PURPOSE: ALK rearrangements predict for sensitivity to ALK tyrosine kinase 
inhibitors (TKIs). However, responses to ALK TKIs are generally short-lived. 
Serial molecular analysis is an informative strategy for identifying genetic 
mediators of resistance. Although multiple studies support the clinical benefits 
of repeat tissue sampling, the clinical utility of longitudinal circulating 
tumor DNA analysis has not been established in ALK-positive lung cancer.
METHODS: Using a 566-gene hybrid-capture next-generation sequencing (NGS) assay, 
we performed longitudinal analysis of plasma specimens from 22 ALK-positive 
patients with acquired resistance to ALK TKIs to track the evolution of 
resistance during treatment. To determine tissue-plasma concordance, we compared 
plasma findings to results of repeat biopsies.
RESULTS: At progression, we detected an ALK fusion in plasma from 19 (86%) of 22 
patients, and identified ALK resistance mutations in plasma specimens from 11 
(50%) patients. There was 100% agreement between tissue- and plasma-detected ALK 
fusions. Among 16 cases where contemporaneous plasma and tissue specimens were 
available, we observed 100% concordance between ALK mutation calls. ALK 
mutations emerged and disappeared during treatment with sequential ALK TKIs, 
suggesting that plasma mutation profiles were dependent on the specific TKI 
administered. ALK G1202R, the most frequent plasma mutation detected after 
progression on a second-generation TKI, was consistently suppressed during 
treatment with lorlatinib.
CONCLUSIONS: Plasma genotyping by NGS is an effective method for detecting ALK 
fusions and ALK mutations in patients progressing on ALK TKIs. The correlation 
between plasma ALK mutations and response to distinct ALK TKIs highlights the 
potential for plasma analysis to guide selection of ALK-directed therapies.