Thyroid Cancer Clinical Trials 2026: Targeted Therapies and Radioiodine Resistance

Thyroid cancer has a split personality that matters clinically. The majority — papillary thyroid cancer with favorable pathology — is curable with surgery and radioiodine, and has a decades-long survival that makes it look like a success story. The minority — radioiodine-refractory differentiated thyroid cancer and anaplastic thyroid cancer — is a different disease in every relevant way, with treatment options that were nearly nonexistent until targeted inhibitors arrived. The story of thyroid cancer drug development since 2018 is largely the story of applying the same oncogenic driver logic that transformed lung cancer and melanoma: BRAF V600E, RET fusions, NTRK fusions — the same mutations, the same targeted drugs, dramatic responses in diseases that had none before. The 2026 trials are working on resistance, combination strategies, and the redifferentiation approaches that may restore radioiodine sensitivity in RAI-refractory patients.

The Molecular Landscape: Why Knowing Your Mutation Matters

Thyroid cancers are classified into well-differentiated (papillary, follicular), poorly differentiated, anaplastic, and medullary subtypes — with distinct driver mutations and prognoses. Papillary thyroid carcinoma (PTC), accounting for ~85% of cases, is dominated by BRAF V600E (~60%), RET/PTC fusions (~20%), and RAS mutations. Medullary thyroid carcinoma (MTC) is driven exclusively by RET — either germline RET mutations in hereditary MEN2A/MEN2B syndromes, or somatic RET mutations in sporadic disease. Anaplastic thyroid carcinoma (ATC) is the most lethal human malignancy, with a median survival historically under five months; it frequently carries BRAF V600E along with TP53, TERT promoter, and PI3K pathway alterations representing dedifferentiation from a pre-existing differentiated thyroid cancer.

This molecular landscape is now directly actionable. Comprehensive molecular profiling at diagnosis is standard of care for any patient with advanced, metastatic, or aggressive thyroid cancer — not a research option, a clinical necessity. The identification of RET, BRAF, NTRK, and ALK alterations has replaced empirical first-line use of poorly tolerated non-selective multikinase inhibitors (sorafenib, lenvatinib) with highly targeted agents that deliver substantially better response rates and quality of life.

Selpercatinib and Pralsetinib: What the Trial Data Shows

Selpercatinib (Retevmo, Eli Lilly) and pralsetinib (Gavreto, Blueprint Medicines/Genentech) are highly selective RET kinase inhibitors — designed to potently inhibit RET while sparing VEGFR2, the primary target of older multikinase inhibitors. Sparing VEGFR2 substantially reduces cardiovascular side effects (hypertension, hepatotoxicity, hand-foot syndrome) that defined tolerability-limited dosing with sorafenib and cabozantinib.

The LIBRETTO-001 basket trial (NCT03157128) established selpercatinib's activity across RET-altered cancers. In RET-mutant MTC previously treated with vandetanib or cabozantinib, selpercatinib achieved an ORR of 69% with median duration of response of 22 months. In treatment-naive RET-mutant MTC, ORR was 73%. The subsequent LIBRETTO-531 Phase 3 trial compared selpercatinib versus cabozantinib or vandetanib in previously untreated RET-mutant MTC — selpercatinib demonstrated a PFS hazard ratio of 0.28 (95% CI 0.16–0.48), representing a dramatic improvement in progression-free survival that definitively established selpercatinib as the preferred first-line standard for this population. In 2026, LIBRETTO-531 extension cohorts are characterizing outcomes after acquired RET resistance mutations — particularly the RET G810 gatekeeper mutation — and testing combination with everolimus to overcome mTOR pathway-mediated resistance.

Pralsetinib, studied in the ARROW trial (NCT03037385), showed an ORR of 71% in previously treated RET-mutant MTC and 89% in treatment-naive patients. In RET fusion-positive differentiated thyroid cancer, pralsetinib demonstrated an ORR of 89% with 7.3% complete responses — responses of unusual depth for differentiated thyroid cancer patients with prior multikinase inhibitor exposure. Both agents are FDA-approved; head-to-head comparison trials are in development to differentiate efficacy and tolerability profiles across specific RET mutation subtypes.

BRAF V600E in ATC: The Drug That Changed a Death Sentence

Before 2018, anaplastic thyroid carcinoma had no approved systemic therapy. Responses to chemotherapy were rare and short-lived. Median survival from diagnosis was approximately 5 months, with most patients dying within a year regardless of treatment. The disease was one of the most demoralizing in oncology from a treatment perspective.

A Phase 2 basket trial (NCT02034110) tested the combination of dabrafenib (BRAF V600E inhibitor) plus trametinib (MEK inhibitor) — already approved in BRAF V600E-mutant melanoma and NSCLC — in BRAF V600E-mutant ATC. Overall response rate: 69%. Complete responses: seen in multiple patients. These results were unheard of in this disease. The FDA granted accelerated approval for dabrafenib plus trametinib in BRAF V600E-mutant ATC in May 2018 — the first systemic therapy ever approved for this indication.

In 2026, the question has shifted to what comes after the initial BRAF/MEK response. ATC is biologically aggressive, and most patients who respond eventually progress. The ATC-COMBO trial is evaluating spartalizumab (anti-PD-1) added to dabrafenib/trametinib, based on evidence that MAP kinase inhibition increases MHC class I expression and tumor immunogenicity. Interim data suggest a higher complete response rate with the triplet versus doublet, though the aggressive biology of ATC complicates completion and follow-up. Optimal surgery timing — when to resect after maximal BRAF/MEK response — is also being studied prospectively.

Redifferentiation: Restoring Radioiodine Uptake in Refractory Disease

This is one of the most conceptually elegant approaches in thyroid cancer — and it's producing real results.

Radioiodine-refractory differentiated thyroid cancer is defined by loss of iodine uptake capacity, which occurs because dedifferentiation (driven by sustained MAPK pathway activation via BRAF or RAS mutations) suppresses expression of the sodium-iodide symporter (NIS, encoded by SLC5A5) and other thyroid-specific proteins required for iodine uptake and organification. If you can transiently suppress the MAPK pathway with a targeted inhibitor, NIS expression can be restored — and the tumor becomes radioiodine-sensitive again. Then you do the RAI treatment.

The MERIDIIAN trial demonstrated that vemurafenib (BRAF inhibitor) prior to RAI dosimetry increased iodine uptake in BRAF V600E-mutant RAI-refractory PTC, with clinically meaningful iodine incorporation restored in approximately 40% of patients. Building on this, the NEWSTART trial (NCT02649465) is evaluating selpercatinib-mediated redifferentiation in RET-altered RAI-refractory PTC — using the same kinase inhibitor that's standard treatment for RET-mutant MTC, but deployed as a bridge to RAI rather than as long-term maintenance. The SEL-RET trial is assessing whether a defined "drug holiday" after initial selpercatinib treatment allows adequate NIS re-expression for effective RAI dosimetry. Cobimetinib (MEK inhibitor) plus RAI is in Phase 2 for RAS-mutant RAI-refractory disease, where BRAF inhibitors are not the primary target.

NTRK Fusions and the Second-Generation TRK Inhibitor Problem

NTRK1/2/3 fusions occur in 2–5% of differentiated thyroid cancers and in a higher proportion (~15%) of pediatric thyroid cancers. Larotrectinib (Vitrakvi, Bayer/Loxo Oncology) and entrectinib (Rozlytrek, Genentech) received the first tumor-agnostic FDA approvals ever — based on NTRK fusion positivity across any solid tumor histology. In the NAVIGATE basket trial, larotrectinib demonstrated a 75% ORR in NTRK fusion-positive thyroid cancer, with remarkably durable responses — many patients remaining on therapy with objective response beyond two years.

The clinical problem that's emerged is resistance. Acquired RET-like kinase domain mutations — specifically NTRK1 G595R and NTRK3 G623R — develop in patients after sustained larotrectinib exposure, conferring resistance through steric clashing that prevents drug binding. Second-generation TRK inhibitors designed to overcome these specific resistance mutations are now in trials: selitrectinib (LOXO-195) and repotrectinib (TPX-0005) take a macrocyclic conformation that maintains binding despite the gatekeeper mutation. The TRIDENT-1 trial for repotrectinib enrolled thyroid cancer patients with NTRK fusion-positive disease after prior TRK inhibitor exposure; Phase 2 data show a 40% ORR in this heavily pre-treated population — a meaningful signal given that larotrectinib-resistant disease previously had no good options. Molecular testing at progression is now as important as molecular testing at diagnosis in thyroid cancer.

Key Takeaways

• Selpercatinib replaced multikinase inhibitors as the preferred first-line treatment for RET-mutant MTC based on LIBRETTO-531 (HR 0.28 for PFS); pralsetinib showed 89% ORR in treatment-naive RET-mutant MTC in ARROW.
• Dabrafenib plus trametinib achieves ~69% ORR in BRAF V600E-mutant ATC — the only FDA-approved systemic therapy for a disease with historically 5-month median survival. The ATC-COMBO trial is adding spartalizumab to test whether immunotherapy improves on this.
• Redifferentiation — using BRAF/MEK/RET inhibitors to restore NIS expression before RAI therapy — restores clinically meaningful iodine uptake in ~40% of RAI-refractory PTC patients. NEWSTART and SEL-RET trials are defining optimal protocols.
• Second-generation TRK inhibitors (selitrectinib, repotrectinib) address acquired NTRK kinase domain resistance mutations after larotrectinib — repotrectinib showed 40% ORR post-larotrectinib in TRIDENT-1.
• Comprehensive NGS at diagnosis and at progression is standard practice in advanced thyroid cancer — it determines both initial targeted therapy selection and second-line options when resistance emerges.