Abstract B110: Heterogeneous genetic alterations emerge during acquired resistance to anti-EGFR therapy in colorectal cancer

Occurence of acquired resistance constitutes one of the major limitations to tumor treatment. In colorectal cancer, the use of anti-EGFR antibodies such as cetuximab and panitumumab is effective in only 15-20% of patients, till secondary resistance to targeted treatment develops. Shedding light on the molecular basis of resistance represents then a mandatory effort in order to foster new lines of therapy. We recently showed that KRAS mutations are responsible for the emergence of secondary resistance to EGFR therapy in a subset of CRC patients (Misale et al., Nature 2012). Here we investigated the role of additional genetic alterations in the development of resistance to EGFR blockade in CRCs. We initially exploited the “liquid biopsy” approach to measure tumor derived DNA mutations in the blood of patients who initially responded and then became refractory to either cetuximab or panitumumab. We found that multiple KRAS and NRAS alleles often emerge during treatment. To better characterize the role of these genetic alterations in CRC, we generated preclinical models, by chronic treatment of several CRC cell lines with cetuximab or panitumumab until resistant derivatives emerged. The resistant populations were a mixture of clones bearing an heterogeneous panel of genetic alterations in KRAS, NRAS and BRAF at various frequencies. Biochemical pathway analyses revealed that, regardless of the genetic alterations, resistant derivatives consistently displayed MEK and ERK activation, which persisted on EGFR inhibition. Collectively, these results identify genetically distinct mechanisms that mediate secondary resistance to anti-EGFR therapies, all of which converge on the reactivation of ERK signaling. With this work, we provide a rational strategy to overcome the multifaceted clonal heterogeneity that emerges when tumors are treated with anti-EGFR drugs; we propose that MEK inhibitors, in combination with cetuximab or panitumumab, should be considered for the treatment of CRC patients who become refractory to anti-EGFR therapies.