Ret Rearrangements Detected By Fish In "Pan-Negative" Lung Adenocarcinoma.

8024 Background: RET rearrangements have recently been reported in NSCLC and there is pre-clinical evidence that RET tyrosine kinase inhibitors (TKIs) block activated RET kinase. Efficient and accurate detection of RET rearrangements are crucial for the success of RET TKIs in clinical trials. RET rearrangements have been detected by specialized sequencing techniques with limited applicability to clinical practice. Methods: A 3-target FISH probe set was developed to detect KIF5B-RET fusions and identify patterns suggestive of RET rearrangements with non-KIF5B partners. 51 lung adenocarcinomas negative for EGFR, KRAS, ALK and ROS1 (36 were also negative for 7 other molecular markers) were investigated. Clinical and demographic characteristics were collected. Results: Eight patients (15%) had rearrangements in the RET gene: 5 with KIF5B-RET fusions, 2 with patterns consistent with the CCDC6-RET fusion, and 1 with extra copies of single 3’RET (loss of 5’RET). Atypical FISH patterns were detected both in RET + and negative specimens suggesting high genomic instability in the KIF5B – RET region. RT-PCR assay determined the exon/fusion variant in 4 cases including 2 patients with K15:R12, 1 with K16:R12 and 1 with C1:R12. Median age at diagnosis was 58.5 in the mutation negative and 63 in the RET+ patients. Both cohorts were predominantly male (66% and 56%, respectively), with a majority of never smokers (59 and 89%, respectively), and stage IV disease at diagnosis (72 and 89%, respectively). Two heavily pretreated RET+ patients had stable disease at their initial restaging scans following treatment with the RET inhibitor vandetanib (radiographic assessment per RECIST 1.1); two others had early radiographic progression with sunitinib. Conclusions: The FISH probe proved efficient to detect RET rearrangements in lung adenocarcinomas, involving KIF5B and non-KIF5B partners. Frequency of RET rearrangements in this enriched lung adenocarcinoma cohort was considerably higher than reported in unselected cohorts. Further molecular analyses are being performed to increase understanding of the natural history of this new molecular subtype of NSCLC. Support: B J Addario Foundation, NCI P50CA058187, NCI CCSG P30CA046934.