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Cardiovascular Research 2007 73(3):443-445; doi:10.1016/j.cardiores.2006.12.005
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Copyright © 2006, European Society of Cardiology

Viral or non-viral angiogenesis gene transfer–New answers to old questions

Sigrid Nikol*

Department of Cardiology and Angiology, University Hospital of Münster, Germany

* Medizinische Klinik und Poliklinik C, Department of Cardiology and Angiology, University Hospital Münster, Albert-Schweitzer-Str. 33, 48149 Münster, Germany. Tel.: +49 251 83 48501; fax: +49 251 83 45101. Email address: nikol{at}uni-muenster.de

Received 27 November 2006; accepted 6 December 2006

See article by Hao et al. [8] (pages 481–487) in this issue.

Therapeutic angiogenesis has come a long way in 15 years of research [1]. Mechanisms of vessel formation are well characterised, and their elucidation has led to numerous clinical trials targeting mostly single factors involved in this process: there are at least 106 registered angiogenesis gene therapy trials with an estimated 2000 patients treated to date [2]. Table 1 summarises phase II angiogenesis gene therapy trials that have been completed for both coronary and peripheral artery disease. Thus, angiogenesis gene therapy has quietly reached the era of phase III clinical trials aiming at the first-time approval for this new class of drug in the Western world.


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  		Table 1 Completed phase II clinical trials of angiogenesis gene therapy

 
Efficacy and safety of angiogenesis gene therapy depend on the optimal combination of target gene, vector, and route of administration [3,4]. Among the dozens of factors involved in the angiogenesis process, only vascular endothelial growth factor (VEGF165, VEGF121) and fibroblast growth factor (FGF-1, FGF-4) have been investigated in completed phase II clinical trials. Other factors tested currently in phase II angiogenesis gene therapy trials are vascular endothelial growth factor 2 (VEGF-2), hepatocyte growth factor (HGF), hypoxia-inducible factor 1{alpha} (Hif-1{alpha}), and developmentally regulated endothelial locus (DEL).

Whereas it has become widely accepted to prefer local or regional administration over systemic to achieve maximum safety and efficacy, two different vector systems are still being evaluated in phase II clinical testing: plasmid and adenoviral vectors (Table 1). In preclinical animal studies, the use of plasmid DNA has been demonstrated to allow for long-term gene expression over several months with rather low transfer efficiencies [4]. In contrast, gene expression is of relatively short duration, lasting from several days to weeks, using adenoviral vectors with relatively high transfer efficiency reported in small study animals. However, transfection efficiencies using adenoviral vectors are inversely related to body weight, allowing for higher transfer efficiencies in small animal models compared with porcine models or humans [3]. In addition, adenoviral antibodies are frequently present in the human adult, further reducing transfer efficiencies to levels of only 0.04 to 5% – levels comparable to those found for non-viral gene transfer [5]. Moreover, viral gene transfer requires special biosafety measures not necessary for non-viral gene transfer. Safety issues are also reflected by a higher event rate in adenoviral clinical gene transfer with transient fever as well as elevation of CRP, liver enzymes, and adenoviral antibody titres [6,7].

While safety and efficacy of adenoviral and plasmid gene transfer has been compared in patients with coronary heart diesease (KAT trial)[6] and patients with peripheral artery disease (PVD trial, Ref. [7]), a systematic animal study for the investigation of myocardial angiogenesis has been lacking. This is now addressed in the paper by Hao et al. published in this issue of Cardiovascular Research [8]. Following ligation of the anterior descending artery in rats, injections of two different human VEGF-A165 gene constructs, adenoviral or plasmid, into the peri-infarct region were performed after a second thoracotomy 7 days later. Both angiogenesis and improvement of cardiac function were observed 4 weeks following gene transfer. For plasmid gene transfer the dose-response curve was flat, suggesting a wide therapeutic window. In contrast, for the adenoviral construct adhVEGF-A165 dose-response curves revealed that the therapeutic window was narrow, being limited by signs of systemic viral infection, minimal expression of VEGF, and increased myocardial apoptosis. Systemic expression of VEGF was higher following adhVEGF-A165, yet angiogenesis did not directly depend on the amount of exogenous VEGF expressed. Consequently, in this experimental model, the capacity to develop angiogenesis seemed to be already saturated with the lower level of expression induced by plasmid gene transfer. This is particularly interesting as improved gene transfer efficiency was the main argument for the introduction of adenoviral vectors, outweighing any concerns regarding safety or more sophisticated infrastructures needed for their use compared to plasmids. Also, transfer efficiency of non-viral gene transfer may be markedly increased by the optimisation of transfer conditions using certain liposomes and adjuvants [9,10].

Despite the difference in gene expression, the degree of vascular growth both at the capillary and arteriolar level as well as left ventricular function improvement were similar for both types of vectors. Importantly, capillary angiogenesis was transient while increased arteriogenesis was maintained over the study period of 4 weeks.

These observations call for future investigations of the mechanisms of vascular growth induction in the ischaemic myocardium. To what extent are endogenous VEGF, its receptors, and other angiogenically relevant factors expressed? How are these factors influenced by the therapeutic over-expression of angiogenic factors such as VEGF? If they are influenced, to what extent is this influence dependent on the amount of expressed exogenous VEGF? The fact that both vectors increased myocardial arteriogenesis indicates that VEGF has trophic effects not only at the capillary level but also on smooth muscle cells that also express VEGFR-2. Data described by Hao et al. also indicate that VEGF induces newly formed capillaries that are transformed into arterioles as capillary densities were increased after 1 week and arterioles after both 1 and 4 weeks. In their previous work this was even more evident with photo documentation of capillaries in transition to arterioles in the treatment group [11]. Thus, the therapeutic effect shown is arteriogenesis within the ischaemic tissue as opposed to the general view on arteriogenesis that commonly is defined as shear-stressed remodelling of conduit arteries proximal to the ischaemic tissue.

Finally, the data of Hao et al. indicate that therapeutic angiogenesis following myocardial infarction leads to improved cardiac function compared to controls. And in a clinical setting, the Euroinject-One trial showed an 80% decrease of reversible ischaemia in the myocardial treatment area combined with improved regional wall motion, presumably as a result of such vascular changes [12]. However, most clinical trials of angiogenesis gene therapy in the myocardium targeting therapy-refractory angina excluded per protocol patients with markedly decreased ejection fractions. Therefore, and as a conclusion of Dr. Hao's work, the application of angiogenesis gene transfer should also be evaluated in patients with ischaemic heart failure and compared with current stem cell and cytokine therapies, which represent alternative therapeutic approaches to this pathology. There are indications that the marginal improvements of cardiac function reported at this timepoint in clinical trials evaluating stem cell and cytokine therapy may be significantly augmented in a synergistic, combined approach with angiogenesis gene therapy [13]. Lastly, while patients with therapy-refractory angina are relatively difficult to identify, which recently led to a halt of GenVec's NOVA gene therapy trial, patients with ischaemic heart failure represent a much larger population.


   References
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