Personalized cancer medicine approaches, inhibiting kinases in tumors driven by defined genomic alterations, have demonstrated striking efficacy in many cancer types. However, the development of drug resistance is a major limitation to the efficacy of targeted therapies in oncology. Identifying and understanding the molecular mechanisms driving resistance may foster opportunities to develop therapeutic strategies to overcome resistance. For example, in the ~10% of colorectal cancer patients whose tumors harbor BRAF V600 mutations, we have found that feedback reactivation of MAPK signaling, often mediated by EGFR, underlies the relative insensitivity of these cancers to RAF inhibitors. The development of targeted therapy combination strategies to block feedback reactivation of MAPK signaling has led to marked improvements in response rates for these patients from ~5% to >30% over the past few years. However, in patients who respond to these targeted combinations, the rapid development of acquired resistance still limits clinical benefit. Acquired resistance in BRAF mutant colorectal cancer and in other molecularly defined tumor types can be marked by the development of extensive molecular heterogeneity due to the selection of sub-clonal tumor cell populations, capable of growing under drug pressures, which poses significant diagnostic and therapeutic challenges. We will present data demonstrating how a single-lesion biopsy at disease progression to diagnose the mechanism of acquired resistance can vastly underrepresent the molecular heterogeneity of resistant tumor clones in an individual patient, and may fail to detect the existence of distinct but important resistance mechanisms that can drive mixed or lesion-specific responses and treatment failure to subsequent targeted therapy. By contrast, liquid biopsy approaches analyzing circulating tumor DNA have the potential to detect the presence of simultaneous resistance mechanisms residing in separate metastases in a single patient and to monitor the effects of subsequent therapies on specific sub-clonal tumor cell populations. These findings highlight the critical role of tumor heterogeneity in driving therapeutic resistance and how convergent targeted therapy strategies blocking common signaling nodes capable of overcoming multiple resistance mechanisms may be needed.

Citation Format: Ryan B. Corcoran. Overcoming drug resistance and tumor heterogeneity in gastrointestinal cancers [abstract]. In: Proceedings of the AACR International Conference held in cooperation with the Latin American Cooperative Oncology Group (LACOG) on Translational Cancer Medicine; May 4-6, 2017; São Paulo, Brazil. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(1_Suppl):Abstract nr IA15.