PHILADELPHIA (TheStreet) -- Spark Therapeutics (ONCE - Get Report) provided more details Thursday about changes made to a late-stage clinical trial involving the company's gene therapy, trying to calm the nerves of jittery biotech investors before the study's results are announced in October.

A wave of heavy selling hit Spark on Aug. 5 after the company disclosed that it was speaking with the FDA about ways to "enrich our analysis" of the data collected from the phase III study. On Thursday's conference call, Spark tried to tamp down investor concerns about the changes, emphasizing that the study's primary endpoint remains the same and modifications made to secondary endpoints were done to provide a stronger overall package of clinical data to eventually present to regulatory agencies like the FDA.

Spark shares are down 6% to $42.31 following the conference call, suggesting (at least in the short term) that investors are still uneasy as they wait to see the outcomes of the study. Spark shares are down 26% since the Aug. 5.

Spark's lead gene therapy, SPK-RPE65, is designed to improve vision in patients with inherited retinal dystrophies (IRD) -- a group of rare eye diseases affecting the retina. Patients with IRDs have decreased sensitivity to light that eventually leads to blindness.

The primary endpoint of Spark's phase III study requires SPK-RPE65 to demonstrate an improvement in functional vision compared to a control. Spark is measuring functional vision using a mobility test in which patients must walk through a maze without bumping into objects placed in their path. The maze is illuminated at different light levels, ranging from the equivalents of a moonless night to an office setting. Spark hopes to show that patients treated with SPK-RPE65 for one year can navigate the maze more successfully at lower light levels than those in the control arm.

On Thursday, Spark assured investors this mobility test of functional vision remains the primary endpoint of the SPK-RPE65 phase III study, as originally designed. Additions and changes to secondary efficacy endpoints were made, however. These changes, taken together, will likely help Spark generate additional positive data for SPK-RPE65 in case the primary endpoint narrowly misses.

An analyst on the call asked Spark what would happen if the study missed on the primary endpoint but other measures of benefit were more positive. Would the company still file SPK-RPE65 for approval?

In response, CEO Jeff Marrazzo said that while it was too early to hypothesize about the study outcome, "based on discussions with regulators, the totality of the data will be important."

The recent clinical setbacks at Avalanche Biotechnologies (AAVL) and Celadon (CLDN) -- also gene therapy firms -- has put a damper on the market's love-a-thon with the emerging technology. It's a mistake to lump all gene therapy stocks together, but it would also be a confidence booster for the market to see positive results from a pivotal gene therapy study.

Spark is next in line. Results from the company's phase III study -- changes and all -- will be released in October.

 

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.