Gene-based drug developer Spark Therapeutics Inc. is taking more office space on a short-term basis near its University City headquarters as it continues to seek a new permanent location to accommodate its rapid growth.

The company plans to occupy 50,000 square feet of newly completed office space in the FMC Tower near 30th Street Station in the spring, and has recently moved into 6,500 square feet at 3711 Market St., part of the University City Science Center complex.

That's in addition to the 42,000 square feet that Spark, which was spun off from nearby Children's Hospital of Philadelphia three years ago, has already been occupying at 3737 Market, another Science Center property.

"We are reviewing our long-term multi-use real estate needs to accommodate our rapidly growing, fully integrated operations," spokeswoman Monique da Silva said in an email late last week. "In the short term, we will tap into additional office space near our current location in West Philadelphia."

Spark's search for new headquarters and lab space comes amid a string of clinical-testing successes that have attracted interest from investors and big pharmaceutical companies.

On Wednesday, Spark said on its website that it had received a $15 million payment from Pfizer Inc. for achieving pre-set milestones in the development of a hemophilia B treatment that the companies are working on together.

Spark also has been developing a therapy that streams genes directly into the retina of the eye to treat rare inherited blindness. It would be among the first U.S. treatments to use genes as medicine if approved by the U.S. Food and Drug Administration.

The drug-development activity has resulted in a burst of hiring, with the company reporting 170 employees as of Sept. 30, compared with fewer than 100 a year earlier.

Brokers with real estate services firm JLL, which is helping Spark vet new locations, said in July that the drug company was considering options in New Jersey and Delaware, as well as various sites in Philadelphia.

Last week, da Silva said that no final decision has been made, but that the company is "very proud of our West Philadelphia roots."

Matt Guerrieri, a managing director at commercial real estate firm Newmark Grubb Knight Frank in Philadelphia, said that Spark's decision to occupy space in both the FMC Tower and the Science Center for the short term may hold clues to its longer-term plans.

FMC Tower owner Brandywine Realty Trust and Science Center partner Wexford Science & Technology LLC are said to be courting Spark as an anchor tenant for the large redevelopment project each has underway in University City, said Guerrieri, who has no direct involvement in the matter.

Brandywine's Schuylkill Yards project is a 14-acre proposal being completed with Drexel University west of 30th Street Station. Wexford and the Science Center are planning their uCity Square project along and north of Market Street between 34th and 39th Streets.

Joseph Reagan, Wexford's senior vice president for development, said his team members are in discussions with Spark "to accommodate their planned expansion and look forward to accommodating them."

Brandywine chief executive Jerry Sweeney did not respond to an email asking about Spark's prospects as a Schuylkill Yards tenant.

Both Wexford and Brandywine are likely offering Spark flexible short-term leases in their existing buildings in hopes of coaxing it onto the larger campuses they have planned, Guerrieri said.

"They both have pad sites ready to go," he said.