Heterogeneity Underlies the Emergence of EGFRT790 Wild-Type Clones Following Treatment of T790M-Positive Cancers with a Third-Generation EGFR Inhibitor

Rociletinib is a third-generation EGFR inhibitor active in lung cancers with T790M, the gatekeeper mutation underlying most first-generation EGFR drug resistance. We biopsied patients at rociletinib progression to explore resistance mechanisms. Among 12 patients with T790M-positive cancers at rociletinib initiation, six had T790 wild-type rociletinib-resistant biopsies. Two T790 wild-type cancers underwent small cell lung cancer transformation; three T790M-positive cancers acquired EGFR amplification. We documented T790 wild-type and T790M-positive clones coexisting within a single pre-rociletinib biopsy. In fact, the pre-treatment fraction of T790M-positive cells impacted response to rociletinib. Longitudinal ctDNA analysis revealed an increase in plasma EGFR activating mutation and T790M heralded rociletinib resistance in some patients, while in others the activating mutation increased but T790M remained suppressed. Together, these findings demonstrate the role of tumor heterogeneity when therapies targeting a singular resistance mechanism are employed. To further improve outcomes, combination regimens that also target T790 wild-type clones are required.