Adenovirus-mediated gene transfer into infarcted myocardium: Feasibility, timing, and location of expression

Gene transfer as a therapeutic modality for the treatment of myocardial ischemia and/or infarction has been proposed as a revolutionary approach to improve collateral circulation, enhance myocardial viability and amplify healing. Our study was undertaken to assess the feasibility, efficiency, anatomic distribution, timing and localization of adenovirus-mediated gene transfer into the vicinity of infarcted myocardium in the adult mammalian heart. We induced myocardial infarction by subjecting rats to 60 min of coronary artery occlusion followed by sustained reperfusion. Gene transfer into the infarction area was performed using direct injection of a replication-defective adenovirus vector encoding the bacterial reporter gene, β-galactosidase. A total of 5.0 x 10⁹ plaque-forming units of virus was delivered into the left ventricular myocardium either immediately (n = 7) or at 7 (n = 6), 22 (n = 5) or 30 days (n = 5) after reperfusion of rat hearts. Control rats received either 50 μl of saline 13 days after myocardial infarction (n = 2) or were not subjected to infarction and received Adenovirus carrying the β-galactosidase gene as described above (n = 4). All rats were killed at 7 days after cardiac injection. Hearts were harvested, frozen and sectioned and stained for β-galactosidase activity and with hematoxylin and eosin. Sections were evaluated by light microscopy. Relative β-galactosidase activity was measured by digital planimetry and expressed as the ratio of the maximal area of β-galactosidase staining relative to the total area of the section examined (% ± S.E.M.). β-galactosidase gene. expression was limited mainly to viable myocytes at the border of the myocardial infarction. The area of transgene expression in the non-infarcted hearts (28 ± 7%) was significantly higher (P = 0.02) than at any time point studied in infarcted tissues (3.4 ± 1.2%, 1.4 ± 1.0%, 2.8 ± 0.8% and 3.4 ± 0.9% at reperfusion and at 7, 22 and 30 days after myocardial infarction, respectively). Hearts injected 7 days after infarction had significantly less transgene activity (P = 0.03) with three of five samples displaying no macroscopically visible β-gal activity. Following viral injection, an inflammatory response consisting of mononuclear cell infiltration was much less intense seven days following injection in non-infarcted control rat hearts than at any of the time points examined for infarcted hearts. Gene transfer into infarcted myocardium, while feasible, was limited by low transfection efficiency when compared to non-infarcted normal myocardium. Transgene expression in the infarcted myocardium appears restricted to residual cardiomyocytes in the periphery. Nevertheless, the ability to introduce genes into these viable peripheral cells might be a useful therapeutic strategy for enhancing neovascularization, collateral flow and healing.