| Summary: | In 2018, Refacto AF R , a B-domain-deleted third-generation FVIII concentrate, became our preferential product. After the introduction, the development of inhibitors was prospectively monitored; retrospectively, we sought for risk factors in the patients who developed a de-novo inhibitor. Over a period of 15 months, 4/19 adult patients with non-severe haemophilia who were treated on demand for surgery, developed high titer antibodies to FVIII after administration of Refacto AF R ; 5/52 mostly severe patients on prophylaxis, developed an inhibitor (3 ≥ 0.1 BU; 1 > 0.6 BU, 1 high titre) after they switched to Refacto AF R ; all were children <14 years of age and with >100 exposure days, none related to surgery or intensive treatment; all received Kovaltry R before. In conclusion: inhibitors were encountered in on demand patients and previously treated prophylaxis patients; this observation might be a coincidental finding, but also risk factors like genotype and surgery and/or that Refacto AF R is more immunogenic should be considered. For the patients on prophylaxis we hypothesize that loss of tolerance by preceding Kovaltry R might have contributed to inhibitor development.
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