Heterogeneity and Coexistence of T790M and T790 Wild-Type Resistant Subclones Drive Mixed Response to Third-Generation Epidermal Growth Factor Receptor Inhibitors in Lung Cancer

Purpose Third-generation epidermal growth factor receptor (EGFR) inhibitors like nazartinib are active against EGFR mutation–positive lung cancers with T790M-mediated acquired resistance to initial anti-EGFR treatment, but some patients have mixed responses. Methods Multiple serial tumor and liquid biopsies were obtained from two patients before, during, and after treatment with nazartinib. Next-generation sequencing and droplet digital polymerase chain reaction were performed to assess heterogeneity and clonal dynamics. Results We observed the simultaneous emergence of T790M-dependent and -independent clones in both patients. Serial plasma droplet digital polymerase chain reaction illustrated shifts in relative clonal abundance in response to various systemic therapies, confirming a molecular basis for the clinical mixed radiographic responses observed. Conclusion Heterogeneous responses to treatment targeting a solitary resistance mechanism can be explained by coexistent tumor subclones harboring distinct genetic signatures. Serial liquid biopsies offer an opportunity to monitor clonal dynamics and the emergence of resistance and may represent a useful tool to guide therapeutic strategies.