Rare cancer, rare alteration: the case of NTRK fusions in biliary tract cancers.

Introduction: For patients with advanced/unresectable biliary tract cancers, cisplatin-gemcitabine combination is the standard first-line treatment. Beyond the first line, the therapeutic arsenal is limited with minimal benefit. Biliary tract cancers exhibit one of the highest frequencies of targetable molecular alterations across cancer types, and several targeted therapies are emerging as treatment options. Areas covered: We discuss neurotrophic tyrosine kinase receptor gene (NTRK) fusions in biliary tract cancers and the use of NTRK inhibitors (now approved in a 'cancer-agnostic' way), mechanisms of resistance, and emerging second-generation NTRK inhibitors. Expert opinion: Despite their rarity in biliary tract cancers, NTRK fusions are promising molecular targets because i) NTRK inhibitors have proven highly effective in NTRK -rearranged cancers and are now approved in a 'cancer-agnostic' way; ii) emerging second-generation NTRK inhibitors may overcome secondary resistance; iii) NTRK rearrangements will be readily detectable with the generalization of next-generation-sequencing in biliary tract cancers, including the detection of other frequent gene rearrangements, such as those involving the fibroblast growth factor receptor 2 gene (FGFR2) . However, more data are necessary regarding the prevalence and characteristics of NTRK fusions in biliary tract cancers and the efficacy of NTRK inhibitors in these patients.