Immune response seen in Spark hemo B study

In updated interim results from an ongoing Phase I/II study to treat hemophilia B, Spark Therapeutics Inc. (NASDAQ:ONCE) and partner Pfizer Inc. (NYSE:PFE) said a single administration of the gene therapy SPK-9001 led to "consistent and sustained Factor IX activity levels" in four evaluable patients with at least 12 weeks of follow-up, but noted an immune response to the therapy's vector in one other patient. Spark slid $6.45 (14%) to $40.51 on the news, and ended the day with a market cap of about $1.2 billion.

Spark said the patient had an immune response to the AAV capsid prior to 12-week follow-up, and experienced an accompanying drop in Factor IX activity. The patient then received a tapering course of corticosteroids, did not have any bleeds and did not require replacement Factor IX...