D898_E901 <i>RET</i> Deletion Is Oncogenic, Responds to Selpercatinib, and Treatment Resistance Can Arise Via RET-Independent Mechanisms.

Abstract

We analyzed the oncogenic potential of RET Δ898-901 mutant and its response to selpercatinib, vandetanib, and cabozantinib in vitro and in a clinical case. A 35-year-old man with a medullary thyroid cancer (MTC) harboring a somatic D898_E901 <i>RET</i> deletion was sequentially treated with vandetanib, selpercatinib, cabozantinib, and fluorouracil (5-FU)-dacarbazine. Functional study of RET Δ898-901 mutant was performed in HEK-293T, NIH-3T3, and Ba/F3 cells. RET C634R and wild-type cells served as positive and negative controls, respectively. The patient showed primary resistance to vandetanib and secondary resistance to selpercatinib after 12 months. Comprehensive next-generation sequencing of a progressing lesion during selpercatinib showed no additional RET mutation but an acquired complete genetic loss of <i>CDKN2A</i>, <i>CDKN2B,</i> and <i>MTAP</i> genes. Subsequent treatment with cabozantinib and 5-FU-dacarbazine had poor efficacy. In vitro, RET Δ898-901 showed higher ligand-independent RET autophosphorylation compared with RET C634R and similar proliferation rates in cell models. Subcutaneous injection of Δ898-901 NIH 3T3 cells in nude mice produced tumors of around 500 mm³ in 2 weeks, similarly to RET C634R cells. Selpercatinib inhibited cell growth of Ba/F3 RET Δ898-901 and RET C634R with a similar half maximal inhibitory concentration (IC₅₀) of approximately 3 nM. Vandetanib was five-fold less effective at inhibiting cell growth promoted by RET Δ898-901 mutant (IC₅₀, 564 nM) compared with RET C634R one (IC₅₀, 91 nM). Cabozantinib efficiently inhibited Ba/F3 RET C634 proliferation (IC₅₀, 25.9 nM), but was scarcely active in Ba/F3 RET 898-901 (IC₅₀ > 1,350 nM). D898_E901 <i>RET</i> deletion is a gain-of-function mutation and responds to tyrosine kinase inhibitors in MTC. RET Δ898-901 mutant is sensitive to selpercatinib and vandetanib, and acquired resistance to selpercatinib may develop via RET-independent mechanisms.