Design of the START-trial: STimulation of ARTeriogenesis using subcutaneous application of GM-CSF as a new treatment for peripheral vascular disease. A randomized, double-blind, placebo-controlled trial

Applications of Ultrasound to Stimulate Therapeutic Revascularization

Many pathological conditions are characterized or caused by the presence of an insufficient or aberrant local vasculature. Thus, therapeutic approaches aimed at modulating the caliber and/or density of the vasculature by controlling angiogenesis and arteriogenesis have been under development for many years. As our understanding of the underlying cellular and molecular mechanisms of these vascular growth processes continues to grow, so too do the available targets for therapeutic intervention. Nonetheless, the tools needed to implement such therapies have often had inherent weaknesses (i.e., invasiveness, expense, poor targeting, and control) that preclude successful outcomes. Approximately 20 years ago, the potential for using ultrasound as a new tool for therapeutically manipulating angiogenesis and arteriogenesis began to emerge. Indeed, the ability of ultrasound, especially when used in combination with contrast agent microbubbles, to mechanically manipulate the microvasculature has opened several doors for exploration. In turn, multiple studies on the influence of ultrasound-mediated bioeffects on vascular growth and the use of ultrasound for the targeted stimulation of blood vessel growth via drug and gene delivery have been performed and published over the years. In this review article, we first discuss the basic principles of therapeutic ultrasound for stimulating angiogenesis and arteriogenesis. We then follow this with a comprehensive cataloging of studies that have used ultrasound for stimulating revascularization to date. Finally, we offer a brief perspective on the future of such approaches, in the context of both further research development and possible clinical translation.

Growth factors for angiogenesis in peripheral arterial disease

BACKGROUND

Peripheral artery disease (PAD) is associated with a high clinical and socioeconomic burden. Treatments to alleviate the symptoms of PAD and decrease the risks of amputation and death are a high societal priority. A number of growth factors have shown a potential to stimulate angiogenesis. Growth factors delivered directly (as recombinant proteins), or indirectly (e.g. by viral vectors or DNA plasmids encoding these factors), have emerged as a promising strategy to treat patients with PAD.

OBJECTIVES

To assess the effects of growth factors that promote angiogenesis for treating people with PAD of the lower extremities.

SEARCH METHODS

The Cochrane Vascular Information Specialist searched the Specialised Register (June 2016) and CENTRAL (2016, Issue 5). We searched trial registries for details of ongoing or unpublished studies. We also checked the reference lists of relevant publications and, if necessary, tried to contact the trialists for details of the studies.

SELECTION CRITERIA

We included randomised controlled trials comparing growth factors (delivered directly or indirectly) with no intervention, placebo or any other intervention not based on the growth factor's action in patients with PAD of the lower extremities. The primary outcomes were limb amputation, death and adverse events. The secondary outcomes comprised walking ability, haemodynamic measures, ulceration and rest pain.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected trials and assessed the risk of bias. We used outcomes of the studies at low risk of bias for the main analysis and of all studies in the sensitivity analyses. We calculated odds ratios (OR) for dichotomous outcomes and mean differences for continuous outcomes with 95% confidence intervals (CI). We evaluated statistical heterogeneity using the I2 statistic and Cochrane's Q test. We conducted meta-analysis for the overall effect and for each growth factor as a subgroup analysis using OR in a fixed-effect model. We evaluated the robustness of the results in a sensitivity analysis using risk ratio (RR) and/or a random-effects model. We also assessed the quality of the evidence for each outcome.

MAIN RESULTS

We included 20 trials in the review and used 14 studies (on approximately 1400 participants) with published results in the analyses. Six published studies compared fibroblast growth factors (FGF), four studies hepatocyte growth factors (HGF) and another four studies vascular endothelial growth factors (VEGF), versus placebo or no therapy. Six of these studies exclusively or mainly investigated participants with intermittent claudication and eight studies exclusively participants with critical limb ischaemia. Follow-up generally ranged from three months to one year. Two small studies provided some data at 2 years and one of them also at 10 years.The direction and size of effects for growth factors on major limb amputations (OR 0.99, 95% CI 0.71 to 1.38; 10 studies, N = 1075) and death (OR 0.99, 95% CI 0.69 to 1.41; 12 studies, N = 1371) at up to two years are uncertain. The quality of the evidence is low due to risk of bias and imprecision (at one year, moderate-quality evidence due to imprecision). However, growth factors may decrease the rate of any limb amputations (OR 0.56, 95% CI 0.31 to 0.99; 6 studies, N = 415). The quality of the evidence is low due to risk of bias and selective reporting.The direction and size of effects for growth factors on serious adverse events (OR 1.09, 95% CI 0.79 to 1.50; 13 studies, N = 1411) and on any adverse events (OR 1.10, 95% CI 0.73 to 1.64; 4 studies, N = 709) at up to two years are also uncertain. The quality of the evidence is low due to risk of bias and imprecision (for serious adverse events at one year, moderate-quality evidence due to imprecision).Growth factors may improve haemodynamic measures (low-quality evidence), ulceration (very low-quality evidence) and rest pain (very low-quality evidence) up to one year, but they have little or no effect on walking ability (low-quality evidence). We did not identify any relevant differences in effects between growth factors (FGF, HGF and VEGF).

AUTHORS' CONCLUSIONS

The results of this review do not support the use of therapy with the growth factors FGF, HGF or VEGF in people with PAD of the lower extremities to prevent death or major limb amputation or to improve walking ability. However, the use of these growth factors may improve haemodynamic measures and decrease the rate of any limb amputations (probably due to preventing minor amputations) with an uncertain effect on adverse events; an improvement of ulceration and rest pain is very uncertain. New trials at low risk of bias are needed to generate evidence with more certainty.

Direct quantitative assessment of the peripheral artery collateral circulation in patients undergoing angiography

BACKGROUND

Despite the fact that numerous studies have pursued the strategy of improving collateral function in patients with peripheral artery disease, there is currently no method available to quantify collateral arterial function of the lower limb.

METHODS AND RESULTS

Pressure-derived collateral flow index (CFIp, calculated as (occlusive pressure-central venous pressure)/(aortic pressure-central venous pressure); pressure values in mm Hg) of the left superficial femoral artery was obtained in patients undergoing elective coronary angiography using a combined pressure/Doppler wire (n=30). Distal occlusive pressure and toe oxygen saturation (Sao2) were measured for 5 minutes under resting conditions, followed by an exercise protocol (repetitive plantar-flexion movements in supine position; n=28). In all patients, balloon occlusion of the superficial femoral artery over 5 minutes was painless under resting conditions. CFIp increased during the first 3 minutes from 0.451±0.168 to 0.551±0.172 (P=0.0003), whereas Sao2 decreased from 98±2% to 93±7% (P=0.004). Maximal changes of Sao2 were inversely related to maximal CFIp (r(2)=0.33, P=0.003). During exercise, CFIp declined within 1 minute from 0.560±0.178 to 0.393±0.168 (P<0.0001) and reached its minimum after 2 minutes of exercise (0.347±0.176), whereas Sao2 declined to a minimum of 86±6% (P=0.002). Twenty-five patients (89%) experienced pain or cramps/tired muscles, whereas 3 (11%) remained symptom-free for an occlusion time of 10 minutes. CFIp values were positively related to the pain-free time span (r(2)=0.50, P=0.002).

CONCLUSIONS

Quantitatively assessed collateral arterial function at rest determined in the nonstenotic superficial femoral artery is sufficient to prevent ischemic symptoms during a total occlusion of 5 minutes. During exercise, there is a decline in CFIp that indicates a supply-demand mismatch via collaterals or, alternatively, a steal phenomenon. CLINICAL TRIAL REGISTRATION-URL: http://www.clinicaltrials.gov. UNIQUE IDENTIFIER: NCT01742455.

Stimulation of collateral artery growth: a potential treatment for peripheral artery disease

In the course of peripheral artery occlusive disease, blood flow to peripheral tissue progressively decreases in a substantial portion of patients, leading to insufficient oxygenation and to the occurrence of claudication or critical limb ischemia. Arteriogenesis (collateral artery growth) is a powerful natural mechanism by which large conductance vessels develop that circumvent sites of obstruction. Promising experimental data on both hypoxia-driven angiogenesis as well as monocyte-orchestrated arteriogenesis have raised high hopes for clinical application. Both endothelial growth factors to stimulate angiogenesis (i.e., capillary growth) and monocyte-attracting or -activating substances to stimulate arteriogenesis, have been proposed as potential new therapeutic agents. However, transferring the promising experimental results into clinical practice has been more cumbersome than initially anticipated. Some recent clinical studies are now focusing more specifically on the stimulation of arteriogenesis. This review will critically evaluate the results of preclinical and clinical investigations on the stimulation of vascular growth, focusing specifically on the peripheral circulation.

Angiogenic therapy for coronary artery and peripheral arterial disease

Neovascularization in chronically ischemic adult cardiac and skeletal muscle results from the processes of angiogenesis, arteriogenesis and vasculogenesis. Therapeutic angiogenesis describes an emerging field of cardiovascular medicine whereby new blood vessels are induced to grow to supply oxygen and nutrients to cardiac or skeletal muscle rendered ischemic as a result of progressive atherosclerosis. Various techniques have been utilized to promote new blood vessel growth in the heart and extremities, including mechanical means such as surgical or percutaneous myocardial laser revascularization, angiogenic growth factor therapies involving members of the vascular endothelial growth factor and fibroblast growth factor families, and more recently, cellular-based therapies using stem cells known as endothelial progenitor cells or angioblasts. The following review discusses each of these treatment strategies in detail including both preclinical and clinical data for their use in peripheral arterial and coronary artery disease.

Granulocyte-macrophage colony-stimulating factor stimulates arteriogenesis in a pig model of peripheral artery disease using clinically applicable infusion pumps

BACKGROUND

A growing number of patients suffer from peripheral artery disease (PAD). Current therapies are often limited by the extent of vascular pathology and the occurrence of restenosis after angioplasty. The stimulatory effect of growth factor administration on collateral vessel formation (arteriogenesis) has evolved as a potential new treatment for this patient group. Granulocyte-macrophage colony-stimulating factor (GM-CSF) was shown to stimulate arteriogenesis in small-animal models and in a pilot study in patients with coronary artery disease. Although a recent clinical study demonstrated disappointing results after subcutaneous GM-CSF application in patients with PAD, we hypothesized that intra-arterial cytokine application using implantable infusion pumps might well stimulate arteriogenesis in a large-species model of peripheral vascular disease. We also aimed to compare continuous and intermittent infusion regimens and to validate experimental and clinically available measurements of collateral artery growth.

METHODS

Twenty-four pigs underwent unilateral occlusion of the right femoral artery and received either GM-CSF continuously, GM-CSF intermittently, or phosphate-buffered saline (PBS). After 1 week, collateral conductance was determined under maximal vasodilatation with adenosine and by using a pump-driven extracorporal shunt system.

RESULTS

Conductance showed a significant stimulatory effect of GM-CSF on arteriogenesis (collateral conductance [mL/min/mm Hg]: PBS, 37.7 +/- 5.4; GM-CSF continuous, 69.2 +/- 12.5; GM-CSF intermittent, 71.5 +/- 11.1). Flow measurements under reactive hyperemia were consistent with these results (flow occluded/non-occluded hind limb: PBS, 40.5% +/- 9.1%; GM-CSF continuous, 48.9% +/- 3.9%; GM-CSF intermittent, 48.7% +/- 4.4%). Measurements of ankle/brachial indices were not sensitive enough to detect the differences in collateral growth between the three groups.

CONCLUSION

These results demonstrate the proarteriogenic properties of GM-CSF in larger animal species, revealing comparable efficacy of continuous and intermittent intra-arterial infusion. Furthermore, we provide evidence that implantable pumps offer a possible means for the intra-arterial application of growth factors. Intra-arterial application of GM-CSF might be a future treatment option for vascular occlusive disease. Finally, we show that in the peripheral circulation, pressure measurements alone have a low sensitivity to determine the effects of proarteriogenic therapy compared with flow or combined flow-pressure measurements.

START Trial: a pilot study on STimulation of ARTeriogenesis using subcutaneous application of granulocyte-macrophage colony-stimulating factor as a new treatment for peripheral vascular disease

BACKGROUND

Granulocyte-macrophage colony-stimulating factor (GM-CSF) was recently shown to increase collateral flow index in patients with coronary artery disease. Experimental models showed beneficial effects of GM-CSF on collateral artery growth in the peripheral circulation. Thus, in the present study, we evaluated the effects of GM-CSF in patients with peripheral artery disease.

METHODS AND RESULTS

A double-blinded, randomized, placebo-controlled study was performed in 40 patients with moderate or severe intermittent claudication. Patients were treated with placebo or subcutaneously applied GM-CSF (10 microg/kg) for a period of 14 days (total of 7 injections). GM-CSF treatment led to a strong increase in total white blood cell count and C-reactive protein. Monocyte fraction initially increased but thereafter decreased significantly as compared with baseline. Both the placebo group and the treatment group showed a significant increase in walking distance at day 14 (placebo: 127+/-67 versus 184+/-87 meters, P=0.03, GM-CSF: 126+/-66 versus 189+/-141 meters, P=0.04) and at day 90. Change in walking time, the primary end point of the study, was not different between groups. No change in ankle-brachial index was found on GM-CSF treatment at day 14 or at day 90. Laser Doppler flowmetry measurements showed a significant decrease in microcirculatory flow reserve in the control group (P=0.03) and no change in the GM-CSF group.

CONCLUSIONS

The present study does not support the use of GM-CSF for treatment of patients with moderate or severe intermittent claudication. Issues that need to be addressed are dosing, the selection of patients, and potential differences between GM-CSF effects in the coronary and the peripheral circulation.

Therapeutic manipulation of the collateral circulation – future directions

Stimulation of collateral artery growth is an attractive alternative treatment modality for patients with coronary or peripheral artery disease. Decades of basic research have led to a reasonable understanding of the mechanisms behind collateral artery growth although 'bench research' is still absolutely warranted for better understanding. It is some 7 years ago that the first clinical trials on therapeutic manipulation of the collateral circulation were published and this field is still relatively new and in large parts unexplored. Arteriogenesis, namely the growth of large collateral conductance arteries, seems to be the best biological substrate candidate for therapeutic manipulation. Future studies in this field will have to cope with problems of substance choice, clinical detection methods and unwanted side-effects.