Gene Delivery to the Atrial Myocardium

A well-administered vector achieves homogeneous delivery at the affected tissue bed and demonstrates minimal accumulation at off-target sites. Vectors can be administered by a wide variety of techniques, but often with an inverse relationship between simplicity and specificity. Route consideration is imperative for patient safety and gene efficacy, and represents an area under active research in parallel with the vector itself.

Intravenous administration (IV)

The least invasive method of administration is intravenous injection of the vector. While IV exposure should offer rapid transit to any well-vascularized target tissue, it is also the least specific route. Upon entering the intravascular compartment, the vector will be systemically dispersed and the tissue beds will be exposed in accordance with the blood flow to each site. AAV and nanoparticle studies have shown that numerous off-target organs, particularly the liver, are transduced following IV administration. This effect poses a clinically relevant concern for decreased efficacy and increased toxicity. To overcome this biodistribution obstacle, site-directed vector engineering including AAV capsid chimerism and nanoparticle targeting ligands may improve specificity and potency.21,38

Cardiac perfusion

The common and well-refined clinical practice of coronary artery catheterization offers intracoronary perfusion as a minimally-invasive modification of IV administration. In this form of delivery, the cardiac vasculature is selectively isolated and perfused with the vector to maximize the potency of a single administration. While the vector still enters the systemic circulation, the cardiac tissues encounter the vector prior to attenuation by dilution or hepatic uptake. However, this high tissue dose is limited by permeability of the coronary endothelium and rapid blood flow clearance through the coronary circulation. These two factors are thought to have contributed to the negative outcome of the CUPID2 trial (AAV1/SERCA2a coronary injection in patients with heart failure).59,60 Vascular permeability enhancers (substance P or thrombin) can be co-administered with a vector to enhance myocardial exposure, although any interference with the coronary arterial tree carries the risk of ischemia and could be unacceptable for patients with pre-existing cardiac disease.34,61
Retrograde infusion via coronary sinus injection may provide a myocardium-targeted approach without the ischemic risks or patient selection criteria of intracoronary techniques. Here, controlled infusion of the venous structures in the setting of obstructed outflow increases the capillary pressure gradient and drives the vector material into the tissue beds. Exposure time can be prolonged, as distribution is not dependent on arterial flow and the coronary sinus occlusion can be safely tolerated for an extended duration. Large animal studies have demonstrated efficacy for both drug and gene delivery utilizing this technique.62-64 While coronary sinus cannulation is generally safe and commonly practiced in routine procedures, trauma to the delicate cardiac veins and myocardial edema are potential complications and necessitate careful injection pressure regulation.34

Epicardial gene painting

Epicardial gene painting combines a vector with a protease and a polymer-forming gel to create a “paintable” gel that can be directly applied to the atrial epicardium.65 Once applied, the polymer vehicle solidifies at body temperature and provides a substrate for strong adsorption of the vector to the tissue bed. The protease component of the paint facilitates transmural gene transfer in the thin atrial myocardium.66
Gene painting is safe and effective in animal models with no significant impact on atrial structure or function.65,67 Though epicardial gene painting can yield homogenous and transmural transduction, the primary drawback is the invasiveness of the surgical procedure required to achieve epicardial access. In addition, structures that are difficult to access via the epicardium (posterior left atrium and pulmonary veins) may preclude delivery to the entire atria, and misapplication of the paint could theoretically result in unintended transmural gene delivery to the ventricle.

Direct myocardial injection with or without reversible electroporation

As a simple and well-studied method, direct injection of vector into myocardial tissue has been extensively explored as a route of administration. Through a surgical approach, the vector can be precisely injected and an intense concentration of gene expression can be achieved. However, gene expression is highly localized to within a few millimeters of the injection site. In this way, injection-mediated delivery to a large area of the myocardium is technically challenging and inefficient.68
Following direct injection, naked plasmid DNA gene therapy vectors require subsequent electroporation for effective myocardial transfection.13,69,70 Electroporation acts by subjecting cells to synchronized electrical pulses, resulting in a transient electrical gradient that alters the structure of cell membranes and forms micropores at the cell surface. These micropores enable diffusion of surrounding plasmid into electroporated cells. The rate of gene uptake in vivo is 15-20 fold higher when electroporation is used versus standard plasmid DNA delivery alone.71 Irreversible electroporation is a developing procedure used in clinical cardiac electrophysiology to ablate specific regions of the myocardium.72 Modification of pre-existing irreversible electroporation techniques and equipment to reduce delivered current from an electroporation device could be used to transduce the myocardium with plasmid DNA.