PRAGUE – Fitusiran, a novel small interfering RNA therapeutic that decreases antithrombin (AT) synthesis and bleeding in patients with hemophilia A or B, with or without inhibitors, is reversible upon dosing cessation and becomes effective again with resumption of dosing. That finding is based on data obtained before, during, and after a phase 2 study dosing suspension.
Will Pass/MDedge News
Dr. Craig Benson
In September 2017, the fitusiran open-label extension study was suspended to investigate a fatal thrombotic event. The suspension was lifted 3 months later, and the trial continued with reduced doses of replacement factors or bypassing agents for breakthrough bleeds.
The pause in the study was used to gather data about the effects of dosing cessation and resumption, which were reported by coauthor Craig Benson, MD, of Sanofi Genzyme in Cambridge, Mass. Dr. Benson presented findings at the annual congress of the European Association for Haemophilia and Allied Disorders on behalf of lead author John Pasi, MD, PhD, of the Haemophilia Centre at the Royal London Hospital and colleagues.
Prior to the suspension, 28 patients with hemophilia A or B, with or without inhibitors, were given 50-80 mg of fitusiran for up to 20 months with a median dose duration of 11 months. During dosing suspension, the investigators measured AT levels, thrombin generation, and annualized bleeding rates (ABR).
“We can see that the antithrombin knockdown effect of fitusiran is reversible with dosing hold,” Dr. Benson said, referring to an upward trend of median AT level.
Within 4 months of stopping fitusiran, median AT level was 60% of normal. This level continued to rise to about 90% by month 7, before returning to normal at month 11.
“This is an AT recovery we had previously seen by a few subjects that had discontinued [fitusiran] prior to the clinical hold,” Dr. Benson said, “but here, with a much larger sample size, we see a fairly consistent AT recovery.”
As expected, while AT levels rose, thrombin generation showed an inverse trend. “We see a similar temporal pattern,” Dr. Benson said. “The bulk of thrombin generation decrease is seen by the 4th month off fitusiran.”
When patients restarted fitusiran after the suspension was lifted, AT levels dropped to about 30% of normal by month 1 and about 20% of normal from month 2 onward. Again, in an inverse manner, thrombin generation increased.
Clinically, changes in AT and thrombin generation before, during, and after study suspension were reflected in median ABR, which rose from 1.43 events per year prior to cessation to 6.47 events per year during treatment hold before falling to 1.25 events per year after restarting fitusiran.
Overall, the results support the efficacy and reversibility of fitusiran. The agent is continuing to be studied in the phase 3 ATLAS trials.
The study was funded by Sanofi Genzyme and Alnylam. Dr. Benson is an employee of Sanofi Genzyme. Other investigators reported financial ties to Sanofi Genzyme, Alnylam, Baxalta, Octapharma, Pfizer, Shire, and others.
SOURCE: Pasi J et al. EAHAD 2019, Abstract OR16.