BAY 43-9006 inhibition of oncogenic RET mutants

Background. Medullary and papillary thyroid carcinomas are often associated with oncogenic activation of the RET tyrosine kinase. We evaluated whether the biaryl urea BAY 43-9006, which is known to inhibit several other tyrosine kinases, blocks RET kinase function and oncogenic activity. Methods: We examined BAY 43-9006 activity against oncogenic RET in vitro and in cellular RET signaling in oncogenic RET-transfected NIH3T3 fibroblasts by using immunocomplex kinase assays and immunoblotting with phospho-specific antibodies. The effects of BAY 43-9006 on proliferation of human TPC1 and TT thyroid carcinoma cells, which harbor spontaneous oncogenic RET alleles, and on RAT1 fibroblasts transformed with oncogenic RET mutants, including mutants that are resistant to other chemotherapeutic agents, were determined using growth curves and flow cytometry. Growth of TT cell-derived xenograft tumors in athymic mice treated orally with BAY 43-9006 or with vehicle was measured. All statistical tests were two-sided. Results: BAY 43-9006 inhibited oncogenic RET kinase activity at half-maximal inhibitory concentrations (IC(50)s) of 50 nM or less in NIH3T3 cells. It also arrested the growth of NIH3T3 and RATI fibroblasts transformed by oncogenic RET and of thyroid carcinoma cells that harbor spontaneous oncogenic RET alleles. Moreover, BAY 43-9006 inhibited the growth of cells carrying RET V804L (IC50 = 110 nM, 95% confidence interval [CI] = 88 to 133 nM) or RET V804M (IC50 = 147 nM, 95% CI = 123 nM to 170 nM), both mutants that are resistant to anilinoquinazolines and pyrazolopyrimidines. After 3 weeks of oral treatment with BAY 43-9006 (60 mg/kg/day), the volume of TT cell xenografts (n = 7) was reduced from 72.5 to 44 mm(3) (difference = 28.5 mm(3), 95% CI = 7 mm(3) to 50 mm(3)), whereas in vehicle-treated mice (it = 7), mean tumor volume increased to 408 mm(3) (difference = 320 mm(3), 95% CI = 180 mm(3) to 460 mm(3); untreated versus treated, P=.02). This inhibition paralleled a decrease in RET phosphorylation. Conclusions: BAY 43-9006 is a powerful inhibitor of the RET kinase. Its potential as a therapeutic tool for RET-positive thyroid tumors, including those expressing V804 mutations merits study.