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Small-molecule MAPK inhibitors restore radioiodine incorporation in mouse thyroid cancers with conditional BRAF activation.

ABSTRACT: Advanced human thyroid cancers, particularly those that are refractory to treatment with radioiodine (RAI), have a high prevalence of BRAF (v-raf murine sarcoma viral oncogene homolog B1) mutations. However, the degree to which these cancers are dependent on BRAF expression is still unclear. To address this question, we generated mice expressing one of the most commonly detected BRAF mutations in human papillary thyroid carcinomas (BRAF(V600E)) in thyroid follicular cells in a doxycycline-inducible (dox-inducible) manner. Upon dox induction of BRAF(V600E), the mice developed highly penetrant and poorly differentiated thyroid tumors. Discontinuation of dox extinguished BRAF(V600E) expression and reestablished thyroid follicular architecture and normal thyroid histology. Switching on BRAF(V600E) rapidly induced hypothyroidism and virtually abolished thyroid-specific gene expression and RAI incorporation, all of which were restored to near basal levels upon discontinuation of dox. Treatment of mice with these cancers with small molecule inhibitors of either MEK or mutant BRAF reduced their proliferative index and partially restored thyroid-specific gene expression. Strikingly, treatment with the MAPK pathway inhibitors rendered the tumor cells susceptible to a therapeutic dose of RAI. Our data show that thyroid tumors carrying BRAF(V600E) mutations are exquisitely dependent on the oncoprotein for viability and that genetic or pharmacological inhibition of its expression or activity is associated with tumor regression and restoration of RAI uptake in vivo in mice. These findings have potentially significant clinical ramifications.

SUBMITTER: Chakravarty D

PROVIDER: S-EPMC3225989 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Background: Oncogenic activation of mitogen-activated protein kinase (MAPK) signaling is associated with radioiodine refractory (RAIR) thyroid cancer. Preclinical models suggest that activation of the receptor tyrosine kinase erbB-3 (HER3) mitigates the MAPK pathway inhibition achieved by BRAF inhibitors in BRAFV600E mutant thyroid cancers. We hypothesized that combined inhibition of BRAF and HER3 using vemurafenib and the human monoclonal antibody CDX-3379, respectively, would potently inhibit MAPK activation and restore radioactive iodine (RAI) avidity in patients with BRAF-mutant RAIR thyroid cancer. Methods: Patients with BRAFV600E RAIR thyroid cancer were evaluated by thyrogen-stimulated iodine-124 (124I) positron emission tomography-computed tomography (PET/CT) at baseline and after 5 weeks of treatment with oral vemurafenib 960 mg twice daily alone for 1 week, followed by vemurafenib in combination with 1000 mg of intravenous CDX-3379 every 2 weeks. Patients with adequate 124I uptake on the second PET/CT then received therapeutic radioactive iodine (131I) with vemurafenb+CDX-3379. All therapy was discontinued two days later. Treatment response was monitored by serum thyroglobulin measurements and imaging. The primary endpoints were safety and tolerability of vemurafenib+CDX-3379, as well as the proportion of patients after vemurafenb+CDX-3379 therapy with enhanced RAI incorporation warranting therapeutic 131I. Results: Seven patients were enrolled; six were evaluable for the primary endpoints. No grade 3 or 4 toxicities related to CDX-3379 were observed. Five patients had increased RAI uptake after treatment; in 4 patients this increased uptake warranted therapeutic 131I. At 6 months, 2 patients achieved partial response after 131I and 2 progression of disease. Next-generation sequencing of 5 patients showed that all had co-occurring telomerase reverse transcriptase promoter alterations. A deleterious mutation in the SWItch/Sucrose Non-Fermentable (SWI/SNF) gene ARID2 was discovered in the patient without enhanced RAI avidity after therapy and an RAI-resistant tumor from another patient that was sampled off-study. Conclusions: The endpoints for success were met, providing preliminary evidence of vemurafenib+CDX-3379 safety and efficacy for enhancing RAI uptake. Preclinical data and genomic profiling in this small cohort suggest SWI/SNF gene mutations should be investigated as potential markers of resistance to redifferentiation strategies. Further evaluation of vemurafenib+CDX-3379 as a redifferentiation therapy in a larger trial is warranted (ClinicalTrials.gov: NCT02456701).

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Small-molecule MAPK inhibitors restore radioiodine incorporation in mouse thyroid cancers with conditional BRAF activation.

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