Updated from 8:42 a.m. EST

Transkaryotic Therapies

(TKTX)

lost almost half its value in the premarket session after saying its Fabry Disease drug candidate Replagal fared no better than a placebo in a phase III study.

The company said a preliminary review of the six-month data didn't show a statistically significant difference between treated and placebo patients for the primary endpoint of renal function.

"Based on our earlier clinical data, we believed that Replagal would demonstrate a statistically significant effect on kidney function after six months of treatment," the company said in a release. "In hindsight, we believe renal benefits might have been observed more readily over a longer treatment period." The company said it expects longer-term data from patients in the study to confirm the drug's efficacy.

The shares fell $5.64, or 40%, to $8.62 on the Instinet premarket session. Shares of

Genzyme

(GENZ)

, which is seeking approval for its own Fabry Disease drug, rose 8% to $32.50 on the news.