Intravascular delivery, the least invasive delivery approach, is the simplest procedure used to deliver drug to solid tissues, including the CNS. Nevertheless, this method fails in the delivery of large molecules or vectors, because channels through the BBB are small enough to inhibit the passage of even small macromolecules. Hence no report exists of robust delivery of virus-sized molecules into the brain after intravascular injection. Additionally, this method does not lend itself to targeting a specific region of the brain.
Another approach for distribution of macromolecules that may enhance distribution of virus-sized particles in solid tissues is CED. Convection-enhanced delivery relies on bulk flow to carry macromolecules through the tissue extracellular space. Instead of being restricted by the BBB, CED uses this barrier to its advantage by restricting the access of infused agents to the systemic circulation.3 Unlike diffusion, which relies on a concentration gradient, CED depends on an externally generated pressure gradient to distribute a relatively homogeneous concentration of compounds to discrete anatomical targets of various sizes.3–5,15,27 This method of delivery should also carry particles significantly farther than can be achieved using diffusion alone.27 Furthermore, by delivering the complete dose at the target site, smaller quantities of agent may be administered than required for systemic dosing, thus reducing systemic exposure and toxicity relative to other modes of delivery. Successful and safe use of convective delivery of macromolecules in the laboratory and clinic has been demonstrated (NIH protocol No. 13415-21). For these reasons, CED is a suitable candidate for the delivery of gene therapy vectors to clinically relevant regions of the CNS.
The objectives of this study were to determine the feasibility of using convective delivery of viral vectors into gray matter and to examine systematically the important variables associated with it. We measured the distributions of virus-sized nanoparticles (polystyrene nanospheres), adeno-associated viral vectors, and adenoviral vectors in rat striata, and compared them with the dispersion of albumin, a 69-kD inert protein that is small relative to viruses. Albumin is a marker compound that can be delivered to large volumes of brain by convection and is transported without significant retardation or short-term loss through the extracellular milieu.21 In this report we demonstrate that convection also can be used to distribute particles the size of AAV and that CED can distribute AAV and adenoviruses into the gray matter of the CNS. The results indicate that, within the size range of these particles, surface properties can be dominant factors for distribution of viruses by CED and, like albumin, perfusion of a relatively large volume of brain with viruses should be achievable with CED. Thus, use of this simple delivery approach can potentially achieve intracerebral distribution of gene transfer vectors for laboratory and clinical use.
We thank Krys Bankiewicz, Edward Muszuka, and Howard Fine for providing the virus; Peter Schuck for the dynamic light scattering; John Harvey for spectroscopy; Christine Piggee for capillary zone electrophoresis; Poonam Mannam and Barbara Ikejiri for histological analysis; and Stuart Walbridge, Eric Curry, and Andrew Bennett for animal procedures and tissue processing.
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