Current Research Going On To Treat Atherosclerosis Using Liposomes
The repercussions of atherosclerotic cardiovascular disease (ASCVD) include myocardial infarction, ischemic stroke, and angina pectoris, which are the main causes of mortality and morbidity worldwide. In atherosclerosis, inflammation in arterial wall is the main cause behind CVD. Due to dysregulated cholesterol metabolism and maladaptive immune response, plaques are formed. ASCVD can be treated by angioplasty and also by administering drugs like anti-inflammatory, fibrates, statins etc. There are various side effects attached to it like in many causes after 6 months of angioplasty plaques are being formed again. Various adverse effects caused by drugs are muscle pain, osteoporosis, fatigue, and weakness. Effective therapeutic approaches to counter atherosclerosis while bypassing the adverse effects of traditional anti-inflammatory drugs will advance the healthcare of CVD patients. Liposomes can be modified to target plaque and also used for the delivery of drugs. Liposomes are biocompatible and non-toxic structures, comprising of phospholipids, which self-assemble in water and form layered vesicles.
Liposome’s implementation mainly depends on its capability to target atherosclerotic lesions/plaques. There are various novel strategies developed to target the endothelium, plaques, and macrophages. By enhanced permeability and retention, plaque can be easily targeted. Damaged endothelium overexpresses some specific biomarkers like vascular cell adhesion molecule 1 (VCAM1), intercellular adhesion molecule 1 (ICAM1), and lectin-type oxidized LDL receptor 1 (LOX-1). Antibodies against biomarkers is attached to target the damaged endothelium. Scientific research found that anti-VCAM1 and anti-ICAM1 liposomes can be used to target endothelial cells and are very effective. LOX-1 receptor is another target for liposomes due to the receptor upregulation on endothelium resulting into secretion of cytokines which leads to inflammation. Specific targeting of liposomes by attaching antibodies creates a difficulty by increasing immune recognitions of liposomes. It causes faster removal of liposomes from the circulatory system. To overcome this problem density of liposomes must be optimized. Despite promising results, it is problematic because blood flow in the vessels creates shear stress at endothelial cells. Macrophages can be used as another approach to treat atherosclerosis. Presence of phosphatidylserine on the surface of an apoptotic cell leads to identification and engulfment of macrophages. Incorporating PS anti liposomes to help identify macrophage brings the macrophages to the site of lesion thus making it a rich site containing macrophages. Receptors like CDR6, gAD (binds due to the presence of adinopectin) and Folate receptor Beta present on the surface of macrophages can be used as a target for liposomes. Fibrous cap is also used as target for liposomes to treat atherosclerosis. Various antifibrinogenic conjugated liposome studies on animal models have shown satisfactory results.
Cholesterol is the main cause of plaque progression. Reducing the cholesterol in the body can used to treat atherosclerosis. In one study of the expression of genes involved in the efflux of cholesterol in an LDLR mouse model in which the mice consumed a cholesterol-rich diet showed that nanoliposomes upregulated ATP binding cassette subfamily A member 1 (ABCA1) and subfamily G member 1 (ABCG1) genes. Negatively charged phospholipids containing liposomes e.g., phosphatidylcholine and diasteroyl phosphatidylglycerols can increase reverse cholesterol transport which helps to decrease the available cholesterol at vascular wall. ABC transporters act as an intermediate in cholesterol efflux pathways, also they control and propagation and mobilization of haematopoietic system and progenitor cells present in bone marrow.
In atherosclerosis liposomes can be used as a potential drug delivery carrier for anti-inflammatory drugs and antiangiogenic drugs. Glucocorticoids are mainly used as anti-inflammatory drugs for ASCVD. Delivering glucocorticoids with liposomes decrease the adverse effects when administered orally. It also increases the half-life of glucocorticoids, so frequency of dose also decreases. Loading glucocorticoids into PEGylated liposomes increases the circulation time and improves their accumulation in atherosclerotic lesions. Research done on Prednisolone phosphate when PEGylated used to target lesions showed good results. Major drawback of prednisolone loaded liposome is lipotoxicity which might worsen ASCVD. First clinical trial done on prednisolone in 2015 has shown some increase in plasma life but it did not show any decrease in inflammation. There are various other anti-inflammatory drugs like corticosteroids and chemokines antagonists and can be incorporated in liposomes to decrease the inflammation. Anti-angiogenic drugs like bevacizumab when loaded with liposomes has shown good results. This drug helps in inhibition of vascular endothelial growth factor VEGF and ceases the development of lesions.
The delivery of some specific genes to the site of plaque formation could stop the progression of atherosclerosis. In one study, liposomes were used as a small interfering (si) RNA delivery system to achieve a reduction in the expression of fatty acid binding protein 4 (FABP4). Immunofluorescent staining demonstrated successful uptake of liposomal siRNA into atherosclerotic plaque and the silencing of FABP4. A very common type is graft atherosclerosis. Graft arteriosclerosis (GA), the major cause of late cardiac allograft failure, is characterized by a diffuse, concentric arterial intimal hyperplasia composed of infiltrating host T cells, macrophages and predominantly graft-derived smooth muscle–like cells that proliferate and elaborate extracellular matrix, resulting in luminal obstruction and allograft ischemia. Gene delivery using liposomes could prevent graft atherosclerosis, a very common problem which limits the long-term success of cardiac transplantation due to the complications developed in the graft. Graft atherosclerosis may be induced due to the decrease in the level of tissue plasminogen activator (tPA) in the arteriolar smooth muscle cells. Gene delivery, with the goal of overexpressing tPA, could reduce the risk of atherosclerosis. On the other hand, some studies have found that when the size of liposomes was increased the MPS uptake was improved, negatively charged liposomes internalize more readily, because they are more easily recognized by macrophages which in turn are the metabolically active regions in an atherosclerotic plaque. Another study investigating the uptake of anionic liposomes into atheromas using Watanabe heritable hyperlipidemic rabbits showed that the uptake of anionic liposomes was improved in the metabolically active sites of the plaque. A study of human coronary artery endothelial cells pre-treated with an inhibitor of clathrin-mediated endocytosis demonstrated that clathrin-mediated endocytosis is the pathway responsible for the uptake. One difference between anionic and cationic liposomes is that cationic liposomes can induce cytotoxicity, cytokine activation, and proinflammatory effects. Despite these limitations, cationic liposomes are more suitable for gene delivery applications. The positive charge on these liposomes facilitates electrostatic interaction with negatively charged proteoglycans on the cell membrane. Therefore, delivery of a gene to the cell is enhanced.
Liposomes have been successfully used to deliver genes, stem cells, and anti-inflammatory or anti-angiogenic drugs to the plaque site. Liposome administration has lowered LDL cholesterol. Liposome administration to the plaque site has been achieved using various methods and techniques, which are discussed above. The targeting of liposomes to macrophages has been extensively studied, but no study till date has investigated the targeting of liposomes to enzymes. Liposomes have many advantages over other materials used in nanomedicine. They have the potential for the clinical management of atherosclerosis. However, much of the research till date has been conducted in laboratory settings, or in small clinical studies. Therefore, the potential to draw major inferences relating to the clinical treatment of patients is limited. Proof-of-concept studies, which demonstrate improved delivery of therapeutic agents to specific sites of atherosclerotic activity, do not necessarily imply improved clinical effectiveness and provides no guarantee against unexpected harm that may result from any new drug or therapeutic technology.