Drug developer Spark Therapeutics Inc said its experimental gene therapy helped improve vision in patients with a type of inherited eye disorder in a late-stage study.

The company said its lead drug, SPK-RPE65, met the goals of improving vision and sensitivity to light in patients who were previously at the risk of complete blindness.
Shares of the company were up about 70 percent in premarket trading.

SPK-RPE65 is a form of gene therapy, a treatment which aims to insert corrective genes into malfunctioning cells. Spark said it would file for marketing approval in the United states in 2016.

The study tested the drug in 31 patients with inherited retinal dystrophies (IRDs), a broad group of hereditary disorders affecting the retina.

Although gene therapy has been touted as a one-time cure for intractable and expensive-to-treat diseases, it has been dogged by a series of disappointments and safety concerns in its more than 20 years of research.

However, investor interest in the field has grown this year, with Bluebird Bio Inc, UniQure NV and Spark all staging successful Nasdaq debuts.

There were no serious adverse events or harmful immune responses observed in the trial, the company said on Monday.

Up to Friday's close, Spark's shares had risen 91 pct since its IPO debut on Jan. 30.