Wolfgang Miesbach/LinkedIn
Jul 17, 2026, 09:36
Wolfgang Miesbach: A Landmark Case of Supratherapeutic FIX Following Gene Therapy Presented at ISTH 2026
Wolfgang Miesbach, Professor of Medicine at Frankfurt University Hospital, shared on LinkedIn:
”Supratherapeutic FIX after gene therapy: a case that deserves attention.
At ISTH 2026, Zoe Marks, Jennifer Curnow and colleagues (Royal Prince Alfred and Westmead, PB2056) report a 52-year-old man with moderately severe haemophilia B treated with fidanacogene elaparvovec:
- From week 5, FIX exceeded 224% by one-stage assay with SynthASil; one-stage with Actin-FSL and the chromogenic assay remained normal
- Intermittent peaks above 150% over two years
- Week 118: exertional dyspnoea and pleuritic chest pain, then bilateral pulmonary emboli and a left leg DVT
- Physically active, no risk factors, negative thrombophilia screen; now on long-term anticoagulation
- The assay discrepancy determines whether overexpression is detected at all.
Precedents exist in both phenotypes:
- Haemophilia B: AV fistula thrombosis at the highest FLT180a dose with supranormal FIX
- Haemophilia A: FVIII above 150% in AFFINE prompted a voluntary pause and FDA clinical hold in 2021, with a protocol amendment for managing elevated FVIII. About half of dosed participants had transient FVIII above 150%, some received anticoagulation, and a below-knee DVT was reported in a participant with elevated FVIII
This appears to be the first thrombotic event after fidanacogene elaparvovec.
We have no agreed threshold, no agreed assay, and no guidance on anticoagulation after gene therapy.
The Sydney group’s local SOP and medical alert card address a real gap.
Thank you for reporting this.”
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