Shares of uniQure slid after patients treated with its Huntington's disease drug in a recent study no longer showed a statistically significant slowdown in disease-induced decline after four years.

The stock fell 47% to $20.91 in premarket trading on Tuesday. As of Monday's market close, shares were up 63% year to date.

At four years, the study's primary endpoint--composite Unified Huntington's Disease Rating Scale (cUHDRS)--showed a 44% slowing of disease progression compared with an external control, but the data did not reach statistical significance, the company said.

cUHDRS had shown 80% slowing of disease progression compared with the external control, with statistical significance, at three years.

The latest study results draw from data on 12 high-dose patients. The company said the treatment effect could be understated by substantial missing data and survivor bias in an updated external control.