257. Selection of Next Generation AAV Gene Therapy Vectors for Specific and Precise Gene Delivery

Abstract

The ultimate goal in human gene therapy is the specific and exclusive modification of the desired target cells upon systemic vector administration. Especially vectors derived from adeno-associated virus (AAV) are among the most promising gene transfer systems for in vivo application and have received broad attention due to substantial clinical benefit. However, AAV specificity for a particular target cell or tissue has been hampered by the broad tropism of different AAV serotypes. Over the last several years, new approaches have been initiated to create and select for more effective and selective recombinant AAV vectors by genetically modifying the capsid protein. These methods include random and/or rationale amino acid substitutions, creation of chimeric capsid variant libraries and various selection screens, and/or peptide insertion. A different approach involves the incorporation of highly specific binding molecules (DARPins) into the AAV capsid. DARPins are derived from ankyrin-repeat proteins that have been developed as alternative to antibody-based scaffolds, which are selected by high-throughput screens from DARPin libraries. At present, the bottleneck remains the cumbersome selection of DARPin molecules and the low number of functional DARPin capsid chimeras, which are able to assemble into a functional vector and still bind their intended extracellular target. Thus after months of screening and analysis the majority of the pre-selected DARPins are found not to be suitable for AAV targeting approaches. Here we describe a novel reliable and faster selection approach. First we eliminate the cumbersome and often misleading prokaryotic selection steps. To do this, we incorporate an entire DARPin library into the AAV capsid as VP2-fusion protein, generating either replication deficient or competent AAV library systems allowing for direct screening on therapeutically relevant cell types in vitro or in vivo. Our studies show that replicating deficient and replicating AAV libraries can be generated with high diversity up to 5×107 and high functional titers retaining their infectivity. We have generated a DARPin-capsid library in AAV-DJ (serotype with diverse cellular tropism) and AAV-LK03 (human selective serotype) and after only two selection rounds, found up to 10% of selected DARPins sharing identical repeat motifs responsible for target receptor binding in the pancreas and the liver. We are currently making vectors from these chimeric capsids and will determine their target specificity both in vivo and in vitro. This new approach takes only six days per selection cycle, allowing discovery of novel DARPin molecules and selection for targeted vectors in substantially reduced time. This approach expands the potential diversity in creating and defining novel rAAV vectors with medically relevant transduction properties. Such viral vectors will not only open the door for an array of new approaches to treat acquired human diseases but also push the development of new AAV-based gene therapy vectors to the next level.