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Background. Despite afatinib as a new first-line treatment for EGFR L858R and exon 19 deletion or other rare EGFR-mutation patients, the acquired resistance or toxic effects associated with it limited its use clinically. The controlling of acquired resistance or optimization of the afatinib dosage in EGFR/T790M mutation-positive non-small-cell lung cancer (NSCLC) is still an important fundamental problem. Ethacrynic acid (EA) has been proved as a dual inhibitor of GST and WNT, and the α, β-unsaturated-keto structure of it is similar to that of irreversible tyrosine kinase inhibitors (TKIs). However, these beneficial effects of EA combined with afatinib have never been reported in NSCLC. Therefore, the antitumor effects of afatinib combined with EA in EGFR L858R/T790M-mutated NSCLC cells and related mechanisms were analyzed. Our in vitro and in vivo results showed that EA has strong synergistic antitumor effects with afatinib in EGFR L858R/T790M-mutated NSCLC cells, but has no cytotoxic effects in NSCLC cells when used it alone, i.e., the cytotoxic effects of afatinib (IC30) plus EA (IC30) were stronger than the effects of afatinib (IC50) alone. Our functional studies found that the antitumor mechanisms of afatinib when combined with EA mainly occurred by inhibiting WNT/β-catenin pathway activation and suppression of the secretion of anti-inflammatory factors. These results revealed that combination of afatinib with EA derivatives not only provided a new therapeutic approach for EGFR/T790M-mutated NSCLC patients but also offered a new idea for developing new drugs or optimizing the dose of afatinib in clinical use in future antitumor therapy. [ABSTRACT FROM AUTHOR]