During the last decades, the knowledge on follicular cell-derived thyroid cancer molecular biology has led to the evolution of a number of novel therapies for these tumors, mainly tyrosine kinase inhibitors. Lenvantinib, sorafenib and recently cabozantinib have been approved for differentiated thyroid cancer (DTC), while larotrectinib and entrectinib for neurotrophic-tropomyosin receptor kinase-fusion thyroid cancer. For radioiodine (RAI) refractory DTCs ongoing research aims to identify agents that may restore RAI-avidity via redifferentiation protocols (vemurafenib or dabrafenib and trametinib) or combination treatments. These treatments are based on the tumor molecular signature. The treatment with targeted therapies has changed the therapeutic strategies and the disease prognosis, however drug resistance remains the main reason for treatment failure. Thus, the understanding of both molecular pathways implicated in tumorigenesis, and tumoral escape mechanisms, are of paramount significance for the development of new therapies for DTC. The present review focuses on the molecular landscape of DTC, the approved targeted therapies as well as the mechanisms of drug resistance. Furthermore, it points to the ongoing research and the future perspectives for the development of more efficient drugs for DTC.

Context Although the majority of RET alterations are single nucleotide variants (SNV), small deletions and/or insertions have been reported at variable prevalence. No information about the efficacy of RET-specific inhibitors in patients harboring RET indels has been provided. Objective We present an update on the prevalence of RET indels in medullary thyroid cancer (MTC) and describe the efficacy of selpercatinib in patients with advanced MTC with RET indels. Methods The MTC tissues of 287 patients were analyzed using an Ion S5 targeted sequencing. The functional role of the reported indels have been evaluated by MutationTaster. Clinical and pathological data of MTC patients harboring a RET indel were collected and analyzed. Two patients with a RET indel were treated with selpercatinib. Results Among 178 RET-positive cases, 147 (82.6%) harbored a SNV and 31 (17.4%) a RET in-frame indel. Nine indels were not previously reported and were found to be disease causing by MutationTaster. Patients harboring an indel were found to have an aggressive disease and 2 of them were treated with selpercatinib, experiencing a good response to the treatment. Conclusion These data show that RET indels are not infrequent and correlate with an aggressive disease. Two RET indel–positive patients showed a partial response to the treatment with a highly selective RET inhibitor; thus, these RET indels can be considered actionable mutations. In order to not miss these alterations, the analysis of the full gene is recommended.

During the last decades, knowledge of the molecular biology in medullary thyroid carcinoma (MTC) and specifically on the role of rearranged during transfection (RET)-activating mutations in tumorigenesis has led to the evolution of novel targeted therapies, mainly tyrosine kinase inhibitors (TKIs). Vandetanib and cabozantinib have been approved for the management of metastatic progressive MTC. Two novel, highly selective RET inhibitors, selpercatinib and pralsetinib, have recently been approved for the treatment of RET-mutant MTCs and RET-fusion differentiated thyroid cancer. The administration of targeted therapies in MTC patients has changed the therapeutic strategies; however, in the majority of cases, there are no real data showing an improvement of prognosis by TKIs in MTC. Drug resistance remains the main reason for treatment failure. Thus, the understanding of the molecular landscape of tumorigenesis and the mechanisms underlying resistance to targeted therapies is of paramount importance for the further development of more efficient therapies for MTC. The present review focuses on the molecular pathways implicated in MTC tumorigenesis, the approved targeted therapies, the tumoral escape mechanisms, as well as the future perspectives for targeted therapy.

Summary: Medullary thyroid carcinoma (MTC) has a varying clinical course; distant metastases are frequently present even at diagnosis. We present two MTC cases with unusual metastatic sites. Two female patients are presented with slow progressive MTC. The first case developed distant metastases 23 years after diagnosis and underwent locoregional therapies. At the same time a breast mass developed representing MTC metastasis. Treatment with vandetanib led to long-term disease stabilization. The second patient is presented with metastases in the pancreas 13 years after diagnosis. Shortly, a painful mass developed in the mandible and metastasis of MTC was diagnosed. Disease progression was recorded 20 months after the initiation of local and systemic therapy. Such cases have only rarely been reported in the literature and highlight the need for prompt recognition of unexpected MTC metastases. Learning points: