Immunomodulatory and antioxidant effects of pentoxifylline and other therapeutic agents in the management of myocardial injury

Myocardial injury, particularly during myocardial infarction (MI), triggers an inflammatory response that is crucial for starting the myocardial healing process but is also responsible for cardiac remodeling and heart failure that can develop in as much as a quarter of the patients post-MI. Pentoxifylline (PTX) is a drug with potential cardiovascular benefits due to its anti-inflammatory and antioxidant properties. The aim of this review is to highlight the immunomodulatory and antioxidant effects of PTX in the management of myocardial injury. A search strategy was designed using medical subject headings (MeSH) terms of antioxidant effect, immunomodulatory, myocardial injury, and PTX for search on PubMed, and MEDLINE databases. PTX has been studied for its potential therapeutic effects in managing myocardial injury. It was extensively investigated in vivo as well as in human clinical trials and has provided promising results. Some of the promising findings include an anti-inflammatory and antioxidant effect with some beneficial clinical outcomes. These findings were evident as decreased levels of inflammatory cytokines such as tumor necrosis factor-α (TNF-α) and certain interleukins such as IL-1, decreased oxidative stress markers such as malondialdehyde (MDA) while increasing antioxidants such as glutathione, improved ejection fraction in some patients and improved mortality profile in one meta-analysis. In conclusion , it can be concluded that PTX has potential immunomodulatory and antioxidant effects in the management of myocardial injury.


Introduction
Myocardial infarction (MI) is a significant cardiovascular ailment that has the potential to result in mortality [1].MI results in cardiomyocyte death in the affected area caused by prolonged ischemia with subsequent cardiac remodeling potentially leading to ventricular dysfunction and heart failure [2,3].
According to its etiology, MI can be a spontaneous MI brought about by atherosclerosis-related coronary incidents known as type 1, MI due to diminished oxygen supply or heightened oxygen demand to the myocardium known as type 2, or MI with sudden cardiac death in the case of unavailable cardiac biomarker results known as type 3.The majority of ST-elevation myocardial infarction (STEMI) patients in addition to many Non-ST-Elevation Myocardial Infarction patients (NSTEMI) fit into type 1 MI [4].In addition to the clinical presentation which could range from asymptomatic patients to patients with cardiac arrest or cardiogenic shock, there are various means which aid in MI diagnosis [5].The electrocardiogram's depiction of the presence of Q waves or alterations in the ST-segment or T segments is a mainstay of diagnosis, in addition to the detection of the cardiac biomarkers released by damaged cardiomyocytes like highsensitivity cardiac troponin and creatine kinase-MB [6].
The immune system is constantly in communication with the heart through messengers such as cytokines, hormones, and neurotransmitters [7].The immune system plays an important role in the repair and remodeling of heart tissue post-MI.The non-specific innate immunity including macrophages, neutrophils, and mast cells as well as the adaptive immunity including T and B cells can all be involved in cardiac injury and repair mechanisms [8,9].However, a continued immune response produces negative outcomes of pathological ventricular remodeling [10].
The search continues for therapies that help regulate the balance between the proinflammatory and the anti-inflammatory processes post-MI to minimize cardiac remodeling that can lead to worse patient outcomes [11].
Reactive Oxygen Species (ROS), at their minimal physiological concentration, act as second messengers for important intracellular signals.Oxidative stress is the state of having higher ROS levels than the antioxidant system can clear [12].The nuclear transcription factor kappa B (NF-κB) is triggered through increased ROS concentrations which results in the transcription of many proinflammatory cytokines and adhesion molecules.Thus, oxidative stress plays a role in the production of cytokines linking it to inflammation and endothelial dysfunction [12,13].Oxidative stress is considered to be a risk factor for coronary artery disease (CAD)

Methods
In the present review, different study designs like clinical trials, systematic reviews, and narrative reviews were included.A search strategy was designed using medical subject headings (MeSH).The MeSH terms of antioxidant effect, immunomodulatory, myocardial injury, and pentoxifylline were used to search PubMed, and MEDLINE databases as shown in Fig. 1.
Studies in the English language and all relevant updated publications up to November 2023 were included.Our inclusion criteria primarily focused on the latest published literature that focuses on the immunomodulatory and antioxidant effects of PTX in the management of myocardial injury with trials before the year 2000 or studies having less than 10 patients excluded as shown in Table 1.

The Role of Immunity and Inflammation in Myocardial Infarction
The release of damage-associated molecular patterns (DAMPs) such as high mobility group box 1 (HMGB1), heat shock protein (HSP), and low molecular weight hyaluronic acid from necrotic myocardial cells results in the induction of numerous inflammatory cytokines.This is what initially drives the innate immune system into action as immune cells such as neutrophils, monocytes, and dendritic cells all start to reach the affected tissue [22].Increased oxidative stress as a result of overwhelmed antioxidant mechanisms after MI also contributes to upregulating the inflammatory signal [23].
Neutrophils are the first to be recruited to the infarcted area which adhere to endothelial cells and penetrate the tissue through the assistance of adhesion molecules such as selectins, integrins, vascular cell adhesion molecule-1 (VCAM-1) and Intercellular adhesion molecule-1 (ICAM-1) Monocytes are recruited to the infarcted myocardium without entering the tissue.In the inflammatory phase, the pro-inflammatory monocytes dominate and differentiate in the proinflammatory macrophage M1.M1 macrophages penetrate the tissue and contribute to inflammation through the release of proinflammatory cytokines such as IL-1 and IL-12.With time the reparative stage ensues where monocytes predominate and differentiate into M2 macrophages which release anti-inflammatory and reparative proteins such as IL-10, vascular endothelial growth factor (VEGF) and transforming growth factor beta 1(TGF-β1).M2 macrophages act to limit the inflammatory process and mediate angiogenesis and myofibroblast growth [9, 22].In normal circumstances, the low levels of ROS produced are equivalent to their removal [28].The phenomenon known as redox signaling, which refers to the specific and reversible oxidation-reduction alterations of components involved in cellular signaling has now been recognized to be involved in both physiological and disease processes.In the heart, these modifications can regulate gene expression, excitation-contraction coupling, as well as processes like cell growth, migration, differentiation, and death [34].

Pharmacology of pentoxifylline
PTX has been utilized in several disease areas.Beyond its FDA-approved indication, PTX is used off-label in diseases such as severe alcoholic liver disease, non-alcoholic fatty liver disease, peripartum, idiopathic and ischemic cardiomyopathy, and chronic kidney disease [35,36].PTX is also employed off-label for the treatment of venous ulcers.A comprehensive literature review conducted by the Cochrane Collaboration revealed that PTX is an efficacious intervention for venous ulcers, regardless of whether compression therapy is administered or not [37].
The hemorheological effects of PTX and its metabolites contribute to a decrease in blood viscosity, thereby augmenting blood flow and peripheral tissue oxygenation.The specific molecular effects through which PTX exerts its pharmacological action are not fully clear.Nevertheless, multiple pathways are hypothesized to be involved [15].Those hypotheses suggest a role for PTX as an immune modulator, anti-TNF-α agent, and hemorheological modulator in addition to affecting the adhesion molecules [38].
Normally, cyclic adenosine monophosphate (cAMP) in platelets modulates the cyclooxygenase enzyme that synthesizes prostaglandins G 1 and H 2 which form thromboxane A2 and thus lead to increased platelet aggregation.As PTX causes the level of cAMP to increase by action of phosphodiesterase (PDE) inhibition, this leads to less activation of cyclo-oxygenase enzyme and thus ultimately less platelet aggregation.Also, PTX raises the prostacyclin level which activates adenylyl cyclase which results in higher cAMP and platelet aggregation inhibition [39].PTX also acts to increase plasminogen activator and plasmin while at the same time decreasing fibrinogen thus further enhancing blood viscosity [38].

Pentoxifylline: Anti-inflammatory and Immunomodulatory Effects
Immunomodulation can take the form of general modulation of more than one inflammatory pathway as with PTX or specific targeting of a certain cytokine or cellular pathway of inflammation as with biologics like canakinumab, etanercept, or anakinra [40].

PTX
possesses immunomodulatory properties that enhance white blood cell (WBC) deformability while simultaneously decreasing neutrophil degranulation, reducing leukocyte adhesion, and decreasing the sensitivity of leukocytes to cytokines [39].
The most common hypothesized mechanism for PTX is the non-selective inhibition of the PDE enzyme.In particular, the inhibition of the isoenzyme PDE-4 is crucial as it is highly expressed in inflammatory cells such as neutrophils, macrophages, T cells, and endothelial cells [41].PDE inhibition leads to elevated levels of cAMP and increased protein kinase A activity and that subsequently results in downregulation of the I kappa B kinase/Nuclear Factor-kappa B (IKK/NF-kB)-mediated transcription of pro-inflammatory cytokines such as TNF-α [17].
The anti-inflammatory effect exerted by PTX is not only due to the inhibition of TNF-α but also due to decreasing the secretion of other inflammatory cytokines such as IL-1 and IL-6 and limiting the expression of adhesion molecules such as VCAM-1 and ICAM-1

Pentoxifylline: Antioxidant Effects
The proposed antioxidant effects of PTX may be attributed to its suppression of neutrophil activation and thus decreased level of neutrophilderived superoxide anions [48].It is also reported that PTX can limit the production of other ROS like hydroxyl and superoxide anions by inhibiting xanthine oxidase [49].
In a study of the antioxidant effects of PTX in mice with arsenic-induced cardiac oxidative damage, PTX was found to augment the activity of antioxidant systems such as superoxide dismutase, catalase, and glutathione peroxidase [50].Using PTX in hypertensive diabetic patients resulted in a significant 20.2% reduction in the level of the oxidative stress marker malondialdehyde in the PTX group which did not occur in the control group [51].In another study, diabetic patients treated with PTX had significantly decreased levels of thiobarbituric reactive substances (TBARS) which is a marker indicative of lipid peroxidation and oxidative stress [52].

Effect of Pentoxifylline in Improving Myocardial Injury
In a study on NSTEMI patients where PTX was administered for 6 months, PTX-treated patients exhibited better outcomes concerning the endpoint of death, reinfarction, or rehospitalization [20].On the contrary, a trial that investigated PTX in 98 STEMI patients who underwent thrombolytic therapy did not conclude a significant improvement in the development of 30-day major adverse cardiac events (MACEs).However, a significant improvement in troponin I concentration was observed in the PTX group when compared to placebo [53].A more recent study that used PTX on 419 NSTEMI patients in the FDA-approved dose also failed to observe an improvement in the 1-year total MACEs of cardiovascular death, reinfarction, and rehospitalization but there was a significantly decreased risk for the need for coronary revascularization in the PTX arm [21].
In patients suffering heart failure secondary to ischemia, the addition of PTX to standard treatment led to the clinical improvement of patients and enhanced ejection fractions in addition to improving some of the biomarkers of inflammatory, prognostic, and apoptotic profiles [46].The use of PTX in severe heart failure patients in addition to guideline-directed medical therapy reduced the levels of TNF-α as a marker of inflammation and Fas/Apo-1 as a marker of apoptosis when compared to the control group at 1 month of treatment.This could imply the potential usefulness of PTX in heart failure patients of severe classification [54].In a metaanalysis consisting of six controlled trials involving 221 patients with cardiomyopathy and heart failure with left ventricular ejection fraction (LVEF) ≤ 40% that administered PTX, the use of PTX was associated with almost a fourfold decrease in all-cause mortality compared to control even though the individual studies examined concluded no such effect [55].
Regarding in-vivo trials, PTX exhibited a potent cardioprotective effect when used to combat Adriamycin-induced cardiotoxicity in rabbits.The conferred benefits in the previous trial included lowering the levels of cardiac biomarkers such as creatine kinase-MB (CK-MB) and lactate dehydrogenase (LDH), oxidative stress markers such as MDA and glutathione as well as inflammatory markers TNF-α and IL-6 [56].Furthermore, PTX when combined with vanillin and used in rats with isoproterenolinduced cardiac injury had resulted in the amelioration of the cardiac abnormalities evident as decreased oxidative stress marker MDA, and increased glutathione, decreased inflammatory markers TNF-α, IL-6, and IL-1β [57].

Repurposed Anti-inflammatory and Immunosuppressive Agents
A comprehensive examination of clinical trials involving pharmacological interventions aimed at the immune system to improve MI or improve its prognosis is provided in Table 2 and Table 3.These interventions range from general immunosuppressive therapy to more specific approaches that target defined pathways and factors.
The measured outcomes in those trials ranged between clinical outcomes as well as serum biomarkers or investigation-related outcomes.These outcomes include angiographyinvestigated restenosis, maximal oxygen uptake, respiratory quotient and exercise time upon exercise testing, New York Heart Association (NYHA) class, MACEs, cardiac magnetic resonance (CMR)-determined myocardial salvage, CMR-assessed infarct size, radionuclide ventriculography, echocardiographic studies including LVEF, left ventricular end-diastolic and end-systolic lengths, endothelium-dependent and endothelium-independent forearm blood flow by venous occlusion plethysmography.Also, serum biomarkers such as TNF-α, TGF-β, IL-6, IL-10, NT-pro-BNP, Fas/Apo-1, albuminuria, fibrinogen, leukocytic count, count of neutrophils, basophils, monocytes and lymphocytes, MDA, glycated hemoglobin, lipid profile, troponin, creatine kinase, von Willebrand Factor (vWF) and flow cytometric measures of platelet activation.However, the outcomes mentioned below are the ones that showed significance only.

Conclusion
It can hence be concluded that PTX offers potential immunomodulatory and antioxidant effects in the management of myocardial injury.PTX can modulate the immune response that starts to occur early post-MI and promote the repair of MI by promoting the formation of granulation tissue and eliminating dead myocardial cells.Moreover, the hemodynamic circulation enhancement effect through the reduction of blood viscosity makes PTX a potential candidate among other repurposed immunomodulatory agents in the management of myocardial injury.

Recommendation
Further large center controlled trials are encouraged to fully elucidate and outline the effects of PTX on different myocardial injury types as well as determine the best duration of use to obtain the desired effect.

Consent to publish
Not applicable
mainly arise via the mitochondrial electron transport chain, xanthine oxidase, NADPH oxidases, and nitric oxide synthases.Example of ROS includes superoxide anion (O 2 •− ) and hydrogen peroxide (H 2 O 2 ) [27].Superoxide anions can give rise to either peroxynitrite (ONOO-) or hydrogen peroxide.Additionally, they can be involved in a reaction with hydrogen peroxide to produce a hydroxyl radical [28].Peroxynitrite itself can be converted to peroxynitrous acid (ONOOH) with protonation.Hydrogen peroxide can give rise to hydroxyl radical when it undergoes a Fenton reaction or to hypochlorous acid (HOCl) via myeloperoxidase activity with hypochlorous acid (HOCl) itself reacting with hydrogen peroxide to form dioxygen ( 1 O 2 ) [29].Fig. 3. Shows reactive oxygen species formation and detoxification.

Fig. 3 .
Fig. 3. Reactive oxygen species formation and detoxification [17].PTX has also been observed to inhibit B lymphocytes and T lymphocytes and inhibit neutrophil activation [18].A meta-analysis demonstrated that treatment with PTX significantly decreased the concentrations of TNF-α and C-reactive protein (CRP) in plasma.However, no significant change in plasma IL-6 concentrations was observed following PTX therapy [42].In the study by Fernandes et.al., treating NSTEMI patients with PTX resulted in a significant reduction of CRP and TNF-α levels [20].PTX has exhibited a positive effect on inflammation in other trials as well [43, 44].CAD patients who were administered PTX for 2 months experienced a significant decrease in the levels of sVCAM-1 as a marker of endothelial dysfunction relating to inflammation [45].The use of PTX in ischemic cardiomyopathy patients resulted in a reduction in plasma concentrations of inflammatory markers TNF-α and CRP and led to an improved left ventricular ejection fraction [46].When PTX use was investigated in type 2 diabetes mellitus patients, it was concluded that PTX resulted in TNF-α reduction in patients on angiotensin II receptor blocker (ARB) therapy and residual proteinuria [47].

Table 3 .
Clinical trials including different repurposed therapeutic agents for the treatment of myocardial injury