Hypercholesterolemia attenuates angiogenesis but does not preclude augmentation by angiogenic cytokines

Angiogenic gene therapy: pre-clinical studies and phase I clinical data

Over the last three decades, significant progress has been made in the management of patients with atherosclerotic coronary and peripheral vascular diseases using medical, surgical, and percutaneous therapies. Despite these advances, there remains a significant population of patients who are not optimal candidates for surgical or percutaneous revascularization. These patients continue to suffer from the debilitating symptoms of their disease and remain at risk for myocardial infarction, limb loss, or death. It was this clinical need, coupled with the advances in the understanding of angiogenesis, that has led to efforts to develop angiogenic therapies for patients with peripheral and myocardial ischemia. In general, these conditions are characterized by local hypovascularity, and the approach to treatment is therefore focused on stimulating neovascularization.

Frequency distribution of collateral flow and factors influencing collateral channel development. Functional collateral channel measurement in 450 patients with coronary artery disease

OBJECTIVES

We sought to determine the pathogenetic predictors of collateral channels in a large cohort of patients with coronary artery disease (CAD).

BACKGROUND

The frequency distribution of collateral flow in patients with CAD is unknown. Only small qualitative studies have investigated which factors influence the development of collateral channels.

METHODS

In 450 patients with one- to three-vessel CAD undergoing percutaneous transluminal coronary angioplasty (PTCA), collateral flow was measured. A collateral flow index (CFI; no unit) expressing collateral flow relative to normal anterograde flow was determined using coronary wedge pressure or Doppler measurements through sensor-tipped PTCA guide wires. Frequency distribution analysis of CFI and univariate and multivariate analyses of 32 factors, including gender, age, patient history, cardiovascular risk factors, medication and coronary angiographic data, were performed.

RESULTS

Two-thirds of the patients had a CFI < 0.25 and approximately 40% of patients had a CFI < 0.15, but only approximately 10% of the patients had a recruitable CFI > or =0.4. By univariate analysis, the following were predictors of CFI > or =0.25: high levels of high-density lipoprotein cholesterol, the absence of previous non-Q-wave myocardial infarction, angina pectoris during an exercise test, angiographic indicators of severe CAD and the left circumflex or right coronary artery as the collateral-receiving vessel. Percent diameter stenosis of the lesion undergoing PTCA was the only independent predictor of a high CFI.

CONCLUSIONS

This large clinical study of patients with CAD in whom collateral flow was quantitatively assessed reveals that two-thirds of the patients do not have enough collateral flow to prevent myocardial ischemia during coronary occlusion, and that coronary lesion severity is the only independent pathogenetic variable related to collateral flow.

Gene therapy for therapeutic angiogenesis in critically ischaemic lower limb - on the way to the clinic

Currently, no effective pharmacological treatment is available for vascularisation defects in lower limbs. Many patients presenting with persistent pain and ischaemic ulcers are not suitable candidates for surgical or endovascular approaches. Further refinement of the available methods will undoubtedly lead to a more active approach towards treatment of peripheral arterial occlusive disease (PAOD). Recently, therapeutic angiogenesis, in the form of recombinant growth factor administration or gene therapy, has emerged as a novel tool to treat these patients. However, improved gene transfer methods and better understanding of blood vessel formation are required to bring therapeutic angiogenesis to clinical practice. Here we review the clinical problem (PAOD), mechanisms of blood vessel formation (angiogenesis, vasculogenesis and arteriogenesis), experimental evidence and clinical trials for therapeutic angiogenesis in critically ischaemic lower limbs. Also, angiogenic growth factors, including vascular endothelial growth factors (VEGFs) and fibroblast growth factors (FGFs), delivery methods, and vectors for gene transfer in skeletal muscle, are discussed. In addition to vascular growth, gene transfer of growth factors may enhance regeneration, survival, and innervation of ischaemic skeletal muscle. Nitric oxide (NO) appears to be a key mediator in vascular homeostasis and growth, and a reduction in its production by age, hypercholesterolemia or diabetes leads to the impairment of ischaemic disorders.

Plaque angiogenesis and atherosclerosis

Therapeutic angiogenesis trials refer to the stimulation of collateral arterioles and new vascular conduits to perfuse ischemic myocardium and limbs. Atherosclerotic lesions responsible for vascular occlusions themselves are associated with angiogenesis within the vessel wall. Plaque neovascularization is comprised of a network of capillaries that arise from the adventitial vasa vasorum and extend into the intimal layer of atherosclerotic lesions and other types of vascular injury. The functions of these plaque capillaries are proposed to be important regulators of plaque growth and lesion instability. The development of agents that are positive and negative regulators of angiogenesis may have potential therapeutic implications in the progression and acute manifestations of atherosclerosis. This review focuses on the role of plaque angiogenesis in atherosclerosis and discusses the potential therapeutic applications of angiogenesis inhibitors in this disease.

Therapeutic angiogenesis for ischemic cardiovascular disease

In animal models of ischemia, a large body of evidence indicates that administration of angiogenic growth factors, either as recombinant protein or by gene transfer, can augment nutrient perfusion through neovascularization. While many cytokines have angiogenic activity, the best studied both in animal models and clinical trials are vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Clinical trials of therapeutic angiogenesis in patients with end-stage coronary artery disease have shown large increases in exercise time and marked reductions in symptoms of angina, as well as objective evidence of improved perfusion and left ventricular function. Larger scale placebo-controlled trials have been limited to intracoronary and intravenous administration of recombinant protein, and have not yet shown significant improvement in either exercise time or angina when compared to placebo. Larger scale placebo-controlled studies of gene transfer are in progress. Future clinical studies will be required to determine the optimal dose, formulation, route of administration and combinations of growth factors, as well as the requirement for endothelial progenitor cell or stem cell supplementation, to provide effective and safe therapeutic myocardial angiogenesis.

Clinical applications of vascular gene therapy

Despite significant advances in prevention, coronary artery disease remains the leading cause of death in the Western world. Surgical bypass and angioplasty are the primary interventional therapies but they are limited by the problems of restenosis and graft occlusions. Natural response to vascular occlusion involves the formation of collateral vessels that bypass obstructions, but they are often inefficient in relieving ischemia. Vascular gene transfer offers a promising new approach to solve these problems. Its potential has been shown in animal models and in first human trials using vascular endothelial growth factor, fibroblast growth factor, and E2F cell-cycle transcription factor decoy. However, further basic research on gene transfer vectors, gene delivery techniques, and identification of effective treatment genes is needed to improve the efficacy and safety of human vascular gene therapy.

Gene therapy for ischemic heart disease: therapeutic potential

Gene therapy is evolving as an alternative mode to pharmacological intervention in the treatment of cardiovascular diseases. Experimental observations indicating that introduction of genes encoding for angiogenic peptide growth factors could result in improvement in perfusion to ischemic myocardium have led to the initiation of a number of preliminary clinical trials to evaluate this therapeutic modality. Sustained expression of the growth factor product from somatic cells transfected with the DNA for that protein has proven to be one of the major advantages of a gene therapy based approach over administration of the recombinant protein. A number of gene therapy vectors have been developed, prominent among these being adenoviral vectors and naked plasmid DNA. Whereas plasmid DNA results in less efficient transfection, its tolerability profile may be superior to adenoviral vectors. Plasmid DNA is particularly suitable when the gene product to be produced is capable of being secreted by the cell which is producing it. Vascular endothelial growth factor (VEGF) is not only essential to the process of angiogenesis, but, because it can be secreted from intact cells, appears to be ideal for gene transfer therapy aimed at improving perfusion to ischemic myocardium. The DNA can be delivered to the myocardium by intra-arterial or intramuscular injection. At present, direct injection into the muscle either via a small thoracotomy incision or by use of a recently developed percutaneous catheter technique appears to be superior to arterial administration. Several clinical trials based on intramyocardial injection of VEGF DNA in patients with otherwise inoperable coronary artery disease and intractable angina pectoris have recently been completed. These phase I trials have documented the tolerability of gene transfer using plasmid DNA and show promise of being able to improve myocardial perfusion and reduce anginal symptoms in the majority of patients treated thus far. While the trials involving gene transfer via a thoracotomy did not allow for randomization to a placebo group, the recent advent of a percutaneous delivery modality has allowed for randomization which should enhance our ability to determine whether angiogenic gene therapy will prove to be as effective as initial results suggest. In the future, results from such randomized placebo-controlled trials, improvement in vectors utilized for gene transfer and innovative new delivery techniques will undoubtedly enhance the potential of this novel approach to myocardial revascularization.

Transplantation of ex vivo expanded endothelial progenitor cells for therapeutic neovascularization

Animal studies and preliminary results in humans suggest that lower extremity and myocardial ischemia can be attenuated by treatment with angiogenic cytokines. The resident population of endothelial cells that is competent to respond to an available level of angiogenic growth factors, however, may potentially limit the extent to which cytokine supplementation enhances tissue neovascularization. Accordingly, we transplanted human endothelial progenitor cells (hEPCs) to athymic nude mice with hindlimb ischemia. Blood flow recovery and capillary density in the ischemic hindlimb were markedly improved, and the rate of limb loss was significantly reduced. Ex vivo expanded hEPCs may thus have utility as a "supply-side" strategy for therapeutic neovascularization.

Favorable effect of VEGF gene transfer on ischemic peripheral neuropathy

Ischemic peripheral neuropathy is a frequent, irreversible complication of lower extremity vascular insufficiency. We investigated whether ischemic peripheral neuropathy could be prevented and/or reversed by gene transfer of an endothelial cell mitogen designed to promote therapeutic angiogenesis. Intramuscular gene transfer of naked DNA encoding vascular endothelial growth factor (VEGF) simultaneously with induction of hindlimb ischemia in rabbits abrogated the substantial decrease in motor and sensory nerve parameters, and nerve function recovered promptly. When gene transfer was administered 10 days after induction of ischemia, nerve function was restored earlier and/or recovered faster than in untreated rabbits. These findings are due in part to enhanced hindlimb perfusion. In addition, however, the demonstration of functional VEGF receptor expression by Schwann cells indicates a direct effect of VEGF on neural integrity as well. These findings thus constitute a new paradigm for the treatment of ischemic peripheral neuropathy.

Transplantation of ex vivo expanded endothelial progenitor cells for therapeutic neovascularization

Animal studies and preliminary results in humans suggest that lower extremity and myocardial ischemia can be attenuated by treatment with angiogenic cytokines. The resident population of endothelial cells that is competent to respond to an available level of angiogenic growth factors, however, may potentially limit the extent to which cytokine supplementation enhances tissue neovascularization. Accordingly, we transplanted human endothelial progenitor cells (hEPCs) to athymic nude mice with hindlimb ischemia. Blood flow recovery and capillary density in the ischemic hindlimb were markedly improved, and the rate of limb loss was significantly reduced. Ex vivo expanded hEPCs may thus have utility as a "supply-side" strategy for therapeutic neovascularization.

Reperfusion-induced changes in capillary perfusion and filtration: effects of hypercholesterolemia

Fluid filtration rate (J(v)/S) and red blood cell velocity (V(RBC)) in individual mesenteric capillaries of normocholesterolemic (NC) and hypercholesterolemic (HC) rats were measured before and after ischemia and reperfusion (I/R). In NC rats, a correlation was found between baseline J(v)/S and the percent of the feeding arteriole length that was paired (<15 micrometer) with a postcapillary venule (A-V pairing), but not in the HC group. Additionally, in NC rats only, a correlation was found between baseline V(RBC) and A-V pairing. In capillaries in which A-V pairing was substantial (>20%), V(RBC) dropped after reperfusion in the HC group (54% of baseline; P < 0.05), but not in the NC group (79%). The decrease in V(RBC) in HC rats could be attenuated by a P-selectin antibody (PB1.3). PB1.3 was also able to attenuate the increase in I/R-induced capillary J(v)/S in HC rats (median increase = 1.26-fold vs. 1.53-fold without PB1.3). These data suggest a role for A-V pairing in capillary perfusion in NC rats and a potential role for P-selectin in I/R-induced microvascular dysfunction in HC rats.

Low density lipoprotein (LDL)-mediated suppression of Lewis lung carcinoma in hypercholesterolemic LDL receptor-deficient mice

An inverse relationship has been reported between cancer risk and cholesterol level, prompting the hypothesis that hypercholesterolemia may be protective against cancer. We tested this hypothesis by evaluating the growth of Lewis lung carcinoma in three different murine models of hypercholesterolemia: Pluronic treated mice, apolipoprotein E (ApoE) deficient mice, and low density lipoprotein receptor (LDL-R) deficient mice. Only the accumulation of LDL-cholesterol in LDL-R deficient mice suppressed tumor growth. Accumulation of chylomicrons, very low density lipoproteins (VLDL), and cholesterol-enriched remnants in the Pluronic treated mice and ApoE deficient mice did not inhibit tumor growth, even though mice in all three models were equally hypercholesterolemic. Taken together, the experimental evidence from our studies indicate that high plasma cholesterol in the form of LDL-cholesterol could have a beneficial effect against cancer in vivo.

Rescue of diabetes-related impairment of angiogenesis by intramuscular gene therapy with adeno-VEGF

Diabetes is a major risk factor for coronary and peripheral artery diseases. Although diabetic patients often present with advanced forms of these diseases, it is not known whether the compensatory mechanisms to vascular ischemia are affected in this condition. Accordingly, we sought to determine whether diabetes could: 1) impair the development of new collateral vessel formation in response to tissue ischemia and 2) inhibit cytokine-induced therapeutic neovascularization. Hindlimb ischemia was created by femoral artery ligation in nonobese diabetic mice (NOD mice, n = 20) and in control C57 mice (n = 20). Hindlimb perfusion was evaluated by serial laser Doppler studies after the surgery. In NOD mice, measurement of the Doppler flow ratio between the ischemic and the normal limb indicated that restoration of perfusion in the ischemic hindlimb was significantly impaired. At day 14 after surgery, Doppler flow ratio in the NOD mice was 0.49+/-0.04 versus 0.73+/-0.06 for the C57 mice (P< or =0.005). This impairment in blood flow recovery persisted throughout the duration of the study with Doppler flow ratio values at day 35 of 0.50+/-0.05 versus 0.90+/-0.07 in the NOD and C57 mice, respectively (P< or =0.001). CD31 immunostaining confirmed the laser Doppler data by showing a significant reduction in capillary density in the NOD mice at 35 days after surgery (302+/-4 capillaries/mm2 versus 782+/-78 in C57 mice (P< or =0.005). The reduction in neovascularization in the NOD mice was the result of a lower level of vascular endothelial growth factor (VEGF) in the ischemic tissues, as assessed by Northern blot, Western blot and immunohistochemistry. The central role of VEGF was confirmed by showing that normal levels of neovascularization (compared with C57) could be achieved in NOD mice that had been supplemented for this growth factor via intramuscular injection of an adenoviral vector encoding for VEGF. We conclude that 1) diabetes impairs endogenous neovascularization of ischemic tissues; 2) the impairment in new blood vessel formation results from reduced expression of VEGF; and 3) cytokine supplementation achieved by intramuscular adeno-VEGF gene transfer restores neovascularization in a mouse model of diabetes.

Treatment of thromboangiitis obliterans (Buerger's disease) by intramuscular gene transfer of vascular endothelial growth factor: preliminary clinical results

PURPOSE

Thromboangiitis obliterans (TAO), or Buerger's disease, a distinct form of vascular occlusive disease that afflicts the peripheral arteries of young smokers, is often characterized by an inexorable downhill course even in patients who discontinue smoking once a stage of critical limb ischemia associated with ulceration or gangrene is reached. As part of a phase I clinical trial to document the safety and efficacy of intramuscular gene transfer of naked plasmid DNA-encoding vascular endothelial growth factor (phVEGF165) in the treatment of critical limb ischemia, we treated TAO in 6 patients.

METHODS

Seven limbs in 6 patients (3 men, 3 women; mean age, 33 years; range, 33 to 51 years) who satisfied the criteria for TAO and had signs or symptoms of critical limb ischemia were treated twice, 4 weeks apart, with 2 or 4 mg of phVEGF165, which was administered by direct intramuscular injection at 4 arbitrarily selected sites in the ischemic limb. The gene expression was documented by enzyme-linked immunosorbent assay that was performed on peripheral blood samples.

RESULTS

The ulcers that were nonhealing for more than 1 month healed completely in 3 of 5 limbs after the intramuscular phVEGF165 gene therapy. Nocturnal rest pain was relieved in the remaining 2 patients, although both continue to have claudication. The evidence of the improved perfusion to the distal ischemic limb included an increase of more than 0.1 in the ankle brachial index in 3 limbs, an improved flow shown with magnetic resonance imaging in 7 of the 7 limbs, and newly visible collateral vessels shown with serial contrast angiography in 7 of the 7 limbs. The adverse consequences of the phVEGF165 gene transfer were limited to transient ankle or calf edema in 3 of the 7 limbs. Two patients with advanced distal forefoot gangrene ultimately required below-knee amputation despite the evidence of improved perfusion. A histologic section disclosed the classic pathologic findings of TAO.

CONCLUSION

Therapeutic angiogenesis with phVEGF165 gene transfer, if instituted before the development of forefoot gangrene, may provide a novel therapy for patients with advanced Buerger's disease that is unresponsive to standard medical or surgical treatment methods.