Effect of nab-rapamycin versus rapamycin in colorectal cancer cell lines and associations with KRAS and PI3K mutations.

Abstract

e13532 Background: The value of mTOR inhibitor therapy in metastatic colorectal cancer (mCRC) has not been established. Activation of the PI3K-AKT-mTOR pathway may be influenced by KRAS and PI3K mutations and may point to therapeutic subgroups for mTOR therapy. The effect of rapamycin and nab-rapamycin, nanoparticle albumin-bound rapamycin, was evaluated in colorectal cancer cell lines for which mutational status was determined. Methods: KRAS (codon 12,13) and PI3K (exon 9,20-1,20-2) mutation status was determined on cell lines (MIP101, RKO, HCT116 and SPARC-overexpressing MIP101 (MIP/SP) and on a tissue microarray (TMA) cohort of 96 patients with mCRC. Immunohistochemical staining was done on the TMA for mTOR, PTEN and pAKT. Univariate analyses were done to estimate association between KRAS, PI3K and protein expression. Cell viability assays were conducted with rapamycin and nab-rapamycin (0, 10, 50 and 100 nM) on CRC cell lines. Results: Among 96 cases included on the TMA, 35% were KRAS mutant and 6% were PI3K mutant. Four of the 5 PI3K mutations occurred in KRAS mutant tumors. There was no association between KRAS status and mTOR expression, or PI3K status and mTOR,pAKT or PTEN. Increasing expression of mTOR (0-3+) was modestly associated with increasing pAKT expression (rho = 0.37, p <0.001). In mutational testing of cell lines, 100% had a PI3K mutation and 100% had either a KRAS or BRAF mutation. Nab-rapamycin and unbound rapamycin were both effective in reducing cell viability in RKO, HCT116 and MIP/SP cells, whereas only nab-rapamycin significantly reduced cell viability in MIP101 cells (p<0.039). In MIP101 cells, nab-rapamycin, but not unbound rapamycin, resulted in a significant decrease levels of phosphorylated p70 S6 kinase. Conclusions: In this study, associations between PI3K, KRAS and IHC expression in the PI3K-AKT-mTOR pathway could not be established. CRC cell lines had a high rate of PI3K and KRAS/BRAF mutations not seen in human tumors. Nab-rapamycin reduces CRC cell viability and decreases downstream signaling.