Shares of Spark Therapeutics surged nearly 20 percent Wednesday after the Philadelphia gene therapy company revealed promising results from a study of its potential one-time therapy for hemophilia A.

Preliminary data from a Phase 1/2 dose-escalation clinical trial of SPK-8011 showed "human proof-of-concept" in three participants, the drug maker said.

"The encouraging start" for hemophilia A "reinforces the strength of our gene-therapy platform and positions us well to potentially transform the current treatment approach for this life-altering disease with a onetime intervention," said Katherine A. High, Spark's president and chief scientific officer.

Hemophilia is a genetic disorder caused by missing or defective factor VIII, a clotting protein. About 20,000 Americans live with hemophilia. The way the medical community has addressed the disorder is to ensure that patients have continuous injections of blood-clotting factors. Patients infuse themselves two to three times a week for the rest of their lives.

In the study, three patients received infusions of "vector genomes" and no serious adverse events were reported, Spark said. One person has been followed for 23 weeks and another for 12 weeks. The initial dose created stable factor VIII levels with no spontaneous bleeds, the company said.

For a third patient, the genome dose was doubled and that person's factor VIII activity level "is tracking proportionally higher, consistent with the dose escalation." So far, the drug has been safe and well tolerated, with no reports of serious adverse events, no thrombotic events, no immune responses, and no elevations of liver enzymes, the company said.

"The data must be considered preliminary and one must be careful not to overinterpret them," said Cowen & Co. analyst Phil Nadeau in a client update. "That being said, we find the results quite encouraging."

Despite a low starting dose, the gene therapy produced "stable and clinically meaningful factor levels" sufficient to prevent spontaneous bleeds in patients, Nadeau said. Moreover, the safety profile is "clean thus far." The results suggest the company may "be able to achieve greater factor levels" at higher doses. "We find SPK-8011's early data encouraging, and think they suggest that Spark has a viable and competitive hemophilia A program."

Spark will present full data at a medical conference in December.

The hemophilia A results, though early, along with previously reported data for the company's hemophilia B candidate, "confirm Spark's thought leadership in hemophilia gene therapy, and the likelihood of achieving a leading position in the overall hemophilia market (currently $7 billion, growing to $14 billion in 2030)," Chardan Capital Markets analyst Gbola Amusa said in a client note. Chardan raised its peak earnings forecast for Spark's hemophilia A therapy to $1.3 billion, up from $397 million.

Spark's lead drug, a treatment for rare inherited blindness, is under priority review with the U.S. Food and Drug Administration, with a possible approval date of Jan. 12, 2018. If approved, it would be the first gene therapy for a genetic disease in the United States.

Spark, which was spun out of Children's Hospital of Philadelphia, reported a second-quarter financial loss of $74.4 million in the quarter ended June 30, or $2.40 per share, on revenue of $1.5 million from its collaboration with Pfizer Inc. for hemophilia B.

Spark's shares have risen 58 percent since Jan. 1 and 37 percent in the last 12 months. The stock closed up 19.72 percent, or $13.13,  to $79.72.