Drugging the catalytically inactive state of RET kinase in RET-rearranged tumors

An examination of the activity profiles of RET inhibitors suggests potential treatment for RET-rearranged cancers. RET-ting out lung tumors Gene fusions and rearrangements serve as oncogenic drivers in a number of tumor types, and some of these can be targeted with existing drugs. RET rearrangements have been identified as drivers in some lung adenocarcinomas, but previous attempts to target RET have not been successful. Plenker et al. determined why the drugs previously proposed for inhibiting RET were not sufficiently potent and showed that successful inhibition of RET requires the ability to bind RET in its catalytically inactive conformation, known as the “DFG-out conformation,” thus locking it in an inactive state. The authors also identified drugs that bind RET in the desired conformation and demonstrated their efficacy in patient-derived xenograft models. Oncogenic fusion events have been identified in a broad range of tumors. Among them, RET rearrangements represent distinct and potentially druggable targets that are recurrently found in lung adenocarcinomas. We provide further evidence that current anti-RET drugs may not be potent enough to induce durable responses in such tumors. We report that potent inhibitors, such as AD80 or ponatinib, that stably bind in the DFG-out conformation of RET may overcome these limitations and selectively kill RET-rearranged tumors. Using chemical genomics in conjunction with phosphoproteomic analyses in RET-rearranged cells, we identify the CCDC6-RETI788N mutation and drug-induced mitogen-activated protein kinase pathway reactivation as possible mechanisms by which tumors may escape the activity of RET inhibitors. Our data provide mechanistic insight into the druggability of RET kinase fusions that may be of help for the development of effective therapies targeting such tumors.