Consistent with this earlier finding, we show here that PP242 BI-D1870 S6 Kinase? inhibitor treatment leads to the incomplete inhibition of 4E-BP1 phosphorylation in colorectal cells. 4E-BP1 is a key effector of the PI3K and mTOR pathway and the incomplete inhibition by PP242 of the 4E-BP1 phosphorylation contributes to the carcinoma resistance to PP242 treatment. The results reported here indicate that PP242 only transiently inhibits mTORC2 kinase activity in colorectal carcinoma cells. Early studies show that PP242 treatment inhibits the AKT phosphorylation at S473, the mTORC2 phosphorylation site on AKT, in colorectal carcinoma cells at 12 hour and pancreatic carcinoma cells at 2 hour of the treatment. In a time course experiment, however, we show that the phosphorylation of AKTS473 resumes gradually up to the untreated cell levels within 24 hours of PP242 treatment and inhibition of PI3K fails to block the AKTS473 phosphorylation. In contrast, rapamycin does not inhibit the AKTS473 phosphorylation R428 side effects through the time course, keeping in line with the notion that rapamycin does not inhibit mTORC2 activity. Our results demonstrate for the first time that the second generation of mTOR kinase inhibitor PP242 can inhibit mTORC2 activity but the inhibition is incomplete and transient in colorectal carcinoma cells. Another salience of our results is that PP242 treatment leads to the increase of EGFR phosphorylation through PI3Kindependent pathway in colorectal carcinoma cells. EGFR can activate downstream PI3K and ERK pathway and thus drives cancer progression. While it remains to be investigated, recent findings that PP242 treatment activates ERK pathway in multiple myeloma and pancreatic carcinoma cells suggest the possibility that the mTOR kinase inhibitor PP242 may activate ERK pathway through the EGFR activation. The feedback activation of PI3K-AKT and ERK pathway undermine the usefulness of the first generation of mTOR inhibitors in clinical treatment of cancers. The data presented here in study of colorectal carcinoma, together with the data in studies of myeloma and pancreatic carcinoma cells suggest the presence of feedback activation of ERK for the second generation of mTOR kinase inhibitors perhaps due the incomplete inhibition of mTORC2 activity and thus raise the concern on whether PP242 is effective in clinical treatment of cancers as single agents. The study presented here provides several lines of evidence in support of the notion that the combination of mTOR kinase and EGFR inhibitors may provide strategy in treatment of colorectal carcinoma. First, the combination treatment of erlotinib and PP242 results in the complete inhibition of mTORC1 and mTORC2 activity in the carcinoma cells.