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J Thorac Cardiovasc Surg 2002;123:1101-1113
© 2002 The American Association for Thoracic Surgery
Cardiopulmonary Support and Physiology (CSP) |
From Evanston Northwestern Healthcare,a Evanston, Ill, and the Institute of Genetic Medicine,b Weill Medical College of Cornell University, New York, NY.
This work was supported in part by grants from the Jeffry M. and Barbara Picower Foundation, Palm Beach, Fla; National Institutes of Health grants RO1 HL 57318 and RO1 HL6698-01; Evanston Northwestern Healthcare Research Institute, Evanston, Ill; and GenVec, Inc, Gaithersburg, Md.
Presented in part at the Seventy-third Annual Meeting of the American Heart Association, November 2000, New Orleans, La.
Received for publication June 18, 2001. Revisions requested Aug 2, 2001; revisions received Aug 20, 2001. Accepted for publication Oct 2, 2001. Address for reprints: Todd K. Rosengart, MD, Evanston Hospital, Division of Cardiothoracic Surgery, Burch 100, 2650 Ridge Ave, Evanston, IL 60201 (E-mail: trosengart{at}enh.org).
| Abstract |
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| Introduction |
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Heart failure caused by myocardial ischemia is typically associated with the occurrence of hibernating myocardium, viable tissue that is nevertheless poorly functioning or noncontractile because of severe ischemia.
4-6 It is thought that this phenomenon serves as a protective mechanism, downregulating energy consumption to preserve cell viability. By definition, however, myocardial hibernation and restoration of myocardial function is reversible with myocardial reperfusion.
Although myocardial ischemia caused by discrete coronary stenosis can usually be treated with coronary angioplasty or bypass graft surgery, diffuse coronary artery disease is commonly not amenable to these conventional therapies. On the basis of these considerations, we hypothesized that improvement in blood flow to the myocardium induced by treatment with angiogenic factors could be used to improve myocardial dysfunction in the setting of ischemic cardiomyopathy.
Vascular endothelial growth factor (VEGF) is a homodimeric 34- to 44-kd heparin-binding glycoprotein that is a potent angiogenic growth factor (angiogen). As a result of posttranscriptional mRNA splicing, VEGF normally exists as 4 major isoforms of 121-, 165-, 189-, and 206-amino-acid residues.
7 VEGF is an attractive candidate for therapeutic angiogenic applications in that, because of localization of its receptors almost exclusively to vascular endothelial cells, it is one of the most specific of the known angiogens.
We and others have used gene transfer as a delivery strategy in which the coding sequence for a selected angiogen, such as VEGF, is delivered to the target tissue to induce angiogenesis.
8-12 Although encouraging initial data have demonstrated improvement in regional myocardial function in the setting of discrete coronary obstructions with this therapy,
8-10 the efficacy of angiogenic therapy in reversing global myocardial dysfunction is largely unknown.
In the present study we compared the efficacy of intramyocardial administration of an adenovirus coding for the 121-amino-acid isoform of VEGF (AdCUVEGF121.1) with that of a vector coding for no genes (AdNull) in enhancing myocardial function in a porcine pacing model of cardiomyopathy. Data generated in this study suggest that AdCUVEGF121.1 administration helps to preserve and enhance recovery of cardiac function in this model of ischemic cardiomyopathy. This may represent a novel strategy for treating cardiomyopathy in the ischemic setting.
| Methods |
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After initial testing of data acquisition and analysis with the Digital Ultrasonic Measurement System and the Sonosoft software program 3.1 (Sonometrics), the sonomicrometry leads were exteriorized for later use. An epicardial pacing lead (Medtronic, Minneapolis, Minn) was placed at the left ventricular apex and attached to a single-chamber pacing generator (Medtronic), which was placed subcutaneously. After a 7-day recovery period, pacing at 230 beats/min was initiated and continued for 7 days. Pacing was well tolerated in all animals. (There were 2 deaths each in the AdNull and the AdCUVEGF121.1 groups: one before and one immediately after pacing in the AdCUVEGF121.1 group and both immediately after pacing in the AdNull group.)
Echocardiographic and sonomicrometric data were collected at the time of the operation, immediately before pacing (baseline; 7 days postoperatively), immediately after pacing (day 0; 14 days postoperatively), and weekly thereafter for 3 weeks before the animals were killed (days 7, 14, and 21). All echocardiographic and sonomicrometric data were recorded for subsequent analysis, except for some instances in which results were uninterpretable or unavailable because of poor image quality or lead displacement, respectively.
Adenovirus vectors
The replication-deficient vector AdCUVEGF121.1 is an E1a-, partial E1b-, partial E3- adenovirus vector based on human adenovirus 5 into which an expression cassette is inserted into the E1 region containing the cytomegalovirus immediate early promoter-enhancer, an artificial splice sequence, the human VEGF 121 cDNA, and the SV40 polyA/stop signal.
15 The vector AdNull (containing no transgene) served as the control. AdCUVEGF121.1-induced VEGF expression was confirmed by means of enzyme-linked immunosorbent assay (R&D Systems Inc, Minneapolis, Minn) assessment of tissue-culture supernatant 48 hours after in vitro infection of A549 (American Type Culture Collection, Rockville, Md).
Intramyocardial vector delivery
In initial experiments naive animals undergoing crystal placement and pacing, as described above, but without vector injection, were analyzed to assess the specific effects of pacing alone on ventricular function (n = 5). Subsequently, at the time of the initial operation, animals were administered either AdCUVEGF121.1 (n = 8) or the control vector AdNull (n = 8) by means of direct myocardial injection of the vector in phosphate-buffered saline solution at 10 sites (10 µL/site) placed at 1-cm intervals in the anterior and lateral territory of the left ventricle (total dose, 1011 pu).
Echocardiographic assessment
Two-dimensional transthoracic images were obtained with an Acuson echocardiographic machine (128 XP/10; Acuson, Mountain View, Calif) and a 3.0/3.5-MHz dual-frequency transducer (Acuson S 3194). During each examination, animals were sedated and placed in the left lateral decubitus position, and right parasternal short-axis midpapillary views of the left ventricle were obtained at rest. Images were recorded for 3 minutes on a standard VHS videotape. Offline analysis of the data was performed by a single experienced investigator, who was blinded to group assignment, using the Image Vue DCR 1.50 System (Nova Microsonics, Mahwah, NJ).
Global ventricular function was estimated by calculating the fractional area change (FAC) of the left ventricle. The percentage of FAC has been shown to correlate well with radionuclide and angiography measurements of ejection fraction.
16 The percentage of FAC was measured by tracing the end-diastolic area (EDA) and the end-systolic area (ESA) of the ventricle from the midpapillary short-axis view. The percentage of FAC was then calculated by using the following equation:
% FAC = [(EDA - ESA)/EDA] x 100.
Regional wall-motion function was evaluated by dividing the left ventricle into 6 equal 60° radial segments (Figure 1). Mean systolic and mean diastolic wall thickness were calculated by averaging 3 single measurements in each segment.
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Sonomicrometric assessment
Heart rate, segmental shortening, stroke volume, and cardiac output were measured with a commercial digital sonomicrometry system (Sonometrics Corp, London), according to the manufacturer's specifications, before (rest) and after (stress) administration of epinephrine (10 µg administered intravenously) at each hemodynamic measurement time point.
17 The scalar distances between omnidirectional sonomicrometers (pairs of 1.3-MHz piezoelectric crystals) were recorded simultaneously at a sampling frequency of greater than 240 Hz. Parameters derived from the intercrystal distances and heart rate were calculated by using a 4-component system consisting of (1) a 486-class personal computer that controls sonomicrometry functions and runs application software, (2) a customized timing board that controls the length of the energizing pulse and the sequence of transducer transmissions, (3) a customized distance-measuring circuit that translates the ultrasonic signal into a distance measurement, and (4) an 8-channel analog-to-digital board that is synchronized with the sonomicrometer distance measurements.
Statistical analysis
Data are reported as means and SDs, and significance measurements were calculated with paired or unpaired Student t tests, as appropriate.
| Results |
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Echocardiographic assessment of ventricular morphology in the treated animals at day 21 after pacing also demonstrated evidence consistent with the presence of congestive failure. For example, end-diastolic area in the AdCUVEGF121.1 animals was 910 ± 200 mm2 versus 1120 ± 230 mm2 in the AdNull group. End-systolic areas were 350 ± 110 mm2 and 560 ± 180 mm2, respectively.
Echocardiographic assessment of global ventricular function
Echocardiographic assessment of FAC at the midventricular level did not differ between the AdCUVEGF121.1 and AdNull groups before initiation of pacing (67% ± 5% vs 66% ± 4%, P = .9; Figure 2). FAC significantly decreased on termination of pacing in both groups but was significantly greater in the AdCUVEGF121.1 group compared with that in the AdNull group at this time point (46% ± 8% vs 29% ± 14%, P = .02). Ventricular function improved in both groups over succeeding time points, but although the FAC in the AdCUVEGF121.1 group recovered to levels approximating prepacing values by day 7 after pacing, FAC in the AdNull group remained significantly decreased compared with AdCUVEGF121.1 levels throughout the 3-week period (day 7: 48% ± 14% vs 62% ± 7%, P < .04; day 21: 52% ± 13% vs 65% ± 5%, P < .04).
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.04) compared with prepacing FWT in the AdNull group in 5 of 6 segments, even at day 21.
Sonomicrometric assessment of hemodynamic performance
The prepacing values for segmental shortening did not differ significantly between the AdCUVEGF121.1 and the AdNull groups (Figure 4, A). Segmental shortening was not significantly impaired in the AdCUVEGF121.1 group at any time after pacing compared with the prepacing values. In contrast, segmental shortening at day 7 was significantly decreased in the AdNull group compared with that in the AdCUVEGF121.1 group (10% ± 4% vs 16% ± 3%, P = .004) and remained significantly impaired (P < .05) in the AdNull group at day 14 and 21 when compared with baseline values. These differences were more dramatically demonstrated when the postpacing function of each animal was expressed as a percentage change from its own baseline (immediate prepacing) values (Figure 4
, B).
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Cardiac outputs were similar for the 2 groups immediately before pacing, although they were greater for the AdCUVEGF121.1 animals compared with those seen in the AdNull animals at the time of the initial operation (Figure 4
, E). Cardiac outputs were well preserved in the AdCUVEGF121.1 group compared with those in the AdNull group throughout the study (day 7: 4.3 ± 1.1 vs 2.7 ± 0.7 L/min, P = .009; day 14: 3.8 ± 1.1 vs 1.8 ± 0.3 L/min, P = .004; day 21: 3.3 ± 1.1 vs 1.0 ± 0.3 L/min, P < .02). Even at day 21 after pacing, cardiac output in the AdNull group remained significantly impaired (P < .03) when compared with each animal's own baseline values (Figure 4
, F).
Assessment of ventricular performance with pharmacologic stress
Sonomicrometry was used to assess the effect of epinephrine administration (10 µg administered intravenously) on cardiac performance (Figure 5, A-F). Before pacing, epinephrine administration resulted in an approximate 30% to 85% increase in cardiac contractility, as measured by changes in cardiac output (Table 1). Administration of epinephrine failed to restore any of the measured indices of ventricular performance in AdNull animals to levels equivalent to those demonstrated by AdCUVEGF121.1 animals in the resting state. Similarly, epinephrine administration resulted in a mean increase in cardiac output in AdCUVEGF121.1 animals that was nearly double that of the increase in cardiac output noted after epinephrine administration in AdNull animals (Table 1
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| Discussion |
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Angiogenic therapy appears to offer promise as an effective means of enhancing the function of ischemic myocardium by improving blood flow to ischemic tissues through increased myocardial vascularization.
8-12 In contrast, the treatment of heart failure with chronic inotropic therapy has been associated with excessive mortality, possibly because this strategy may exacerbate preexistent ischemia.
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Evidence supporting the ability of growth factor administration to induce therapeutic angiogenesis has been provided by a wide variety of animal studies and a limited number of early clinical trials.
8-12,19-30 Gene therapy, whereby the angiogenic agent is transferred to the myocardium by means of a viral or plasmid vector as the cDNA for the corresponding protein angiogen, is a promising delivery strategy for angiogenic therapy.
19 In the present study we demonstrate that adenoviral-mediated transfer of VEGF 121 as an angiogenic mediator appears to minimize the development of, and speed recovery from, ventricular dysfunction induced by rapid ventricular pacing. The findings of the present study suggest that this intervention may represent an effective treatment for cardiomyopathy.
Enhancement of myocardial performance after delivery of the AdCUVEGF121.1 vector was demonstrated by means of both echocardiographic and sonomicrometric assessment of ventricular function, by using both global and regional analyses. In comparison with prior studies demonstrating that angiogenic therapy improves regional myocardial perfusion and function,
8-10,20-22 angiogenic therapy in the present study was used to treat global dysfunction and is thus more relevant for the treatment of ischemic cardiomyopathy. In contrast to the 2 previous studies assessing the treatment of global dysfunction with intracoronary delivery of a growth factor,
11,31 the present study demonstrates the efficacy of an intramyocardial delivery technique. This approach takes advantage of the potentially beneficial localizing effects of intramyocardial delivery
9,32 and suggests that such a strategy is a viable means of homogeneously enhancing global function, despite the use of multiple, isolated injections.
Several limitations of this study must be addressed. First, we did not specifically document the induction of angiogenesis in the current study, although clear evidence of enhanced vascularization, perfusion, function, or a combination thereof has been previously established after myocardial delivery of angiogenic therapy,
8-12,24,26,27,33,34 including our recent demonstration of enhanced myocardial perfusion after AdCUVEGF121.1 delivery in the nonischemic rabbit myocardium (G. F. Rahman, unpublished observations). Although it is possible that nonangiogenic effects of VEGF, such as vasodilator or cytoprotective properties,
12 may have played a role in the hemodynamic changes noted in this study, the pharmacokinetics of adenovirus delivery, whereby transgene expression beyond 2 weeks is negligible,
9,32 suggest that this alternative mechanism is unlikely.
A second limitation is that the rapid ventricular pacing model we used does not duplicate the diffuse coronary occlusions and multiple myocardial infarctions typically associated with ischemic cardiomyopathy in the clinical setting. Myocardial ischemia in our model is likely induced by excessive myocardial demand induced by rapid pacing rather than the limited perfusion-blood supply associated with coronary disease.
34 The relevance of such a supply-demand mismatch as a basis for ischemic cardiomyopathy has been previously established, specifically in the porcine model, in which collateral vessel formation remains limited.
13,14,35 Evidence supporting the use of this preparation as a relevant model of cardiomyopathy is further suggested in the present study by the blunted response to epinephrine observed in the AdNull-treated animals, which is consistent with the ß-adrenergic downregulation characteristic of a cardiomyopathic state.
13,14,36 Furthermore, demonstration of decreased ventricular function in naive animals paced without vector administration suggests that this effect is the direct result of pacing rather than a toxic effect of vector administration. Given these considerations, it seems plausible that angiogenic therapy is effective in this model because of its ability to enhance myocardial blood supply through increased vascularization.
In conclusion, we demonstrate that adenovirus-mediated transfer of VEGF 121 in a porcine model of myocardial dysfunction helps to preserve ventricular function and speed recovery of myocardial contractility after rapid ventricular pacing. This form of gene therapy may be useful for the treatment of ischemic cardiomyopathy clinically.
| Acknowledgments |
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| References |
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