A novel approach to quantitate biodistribution and transduction of adeno-associated virus gene therapy using radiolabeled AAV vectors in mice

An understanding of recombinant adeno-associated virus (AAV) biodistribution profiles is an important element of a preclinical development program. Here, we have developed a radiolabeling strategy utilizing the co-delivery of ¹²⁵I (non-residualizing) and ¹¹¹In (residualizing) radionuclide-conjugated AAVs to provide a detailed distribution quantification at tissue level delineating between the cellular internalized AAV (degraded, ¹¹¹In-¹²⁵I) and AAV remaining in the extracellular matrix (intact, ¹²⁵I). This labeling method has been successfully applied to AAV9 and AAV-PHP.eB as tool molecules without altering the physical properties and biological activities of the AAVs. Upon labeling with either of the radioactive probes, these molecules were systemically injected into C57BL/6 mice. The biodistribution results indicate that AAVs, with a fast distribution profile, were mainly located in the extracellular matrix of highly perfused organs such as liver and spleen at early time points, leading to a difference between capsid quantification and vector genome quantification. The results suggest that the ¹²⁵I-AAV/¹¹¹In-AAV co-delivery approach offers a robust and efficient analytical strategy to investigate the detailed tissue distribution of AAV vectors, including both vector genome and protein capsids. This novel method has the potential to be applied to capsid optimization, selection, and lead candidate development.