Background: Medullary thyroid cancer (MTC) accounts for 1 to 2% of thyroid cancers in the United States; RET alterations occur in >95% of hereditary and 50% of sporadic forms. Up to 80% of patients with sporadic MTC have somatic M918T RET mutations, which is associated with poor prognosis (1). The tyrosine kinase inhibitors (TKIs) cabozantinib and vandetanib are approved to treat patients with MTC regardless of RET status; however, retrospective analyses have suggested that there may be greater benefit in patients with M918T mutations (1,2). Newly approved therapies selpercatinib and pralsetinib, developed for patients with RET mutations, have demonstrated higher response rates than previous first line therapies. In this analysis, we examine the differences in overall response rate (ORR) between patients with MTC with RET M918T non-RET M918T mutations. Methods: An analysis of ORR in patients with MTC with RET M918T mutations with non-M918T mutations was conducted using the efficacy populations used to support the approvals of pralsetinib and selpercatinib using the following groups: Patients who received prior cabozantinib or vandetanib (referred to as “previously treated”). Patients with no prior cabozantinib or vandetanib (“TKI naïve”). All patients regardless of prior therapy. Results: Exploratory analysis of ORR of pooled population of Selpercatinib and Pralsetinib in patients with MTC with RET M918T mutations and non-M918T mutations. ¹ Prior vandetanib or cabozantinib. ² No prior vandetanib or cabozantinib. Two groups of patients were analyzed (RET M918T mutation and RET non-M918T mutation), with subgroups with respect to prior treatment. Among all patients regardless of prior therapy, the ORR was similar between M918T non-M918T groups. Among previously treated patients, the ORR was lower in the M918T group vs. the non-M918T group, while in the TKI naïve group the ORR was higher in the M918T groups vs the non-M918T group although the 95% CIs overlap in both comparisons. Conclusions: There were no major differences in ORR among mutational subtypes in patients with MTC treated with RET inhibitors, regardless of prior therapy. ORR was similar between patients with M918T and non-M918T mutations. Additional experience in ongoing clinical studies may provide additional data regarding responses across specific mutation types. References: 1.Sherman SI et al “Correlative analyses of RET and RAS mutations in a phase 3 trial of cabozantinib...” Cancer. 2016;122(24):3856-3864. 2.Wells SA Jr et al “Vandetanib in patients with locally advanced or metastatic medullary thyroid cancer...” J Clin Oncol. 2012;30(2):134-41.