RET and medullary thyroid gland carcinoma: LIBRETTO-531 is a global, multi-center, randomized (2:1), open-label, Phase 3 study comparing selpercatinib to physician’s choice of cabozantinib or vandetanib in patients with progressive, advanced, tyrosine kinase inhibitor (TKI)-naïve, rearranged during transfection (RET)-mutation positive medullary thyroid cancer (MTC) [6].