NovoCure (NVCR) said Thursday a phase 3 trial evaluating its tumor treating fields therapy in patients with newly diagnosed glioblastoma failed to meet its primary endpoint.

The company said initiating TTFields therapy at the start of chemoradiation did not result in a statistically significant improvement in overall survival compared with starting the therapy during the maintenance phase.

In the intent-to-treat population, median overall survival was 17.7 months in the early-start arm, compared with 17.5 months in the maintenance-start arm, NovoCure said.

The company added that long-term survival rates were similar between the two groups, with three-year survival rates being 22.5% in the early-start arm and 18.4% in the maintenance-start arm.

Shares of the company were down more than 21% in Thursday's premarket trading.