Systemic delivery of adeno-associated viral vectors

Abstract

For diseases like muscular dystrophy, an effective gene therapy requires bodywide correction. Systemic viral vector delivery has been attempted since early 1990s. Yet a true success was not achieved until mid-2000 when adeno-associated virus (AAV) serotype-6, 8 and 9 were found to result in global muscle transduction in rodents following intravenous injection. The simplicity of the technique immediately attracts attention. Marvelous whole body amelioration has been achieved in rodent models of many diseases. Scale-up in large mammals also shows promising results. Importantly, the first systemic AAV-9 therapy was initiated in patients in April 2014. Recent studies have now begun to reveal molecular underpinnings of systemic AAV delivery and to engineer new AAV capsids with superior properties for systemic gene therapy.

Figure 1. Systemic AAV delivery results in bodywide gene transfer in rodents and large mammals

Peripheral vascular delivery provides a method that allows an AAV vector to reach most, if not every, part of the body. A, Bodywide muscle transduction in mice following tail vein delivery of an alkaline phosphatase (AP) reporter gene AAV-9 vector. B, Robust and persistent (up to one year) skeletal muscle and myocardial transduction after jugular vein injection of an AAV-8 AP vector in a neonatal dog. C, Tyrosine mutant AAV-9 results in whole body striated muscle transduction in young adult dystrophic dogs. Top panel, representative full-view images from selected skeletal muscles; middle panel, representative high-power images from selected skeletal muscles, heart and internal organs; bottom panel, quantification of the AAV genome and AP expression in selective tissues. BB, biceps brachii; Bra, brachialis; Dia, diaphragm; CT, cranial tibialis; ECR, extensor carpi radialis; FCU, flexor carpi ulnaris; FD, flexor digitorum; Gas, gastrocnemius; IS, interstitial septum; LV, left ventricle; LVa, left ventricle anterior portion; LVx, left ventricle apex; PM, papillary muscle; Sep, septum; TB, triceps brachii; Ter, teres; Ton, tongue; VM, vastus medialis.

Figure 1. Systemic AAV delivery results in bodywide gene transfer in rodents and large mammals

Peripheral vascular delivery provides a method that allows an AAV vector to reach most, if not every, part of the body. A, Bodywide muscle transduction in mice following tail vein delivery of an alkaline phosphatase (AP) reporter gene AAV-9 vector. B, Robust and persistent (up to one year) skeletal muscle and myocardial transduction after jugular vein injection of an AAV-8 AP vector in a neonatal dog. C, Tyrosine mutant AAV-9 results in whole body striated muscle transduction in young adult dystrophic dogs. Top panel, representative full-view images from selected skeletal muscles; middle panel, representative high-power images from selected skeletal muscles, heart and internal organs; bottom panel, quantification of the AAV genome and AP expression in selective tissues. BB, biceps brachii; Bra, brachialis; Dia, diaphragm; CT, cranial tibialis; ECR, extensor carpi radialis; FCU, flexor carpi ulnaris; FD, flexor digitorum; Gas, gastrocnemius; IS, interstitial septum; LV, left ventricle; LVa, left ventricle anterior portion; LVx, left ventricle apex; PM, papillary muscle; Sep, septum; TB, triceps brachii; Ter, teres; Ton, tongue; VM, vastus medialis.

Figure 2. Rate-limiting steps in systemic AAV delivery

The major rate-limiting steps include interaction with serum proteins (such as neutralizing antibodies), blood clearance, vessel escape, attachment, endocytosis, intracellular processing, nuclear entry and vector genome conversion. Capsid engineering can yield new AAV variants with enhanced systemic delivery properties. Numerical numbers highlight five rate-limiting barriers. Capillaries in the central nerve system (CNS) are sealed by the blood-brain barrier. Transcytosis is the only way for AAV to exit the vasculature in CNS. Capillaries in the liver and spleen are fenestrated and discontinuous. This allows for efficient paracellular diffusion of AAV into the parenchyma. Capillaries in muscles may allow for limited paracellular transport of AAV. However, transcytosis may likely be the primary pathway for AAV to get to muscle.