Mechanisms mediating the negative interaction between oxaliplatin (Ox) and epidermal growth factor receptor (EGFR) inhibitors in patients (pts) with KRAS mutant (MT) colorectal cancer (CRC).

Abstract

3580 Background: Phase 3 trials have shown that pts with MT KRAS tumors have shorter progression-free survival and overall survival when an EGFR inhibitor is added to Ox-containing chemotherapy (CT) (Bokemeyer et al 2011, Douillard et al 2010). However, KRAS status alone is not predictive of pt outcomes in this setting. The negative interaction has not been observed in pts with wild-type (WT) KRAS tumors receiving an EGFR inhibitor and platinum-CT or with MT KRAS tumors receiving an EGFR inhibitor with irinotecan (Peeters et al 2010, Van Cutsem et al 2009). Our goal was to understand the pharmacodynamic interaction between EGFR inhibitors and Ox in KRAS MT CRC cells. Methods: Isogenic MT and WT KRAS-expressing HCT116 CRC cells were treated with Ox, SN-38 (the active metabolite of irinotecan), panitumumab, gefitinib, or inhibitors of MEK, PI3K or Src as single agents or in combination. Viability was measured using an ATPlite assay. Cellular distribution of EGFR was visualized by a confocal microscopy. Downstream activation of MAPK and PI3K was detected by Western blotting. Results: Treatment (tx) with gefitinib, but not panitumumab, reversed the anti-proliferative effects of Ox in the MT KRAS-expressing CRC cells vs Ox alone (p<0.0005) despite no differences in the number of platinum-DNA adducts. Neither tx reversed the anti-proliferative effects of Ox observed in the WT KRAS-expressing cells or SN-38 in either the MT or WT KRAS-expressing cells. EGFR was predominantly expressed in the cytoplasm of KRAS MT cells vs on the cell surface in the KRAS WT cells, which may explain the inability of panitumumab to reverse the anti-proliferative effects of Ox in the CRC cells. Combination tx of MT KRAS CRC cells with Ox and gefitinib increased pAKT which was not observed with Ox alone. Tx of the MT KRAS-expressing cells with a MEK, PI3K or Src inhibitor reversed the negative interaction between Ox and gefitinib (p<0.0005). Conclusions: Feedback though AKT may contribute to the increased resistance to Ox in KRAS MT cells. Inhibitors of MEK, PI3K, or Src reversed the negative interaction between the EGFR inhibitors and Ox in a MT-KRAS genetic background.