Nearly 50% of patients with the most aggressive form of thyroid cancer had long-lasting responses to pazopanib in a phase 2 study by Mayo Clinic researchers.
Keith Bible, MD, and his coworkers evaluated the safety and efficacy of pazopanib, a tyrosine kinase inhibitor approved to treat kidney cancer, in 37 patients with metastatic, rapidly progressive, radioiodine-refractory differentiated thyroid cancers. Patients received a median of 12 cycles of therapy. Of the 37 patients, 18 (49%) had confirmed partial responses, and 12 patients are still alive without disease progression. The median progression-free survival was 11.7 months, with an overall survival rate of 81% at 1 year.
According to the authors, to their knowledge, these findings represent by far the highest response rate yet reported in patients with such an aggressive form of differentiated thyroid cancer. Their findings are reported in the September 20 online issue of The Lancet Oncology.
Despite the promising results, the researchers caution that pazopanib should not be used in patients with slow-growing differentiated thyroid cancers because of the potential for serious side effects. In this study, the dose of the drug had to be reduced in 16 patients because of side effects judged to be potentially threatening or debilitating, and two patients had significant bleeding. Two patients died in association with pre-existing disease during the study, but pazopanib could have contributed to their deaths, Bible said.
Further studies of pazopanib in advanced thyroid cancer are under way at Mayo Clinic and a larger phase 3 trial is planned.