Abstract 918: A secondary RET mutation allosterically conferring resistance to vandetanib

Targeted therapy against oncogenic ALK and ROS1 fusion using type I tyrosine kinase inhibitors (TKIs), which bind to the ATP-binding cleft of kinases, is highly effective in lung adenocarcinoma (LADC); however, such cancers inevitably acquire resistance, which severely limits the efficacy of cancer treatments. The identification of these resistance mutations led to the development of novel TKIs to overcome acquired resistance. Here we report resistance to vandetanib, a type I RET kinase inhibitor, developed in a patient with metastatic LADC harboring a CCDC6-RET fusion that initially exhibited a response to treatment. The resistant tumor acquired a secondary mutation resulting in substitution of an amino acid located outside the nucleotide binding cleft. The mutation conferred resistance to vandetanib by increasing the ATP affinity and auto-phosphorylation activity of RET kinase. A crystal structure of the mutant revealed a small hydrophobic core around the mutated amino acid likely to enhance basal kinase activity by stabilizing an active conformer. In addition, a thermal shift assay suggested that the mutation was less able to bind vandetanib. Molecular dynamics simulation supported the mutant underwent conformational changes decreasing the binding affinity of vandetanib. Our findings indicate that the missense mutation is able to increase kinase activity and confers drug resistance through allosteric effects. Citation Format: Takashi Nakaoku, Takashi Kohno, Mitsugu Araki, Seiji Nino, Rakhee Chauhan, Phillip P. Knowles, Katsuya Tsuchihara, Shingo Matsumoto, Yoko Shimada, Sachiyo Mimaki, Genichiro Ishii, Hitoshi Ichikawa, Yasushi Okuno, Kiyotaka Yoh, Neil Q. McDonald, Koichi Goto. A secondary RET mutation allosterically conferring resistance to vandetanib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 918.