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The continued osimertinib plus chemotherapy overcoming resistance after histologic transformation in EGFR-mutant lung adenocarcinoma treated with osimertinib.

e20615 Background: Although the third-generation Epidermal Growth Factor Receptor (EGFR) tyrosine kinase inhibitor (TKI) osimertinib is more potent, histologic transformation from lung adenocarcinoma to other histological types is still a mechanism of resistance in EGFR-mutant adenocarcinoma. Those with histologic transformation usually have poor prognosis. Hence, we focused on the unique molecular mechanism of histologic transformation after osimertinib and explore the strategy of treatment. Methods: Pathology was confirmed in 55 EGFR-mutant patients treated with osimertinib at baseline and disease progression. They were all lung adenocarcinoma at baseline. We established the patient-derived xenograft (PDX) mouse model from an EGFR-mutant adenocarcinoma transforming to neuroendocrine carcinoma. Preclinical efficacy of erlotinib versus osimertinib was evaluated in vivo. All the clinical data of these patients were analyzed. Results: The frequency of histologic transformation after resistance to osimertinib was 14.5% (8/55). Three of them were transformed to small-cell lung cancer totally, two transformed to complex small-cell lung cancer, two transformed to adenosquamous cell carcinoma and one transformed to neuroendocrine carcinoma without T790M mutation in which an PDX mouse model was successfully established. The genetic profiles of those transformed to small-cell lung cancer were characterized by Rb1, TP53 mutations, and PI3K/AKT/MTOR aberrances. Those transformed to adenosquamous carcinoma were characterized by KRAS amplification and EGFR amplification. The median progression-free survival (PFS) was not significantly different between the patients with histologic transformation and those without during the treatment of osimertinib (7.7 VS. 5.7 months, P = 0.763). One went on first-line osimertinib, added etoposide/ cisplatin (EP) chemotherapy meanwhile after progressive disease, and achieved minor response after 2 cycles. The other one received the treatment of EP + osimertinib + erlotinib, and acheived partial response with a PFS of 7 months without obvious toxicities. The PDX model showed more sensitivity to erlotinib than osimertinib. Conclusions: Continued osimertinib plus chemotherapy might be effective in overcoming the resistance. Particularly, the first-generation EGFR-TKI seems to be efficacious in histologically-transformed EGFR-mutant patients without T790M mutation. Further investigations of these patients and PDXs are warranted.