Philadelphia drugmaker Spark Therapeutics, a for-profit company spun out of non-profit Children's Hospital of Philadelphia, said Monday it has begun a public offering of 3,500,000 shares of its common stock, which closed Monday at $52.55.

Spark said the offering includes 2,500,000 shares offered by Spark and 1,000,000 shares offered by Children's Hospital, the largest shareholder in the publicly traded company.

Spark said it will grant the underwriters of the offering an option for 30 days to purchase up to an additional 525,000 shares. Spark, with offices at 3737 Market St., said it will not receive proceeds from the sale of shares by CHOP.

Spark's leading drug candidate, SPK-RPE65, if approved by the Food and Drug Administration, would be the first U.S. gene treatment that uses genes as medicine. SPK-RPE65 streams genes directly into the retina of the eye to treat rare inherited blindness.

Spark announced last month, with partner Pfizer Inc., encouraging data on four patients treated with the investigational compound, SPK-9001, for hemophilia B, in which the body does not produce enough factor IX, a protein that clots blood.

Spark said Monday it has also seen strong pre-clinical data for SPK-8011 to treat hemophilia A.

Spark has "successfully delivered gene therapies with proof of concept data in both the eye and the liver," said Jeffrey D. Marrazzo, co-founder and CEO."We have made strong, tangible progress throughout our entire hemophilia franchise."

Clinical data from the hemophilia B program, and preclinical data "from our rapidly progressing program in hemophilia A, give us early confidence in achieving our goal of eliminating the need for regular infusions to control and prevent bleeding episodes in patients with these diseases through a potentially one-time, intravenous administration of highly optimized gene therapies," Marrazzo said.