Effect of PIK3CA and KRAS mutations on sensitivity to ATP-competitive mTOR inhibitors in a primary xenograft model of colorectal carcinoma.

Abstract

483 Background: The mammalian target of rapamycin (mTOR) regulates cell growth by integrating nutrient and growth factor signaling and has been strongly implicated in cancer. Mutations in KRAS and the PI3K pathway are upstream of mTOR and are common in colorectal carcinoma (CRC). Currently approved mTOR inhibitors are derivatives of the natural product rapamycin and have shown little clinical efficacy in CRC. A new and potentially more efficacious class of ATP-competitive mTOR inhibitors has now entered clinical trials. Methods: Seeking a more representative preclinical model of CRC, we generated primary xenografts in nude mice of surgically resected specimens of human hepatic colorectal cancer metastases. We then treated xenograft tumors with the selective ATP-competitive mTOR inhibitor PP242 and monitored response by inhibition of tumor growth, changes in histopatholgy, and alterations in signaling pathways. Cell line experiments were performed to support observations made in the patient derived xenografts. Results: We demonstrate that in contrast to rapamycin, the mTOR inhibitor PP242 is highly effective at inhibiting tumor growth in both the primary xenograft model and in colorectal cancer cell lines. The inhibition of tumor growth in the xenografts and cell lines depended upon the strong inhibition of phosphorylation of mTOR substrate eIF4E binding protein 1 (4EBP1) but was not correlated with inhibition of phosphorylation of S6 kinase (S6K). Cells with mutant KRAS were relatively resistant to PP242 induced growth inhibition and this correlated with reduced inhibition of 4EBP1 phosphorylation. However, these effects were partially rescued in cells in which a co-mutation in PIK3CA resulted in AKT activation. Conclusions: Our studies reveal the first mTOR inhibitor resistant cell line, the genetic basis for its resistance and most importantly, these findings were revealed through the use of a primary xenograft mouse model which recapitulates morphologically the features of the tumors isolated from patients. We believe ATP-competitive inhibitors may be of limited clinical utility in mutant KRAS tumors, except for those that have concomitant activation of AKT.