A Case Report of Poor Response to Selpercatinib in the Presence of a 632_633 RET Deletion

Background: Genomic deletions in medullary thyroid cancer (MTC) are rare. Selpercatinib is a highly selective RET inhibitor for treatment of metastatic RET-altered MTC. We report a 35-year-old male with an aggressive metastatic MTC harboring p.632_633del RET that was poorly responsive to RET kinase inhibitor selpercatinib. Objective: Our objective was to understand the clinical phenotype of p.632_633del RET in MTC in the context of novel RET kinase inhibitor treatment. Methods: Wild-type and p.632_633del RET sequences were modeled using a lighter version of the AlphaFold2 (AF2) software. Functional studies were performed on transfected HEK 293 cells (pCMV6-Entry, pCMV6-RET, or pCMV6-RET(p.632_633del) treated with inhibitors for 24 hours and analyzed on luciferase assays. Results: Structural modeling revealed a paucity of disulfide bridge between Cys630-Cys634 in p.632_633del RET sequences, apparent in wild-type, while forming an intermolecular disulfide bridge between two Cys656. Proximity of juxtamembrane segments of each dimer may impede Tyr687 phosphorylation and stable conformation of intracellular RET that hosts selpercatinib. In vitro experiments confirmed a reduction in efficacy of selpercatinib upon p.632_633del RET compared with wild-type RET control. Conclusion: Clinical presentation together with structural modeling and functional studies suggests that p.632_633del RET results in poor response to selpercatinib.