Spark hemophilia B gene therapy clears another early test

Spark's intervention appears to have managed the possible immune reactions to SPK-9001.

Spark Therapeutics has posted fresh data from a phase 1/2 trial of its hemophilia B gene therapy, SPK-9001. The data suggest Spark’s tapering courses of steroids successfully arrested the factor IX (FIX) declines seen in two patients who appeared to have an immune reaction to the Spark100 vector capsid.

Philadelphia-based Spark first reported on the two patients in December. At that time, the fall in FIX activity—from 71% to 68% and, more dramatically, 32% to 12%—raised concerns about the safety of the treatment. Spark downplayed these concerns, noting that the patient who received steroids more promptly only experienced a 3% decline in FIX activity. The latest set of data backed up Spark’s confidence.

Both patients have now completed their tapering courses of steroids. After completing the courses of steroids, both patients saw their alanine aminotransferase levels return to baseline. The FIX activity of the subject whose level fell to 12% has since stabilized. Spark said the patient has been at around 15% over the nine weeks since finishing the course of steroids, a level likely to result in improved clinical outcomes. The other patient’s FIX activity is back up above 70%.

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“Our analysis suggests that a tapering course of oral corticosteroids has been well-tolerated and may help control potential capsid immune responses following SPK-9001 infusion,” Spark CSO Katherine High, M.D., said in a statement.

Those data could lessen concerns that immune responses will scuttle the gene therapy, which has consistently delivered higher FIX activity levels than its main rival, uniQure’s AMT-060. UniQure sees safety as an area in which it may have an edge over Spark, although question marks hang over the elevated liver enzymes seen in some patients in the study.

Other aspects of the Spark data drop are free from red flags. Mean steady-stage FIX activity in the 10 participants who are 12 weeks or more post treatment stood at 33% at the last count. That is up on the 28% Spark reported in its December update. And with the range of responses spanning from 14% to 81%, it is also well clear of the 3% to 12.7% uniQure recorded in its high-dose cohort in December.

UniQure has argued FIX activity levels are less important than clinical benefits, such as cessation of bleeding and reduction in infusions. Spark’s latest data show SPK-001 continuing to perform well against these measures. At the last count, the annualized bleeding rate was down 96%. The fall in the annualized infusion rate was 99%.