Mechanisms, Pathophysiology and Currently Proposed Treatments of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is one of the leading global causes of morbidity and mortality. A hallmark of COPD is progressive airflow obstruction primarily caused by cigarette smoke (CS). CS exposure causes an imbalance favoring pro- over antioxidants (oxidative stress), leading to transcription factor activation and increased expression of inflammatory mediators and proteases. Different cell types, including macrophages, epithelial cells, neutrophils, and T lymphocytes, contribute to COPD pathophysiology. Alteration in cell functions results in the generation of an oxidative and inflammatory microenvironment, which contributes to disease progression. Current treatments include inhaled corticosteroids and bronchodilator therapy. However, these therapies do not effectively halt disease progression. Due to the complexity of its pathophysiology, and the risk of exacerbating symptoms with existing therapies, other specific and effective treatment options are required. Therapies directly or indirectly targeting the oxidative imbalance may be promising alternatives. This review briefly discusses COPD pathophysiology, and provides an update on the development and clinical testing of novel COPD treatments.


Introduction
A hallmark of chronic obstructive pulmonary disease (COPD) is the chronic obstruction of the airways. COPD is a progressive condition caused by inhalation of toxic particles or gases [1,2]. Tobacco smoking and inhalation of other pollutants are the leading causes of COPD [3][4][5].
COPD is a major cause of global morbidity and mortality, resulting in increased economic and social burden [1,2,6]. Variance among countries and between different groups in the prevalence of this disease is often directly related to smoking prevalence, although environmental pollution is also a significant risk factor in many countries. The prevalence and burden of COPD will increase in the coming decades due to continued exposure to risk factors and aging of the world population [5,7]. There are many pulmonary and systemic comorbidities in COPD patients, such as bronchiectasis, asthma, heart failure, cardiovascular diseases, sleep apnea, malnutrition, and frailty [8].

Diaphragm Dysfunction and COPD
The dynamics hyperinflation results in diaphragm mechanical disadvantage leading to dysfunction [43,44]. Clinical studies using ultrasonography in in-hospital patients have shown its ability to detect diaphragm weakness, resulting in increased hospital length of stay [45,46]. The diaphragm weakness acquired during exacerbation can be explained by: (1) elevated number of inflammatory cells in the lungs [47,48]; (2) oxidative stress and damage within the diaphragm [49]; (3) diaphragm remodeling [44]; (4) maintenance of hyperinflated areas, which jeopardize diaphragm performance [50,51]; and (5) changes in mitochondrial dynamics. Several studies have shown that mitochondria are dynamic organelles with the ability to change morphology and function according to the pathologic situations through fusion and fission processes [52]. Mitochondrial fusion is mediated by proteins located at the external mitochondria membrane, such as mitofusin 1 (MFN1), mitofusin 2 (MFN2), and optic protein factor 1 (OPA 1). These proteins hydrolases GTP and promote mitochondria fusion, which allows DNA, protein, and metabolites sharing. Mitofusins act toward the external membrane forming homo-and heterodimers [53,54], while OPA1 acts toward the internal membrane. Thus, the loss of these proteins may lead to mitochondrial DNA damage [55], affecting bioenergetics function [56]. The mitochondrial fission is characterized by mitochondria fragmentation, and the main objectives are: (1) to increase the mitochondria numbers to distribute to new cells during mitosis [57]; (2) to transport to other regions of the cell; and (3) to signalize injured cells and forward them to mitophagy [58] and apoptosis. First, fission occurs through the inhibition of mitochondrial fusion protein. Second, the fission process demands the presence of mitochondrial fission, such as dynamin-related protein 1 (DRP1) [59], which interacts with human fission factor (Fis 1) and mitochondria fission factor (MFF). Thus, the extensive activation of DRP1 may increase mitochondrial fragmentation, increasing reactive oxygen species followed by decreased ATP production [60,61].

Pulmonary Arterial Hypertension and COPD
Pulmonary arterial hypertension (PAH) and diaphragm dysfunction are commonly observed during COPD progression, contributing to exacerbations [62,63]. Exacerbations are acute episodes caused by viral and bacterial infections, which worsen airway inflammation, cause lung function decline followed by hospitalization, and increase mortality [64,65]. COPD patients who present PAH have decreased survival rate compared to COPD patients at similar severity but without PAH [62]. One probable explanation is that PAH is associated with vascular remodeling, likely due to collagen fibers accumulation beneath pulmonary vessels, which leads to vessel narrowing overloading the right ventricle [66][67][68]. Furthermore, during PAH associated with COPD development, there is an increase in the pulmonary inflammatory process and the release of vasoactive agents, such as thromboxane A2. It can induce vascular constriction and further increase vascular resistance [69].

Reactive Oxygen Species and COPD
In addition to inflammation, COPD is characterized by an imbalance between proteases and their inhibitors, oxidative stress, and infections that generate disease symptoms [70,71]. Prognosis for COPD patients depends on different factors, including disease severity, body mass index, and age [10]. Patients display increased numbers of neutrophils, macrophages, and T cells in the lungs, increasing chemotactic mediators [71]. In addition to pulmonary inflammation, there is also systemic inflammation with increased levels of fibrinogen, C-reactive protein (CRP), serum amyloid A (SAA), and pro-inflammatory cytokines tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and IL-8 in the serum [72]. Cigarette smoke (CS) is the primary source of oxidant agents in the lungs, but inflammatory cells and phagocytes residing in the respiratory tract also generate reactive oxygen species (ROS) in the lungs [70,73]. Increased nicotinamide adenine dinucleotide phosphate (NADPH) activity in epithelial cells, phagocytes, and myeloperoxidase in neutrophils is responsible for ROS production in patients with COPD [73]. Oxidative stress generated by CS results in nuclear kappa B (NF-κB) activation, producing inflammatory mediators that foster tissue damage [70,73]. NF-κB activation induces cytokines, chemokines, and cell adhesion molecules, which are boosted by bacterial or viral infections, exacerbating disease symptoms [74]. Oxidative stress is the primary cause of COPD pathogenesis, triggering apoptosis, extracellular matrix remodeling, inactivation of protease inhibitors, mucus secretion, NF-κB activation, mitogen-activated protein kinase (MAPK) activation, chromatin remodeling, and pro-inflammatory gene transcription [71,74,75].
Healthy lungs possess enzymatic and non-enzymatic antioxidant mechanisms that counteract oxidative stress. Non-enzymatic mechanisms involve glutathione (GSH), vitamin C, uric acid, vitamin E, and albumin. Enzymatic mechanisms rely on superoxide dismutase (SOD), catalase, and glutathione peroxidase (Gpx) [72]. Exposure to CS decreases intracellular GSH levels, boosting oxidative stress in COPD patients [76]. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is crucial for regulating the cellular antioxidant response and preventing ROS-induced injury. Nrf2 regulates the expression of genes encoding enzymes that regulate oxidative stress, including the typical phase 2 detoxifying enzyme hemoxygenase-1 (HO-1). Decreased Nrf2 pathway stimulation in peripheral lung tissue and alveolar macrophages is associated with increased susceptibility to and severity of COPD [77].
In the next section, we discuss both immune and structural cells in COPD pathophysiology.

Inflammatory Cells and Mediators
The terminal bronchioles and lung parenchyma are the main regions affected by COPD inflammation, and are characterized by infiltrating macrophages and CD8 + Tcells. Macrophages are primarily present within the lungs, while CD8 + T-cells cause Pharmaceuticals 2021, 14, 979 5 of 32 alveolar epithelial cell apoptosis and destruction through release of perforins and TNFα [9]. Macrophages and neutrophils are involved in ROS generation during COPD [70]. In response to macrophages and neutrophils, alveolar epithelial cells release leukotriene B4 (LTB4), a chemotactic factor that attracts immune cells [9,78]. Macrophages and pulmonary cells also produce IL-8/CXCL8 [79] and growth-related oncogene (GROα)/CXCL1, which amplify the inflammatory response by attracting more leukocytes from the blood to the inflammatory site [80]. Patients with COPD display smoking-linked ICAM-1 augmentation in epithelial cells. ICAM-1 is an adhesion molecule that is crucial for leukocyte migration. It is highly expressed in patients with severely limited airflow, and is associated with increased risk of viral and bacterial infections [81].
The imbalance between proteases and their inhibitors plays a crucial role in COPD pathogenesis. Proteases, including neutrophil elastase (NE) and proteinase 3, degrade connective tissue components, especially elastin, leading to emphysema [78,82]. Elastin is detected in the serum of patients with COPD due to massive tissue destruction [82]. α1-antitrypsin can inhibit NE, but it is reductively inactivated in COPD patients [83]. Elastic fiber damage may lead to collagen deposition in the pulmonary parenchyma, leading to alveolar septa destruction and alveolar distention [3]. Metalloproteinases (MMPs) attack the extracellular matrix, causing the release of elastin fragments that attract monocytes to the lungs. MMPs are also involved in recruiting pulmonary macrophages, thus raising proteolytic and inflammatory activity, thereby playing an essential role in COPD progression [11].
Therefore, ROS, inflammatory mediators, and proteolytic enzyme production can initiate, enhance, and aggravate tissue damage, and exacerbate lung injury, resulting in COPD development and progression ( Figure 2).

Alveolar Epithelial Cells
Alveolar epithelial cells serve as a mechanical barrier to harmful stimuli [88]. Exposure of these cells to CS and other pollutants activates several intracellular signaling pathways, which induce pro-inflammatory mediators, including CXCL8/IL-8, GM-CSF, ICAM-1, and TNF-α, regulating the influx of inflammatory cells [89].
Vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) are vital for maintaining alveolar epithelial cell integrity [90]. Low VEGF and HGF levels have been associated with alveolar epithelial cell apoptosis in COPD patients [91]. After exposure to inhaled irritants, small airway epithelial cells increase TGF-β expression, which stimulates fibroblasts to differentiate into myofibroblasts that produce extracellular matrix (ECM), leading to local fibrosis [92]. While MMP-9 is released by cells involved in immune defense, such as macrophages, MMP-2 is synthesized by fibroblasts, and has been associated with chronic tissue remodeling, leading to abnormal tissue changes [93]. Furthermore, intense ROS production can disrupt surfactant secretion by alveolar epithelial cells, leading to alveolar collapse and high airway resistance due to interdependence, a typical sign of emphysema [89,94].
ROS production is also associated with mitochondrial DNA damage, causing mitochondrial dysfunction [95]. Such dysfunction has been reported in airway smooth muscle cells in COPD patients [96]. CS inhibits mitochondrial respiratory function, reducing ATP production, which induces mitophagy in alveolar epithelial cells [97]. Mitochondrial ROS generation has been associated with surfactant changes, thus interfering with alveolar epithelial cell stability [98].

Alveolar Epithelial Cells
Alveolar epithelial cells serve as a mechanical barrier to harmful stimuli [88]. Exposure of these cells to CS and other pollutants activates several intracellular signaling pathways, which induce pro-inflammatory mediators, including CXCL8/IL-8, GM-CSF, ICAM-1, and TNF-α, regulating the influx of inflammatory cells [89].
Vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) are vital for maintaining alveolar epithelial cell integrity [90]. Low VEGF and HGF levels have been associated with alveolar epithelial cell apoptosis in COPD patients [91]. After exposure to inhaled irritants, small airway epithelial cells increase TGF-β expression, which stimulates fibroblasts to differentiate into myofibroblasts that produce extracellular matrix (ECM), leading to local fibrosis [92]. While MMP-9 is released by cells involved in immune defense, such as macrophages, MMP-2 is synthesized by fibroblasts, and has been associated with chronic tissue remodeling, leading to abnormal tissue changes [93]. Furthermore, intense ROS production can disrupt surfactant secretion by Figure 2. COPD pathophysiology. The toxins present in cigarette smoke lead to the recruitment of inflammatory cells and the release of inflammatory mediators. Macrophages release CXCL1, CXCL8, and LTB4, which attract neutrophils, and CCL2 and CXCL1, which attract monocytes. Neutrophils release ROS, enhancing inflammation and reductively inactivating α1 antitrypsin. They also release proteases, such as NE, leading to tissue damage. Epithelial cells and macrophages release CXCL9, CXCL10, CXCL11, and CXCL12, which attract Th1 and Tc1 lymphocytes. They also release IFN-γ, leading to alveolar destruction. Epithelial cells release CXCL8, recruiting and activating neutrophils, and TGF-β and FGF, recruiting fibroblasts that promote tissue fibrosis. Epithelial cells also attract CD8 + T cells believed to foster inflammation. Macrophages release TNF-α, IL-1β, and ROS, inducing MMP secretion by epithelial cells, macrophages, and neutrophils, causing tissue remodeling. CXCL1, chemokine (C-X-C motif) ligand 1; LTB4, leukotriene B4; CC, Chemokine (C-C motif); IFNγ, interferon gamma; TGF, transforming growth factor; FGF, fibroblast growth factor; TNF-α, tumor necrosis factor alpha; IL, interleukins; ROS, reactive oxygen species; MMPs, metalloproteinases. Created with BioRender.com.

Goblet Cells
Goblet cells are essential to the immune system because of their ability to secrete mucus, antioxidants, protease inhibitors, and defensins, to maintain the epithelial barrier against infectious agents [90]. The epidermal growth factor receptor (EGFR) induces cell proliferation by promoting production of transforming growth factor-alpha (TGF-α), resulting in mucus production [102,103]. EGFR activation is increased in COPD patients [104], elevating their cell proliferation and mucus production [14]. COPD patients display high production of intracellular mucin, mainly MUC5AC [105], and high NF-κB activation and cytokine release [106]. CS and ROS also amplified MUC5AC expression via EGFR activation. MUC5AC production leads to respiratory tract hypertrophy and hyperplasia [70], facilitating disease progression [107]. Mucus hypersecretion also increases viscosity, decreases antibacterial molecule production, aggravates airflow restrictions, and increases the risk of lung infections [108].

Alveolar Macrophages
Alveolar macrophages play a crucial role in innate and adaptive immune responses. They are involved in capturing and processing inhaled harmful agents, and stimulating the immune response by releasing inflammatory mediators, including TNF-α, CXCL1, CXCL8, CXCL9, CXCL10, CCL2, leukotriene B4 (LTB4), and ROS [90]. These mediators also recruit monocytes, neutrophils, and lymphocytes to the inflammatory site. Macrophages also secrete elastolytic enzymes, including MMPs and cathepsins [109]. Similar to neutrophils, macrophages generate ROS through NADPH oxidase (NOX) when stimulated by CS oxidants [110]. In functional NOX2-deficient mice, decreased ROS production provided protection against emphysema [111].
Macrophages can be polarized into either M1 or M2. M1 macrophages display classic activation and have a more inflammatory profile, secreting pro-inflammatory cytokines [112]. They are potent effector cells that specialize in killing microorganisms [113]. M2 macrophages are considered less inflammatory, due to their release of anti-inflammatory cytokines, such as IL-10, and participate in tissue remodeling and repair [114]. In COPD, type M1 is more prevalent than M2, indicating a more accentuated inflammatory response [115], denoted by the release of pro-inflammatory mediators including CCL2 and CXCL1, which enhance cell recruitment, and explaining the higher number of macrophages found in the pulmonary parenchyma, BALF, and sputum of COPD patients [90,116]. Despite their high numbers, macrophages in COPD have low phagocytic and efferocytotic abilities [117]. This altered behavior is associated with exogenous ROS-induced oxidative stress, which is also responsible for altering mitochondrial function, resulting in uncontrolled ROS production [118]. An inadequate macrophage response favors bacterial infection, with increased risk of developing pneumonia and exacerbated symptoms. Therefore, the decrease in phagocytic and efferocytotic functions is related to COPD progression [119]. Macrophage dysfunction is linked to oxidative stress caused by ROS [120]. In healthy individuals, upon oxidative stress, Nrf2 activates a cellular antioxidant response, but Nrf2 activation in macrophages of COPD patients is attenuated [77].
The increase in elastolytic enzymes, such as MMPs, induces ECM degradation and alveolar wall destruction [121]. Alveolar macrophages are pivotal to COPD development, because even after smoking cessation, their dysfunction may continue to contribute to disease progression [122]. These cells play a primary role in the pathology of COPD, and the integrity of their functions is directly related to the degree of inflammation and disease severity.
CD4/CD28 null T cells are a pro-inflammatory subset of T helper lymphocytes, whose main characteristic is loss of CD28, a necessary co-stimulatory receptor for CD4+ T cell activation, proliferation, and survival [129]. The increase in these T cells in COPD patients is linked to impaired lung function. Chronic exposure to CS causes a reduction in the costimulatory molecule CD28, increasing expression of perforin, granzyme B, and receptors in NK cells and T cells [130]. COPD patients are resistant to immunosuppression induced by corticosteroids compared to control individuals with higher counts of CD8/CD28 T cells [131]. T cell senescence in COPD patients may be associated with reduced histone deacetylase 2 (HDAC2) expression in CD8/CD28 null T cells [132]. T cell receptor downregulation leads to an inadequate response to infection, causing either autoimmune disease [133] or increased susceptibility to COPD complications [125].
Regulatory T cells (Tregs) cause immune suppression at the inflammatory site, producing IL-10 and TGF-β1 [134]. FOXP3 is considered a specific marker of Treg cells [135]. In COPD patients, increased levels of Tregs have a pronounced effect, disrupting effector T cell response and tolerance to self-antigens [136].
Patients with severe COPD have a higher number of B cells in their small airways [137]. They also have higher levels of B cell activation factors in lymphoid follicles [138], and B cells secrete autoantibodies against carbonylated proteins, which form due to oxidative stress [139]. Antibodies against elastin, observed in patients with emphysema, were the first evidence of a relationship between autoimmunity and COPD [140]. COPD patients also present with high levels of anti-endothelial and anti-epithelial antibodies [141]. Similar to rheumatoid arthritis patients, they have citrullinated proteins in the lungs, which can induce autoantibodies [142]. Furthermore, COPD patients display a high percentage of apoptotic cells in follicles, suggesting immune dysfunction [143].
Interestingly, the increased number of neutrophils and B lymphocytes correlates with disease severity [144][145][146]. Lymphocytes play an essential role in the autoimmune effects in COPD patients. The ratio of neutrophils to lymphocytes has been suggested for use as a prognostic marker for predicting exacerbations in COPD patients [147,148]

Neutrophils
COPD patients have increased neutrophil numbers in sputum and BALF [149]. Smoking stimulates the release of granulocytes from the bone marrow and their survival in the respiratory tract [150]. Chemotactic factors, including LTB4, CXCL1, CXCL5, and CXCL8 derived from alveolar macrophages, epithelial cells, and T cells, can boost neutrophil migration in COPD patients [90].
Expression of adhesion molecules such as E-selectin is increased in COPD patients. E-selectin is expressed on endothelial cells and is critical for neutrophil recruitment [151]. Likewise, myeloperoxidase (MPO) and human neutrophilic lipocalin (HNL) are high in the airways of COPD patients [152]. Upon arrival into the lung inflammation site, activated neutrophils secrete serine proteases (including neutrophil elastase (NE)), cathepsin G, protein-3, MMP-8, and MMP-9, causing alveolar damage [90,153]. Neutrophils also produce neutrophil extracellular traps (NETs), increasing lung tissue damage in COPD patients [154,155].
Neutrophil gelatinase-associated lipocalin (NGAL) has been suggested as a systemic marker for COPD [161] because its levels are high in induced sputum and bronchiolar lavage fluid from COPD patients [162]. NGAL is secreted by neutrophils and other cells and possesses antimicrobial properties that can reduce bacterial growth. However, when bound to MMP-9, NGAL extends MMP9 enzyme activity, promoting tissue destruction [163]. Granulocyte-colony-stimulating factor (G-CSF) stimulates neutrophil production in the bone marrow, and promotes their survival, priming, and function. Neutralization, inactivation, or blocking of G-CSF causes inflammation and tissue damage, controls monocyte influx into the lungs, initiates neutrophil apoptosis, and mitigates COPD symptoms [164,165]. Neutrophil-mediated inflammation is critical for COPD development and progression [166]. Studies investigating neutrophil infiltration and activity may shed light on the role of these cells in COPD pathogenesis.

Genetic and Epigenetic Regulation
CS and oxidative stress cause alterations in histones, including acetylation/deacetylation and methylation/demethylation patterns, resulting in DNA damage, cellular senescence, and pulmonary cell apoptosis, in addition to pro-inflammatory gene expression [167]. Studies have shown that DNA double-strand breaks (DSBs) are among the most lethal forms of DNA damage caused by smoking and oxidative stress. If not repaired, they cause cellular senescence and apoptosis [168]. Oxidant enzyme encoding genes, including cytochrome P450 family 2 subfamily C member 18 (CYP2C18) and aryl hydrocarbon receptor nuclear translocator-like 2 (ARNTL2), are upregulated in COPD. Other antioxidant genes may undergo mutations, including polymorphisms in glutathione S-transferase (GST) M1, glutathione S-transferase pi 1 (GSTP1), superoxide dismutase 3 (SOD3), and epoxide hydrolase 1 (EPHX1), being related to lowering lung function and COPD severity [70]. Furthermore, the correlation between epigenetics and the production of inflammatory cytokines may also be linked to disease progression [169].
HDACs play a vital role in regulating the inflammatory response. HDACs downregulate oxidative stress sensitive inflammatory gene expression [170]. Macrophage accumulation in the lungs of COPD patients increases the secretion of inflammatory mediators and elastolytic enzymes due to NF-κB activation [171] and the reduction of HDAC2 activity [172]. HDAC2 activity is reduced in alveolar macrophages and lung tissue in COPD patients [77], and this reduced activity is linked to increased histone acetylation at the IL-8 promoter (FISCHER; VOYNOW; GHIO, 2015). CS, oxidative stress, nitrative stress, and aldehyde reduce HDAC2 expression in the lungs [167,173]. Decreased HDAC2 in the lungs further impairs Nrf2 activation, decreasing its half-life, impairing its orchestrated antioxidant defense [70,174], and increasing NF-κB RelA/p65 subunit activation, and thus, increasing the transcription of pro-inflammatory genes [167].
Mucus hypersecretion observed in COPD patients involves epigenetic mechanisms as DNA methylation and histone modification. COPD downregulates HDAC2, causing upregulation of the MUC5AC gene, leading to mucin production and mucus hypersecretion. Conversely, upregulation or increased HDAC2 activity can downregulate MUC5A, diminishing mucus secretion [175].
CS exposure also dysregulates in the expression of small non-coding RNA microRNA (miRNA). Izzotti et al. reported the first evidence of miRNA expression alterations caused by CS, namely downregulation of 24 miRNA involved in apoptosis, proliferation, and angiogenesis in lung [176]. The analysis of the miRNA pattern is important because miRNA senses the environmental stresses, causes phenotype changes in a cell-and tissue-specific way, being potentially used in prognostics, and contributes to the COPD pathogenesis [177,178].
Advances in studying miRNA-based treatment in COPD are promising. Corticosteroids function partially through epigenetic mechanisms as miRNAs. miR-708 and miR-155 were downregulated and miR-320d and miR339-3p can be upregulated by corticoids. miR320d-increased expression diminished the activation of NF-kB signaling. miR-NAs affected by corticosteroid treatment in patients with moderate to severe COPD can be considered therapeutic targets in COPD. The miR-223 plays the opposite role. miR-223 directly targets HDAC2 because miR-223 overexpression represses the activity of total HDAC and HDAC2 in pulmonary endothelial cells. COPD population has an inverse correlation between HDAC2 and miR-223 levels. The increase of HDAC2 could diminish the insensitivity GC. This miR-223 can decrease treatment efficacy in COPD patients. Several miRNAs have been modified in COPD and by classical COPD treatments. The identification of these miRNAs and description of their roles through their up-or downregulation could contribute to treatment in the future [179].
Extracellular vesicles carrying extracellular miRNA can be used to diagnose and treat COPD because miRNAs can be delivered in the specific site of action. Exosomal miRNA can be considered biomarkers for diagnosis or prognosis for COPD. A specific miRNA that is important to the better disease outcome can be delivered to the disease site through extracellular vesicles [180].

Treatments and New Therapeutic Approaches
Although new drug searches target different mechanisms, most drug candidates fail to reach the clinical stage of development, or fail in this phase. Therefore, management of COPD still depends on the use of bronchodilators and corticosteroids [181]. Airflow limitation occurs due to loss of elastic recoil and augmented airway resistance. Despite airflow limitation being the hallmark COPD characteristic, the primary symptom is dyspnea [182], which is related to increased resistive work. Disease progression and dynamic lung hyperinflation progressively increases residual volume after expiration, complicating the inspiratory process [183,184]. Bronchodilators relieve dyspnea by reducing resistive work and airway resistance [183,185]. Spirometry provides a more global assessment of airflow limitation, while computed tomography allows visualization of the anatomical location of the disease, enabling morphological characterization and quantitative analysis of severity contributing to phenotyping [186]. The focus on specific individual characteristics has stimulated research into treatments targeting fundamental disease mechanisms [169]. There are no specific effective pharmacological treatments for emphysema, except for those targeting α1 antitrypsin deficiency [187]. Modes of therapy administration include self-infusion, aerosol, and subcutaneous administration. Gene and recombinant therapies are under development [188,189], and intravenous therapy using α1 antitrypsin derived from human donor plasma has proven to be safe [187].
Corticosteroids (one of the main treatments used in COPD), delivered by oral administration or inhalation, are highly effective anti-inflammatory drugs for asthma. Gene transrepression caused by corticosteroids decreases NF-κB activity [74,190]. Nevertheless, corticoid treatment displays no anti-inflammatory effects in COPD patients. Corticosteroid resistance is primarily caused by inactivation of HDAC2, which is essential for glucocorticoid receptor (GR) repressor activity, which mediates the anti-inflammatory effect of corticosteroids [74,167,190]. HDAC activity represses several activated inflammatory genes, thereby inhibiting oxidative stress [191]. Reduced HDAC2 activity is observed in COPD patients [192,193]. Restoration of HDAC2 and Nrf2 levels overcomes corticosteroid resistance in COPD. Inhibition of the PI3K/Akt/p70S6K signaling pathway restores nuclear HDAC2 expression and activity. Increasing nuclear Nrf2 levels also enhances HDAC2 levels, indicating HDAC2 and Nrf2 involvement in restoring corticoid sensitivity in COPD [194].
Antioxidants and nitric oxide synthesis inhibitors can restore corticosteroid sensitivity in COPD [173]. Corticosteroids associated with bronchodilator therapy are used to prevent exacerbations [20,195]. Bronchodilators, such as long-acting β2 agonists (LABA) and longacting muscarinic antagonists (LAMA), have beneficial effects against airflow limitation and exercise intolerance. During COPD exacerbations, LAMA is better than LABA, even when LABA is associated with an inhaled corticosteroid [20]. Unfortunately, chronic corticosteroid use can increase the risk of pneumonia in patients with severe COPD, advanced age, comorbidities, such as cardiovascular disease, skeletal muscle wasting, lung cancer, and osteoporosis, or a history of recently diagnosed pneumonia [196,197], because they are immunomodulators and immunosuppressors.
COPD treatment using anti-inflammatory compounds remains a challenge due to the complexity of inflammation and related comorbidities. Bronchodilators can reduce inflammation, but they can only be used for a short time, and are not effective in COPD patients [20]. Currently, no therapies effectively reverse COPD pathology. Minimizing COPD progression is an alternative therapeutic strategy. Decreasing oxidative stress and inflammation can improve quality of life and increase survival [72]. Supplementation, therapeutic administration, and/or the use of multiple antioxidants may benefit COPD patients by increasing endogenous antioxidant levels [74,219].

Role of Medications of Each Drug in Patients with COPD
Bronchodilator drugs are currently used to treat patients with COPD to improve symptoms of the disease. β2-adrenergic receptors are present in the bronchi smooth muscles and are G protein-coupled in the cell membrane; when stimulated, they increase the activity of adenyl cyclase, an enzyme that catalyzes the conversion of ATP to cyclic adenosine monophosphate (cAMP). cAMP inhibits intracellular calcium release, decreasing the influx of calcium through the membrane, relaxing smooth muscles, and dilating the airways. Adrenergic receptor agonists may be short-acting bronchodilators (SABA), such as albuterol, used for rapid relief of acute symptoms, or long-acting bronchodilators (LABA), such as formoterol, indacaterol, salmeterol, and tiotropium, used to relieve the most common and persistent symptoms, such as cough and dyspnea [216,220]. Muscarinic and anticholinergic antagonists are short-acting (SAMA), such as levalbuterol, or long-acting (LAMA), such as glycopyrrolate, umeclidinium, arformoterol, and revefenacin. These drugs regulate bronchomotor tonus by stimulating their bronchi muscles-specific receptors. These receptors are G protein-coupled and have five subtypes, among them M1 and M3, which are the primary drugs targets for presenting effects of improving bronchoconstriction and mucus secretion, resulting in improved lung function and dyspnea [221,222].
Corticosteroids can suppress mucus production and decrease airway obstruction due to the suppression of mRNA expression of proteins encoding the MUC5AC gene [222]. Prednisolone and budesonide are used for COPD treatment in combination therapy with bronchodilators and improved symptoms in patients with a history of multiple severe exacerbations [223]. The activation of β2-adrenergic receptors potentiates the anti-inflammatory effect of corticosteroids by increasing the glucocorticosteroid receptor translocation from the cytoplasm to the nucleus [224,225]. Combining corticosteroid therapy with bronchodilators or double-acting bronchodilators (muscarinic antagonists and β2-adrenergic agonists-MABA) has proven beneficial to treat COPD patients' symptoms and exacerbation [225][226][227]. Viral and bacterial infections are the most frequent cause of exacerbation, which include Haemophilus influenzae (NTHi), Moraxella catarrhalis, Streptococcus pneumoniae, Pseudomonas aeruginosa, human Rhinovirus (HRV), Influenza virus, Coronavirus, and Respiratory syncytial virus (RSV) [65]. Antibiotics are being tested to treat bacterial origin exacerbation, such as aismigen, levofloxacin, and ciprofloxacin, already used in tuberculosis and sinusitis treatment. Azithromycin is being tested in viral exacerbations in a mechanism involving interferon response and decreased inflammatory mediator production. Table 1 shows updated research on new drugs for COPD treatment. The tables include drug names, administration form, drug target, registration study number, phase of development and current status, and whether the drug described has already been approved and used or not for the treatment of COPD or other disease COPD comorbidities, as exacerbation because of viral and bacterial infections is the leading cause of hospitalizations and worsening of symptoms. Table 2 shows the current studies treating exacerbations due to bacterial and viral infection. Table 3 shows the different methodologies used in pre-clinical studies. The methodology shows how studies of current clinical treatments for COPD were caried out. The results of these studies are shown in the tables below. We will discuss some of these next.   Atorvastatin and simvastatin decrease cytokine and leukocyte levels, reduce oxidative stress markers, and improve lung repair [228,229]. Statins can modulate the lung's extracellular matrix composition because statins may directly regulate MMPs or their biological inhibitors, the TIMPS (inhibitors of matrix metalloproteinases), improving lung function through structural changes [230].
Curcumin obtained from turmeric (Curcumina longa) is a polyphenol with antioxidant and anti-inflammatory properties [206,207] that modulates glutathione levels and inhibits IL-8 release in lung cells [198]. Studies have shown that curcumin treatment inhibits the increases in neutrophils and macrophages in the BALF of mice exposed to CS and attenuates increases in air space in mice exposed to CS or porcine pancreatic elastase [207].
LJ-2698, an adenosine A3 receptor antagonist, significantly attenuated increases in air space, improved lung function, inhibited matrix metalloproteinase activity and lung cell apoptosis, induced increases in anti-inflammatory cytokines produced by macrophages, and significantly increased the number of M2 macrophages [236]. LJ-529, a partial peroxisome proliferator-activated gamma receptor (PPARγ) agonist, showed similar results, and induced expression of PPARγ target genes, which play a role in regulating inflammation [237].
Therapy with mesenchymal stromal cells (MSCs) is a thoughtful approach to treating pulmonary diseases, including COPD, mainly based on the immunosuppressive role of MSCs. MSCs are promising adjuvants, used in combination with other treatments, that can improve pulmonary function and decrease inflammation through their anti-inflammatory, antioxidant, microbicidal, and angiogenic action [238,239].
Thioredoxin (Trx) is an essential regulator of the body's redox balance, which can benefit COPD patients through varied mechanisms of action, either as a primary treatment or as a coadjuvant with other treatments [240,241]. Trx also improves resistance to corticosteroids [240], inhibits elastase-induced emphysema [242], decreases neutrophilic inflation [243], and blocks the production of inflammatory cytokines [241]. Further clinical studies are required to verify its effectiveness in COPD treatment [240].
New treatment research focuses on LABA [244] and LAMA [245], such as vilanterol and umeclidinium, including inhibitors of inflammatory mediators, such as canakinumab [246], infliximab [247], and mepolizumab [248]. Phosphodiesterase (PDE) inhibitors (roflumilast and the M3 receptor antagonist glycopyrrolate) [249] exert anti-inflammatory and bronchodilator effects by inhibiting an enzyme involved in the degradation of second messengers [250]. Nevertheless, few clinical trials are currently assessing decreases in oxidative stress, which is a significant factor in COPD and its comorbidities. Figure 3 shows the main mechanisms of action of drugs under development for COPD treatment.

COPD and COVID-19
SARS-CoV-2 has affected more than 150 million people worldwide, and has caused more than 3 million deaths [251]. Patients with pulmonary comorbidities, such as COPD, belong to the high-risk group [252,253]. High-risk group patients are more likely to develop COVID-19 with worse progression and prognosis [253,254]. They present a four times greater risk of developing the severe form of the disease [254], and smokers have a higher risk of severe complications and a higher mortality rate [253] SARS-CoV-2 uses the ACE-2 receptor (angiotensin-converting enzyme 2) to enter cells [255,256]. Increased airway expression of the ACE-2 receptor in COPD patients and smokers correlates with their increased risk of developing COVID-19 [252,257]. COPD patients also display changes in their renin-angiotensin-aldosterone system (RAAS), which positively regulates ACE and angiotensin II expression, potentially aggravating SARS-CoV-2 infection [258]. Additionally, the remodeling and tissue repair caused by Figure 3. Mechanism of action of drugs for the treatment of COPD. Pollutants and CS initiate an inflammatory response by attracting inflammatory cells and releasing inflammatory mediators. mAChR antagonists act as bronchodilators, promoting the release of CXCL8 and LTB4. AMPK and HMG-CoA reductase stimulants decrease inflammation. Inhibitors of inflammatory mediators, such as CXCL1, CXCL8, and CXCL5, act by decreasing chemoattraction of neutrophils and macrophages to the lung (underlying inflammation). Nrf2 stimulants increase the transcription of antioxidant genes, and block the release of pro-inflammatory mediators. Interleukin, EGFR, and TNF-α inhibitors antagonize the activation of MAPKs and PI3K, and attenuate the release of pro-inflammatory mediators, such as IL-6 and IL-1. ADRB2 agonists inhibit the release of inflammatory mediators, cause smooth muscle relaxation, and increase cAMP and cGMP. PDE4 inhibitors prevent cAMP degradation, increasing intracellular cAMP levels, leading to smooth muscle relaxation, and enhancing the bronchodilator effects of β-agonists. CS, cigarette smoke; mAChR, Muscarinic ACh receptors AMP, adenosine monophosphate, AMPK, AMP-activated protein kinase; ADRB2, beta-2-adrenergic receptor; CXCL, chemokine; IL, interleukin; ERK, extracellular signal-regulated kinases; GMP, guanosine monophosphate; HMG-CoA, 3-hydroxymethylglutaryl CoA reductase; LOX, lipoxygenase; LTB4, leukotriene B4; MMPs, matrix metalloproteinases; ADRB2, adrenoceptor Beta 2; NFkB, factor nuclear kappa B; Nrf2, nuclear factor erythroid 2-related factor 2; PDE4, phosphodiesterase 4, PI3K, phosphoinositide 3-kinases; PKC, protein kinase C; TNF-α, tumor necrosis factor alpha. Created with BioRender.com.

COPD and COVID-19
SARS-CoV-2 has affected more than 150 million people worldwide, and has caused more than 3 million deaths [251]. Patients with pulmonary comorbidities, such as COPD, belong to the high-risk group [252,253]. High-risk group patients are more likely to develop COVID-19 with worse progression and prognosis [253,254]. They present a four times greater risk of developing the severe form of the disease [254], and smokers have a higher risk of severe complications and a higher mortality rate [253] SARS-CoV-2 uses the ACE-2 receptor (angiotensin-converting enzyme 2) to enter cells [255,256]. Increased airway expression of the ACE-2 receptor in COPD patients and smokers correlates with their increased risk of developing COVID-19 [252,257]. COPD patients also display changes in their renin-angiotensin-aldosterone system (RAAS), which positively regulates ACE and angiotensin II expression, potentially aggravating SARS-CoV-2 infection [258]. Additionally, the remodeling and tissue repair caused by COPD alters ACE2 expression in epithelial cells [259]. SARS-CoV-2 has a spike protein (S) in its envelope that is activated for ACE-2 binding by serine protease TMPRSS2 (transmembrane protease, serine 2)-mediated cleavage. This protease is essential for viral infectivity and pathogenesis. The action of TMPRSS2 on the envelope protein facilitates viral entry into cells by facilitating the association with the ACE-2 receptor [260].
There are no definitive data on how COPD patient health should be managed during the current COVID-19 pandemic. Nevertheless, patients should be encouraged to continue standard therapy with inhaled corticosteroids and bronchodilators [261]. In COPD patients that develop COVID-19, the use of corticosteroids is controversial. In addition to uncertainty about their effectiveness, some studies claim that corticosteroids are contraindicated [262]. However, dexamethasone has shown a decrease in mortality in patients with COVID-19 [206], which seems to suggest their continued use for treatment of patients with pre-existing COPD who develop COVID-19.

Prevention
Quitting smoking is the best way to prevent COPD progression [182] and pathologies related to COPD, such as lung infections, lung cancer, and cardiovascular disease [20]. COPD patients report low self-esteem, low motivation to smoking cessation, and depression. Interventions that help the patient to smoking cessation, such as treatment with varenicline or bupropion (drugs that act on nicotine dependence) are fascinating [20]. Secondary prevention includes increasing physical activity in daily life, effectively preventing morbidity and mortality in COPD patients [20]. Increasing physical activity by at least 600 steps per day is associated with decreased hospitalization and COPD patient admission [263]. Secondary prevention also includes a healthy and nutritious diet, such as the Mediterranean diet, which has protective effects against respiratory diseases [264], possibly because the Mediterranean diet incorporates a balanced lipid composition with low inflammatory potential [265].
Vaccination prevents diseases in the overall population at any age. In COPD patients, vaccine in conjunction with smoking cessation, increased physical activity, and a healthy diet can improve quality of life and prevent onset of comorbidities. Influenza virus infections cause increased morbidity and mortality in COPD patients. Evidence shows decreased risk of exacerbations in patients who received influenza vaccination as a means of prevention [266]. The pneumococcal vaccine helps to keep the disease stable if administered early upon development of COPD [267].

Methodology
An initial literature research was performed by searching in the database Clinical Trials with the keyword COPD. All comparative or complementary articles as well as those with closed, retired, or unknown recruitment status were excluded. Articles selected for inclusion in this work were randomized and masked and were described in two tables, one for those intended for COPD treatment and one for those intended for COPD viral and bacterial exacerbation treatment or those aimed at improving acute exacerbations. Figure 4 shows how studies of current clinical treatments for COPD were carried out. The results of these studies are shown in the tables below.

Conclusions
COPD presents a high mortality rate because it causes organ damage and alter lung function. An imbalance between oxidants and antioxidants is a primary characteris tic of COPD. Oxidative stress plays a critical role in the inflammatory response in th lungs, leading to the activation of transcription factors that amplify the inflammatory re sponse with cell infiltration and activation and inflammatory mediators' production Current therapy consists of inhaled corticosteroids, bronchodilators, and both of them together. However, this is not fully effective in treating COPD or prevent exacerbations Thus, studies aiming at the development or repurposing of new effective molecules ar vital to treating COPD. Therapies to decrease oxidative stress and inflammation ma improve lung function and increase patient survival. Herein, we discussed approache focusing on prevention and treatment at a molecular level. Certain therapies, includin various natural or synthetic antioxidants, can be effective because they can attenuat mucus hypersecretion, inflammation, matrix remodeling, and corticosteroid resistance Current clinical and pre-clinical treatments under analysis include: (1) inhibitors of in flammatory mediators, phosphodiesterase, metalloprotease, neutrophil elastase, lipoxy genases, intracellular pathways (p38 MAPK, kinase inhibitor, and PI3K), EGFR, an HMG-CoA; (2) antagonist of mAChR, CRTH2, AT1R, CCR2, epithelial cells, sodium channels, CXCR2, bradykinin B1, and adenosine receptors; (3) new LAMA and LABA compounds; (4) agonists of ADRB2, lipocortin synthesis, RAR, and PPAR; and (5) stem cells therapies, immunostimulants, and gene therapies. The major challenge in COPD o exacerbation treatment is the diversity of COPD origin and time frame of intervention too soon versus too late. Therefore, novel treatments focusing on antioxidant and ant

Conclusions
COPD presents a high mortality rate because it causes organ damage and alters lung function. An imbalance between oxidants and antioxidants is a primary characteristic of COPD. Oxidative stress plays a critical role in the inflammatory response in the lungs, leading to the activation of transcription factors that amplify the inflammatory response with cell infiltration and activation and inflammatory mediators' production. Current therapy consists of inhaled corticosteroids, bronchodilators, and both of them together. However, this is not fully effective in treating COPD or prevent exacerbations. Thus, studies aiming at the development or repurposing of new effective molecules are vital to treating COPD. Therapies to decrease oxidative stress and inflammation may improve lung function and increase patient survival. Herein, we discussed approaches focusing on prevention and treatment at a molecular level. Certain therapies, including various natural or synthetic antioxidants, can be effective because they can attenuate mucus hypersecretion, inflammation, matrix remodeling, and corticosteroid resistance. Current clinical and pre-clinical treatments under analysis include: (1) inhibitors of inflammatory mediators, phosphodiesterase, metalloprotease, neutrophil elastase, lipoxygenases, intracellular pathways (p38 MAPK, kinase inhibitor, and PI3K), EGFR, and HMG-CoA; (2) antagonist of mAChR, CRTH2, AT1R, CCR2, epithelial cells, sodium channels, CXCR2, bradykinin B1, and adenosine receptors; (3) new LAMA and LABA compounds; (4) agonists of ADRB2, lipocortin synthesis, RAR, and PPAR; and (5) stem cells therapies, immunostimulants, and gene therapies. The major challenge in COPD or exacerbation treatment is the diversity of COPD origin and time frame of intervention, too soon versus too late. Therefore, novel treatments focusing on antioxidant and anti-inflammatory activities, a new bronchodilator, a particular population cohort, targeting COPD at early or late stages, and lifestyle changes could provide new possibilities for the treatment or prevention of this noxious disease.