BACKGROUND: Anaplastic lymphoma kinase (ALK) is a validated molecular target in 
non-small-cell lung cancer (NSCLC). However, the clinical benefits of ALK 
inhibitors are almost universally limited by the emergence of drug resistance.
METHODS: We monitored the plasma circulating tumor DNA (ctDNA) using 
captured-based ultra-deep sequencing analysis of one patient with metastatic 
ALK-positive NSCLC who had received therapies including first-, second- and 
third-generation ALK inhibitors. Functional in vitro studies were further 
undertaken to elucidate the mechanism of resistance.
RESULTS: ALK T1151Sins mutation was detected when the patient developed 
resistance to ceritinib, and undetectable when she responded to lorlatinib. MET 
amplification was present when the tumor developed resistance to lorlatinib, and 
reduced when the patient received combination therapy of lorlatinib with 
crizotinib, which corresponded to clinical radiologic responses. In addition, 
further functional in vitro studies demonstrated that ALK harboring the 
T1151Sins mutation, while conferring resistance to ceritinib, was inhibited by 
lorlatinib.
CONCLUSIONS: Clinical evidence and in vitro validation revealed the clinical 
usefulness of captured-base ultra-deep sequencing on longitudinal plasma ctDNA 
in revealing the underlying resistance mechanism and guiding the precise 
administration of ALK inhibitors in patients with advanced ALK-positive NSCLC.