TRK inhibitors in TRK fusion-positive cancers

TRK fusions are oncogenic drivers of various adult and paediatric cancers. The first-generation TRK inhibitors, larotrectinib and entrectinib, were granted landmark, tumour-agnostic regulatory approvals for the treatment of these cancers in 2018 and 2019, respectively. Brisk and durable responses are achieved with these drugs in patients, including those with locally advanced or metastatic disease. In addition, intracranial activity has been observed with both agents in TRK fusion-positive solid tumours with brain metastases and primary brain tumours. While resistance to first-generation TRK inhibition can eventually occur, next-generation agents such as selitrectinib (BAY 2731954, LOXO-195) and repotrectinib were designed to address on-target resistance, which is mediated by emergent kinase domain mutations, such as those that result in substitutions at solvent front or gatekeeper residues. These next-generation drugs are currently available in the clinic and proof-of-concept responses have been reported. This underscores the utility of sequential TRK inhibitor use in select patients, a paradigm that parallels the use of targeted therapies in other oncogenic driver-positive cancers, such as ALK fusion-positive lung cancers. While TRK inhibitors have a favourable overall safety profile, select on-target adverse events, including weight gain, dizziness/ataxia and paraesthesias, are occasionally observed and should be monitored in the clinic. These side-effects are likely consequences of the inhibition of the TRK pathway that is involved in the development and maintenance of the nervous system.