Early radiotherapy delays acquired gefitinib resistance in EGFR-mutant NSCLC by suppressing c-FOS-associated survival signaling.

BACKGROUND: Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) have been widely used in the treatment of non-small cell lung cancer (NSCLC) harboring EGFR-sensitive mutations. However, acquired resistance remains a major clinical challenge. In a previous phase II clinical trial conducted by our group, early radiotherapy significantly delayed acquired resistance to EGFR-TKIs. Nevertheless, the optimal timing of radiotherapy and the underlying molecular mechanisms remain unclear.

METHODS: We compared gefitinib alone with gefitinib plus radiotherapy in EGFR-mutant NSCLC xenograft models and evaluated early versus late radiotherapy intervention. RNA sequencing of xenograft tumors was performed to identify timing-associated mediators of radiotherapy-delayed gefitinib resistance. Based on these findings, we used gefitinib-resistant NSCLC cell lines, radiotherapy, c-FOS perturbation, and pathway inhibition assays to validate the underlying mechanisms and assess treatment efficacy in vitro.

RESULTS: Gefitinib combined with radiotherapy showed enhanced antitumor activity in the xenograft model. Compared with gefitinib alone, early radiotherapy reduced tumor volume by approximately 70%-75% at the experimental endpoint, whereas late radiotherapy reduced tumor volume by approximately 50%. Early radiotherapy was associated with improved survival during the observation period compared with late radiotherapy; notably, median survival was not reached in the early radiotherapy group during follow-up. RNA-seq identified c-FOS as a timing-associated differentially expressed gene in xenograft tumors. In vitro, radiotherapy enhanced the sensitivity of gefitinib-resistant cells and reduced c-FOS expression. Mechanistically, c-FOS promoted survival and proliferation while inhibiting apoptosis in resistant cells, in association with increased PI3K/AKT and JNK/p38 signaling activity, thereby contributing to gefitinib resistance.

CONCLUSIONS: This study indicates that early radiotherapy delays acquired gefitinib resistance, at least in part, by suppressing c-FOS in the tested model system. c-FOS may serve as a candidate biomarker for radiotherapy timing and as a hypothesis-generating therapeutic target for combination strategies, although further validation in other EGFR-mutant subtypes and with later-generation EGFR-TKIs is required.