Examination of mRNA levels of PC9 parental, drug-tolerant, PC9-GR2 and PC9-GR3 cells after treatment with vehicle, gefitinib or WZ4002 for 24 hours.. Evolution of acquired resistance to EGFR inhibitors from drug tolerant cells

Acquired drug resistance to targeted cancer therapies remains a significant clinical problem. Although mechanisms of acquired resistance of EGFR mutant non-small cell lung cancers to EGFR inhibitors have been identified, little is known about how resistant clones evolve during drug therapy. Herein, we observe that acquired resistance caused by the T790M gatekeeper mutation can occur either by selection of pre-existing T790M clones or via evolution of T790M-negative drug tolerant cells that develop the mutation during drug treatment. Additionally, the path to resistance impacts the biology of the resistant clone, as those that evolved from drug tolerant cells have a diminished apoptotic response to third generation EGFR inhibitors that target T790M EGFR and can be sensitized by BCL-XL inhibitors. This observation is corroborated in cell cultures derived directly from resistant patient biopsies. These findings provide evidence that clinically relevant drug resistant cancer cells can both pre-exist and evolve from drug tolerant cells, and point to new therapeutic opportunities to prevent or overcome resistance in the clinic.