Spark Therapeutics shares plunged Monday while those of competitor BioMarin Pharmaceutical Inc. climbed after both reported data on experimental treatments for hemophilia A.

Philadelphia-based Spark released data at a medical meeting in Atlanta on four patients. Two received low-dose infusions of SPK-8011 treatment, aimed at improving a blood clotting protein that hemophilia patients lack. The first two patients, after 40 weeks and 30 weeks, had a blood clotting or factor activity level of 10 percent and 16 percent of normal, respectively.

Spark doubled the dose for two more patients, and after 19 weeks and 14 weeks, the factor or clotting level remained about 10 percent and 16 percent.

Investors assumed that when the dose was doubled, Spark's results would show twice the factor or clotting level – that dose potency would improve clotting. When that didn't happen, the stock took a hit.

BioMarin also released data at the American Society of Hematology (ASH) meeting on six of seven patients, and the factor or clotting levels were higher.

Spark, spun out of research at Children's Hospital of Philadelphia, is also developing treatments for hemophilia B and for a rare inherited blindness. Spark expects the Food and Drug Administration's approval of its gene therapy for inherited blindness on Jan. 12.

Katherine A. High, Spark's president and head of research and development, said that BioMarin's doses for hemophilia A were 120 times higher than Spark's initial dose. BioMarin also followed its patients for a longer time. Levels of the protein are known to increase with time, High said.

"This is a dose escalation study," High said.  "The really important thing, all these patients took clotting factor infusions one to three times a week," but after getting Spark's treatment, "nobody needed clotting factor for this disease anymore," she said. Spark's hemophilia A gene therapy is a one-time infusion.

The four Spark patients studied had no spontaneous bleeds. "They went from having a defined level of bleeds every year to having no bleeds," High said. "It's working great at this dose."

Going forward, Spark announced that it plans to increase the dose, and to study more people. "We have confidence in this vector, are very pleased with the clinical results to date, and look forward to continuing the study," High said.

Leerink Partners analyst Joseph Schwartz said Monday that the Spark patients had "very inconsistent" responses to the drug. "Although the enrollment size is small so far, early signs raise more caution than encouragement," he said in an update, reported by Bloomberg News.

Spark dropped 34.97 percent, or $25.66, to $47.72 on Monday. The stock closed at $73.38 on Friday.

BioMarin shares closed Monday up 7.46 percent, or $6.12, to $88.11.