First phase 3 trial backs dabrafenib–trametinib in BRAF-mutated thyroid cancer

The first phase 3 trial of dabrafenib plus trametinib in radioactive iodine-refractory, BRAF V600E-mutated differentiated thyroid cancer found that the targeted combination more than tripled median progression-free survival — 12.8 months versus 3.7 months with placebo (hazard ratio 0.38; 95% CI 0.25 to 0.57; p<0.0001) [s1]. The tumour response rate was 57% against 4%, a large effect, but overall survival is not yet statistically significant and the trial population was predominantly Asian, both of which temper how far the result should be read [s1].

The two drugs are not new. Dabrafenib blocks the mutated BRAF protein and trametinib blocks MEK, the next relay in the same growth-signalling pathway; together they are already approved by the US Food and Drug Administration for any solid tumour that carries a BRAF V600E mutation, a tissue-agnostic clearance granted in 2022 on the strength of basket trials [s2]. What was missing was a randomised phase 3 trial in thyroid cancer specifically — the kind of evidence that turns a mechanism-based approval into a settled standard [s1][s2].

Why this cancer needs it

Most differentiated thyroid cancer is curable with surgery and radioactive iodine. The problem is the minority whose disease stops taking up iodine — "iodine-refractory" disease — for whom the options narrow to the multi-target kinase inhibitors lenvatinib and sorafenib, drugs with substantial toxicity and no cure. Roughly a third to a half of these cancers carry a BRAF V600E mutation, the same driver targeted in melanoma, which is what made dabrafenib plus trametinib a rational thing to test here [s1].

What the trial did

The trial enrolled 153 previously treated patients with locally advanced or metastatic, iodine-refractory BRAF V600E-mutated differentiated thyroid cancer at 42 sites in 11 countries, and randomly assigned them 2:1 — 101 to dabrafenib (150 mg twice daily) plus trametinib (2 mg once daily) and 52 to matching placebos [s1]. It was double-blind, and the primary endpoint, progression-free survival, was judged by a blinded independent review committee [s1]. Enrolment ran from December 2021 to May 2024, and the analysis had a median follow-up of 17.4 months [s1]. Of the participants, 131 (86%) were Asian and 18 (12%) were White, with a mean age of 62.6 years [s1].

What it found

Median progression-free survival was 12.8 months (95% CI 10.2 to 21.2) with the combination versus 3.7 months (95% CI 2.3 to 7.5) with placebo — the hazard ratio of 0.38 quoted above [s1]. The objective response rate was 57% (58 of 101 patients) against 4% (2 of 52), a stratified difference of 53 percentage points (95% CI 42 to 64; p<0.0001) [s1].

Overall survival, though, has not yet declared. An interim analysis gave a hazard ratio of 0.66 (95% CI 0.36 to 1.19; p=0.083) — a direction of benefit, but not a statistically significant one [s1]. On safety, the most common grade 3 or worse event was pneumonia, in 8% of the combination group versus 2% of placebo; serious adverse events occurred in 43% versus 25%; the most common event of any grade was fever, at 48%; and one treatment-related death, a stroke, occurred in the combination group [s1].

How to read it

Two features set the ceiling on the claim. First, the comparator was placebo, not an active drug — so the trial shows that dabrafenib plus trametinib beats nothing, not that it beats lenvatinib, the current standard [s1]. That is a legitimate design for a group with few options, but it means the result does not rank the combination against its real-world competitor. Second, the immature overall-survival signal is a genuine caveat: progression-free survival and response rate are the endpoints that moved, and in slow-growing cancers those do not always translate into longer life [s1].

The population skew is the third caveat. With 86% of participants Asian, the trial's generalisability to other populations rests on the assumption that a BRAF-driven cancer behaves the same way regardless of ancestry — plausible for a mutation-defined disease, but an assumption the data cannot itself confirm [s1]. The trial was funded by Novartis, which markets both drugs [s1].

Why it matters

This is a template that precision oncology keeps repeating: take a driver mutation shared across cancers, and a drug pair already proven against it, and confirm the match in a randomised trial in a new tumour type [s1][s2]. It sits alongside other driver-matched strategies now being tested tumour by tumour, such as selpercatinib against RET-altered lung cancer. For patients with iodine-refractory thyroid cancer, a genuinely active oral option is meaningful; whether it becomes a first choice depends on data the trial did not generate.

What to watch

The overall-survival readout is the number that will decide the drug's standing. Beyond that, the practical questions are sequencing against lenvatinib, whether BRAF testing becomes routine at the point iodine resistance emerges, and how durable the responses prove to be — the median duration of response was not reached at this analysis [s1].

This article describes research and regulatory developments and is not medical advice. Treatment decisions are for patients and their treating clinicians.

Sources

• Efficacy and safety of dabrafenib plus trametinib in adults with differentiated thyroid cancer: a phase 3 trial — The Lancet Oncology, online 13 July 2026
• Drugs@FDA: Tafinlar (dabrafenib), NDA 202806 — BRAF V600E solid-tumour (tissue-agnostic) approval — U.S. Food and Drug Administration, approved 22 June 2022