Background: A pilot study showed MEK inhibition could enhance radioiodine (RAI) avidity/efficacy in 5 RAS mutant (MUT), RAI-refractory (RAIR) thyroid cancer (TC) patients (pts). This phase 2 trial with the MEK 1/2 inhibitor trametinib (tram) was conducted to define the efficacy of this “redifferentiation” strategy in RAS MUT RAIR pts and separately in a RAS wild-type (WT) cohort. Methods: Recurrent and/or metastatic, RAIR TC pts w/ RAS MUT (Cohort A) or RAS WT (excluding BRAF^V600E) (Cohort B) tumors were treated w/ tram (2 mg orally daily). Progressive disease or new/worsening disease-related symptoms was required for Cohort A pts. ¹²⁴I PET was performed at baseline and the fourth week of tram. If the second ¹²⁴I PET showed increased RAI avidity allowing > 2000 cGy to be delivered to a tumor w/ < 300 mCi ¹³¹I, pts were treated w/ ¹³¹I, guided by whole body and blood dosimetry. Tram was continued through 2 days s/p ¹³¹I. Pts who did not qualify for ¹³¹I from A/B were taken off study or continued tram alone (Cohort C). For Cohort A (n = 25), the two co-primary endpoints were objective response rate (ORR) and progression-free survival (PFS) 6 months (mos) s/p ¹³¹I. Observing either >4 pts w/ confirmed complete or partial response (cCR or cPR) or > 9 progression-free at 6 mos would be considered promising. Secondary endpoints were the proportion of pts w/ increased ¹²⁴I, safety/tolerability of tram and thyroglobulin changes s/p RAI. The Cohort B primary endpoint was the proportion of pts whose tumors exceeded the lesional dosimetry threshold for ¹³¹I w/ tram. An exploratory endpoint for Cohort C was best objective response (BOR) w/ tram. Results: 25 RAS MUT pts enrolled in Cohort A. 23 had at least one (> 1) ¹²⁴I (-) lesion, 21 had >1 ¹²⁴I (+) lesions and 4 pts had tumors lacking any ¹²⁴I uptake. After tram treatment, 22/25 had increased ¹²⁴I uptake; 17/23 had ¹²⁴I (-) tumors convert positive. Importantly, 15/25 (60%) pts had increased ¹²⁴I uptake and met lesional dosimetry criteria for ¹³¹I on tram. Of 14 pts treated w/ ¹³¹I, 8 (57%) achieved cPR, 3 (21%) stable disease (SD) and 3 (21%) progression of disease (PD) 6 mos s/p RAI, translating to 32% ORR and 44% 6-month PFS among all 25 pts. Cohort B had 9 pts (4 Class II BRAF alterations, 4 RET rearrangements, 1 STK11 mutation). 3/4 pts w/ Class II BRAF altered tumors qualified for ¹³¹I, leading to 1 cPR, 2 SD 6 mos s/p ¹³¹I. 1/4 pts w/ RET rearranged tumors qualified for ¹³¹I, producing SD at 6 mos. The STK11 MUT pt did not have increased ¹²⁴I uptake w/ tram. 7 ¹³¹I-ineligible pts enrolled to continue tram (Cohort C). Two serious adverse events (grade 3 anemia [Cohort A], grade 3 ejection fraction decrease [Cohort C]) and 3 grade 1 blurred vision/decreased visual acuity AEs were related to tram. Conclusions: Trametinib enhanced RAI uptake/efficacy in a subset of RAS MUT and Class II BRAF altered tumors. Further study to define the efficacy and optimal application of this therapeutic strategy is warranted. Clinical trial information: NCT02152995.