Little safety data is available from angiogenesis trials having used highly efficient vector constructs and shown clear angiogenesis in treated tissues. Detailed understanding of the side-effects caused by efficient angiogenic gene therapy would be essential as manifold increased perfusion and vascular permeability in the treated tissues might for example enhance the release of the therapeutic protein from the target tissues. Structural reorganization of the vascular network, increased perfusion and changes in local blood pressure gradients related to efficient angiogenesis might also cause haemodynamic changes that could reflect to the whole body. In this study, the effects of increasing doses of adenoviral vascular endothelial growth factor A (AdVEGF-A) were evaluated on vascular growth, metabolic benefits and systemic side-effects.
AdVEGF-A or AdLacZ control were injected intramuscularly (10e9-10e11vp/ml) into rabbit (n = 102) hind limb muscles and examined six or 14 days later. Blood flow, tissue oedema, metabolic benefits and the structure of angiogenic vessels were assessed using ultrasound imaging, modified miles assay, arterial blood gas- and metabolite analyses, and light and confocal microscopy, respectively. Safety analyses included cardiac ultrasound, electrocardiograms, and blood and tissue samples.
It was found that metabolic benefits, such as improved aerobic energy metabolism and decreased metabolic acidosis during exercise, after AdVEGF-A administration were highly correlated to the level of capillary enlargement but not on sprouting angiogenesis. Several systemic dose-dependent side-effects, including transient increases in liver, kidney and pancreatic enzymes, and also signs of cardiac failure, such as decreased ejection fraction in cardiac echo and left ventricular strain in ECGs, were observed with the highest AdVEGF-A doses. Both blood and lymphatic routes were found to mediate the spread of the transgene in several parts of the body after intramuscular gene transfer.
Comparing our evidence to previously published safety data, we conclude that increased perfusion and vascular permeability in the angiogenic tissues might facilitate the release of the transgene from the target muscles into the systemic blood and lymphatic circulation. Thus, as more efficient gene constructs are being developed and tested, careful attention should be paid to systemic side-effects as also peripheral, intramuscular gene transfers can have wide systemic side-effects possibly also including cardiac failure.