In the setting of recent exciting clinical results and numerous ongoing trials, Piotrowska and colleagues explore mechanisms of acquired resistance to the mutant-specific EGFR inhibitor rociletinib, and demonstrate that loss of T790M, EGFR amplification, and small-cell transformation are all clinically relevant mechanisms of drug resistance. The authors provide a new paradigm for using quantitative assessment of the EGFR T790M:activation mutation allele frequency ratio to prognosticate responses to rociletinib and also demonstrate that plasma-based assessments of circulating tumor DNA can be used to monitor drug response and the emergence of drug resistance.
©2015 American Association for Cancer Research.