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CHICAGO -- The targeted agent lenvatinib extended progression-free survival (PFS) among patients with treatment-refractory thyroid cancer, researchers said here.

Patients who received lenvatinib had a median PFS of 18.3 months compared with patients on placebo who had a PFS of 3.6 months -- a significant 14.7-month difference (P<0.0001), reported of the University Paris Sud, and colleagues.

In addition, the response rate to the investigational tyrosine kinase inhibitor (TKI) was 65% of the 261 patients in the SELECT trial compared with 2% of the 131 placebo patients (P<0.0001), he said during a press briefing at the American Society of Clinical Oncology annual meeting.

While nearly all of the patients on the study drug experienced adverse events, the events were manageable and there were no unexpected toxicities, the authors reported.

"Patients with progressive, relapsed, refractory, differentiated thyroid cancer have a 10-year survival rate of 10% from time of metastasis detection," Schlumberger said. "Until recently, treatment options for these patients have been limited."

"We are confident that, based on our findings, lenvatinib will eventually become a standard treatment for radioiodine-resistant thyroid cancer," he explained. "As little as a year ago, this group of patients had no effective treatment options. It's remarkable that we now have two active drugs in this setting, both of them tyrosine kinase inhibitors." The other drug, sorafenib (Nexavar), was shown effective in a study presented at last year's ASCO meeting and received FDA approval in November 2013 for the treatment of metastatic, differentiated thyroid cancer.

Lenvatinib is a selective TKI with a novel binding mode including sites at KDR (VEGFR-2), Flt-1 (VEGFR-1), RET, FGFR1, PDGFR-Beta and c-kit, which are involved in angiogenesis and tumor proliferation. Eisai submitted an application for organ designation in December 2012.

Schlumberger's group enrolled patients in SELECT with differentiated thyroid cancer who had refractory and measurable disease with independent radiological evidence of progression within the previous 13 months. They were assigned to receive either lenvatinib (24 mg a day on a 28-day cycle) or a matching placebo (24 mg) tablet. They were treated until confirmed disease progression. Patients were eligible to cross over to lenvatinib treatment if disease progressed.

Schlumberger said that 97% of the patients on lenvatinib experienced some adverse events, most often hypertension, diarrhea, fatigue, and decreased appetite. He said that 68% of the patients on lenvatinib required dose reduction, 82% of patients required dose interruption, and 14% of patients had to be removed from the drug.

Twenty people in the lenvatinib group died compared with five patients on placebo.

"Six of the 20 treatment-emergent deaths were considered by the researchers to be treatment-related," Schlumberger said. One person died as the result of a pulmonary embolism, one died due to hemorrhagic stroke, and four others died due to general health deterioration, he explained.

"In view of these deaths, I think we do have to proceed with caution," said ASCO press briefing moderator of the Helen F. Graham Cancer Center in Wilmington, Del.

"Treating cancer patients with cancer therapies is toxic," Masters said. "For example, I had a patient who came to see me to talk about sorafenib. She had progression of her disease but was feeling relatively well. We had a discussion about the risks of sorafenib, which probably are not as severe as lenvatinib, and she decided to wait because you don't necessarily have to go on the drug immediately. When a patient starts to have more and more effects of the cancer such as breathing problems or bone pain, then you are more likely to put up with those potential side effects. This is a fatal cancer but you have to balance that, and often the best way to go is to be patient until you really need to use it. That's the art of our practice."

UPDATE: This article, originally published on Jun. 1 at 00:00 EDT, was updated with new material on Jun. 4 at 11:18 EDT.