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.