, Hep3B, PLC, and SKHep) and in vivohuman tumor xenografts. PD184161 inhibitedMEK activity (IC50 = 10–100 nM) in a time- and concentration-dependent manner more effectively than PD098059 or U0126. PD184161 inhibited cell proliferation and induced apoptosis at concentrations of –> 1.0 MM in a time- and concentration-dependent manner. In vivo, tumor xeno-graftP-ERKlevelsweresignificantly reduced3 to12hours after an oral dose of PD184161 (P <.05). Contrarily, tumor xenograft P-ERK levels following long-term (24 days) daily dosingof PD184161were refractory to this signaling effect. PD184161 significantly suppressed tumor engraft-ment and initial growth (P <.0001); however, established tumors were not significantly affected. In conclusion, PD184161hasantitumoreffects inHCC in vitroand in vivo that appear to correlate with suppression of MEK activity. These studies demonstrate that PD184161 is unable to suppressMEKactivity inHCCxenografts in the long term. Thus, we speculate that the degree of success of MEK-targeted treatment in HCC and other cancersmay, in part, depend on the discovery of mechanisms governing MEK inhibitor signaling resistance