Response to Cabozantinib in patients with RET fusion-positive lung adenocarcinomas.

The discovery of RET fusions in lung cancers has uncovered a new therapeutic target for patients whose tumors harbor these changes. In an unselected population of non-small cell lung carcinomas (NSCLCs), RET fusions are present in 1% to 2% of cases. This incidence increases substantially, however, in never-smokers with lung adenocarcinomas that lack other known driver oncogenes. Although preclinical data provide experimental support for the use of RET inhibitors in the treatment of RET fusion-positive tumors, clinical data on response are lacking. We report preliminary data for the first three patients treated with the RET inhibitor cabozantinib on a prospective phase II trial for patients with RET fusion-positive NSCLCs (NCT01639508). Confirmed partial responses were observed in 2 patients, including one harboring a novel TRIM33-RET fusion. A third patient with a KIF5B-RET fusion has had prolonged stable disease approaching 8 months (31 weeks). All three patients remain progression-free on treatment.