Another obstacle to AAV-mediated gene transfer for HA gene therapy is the size of the FVIII coding sequence, which at 7.0 kb far exceeds the normal packaging capacity of AAV vectors. Packaging of large expression cassettes into AAV vectors has been reported but this is a highly inconsistent process resulting in low yields of vector particles with reduced infectivity [49, 50]. AAV vectors encoding the BDD-FVIII variant that is around 4.4 kb in size show promising results using canine FVIII but further evaluation of this approach using human BDD-FVIII is required. Other approaches include the co-administration

of two AAV vectors separately encoding the FVIII heavy- and light chains whose intracellular association in vivo leads to the formation of a functional molecule. The alternative two AAV vector approach exploits the tendency of these vectors GS-1101 cell line to

form head to tail concatamers. Therefore, by splitting the expression cassette such that one AAV vector contains a promoter and part of the coding sequence, as well as a splice donor site, whereas the other AAV vector contains the splice acceptor site and the remaining coding sequence. Following in vivo head to tail concatemerization a functional transcript is created that is capable of expressing full-length FVIII protein [51-53]. We have developed an AAV-based gene transfer approach that addresses both the size constraints and inefficient FVIII expression. Expression of human FVIII was improved 10-fold by re-organization of the wild-type medchemexpress cDNA of human http://www.selleckchem.com/products/LDE225(NVP-LDE225).html FVIII according to the codon usage of highly expressed

human genes [54-56]. Expression from B-domain-deleted codon optimized FVIII molecule was further enhanced by the inclusion of a 17 amino-acid peptide that contains the six N-linked glycosylation signals from the B domain required for efficient cellular processing. These changes have resulted in a novel 5.2 kb AAV expression cassette (AAV-HLP-codop-hFVIII-V3), which is efficiently packaged into recombinant AAV vectors and capable of mediating supraphysiological level of FVIII expression in animal models over the same dose range of AAV8 that proved to be efficacious in subjects with haemophilia B. Our novel AAV-HLP-codop-hFVIII-V3 cassette substantially improves the prospects of safe and effective gene transfer for haemophilia A. We are currently in the process of developing clinical grade AAV-HLP-codop-hFVIII-V3 for use in human subjects in the context of a clinical trial, which we hope will open in early 2015. The design of this clinical trial will be discussed in greater detail during the meeting. Recombinant retroviruses used in clinical gene therapy applications have been extensively engineered for efficient transfer of nucleic acid sequences into human cells. The most significant modification is the creation of replication incompetent viruses.