Researchers reported that increases in factor IX expression levels were associated with decreases in annualized bleeding rates among men with hemophilia B in the United States and Europe. The study was published in Haemophilia.

Hemophilia B is caused by factor IX deficiency and affects around 3.8 per 100,000 male individuals globally. Clinical condition is largely determined by residual factor IX levels; less than 1% is considered severe. The current standard of care for hemophilia is prophylactic prevention of bleeding using factor replacement therapy and on-demand treatment when breakthrough bleeding occurs. 

“A spectrum of options exists between continuous prophylaxis and on-demand therapy, including ‘intermittent’ prophylaxis and ‘individualized’ prophylaxis, meant to be optimized based on the patient’s pharmacokinetic profile and lifestyle,” the authors of the study wrote. 

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Moreover, gene therapy has emerged as a potential breakthrough treatment to provide continuous endogenous production of factor IX; if this occurs, this therapeutic option may be curative. Scientists’ focus at present is to maintain steady factor IX expression levels in the hopes that this reduces bleeding. The authors of the study hence sought to examine the relationship between factor expression levels and annualized bleeding rates. 

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They used data from 2 studies: the “Cost of Hemophilia in Europe: a Socioeconomic Survey” (CHESS EU I-II) and the “Cost of Severe Hemophilia across the US: a Socioeconomic Survey” (CHESS US/US+). The CHESS EU I-II study encompassed 5 European countries—France, Germany, Italy, Spain, and the United Kingdom—and included male patients aged 18 years and above with hemophilia A or B. The CHESS US study was a 12-month retrospective study involving adult male patients with severe hemophilia.

From both studies, the research team identified 407 male patients with hemophilia B who did not have inhibitors and were receiving on-demand treatment. 

“The results of this study . . . suggest that factor IX expressions within the upper mild hemophilia range (ie, >25% factor IX levels) following gene therapy administration could be sufficient to eradicate all bleeding events and eliminate the need for prophylactic factor IX replacement therapy, based on typical [annualized bleeding rates] reported in severe and moderately severe [hemophilia B] patients within studies examining outcomes of [patients with hemophilia B] receiving prophylaxis,” the authors of the study concluded.


Burke T, Shaikh A, Ali TM, et al. Association of factor expression levels with annual bleeding rate in people with haemophilia BHaemophilia. Published online November 4, 2022. doi:10.1111/hae.14675