Vertex Pharmaceuticals (VRTX) shares sank Thursday morning after the company reported mixed results in a Phase 2 drug trial to treat lumbosacral radiculopathy (LSR), which causes leg and back pain.

The company said the trial of suzetrigine "met the primary endpoint with a statistically significant and clinically meaningful 2.02 point within-group reduction from baseline in the Numeric Pain Rating Scale (NPRS)."

However, the placebo reference arm "showed a similar within-group reduction from baseline in pain with a mean change in NPRS at Week 12 of -1.98," Vertex said. It added that "the study was not designed nor powered for statistical comparison between suzetrigine and placebo."

Vertex shares fell 14% shortly after the opening bell Thursday, bringing them into negative territory for the year.