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Haemophilia is an excellent candidate for gene therapy: it is caused by a single gene, the gene has been cloned, tests to measure factor levels (and thus the response to the gene therapy) are readily available and, most importantly, the level of factor VIII does not need to be tightly regulated and there are beneficial effects over a wide range of factor VIII activity levels: from a few percent to over 150%. This latter point is important because an individual's response to gene therapy cannot be accurately predicted.

BioMarin Pharmaceutical Inc.’s Roctavian (valoctocogene roxaparvovec-rvox), is the first gene therapy to be marketed for adults with severe hemophilia A. Roctavian was granted conditional marketing authorization by the European Commission on August 24, 2022 and approved by the US Food and Drug Administration (FDA) on June 29, 2023.

Roctavian uses a vector (a virus modified to deliver a gene therapy product), derived from an adeno-associated virus serotype 5 (AAV5), which has been stripped of most of its DNA so as to not cause disease. The vector delivers a working copy of the FVIII gene to liver cells, enabling the liver to produce FVIII on its own. Roctavian involves gene-addition: it adds a working copy of the FVIII gene to cells, but unlike gene editing, it does not modify an individual’s DNA and is not incorporated into the cells’ genome.