Activity of transgene-produced B-domain-deleted factor VIII in human plasma following AAV5 gene therapy
Steffen Rosen, Stefan Tiefenbacher, Mary Robinson, Mei Huang, Jaydeep Srimani, Donnie Mackenzie, Terri Christianson, K John Pasi, Savita Rangarajan, Emily Symington, Adam Giermasz, Glenn F Pierce, Benjamin Kim, Stephen J Zoog, Christian Vettermann, Steffen Rosen, Stefan Tiefenbacher, Mary Robinson, Mei Huang, Jaydeep Srimani, Donnie Mackenzie, Terri Christianson, K John Pasi, Savita Rangarajan, Emily Symington, Adam Giermasz, Glenn F Pierce, Benjamin Kim, Stephen J Zoog, Christian Vettermann
Abstract
Adeno-associated virus (AAV)-based gene therapies can restore endogenous factor VIII (FVIII) expression in hemophilia A (HA). AAV vectors typically use a B-domain-deleted FVIII transgene, such as human FVIII-SQ in valoctocogene roxaparvovec (AAV5-FVIII-SQ). Surprisingly, the activity of transgene-produced FVIII-SQ was between 1.3 and 2.0 times higher in one-stage clot (OS) assays than in chromogenic-substrate (CS) assays, whereas recombinant FVIII-SQ products had lower OS than CS activity. Transgene-produced and recombinant FVIII-SQ showed comparable specific activity (international units per milligram) in the CS assay, demonstrating that the diverging activities arise in the OS assay. Higher OS activity for transgene-produced FVIII-SQ was observed across various assay kits and clinical laboratories, suggesting that intrinsic molecular features are potential root causes. Further experiments in 2 participants showed that transgene-produced FVIII-SQ accelerated early factor Xa and thrombin formation, which may explain the higher OS activity based on a kinetic bias between OS and CS assay readout times. Despite the faster onset of coagulation, global thrombin levels were unaffected. A correlation with joint bleeds suggested that both OS and CS assay remained clinically meaningful to distinguish hemophilic from nonhemophilic FVIII activity levels. During clinical development, the CS activity was chosen as a surrogate end point to conservatively assess hemostatic efficacy and enable comparison with recombinant FVIII-SQ products. Relevant trials are registered on clinicaltrials.gov as #NCT02576795 and #NCT03370913 and, respectively, on EudraCT (European Union Drug Regulating Authorities Clinical Trials Database; https://eudract.ema.europa.eu) as #2014-003880-38 and #2017-003215-19.
Conflict of interest statement
Conflict-of-interest disclosure: S. Rosen is chairman of Rossix AB. S.T. and M.R. are employees and hold stock in Laboratory Corporation of America Holdings. S. Rosen and G.F.P. are consultants for BioMarin Pharmaceutical Inc. M.H., J.S., D.M., T.C., B.K., S.J.Z., and C.V. are employees and own stock in BioMarin Pharmaceutical Inc. The remaining authors declare no competing financial interests.
© 2020 by The American Society of Hematology.
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Source: PubMed