The investigational drug (motesanib diphosphate) is a VEGF inhibitor, a biologic agent that targets receptors on a protein known as vascular endothelial growth factor, or VEGF. The researchers said VEGF is instrumental in angiogenesis -- formation of new blood vessels -- a process that allows tumors to grow and spread.

"There is no standard accepted chemotherapy for advanced metastatic differentiated thyroid cancer, and response rates have typically been 25 percent or less," said Dr. Steven Sherman, a professor at the University of Texas M.D. Anderson Cancer Center.

Sherman, with colleagues in 10 nations, planned and conducted one of the largest clinical trials ever done for metastatic thyroid cancer.

Of the 93 patients with rapidly progressing cancer who were enrolled in the study, 49 percent had a positive response. From that group 14 percent experienced tumor shrinkage while 35 percent experienced tumor stabilization for more than 24 weeks. Median progression-free survival was estimated to be 40 weeks.

The study is detailed in the July 3 edition of The New England Journal of Medicine.