Vertical Pathway Inhibition Overcomes Adaptive Feedback Resistance to KRASG12C Inhibition.

PURPOSE: Although KRAS represents the most commonly mutated oncogene, it has long been considered an "undruggable" target. Novel covalent inhibitors selective for the KRASG12C mutation offer the unprecedented opportunity to target KRAS directly. However, prior efforts to target the RAS-MAPK pathway have been hampered by adaptive feedback, which drives pathway reactivation and resistance. EXPERIMENTAL DESIGN: A panel of KRASG12C cell lines were treated with the KRASG12C inhibitors ARS-1620 and AMG 510 to assess effects on signaling and viability. Isoform-specific pulldown of activated GTP-bound RAS was performed to evaluate effects on the activity of specific RAS isoforms over time following treatment. RTK inhibitors, SHP2 inhibitors, and MEK/ERK inhibitors were assessed in combination with KRASG12C inhibitors in vitro and in vivo as potential strategies to overcome resistance and enhance efficacy. RESULTS: We observed rapid adaptive RAS pathway feedback reactivation following KRASG12C inhibition in the majority of KRASG12C models, driven by RTK-mediated activation of wild-type RAS, which cannot be inhibited by G12C-specific inhibitors. Importantly, multiple RTKs can mediate feedback, with no single RTK appearing critical across all KRASG12C models. However, coinhibition of SHP2, which mediates signaling from multiple RTKs to RAS, abrogated feedback reactivation more universally, and combined KRASG12C/SHP2 inhibition drove sustained RAS pathway suppression and improved efficacy in vitro and in vivo. CONCLUSIONS: These data identify feedback reactivation of wild-type RAS as a key mechanism of adaptive resistance to KRASG12C inhibitors and highlight the potential importance of vertical inhibition strategies to enhance the clinical efficacy of KRASG12C inhibitors.See related commentary by Yaeger and Solit, p. 1538.

Convergent Therapeutic Strategies to Overcome the Heterogeneity of Acquired Resistance in BRAFV600E Colorectal Cancer.

Clonal heterogeneity associated with acquired resistance presents a critical therapeutic challenge. Whole-exome sequencing of paired tumor biopsies and targeted sequencing of cell-free DNA (cfDNA) from patients with BRAFV600E colorectal cancer receiving BRAF inhibitor combinations identified 14 distinct alterations in MAPK pathway components driving acquired resistance, with as many as eight alterations in a single patient. We developed a pooled clone system to study clonal outgrowth during acquired resistance, in vitro and in vivoIn vitro, the dynamics of individual resistant clones could be monitored in real time in cfDNA isolated from culture media during therapy. Outgrowth of multiple resistant clones was observed during therapy with BRAF, EGFR, and MEK inhibitor combinations. However, ERK inhibition, particularly in combination with BRAF and EGFR inhibition, markedly abrogated clonal outgrowth in vitro and in vivo Thus, convergent, up-front therapy may suppress outgrowth of heterogeneous clones harboring clinically observed resistance alterations, which may improve clinical outcome.Significance: We observed heterogeneous, recurrent alterations in the MAPK pathway as key drivers of acquired resistance in BRAFV600E colorectal cancer, with multiple concurrent resistance alterations detectable in individual patients. Using a novel pooled clone system, we identify convergent up-front therapeutic strategies capable of intercepting multiple resistance mechanisms as potential approaches to suppress emergence of acquired resistance. Cancer Discov; 8(4); 417-27. ©2018 AACR.See related commentary by Janku, p. 389See related article by Corcoran et al., p. 428This article is highlighted in the In This Issue feature, p. 371.