Cardiovascular diseases are one of the leading causes of death in the world. The inflammation or accumulation of a substance in arteries generates the formation of atherosclerosis, stenosis, coronary artery disease, coronary thrombosis, and occlusion. The present study offers some solutions under investigation for the mentioned pathologies. Traditionally this condition is treated mainly by a percutaneous coronary intervention, bypass, or by specific medications.

In the present review, based on original articles, the more suitable treatments were chosen. These angiogenic treatments show the best assessments on coronary artery occlusions. All these treatments focus on the generation of new blood vessels from the already existing vasculature.

Keywords: occlusion, angiogenesis, angiogenic factors, collateral formation.

Despite advances in medicine, mortality from cardiovascular diseases is one of the leading causes. For example, according to the statistical analysis of the European society of cardiology, deaths due to coronary heart disease is amount to 1.8 million in Europe alone¹. The disease develops when the narrowing of the coronary artery reduces the flow of essential nutrients to the heart due to the deposit of cholesterol, calcium, inflammatory cells, and other substances². The total or partial occlusion of the coronary artery is commonly known as atherosclerosis, stenosis, coronary arteriopathy, coronary artery disease, and coronary thrombosis. While, for the diagnosis of this pathology, it is done through the use of angiograms with the evaluation of the frames generated by it. In addition, there are other forms of exploration, such as electrocardiograms that can evaluate a previous or current heart attack; by computerized tomography in which the deposits of substances in the arteries like calcium are observed; by echocardiograms in which the waves generated by the physical effort of the patient will be evaluated; also, intravascular ultrasound, radiography and nuclear exploration of heart³.

Percutaneous coronary intervention (PCI) is one of the main procedures traditionally used to improve and restore blood flow. This procedure uses a catheter for the placement of a stent to allow passage of flow through the arteries affected by the occlusion of any of the substances mentioned above⁴. Another of the interventions to treat this pathology is the bypass that, like the stent, helps improve blood flow. However, this treatment is carried out employing a surgical surgery, which consists of creating a new route or derivation around the obstruction of the artery. The material used for the new route is a piece of a blood vessel from the leg, chest, or another part of the body to graft it into the coronary artery of interest^5,6. Apart from this, it is possible to treat coronary occlusion with drugs such as cholesterol modifying, aspirin, Beta-blockers, calcium channel blockers, ranolazine, nitroglycerine, angiotensin-converting enzyme inhibitors, and angiotensin II receptor blockers⁷.

Therapeutic angiogenesis is a procedure made to treat ischemic diseases by generating new blood vessels from already existing vasculature⁸. Its primary focus is to induce, develop, and control the angiogenic responses of the subject, to revascularize ischemic tissues⁹. This therapeutic angiogenesis is based on the delivery of exogenous factors, which are the ones that will stimulate the formation of new vasculature, these factors can be either genes, proteins or cells, due to the best efficacy showed in animal studies⁸. Growth factors, such as the vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) can be delivered as proteins or genes encoding target proteins. The underlying hypothesis of therapeutic angiogenesis is to apply angiogenic factors in ischemic tissues to guide angiogenic cellular and tissue behavior⁹.

Erythropoietin (EPO) is a glycoprotein cytosine that regulates the proliferation and differentiation of erythrocyte progenitor cells¹⁰. Westenbrink et al. demonstrated that EPO induces neovascularization in ischemic hearts. In addition to improving micro vascularization and cardiac function in patients with ischemia¹¹. EPO stimulates vessel growth known as angiogenesis through the use of bone marrow cells and increased myocardial expression of VEGF¹².

According to the study of Jaquet et al. several angiogenic factors were used, mainly EPO which gave favorable results with a maximum effect on the endothelial cells developed from myocardial tissue. While with the continuous supply of recombinant human erythropoietin (rHuEpo) it resulted in a 220% increase in the length of the endothelium. Besides, they obtained that EPO and VEGF165 demonstrate an equal angiogenic potential. Additional experiments revealed that when they were applied at the same time as growth factors, they do not generate a further increase in proliferative capacity. The combination of VEGF and bFGF resulted in a slight increase in the length of the endothelium growths compared to VEGF. Additionally, it was observed that by combining VEGF and aFGF, they mutually inhibit each other¹³.

In conclusion, EPO has angiogenic potential in human myocardial tissue. Angiogenesis and among others are essential functions of the EPO¹⁴, which makes it a substance considered for the treatment of coronary heart diseases.

The use of gene therapy has a high success rate in stimulating angiogenesis. Various cytokines have been found to be mediating agents for collateral growth and angiogenesis, between these agents we have VEGF, platelet-derived growth factor (PDGF), and basic fibroblast growth factor (bFGF)^15,16.  Nevertheless, there are still problems related to this kind of therapy, for example, a poor transfection efficiency, nonspecific effects of the transfection, and the organ-specific transfection. Moreover, as the technique has been based on direct injection of the vector (plasmid or viral) in the myocardium, more issues come with it, such as the non-uniformity of the transfection and the possibility that a substantial fraction of the vectors can reach the blood circulation, thus transfection other organs.

A proposal to go past these problems is to use autologous cells, more specific vascular smooth muscle cells (VSMC) that can transfect the genes in the myocardium in a uniform way via intracoronary administration. The main reasons to work with this type of cells is because of their ease to obtain, their division is quickly, and their limited plasticity will not let them differentiate or dedifferentiate^16,17. VSMC are administered via intracoronary injection to avoid a non-organ-specific transfection, as the cells due to its larger size are going to stay at the precapillary portions.

In the most relevant study found, they use autologous VSMC transfected with the VEGF gene and the enhanced green fluorescent protein (EGFP) gene, delivered via an intracoronary pathway to accelerate collateral development. The model of the study were dogs with induced repetitive ischemia over 21 days. The study had three groups, the animals which were injected with the VSMCs transfected with the VEGF gene and the EGFP gene, the control group which only had the enhanced green fluorescent protein-transfected cells, and the sham group (no cell transplantation). The collateral flow showed an increase in the VEGF group; however, it never reached the natural collateral flow levels, and the suspicions were a diminish of the VEGF expression in the transfected group or that VEGF needs to work with other factors. Implying that VEGF is essential and useful for the initiation of collateral development, but it needs other growth factors to complete the process. Another consideration found to be needed for a better action of VEGF is existing ischemia¹⁶.

FGF is a powerful and widely used angiogenic factor; this family of growth factors is conformed by nine factors named basic FGF (bFGF), acidic FGF, FGF 3-9^8,18.  b(FGF) and acidic (FGF) are the most characterized of this family of FGF¹⁸. b(FGF) is one of the first discovered angiogenic factors that have angiogenic and arteriogenic properties⁹. This member of the FGF family is a monomeric polypeptide which induces angiogenesis in vivo studies, induces the proliferation of endothelial cells and smooth muscle cells due to the expression of FGF receptors, and also it is involved in the regulation of homeostasis and proliferation of blood vessels^9,19. Nevertheless, as many of the other growth factors, preclinical data have established that eventually the delivery of bFGF results in unstable vessel growth, this can be addressed to different factors, such as its rapid diffusion, impaired biostability, and short half-live⁹.

Inflammation influences the formation of atherosclerosis, and other diseases produced in the coronary arteries. In the first place, chemokines are inflammatory mediators which suppress certain biological functions, such as the migration of leukocytes with inflammatory signals. Based on the study of Vahid et al., they evaluate the potential correlation between serum levels of the chemokines (CXCL-10 and CXCL-12) and the degree of coronary stenosis, and also the degree of occlusion of the coronary artery. The processes of angiogenesis and angiostasis can be attributed to chemokines, which intervene in the development of collateral circulation²⁰.

Chemokines are part of a subcategory of cytokines. Something to note about these proteins is that they are the first to transmigrate to infection, inflammation, and traumatic tissue injury sites²¹. Additionally, chemokines influence angiogenesis, initiation of the formation of new blood vessels, or angiostasis, inhibition of the formation of new blood vessels²². The mentioned chemokines CXCL-10 and CXCL-12 belonging to CXC, one of the four significant subclasses of chemokines, are an example of this²³.

According to Vahid et al., there was an increase in CXCL-10 chemokines, such as CXCL-12 in men, which represents the interaction between the balance of angiogenesis, inflammation, and atherosclerosis²⁰. It has been found that male hormones and globulin, through their impact on angiogenic CXCL-12, affect the pathogenesis of coronary artery diseases that induces the expression and production of chemokines mentioned²⁴. Moreover, the older the person, the higher the risk of arterial calcification and rigidity. Also, it was determined that diabetes, endothelial and acute cell dysfunction contribute to vascular inflammation and the development of atherosclerotic coronary arteries and that chemokines possess antisclerotic functions²⁵.

One of the striking findings in their study is the correlation between CXCL10 and CXCL12 chemokine concentrations and coronary artery occlusion. Where they found intense competition between these two chemokines and each chemokine increases in an attempt to influence the balance of angiogenesis/angiostasis²⁰. It is worth mentioning that in other studies, cytokines such as TNF-α, IL-27, CXCL-8, and CRP play a role in the degree of occlusion of the coronary artery²⁶.

Thus, if an equilibrium will be induced between the two chemokines, and the influence of the angiogenic chemokine was more significant than the angiostatic one, satisfactory results could be generated for the improvement and efficiency of these blood vessels.

Nowadays, one of the most significant challenges for future medicine is to search ways to treat coronary artery diseases, without the surgical intervention of implants like stents and grafts like the bypass, that do not necessarily ensure a long and reliable solution, as the aforementioned treatments come with risks like heart rhythm problems, blood clots, and many other problems²⁷. Thus, the creation and implementation of reliable solutions are needed to improve the management of patients with artery coronary occlusions. It has been demonstrated that certain angiogenic factors, such as VEGF and bFGF, increase the collateral flow and formation, even enhance the cellular proliferation on the damaged zones by the occlusion. As well as that EPO is an angiogenic growth factor as it induces neovascularization. We have also to consider, the different ways of monitoring and control the angiogenic process, researchers have found two types of chemokine (CXCL10 and CXCL12) that by concentrations level it is possible to measure the grade of angiogenesis and angiostasis produced in areas of a coronary artery.

Even though there are strong pieces of evidence that these therapies using angiogenic factors are useful and have good results, there are still some issues that are needed to be addressed, like the optimal dose, time course, routes of administration. Also, the comprehension of the exact mechanism of how these factors work to correctly combine these components in a single complemented treatment between them is an issue.

Our review cites several research papers, in which various authors evaluate and demonstrate the different new techniques to treat and rehabilitate damages caused by occlusions on the coronary artery. All these treatments have shown good efficacy to treat the problem by increasing the collateral flow in the coronary arteries, and are suitable solutions to treat this problem on humans.