Acquired resistance to AZD9291 as an upfront treatment is dependent on ERK signaling in a preclinical model.

AZD9291 (osimertinib) is approved for standard care in patients with EGFR T790M-positive non-small cell lung cancer (NSCLC) after prior EGFR TKI progression. Furthermore, AZD9291 is now being evaluated as a first-line treatment for NSCLC patients with activation EGFR mutations. Based on previous experiments, resistance to AZD9291 as a first-line treatment may also emerge. Thus, identification and understanding of resistance mechanisms to AZD9291 as a first-line treatment can help direct development of future therapies. AZD9291-resistant cells (PC9/AZDR) were established using EGFR inhibitor-naïve PC9 cells. Resistance mechanisms were analyzed using next-generation sequencing (NGS) and a proteome profiler array. Resistance to AZD9291 developed through aberrant activation of ERK signaling by an EGFR-independent mechanism. The combination of a MEK inhibitor with AZD9291 restored the sensitivity of PC9/AZDR cells in vitro and in vivo. PC9/AZDR cells also showed increased MET expression and an HRAS G13R mutation. In addition, maspin expression was higher after AZD9291 treatment in PC9/AZDR cells. Sustained ERK activation confers resistance to AZD9291 as a first-line therapy. Thus, co-targeting EGFR and MEK may be an effective strategy to overcome resistance to AZD9291.