Background: Small cell lung cancer (SCLC) is a disease with a poor prognosis. Besides combination strategy with immune checkpoint inhibitors (ICIs) and cytotoxic agents, antibody-drug conjugates (ADCs) and bispecific T-cell engagers (BiTE) are being developed. These drugs target tumor-associated surface protein. Whether prior treatment affect the transition of protein expression has not been fully investigated especially in the era of ICIs. The purpose of this study is to evaluate changes in protein expression in SCLC tissues before and after treatment. Methods: We included patients diagnosed with SCLC by surgery or biopsy and underwent re-biopsy after treatment (non-transformed group) at the National Cancer Center Hospital between 2014 and 2023. Patients who were initially diagnosed with non-small cell lung cancer and underwent transformation to SCLC (transformed group) were also included. Tissues were obtained by surgery, bronchoscopy, or CT-guided biopsy with an interval of at least 100 days. We investigated clinicopathological features in paired specimens focusing on DLL3, ASCL1, NEUROD1, POU2F3, and YAP1, which have been reported to be expressed in SCLC, as well as tumor-associated surface proteins potentially related to novel drug development. Results: Fourteen non-transformed group cases and 9 transformed group cases were included. Of these, pathology specimens were evaluable in a total of 15 cases: 9 for the non-transformed group and 6 for the transformed group. The median age was 68 years, 11 (73%) were male, 11 (73%) were smokers, 8 (53%) were in clinical stages I-IIIA, 1 (7%) in stages IIIB and 6 (40%) in stage IV. Among the transformed group, 4 of them were treated by Osimertinib before transformation. Progression free survival from first-line therapy was 298 days (255-446) vs. 314 days (245-NA) and overall survival was 1239 days (567-NA) vs. NA days (517-NA), respectively for non-transformed group and transformed group. For both these group of cases, we have evaluated several protein expressions as described above. The transition of these proteins within each group is presented in the conference. Conclusions: In this study, we focused not only on proteins already reported to be expressed in SCLC, but also on proteins that are potential new therapeutic targets and evaluated changes in protein expression.

TKI: tyrosine kinase inhibitors, ICI: immune checkpoint inhibitors, Osi: Osimertinib.