Different response to ALK inhibitors in EML4-ALK positive mediastinal cancer of unknown primary.

104 Background: With the development of next-generation sequencing (NGS) and precision medicine, targeted therapy especially molecule-driven therapy irrespective of tumor site may play an important role in cancer patients with druggable targets. Cancer of unknown primary (CUP) especially mediastinal CUP (MCUP) has poor prognosis with empirical chemotherapy but may harbor targetable genetic alterations. In the study, we evaluate the response to ALK inhibitors in ALK-positive MCUP. Methods: We queried the cancer database of the Second Xiangya hospitalfrom 2018.10 to 2019.12 for patients with ALK-positive cancers regardless of tumor site.Then we selected ALK-positive MCUP patients treated with ALK inhibitors, on which both NGS and programmed death-ligand 1 (PD-L1) testing were performed. Results: Forty-eightpatients with ALK-positive cancers were registered in the database. Two MCUP patients were found and showed different response. The first MCUP patient obtaining remarkable response to alectinib harbored ALK/TP53 co-mutation and TERT wild type accompanied by low PD-L1 expression, while the second one showing stable disease with crizotinib carried ALK/TP53/TERT co-mutation and high PD-L1 expression. Conclusions: We reveal for the first time that ALK inhibitors are applied to MCUP as first-line therapy and alectinib is firstly reported to treat MCUP patients. ALK inhibitor alectinib is expected to improve outcomes of patients with ALK-positive MCUP. NGS and PD-L1 testing should be recommended for MCUP to get more information about therapy and efficacy. [Table: see text]