Three years after spinning off from Children's Hospital of Philadelphia, gene-based drug developer Spark Therapeutics is planning an expansion that could take it out of the city.

While Spark, currently based in University City, has some Philadelphia sites on its radar, it's also considering locations in New Jersey and Delaware, according to brokers at real estate services firm JLL, which is aiding the company in its search.

The move to expand comes as Spark - among the most successful companies to emerge from research originating at Children's Hospital during its 160-year history - enjoys a string of clinical-testing successes that could yield one of the country's first approved gene therapies.

Spark's ability to translate research from one of Philadelphia's many university labs into a successful business makes it a model the city should strive to retain as it seeks to upgrade its economy, said former commerce director Alan Greenberger.

"I'm sure everybody's eager to keep them here," he said. "You don't ever like to lose an emerging business, especially one in a field like this, where there's such strong growth potential."

Spark and partner Pfizer Inc. announced Thursday that the U.S. Food and Drug Administration had granted "breakthrough therapy" designation to a treatment being developed to fight hemophilia B.

Breakthrough designation, granted to investigational treatments with the potential to surpass existing therapies, is intended to speed development and FDA review of drugs to treat serious medical conditions.

The company also published findings this month showing the safe and effective use on 11 test subjects of its leading drug candidate, a treatment that streams genes directly into the retina of the eye to treat rare inherited blindness.

The therapy would be among the first U.S. treatments to use genes as medicine if approved by the FDA.

Those successes have prompted Spark to seek a "significant increase" beyond the 48,000 square feet it now occupies in buildings within University City's Science Center business-development and research complex, Tyler Vandegrift, a JLL executive vice president, said Wednesday.

The company had 128 full-time employees as of May 4, when it announced financial results for the quarter ended March 31, up from 50 at the end of 2014. Spark spokesman Dan Quinn did not immediately respond to an email seeking comment on the implications of a move out of Philadelphia.

Spark's new location - which will accommodate administrative offices, research-and-development labs, and drug-production facilities - may be in a building shared with other firms or in a new purpose-built structure, said JLL managing director Ron Cariola.

Two of the Philadelphia locations under consideration are in University City: the uCity Square development planned by Wexford Science and Technology and the Science Center, and the Schuylkill Yards project by Brandywine Realty Trust in partnership with Drexel University.

South Philadelphia's city-owned Navy Yard complex, being developed by Liberty Property Trust, is also being mulled, Cariola said.

In New Jersey, the focus is on Brandywine's Knights Crossing development in Camden, where the developer is building a new headquarters for Subaru of America, he said. The potential Delaware location is in the Wilmington area.

JLL is in conversation with officials in Pennsylvania, New Jersey, and Delaware regarding financial incentives for Spark, Cariola said.

"This is a pretty comprehensive review of locations where they can continue to grow," Cariola said. "Spark is a great Philadelphia growth story."

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