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Wolfgang Miesbach։ Thrombosis After Haemophilia B Gene Therapy

Wolfgang Miesbach/LinkedIn

Aug 20, 2026, 14:22

Wolfgang Miesbach։ Thrombosis After Haemophilia B Gene Therapy

Wolfgang Miesbach, Professor of Medicine at Frankfurt University Hospital, shared post on LinkedIn:

”Newly reported at ISTH2026: bilateral pulmonary emboli after gene therapy for haemophilia B.

A 52-year-old man with moderately severe haemophilia B (FIX less than 2 percent), physically active, with no thrombotic risk factors, received 1 times 10 to the 12th vg per kg fidanacogene elaparvovec in a Pfizer-sponsored trial in May 2023 — twice the licensed dose of 5 times 10 to the 11th vg per kg.

For two years, his FIX activity fluctuated but was repeatedly supranormal. Then, at week 108: DVT and bilateral pulmonary emboli on CTPA, now requiring long-term anticoagulation.

Not the first thrombotic event reported after haemophilia B gene therapy: B-AMAZE (FLT180a) reported an AV fistula thrombosis at 1.28 times 10 to the 12th vg per kg, with FIX levels reaching approximately 310 percent.

Assay discrepancy, followed through week 120:

One-stage SynthASil, local: plateau 120 to 160 percent, peaks approximately 180 percent.
One-stage SynthASil, central: approximately 220 percent early and again near week 104, approximately 185 percent at the time of the event.
One-stage Actin-FSL: mostly 80 to 110 percent, but three discordant spikes of 310 percent or higher, and only approximately 130 percent on the day of thrombosis.
Chromogenic: 0.6 to 1.0 IU per mL throughout, never above the upper limit.

Wolfgang Miesbach։ Thrombosis After Haemophilia B Gene Therapy