Heart disease remains the number one cause of death in the United States, with an incidence of approximately 800,000 per year. Coronary artery disease (CAD), which includes myocardial infarction (MI), is the major cause of mortality, with an incidence of approximately 370,000 per year.1 Atherogenesis refers to the development of plaque in the inner lining of arteries. Atherogenic plaque can cause clinical manifestations by producing flow-limiting stenosis or by provoking thrombi that interrupts blood flow locally or embolizes to distal arteries. Paradoxically, complications often arise after physical disruption of the plaque rather than at the sites of the most severe arterial narrowing.2 The typical plaque that ruptures and causes fatal coronary thrombi has a thin, collagen-poor fibrous cap with few smooth muscle cells and many inflammatory cells.3 Inflammation plays a dual detrimental role in collagen metabolism. It impairs collagen synthesis through inhibiting smooth muscle cells from generating new collagen. In addition, inflammation increases collagen breakdown through overexpression of interstitial collagenases by plaque macrophages.4 Plaque macrophages contribute to the thrombogenicity of the lipid core by producing procoagulant tissue factor.2 CD40-ligand, a cell-surface–associated inflammatory cytokine, further heightens the thrombogenicity by increasing the production of tissue factor.5
Despite the understanding that inflammation can play an important role in atherogenesis and the pathogenesis of acute coronary syndrome (ACS), the question remains as to whether targeted inhibition of pathways of inflammation will translate into clinical benefit.6 Many intracellular signaling pathways are involved in the myocardial response to ischemia. Of interest, multiple mitogen-activated protein kinases (MAPK), including p38 MAPK, are activated during ischemia and may contribute to structural and functional changes.7 In this article, we will review whether targeted inhibition of p38 MAPK by losmapimod has cardiovascular benefit in patients with MI.
One of the key mediators of cellular stress is p38 MAPK, which is activated by inflammatory and environmental stresses and coordinates the cellular responses needed for adaptation and survival. Yet, in certain disease states, including cardiovascular diseases, this same system can provoke maladaptive responses that worsen the disease.8 The p38 MAPKs, the extracellular signal-regulated kinases, the c-Jun N-terminal kinases, and extracellular signal-regulated kinase 5 make up the MAPK system.9
Four p38 MAPK isoforms (α, β, δ, and γ) exist, and they have a preserved structure, a Thr180-Gly181-Tyr182 (TGY) dual phosphorylation motif.10 The α and β isoforms share 74% sequence identity, whereas the other two isoforms, γ and δ, are about 70% homologous to each other and about 60% homologous to the α isoform.8 P38α MAPK and p38β MAPK are ubiquitously expressed; the other two isoforms are less ubiquitous and are tissue-specific. P38γ MAPK is predominantly expressed in skeletal muscle, whereass p38δ MAPK is predominantly expressed in lungs, kidney, testis, spleen, pancreas, and small intestine.11 , 12 The predominant isoform in humans is p38α MAPK, and it appears to be the most relevant to cardiovascular biology.8 , 13
The p38 MAPK cascade includes a MAPK kinase kinase (MAPKKK) such as Ask1, a MAPKK such as MKK3 or MKK6, and a MAPK, such as p38α MAPK.14 Phosphatases such as MAPK phosphatase-1 (MKP-1) attenuates the activity of p38 MAPK.15 Interestingly, glucocorticoids upregulate MKP-1 and suggests that their anti-inflammatory properties may be due to p38 MAPK inhibition.16 P38 MAPKs are activated by a number of extracellular influences such as radiation, ultraviolet light, proinflammatory cytokines and myocardial ischemia.17 , 18 Activation of p38 MAPKs leads to amplification of the inflammatory cascade through stabilization of mRNA and consequent enhanced translation of multiple inflammatory cytokines, including tumor necrosis factor α, interleukins 1 (IL-1), interleukins 6 (IL-6), cyclooxygenase 2, and metalloproteinases (Fig. 1).19 Epidemiologic data have consistently demonstrated an association between biomarkers of inflammation, such as C-reactive protein (CRP) and IL-6, and the risk of subsequent cardiovascular events.20 It has been hypothesized that part of the benefits of statin therapy in cardiovascular disease is due its anti-inflammatory role, as exhibited by the lowering of CRP with statin use.21
P38 MAPK AND ITS ROLE IN CAD
Ischemia and reperfusion in the heart regulate the activity of p38α MAPK and p38β MAPK.18 The two isoforms are differentially activated during ischemia; ischemia is accompanied by an increase in p38α MAPK phosphorylation and a decrease in p38β MAPK phosphorylation. It has been shown that specific inhibition of p38α MAPK in rat neonatal cardiomyocytes, but not p38β MAPK, reduces injury during ischemia.22 Furthermore, activation of p38α MAPK in cardiac myocytes is sufficient to cause apoptosis during myocardial ischemia, whereas activation of the p38β MAPK leads to protection and hypertrophy.23 In 2005, Ren et al.24 demonstrated that mice with dominant negative p38α subjected to MI, through occlusion of the left coronary artery, had reduced infarct size and increased ventricular systolic function 7 days after the MI compared with wild-type mice. Additionally, there was less cardiomyocyte apoptosis in dominant negative p38α mice compared with wild-type in the infarct border zone.
P38 MAPK amplifies the production of reactive oxygen species, which reduces the levels of nitric oxide and soluble guanylate cyclase. Consequently, there is a reduction in the production of cGMP in the vascular smooth muscle leading to vasoconstriction, suggesting that p38 MAPK plays a role in vascular inflammation as well.25 There is an interesting concept of the role of p38 MAPK in response to ischemia. Although the activation of p38 MAPK is necessary during the preconditioning stimuli to establish its protective effect, its inhibition during lethal stress protects against severe insult.26 Ischemic preconditioning refers to an increase in protection against lethal ischemic injury after a brief, sublethal, period of ischemia.8 It has been suggested that ischemic preconditioning is the result of selective activation of p38β and that selective inhibition of p38α MAPK during lethal stress promotes protection.9 Overall, it appears that activation of p38α MAPK plays a detrimental role in CAD and it is of interest to investigate whether inhibition of p38 MAPK will have clinical benefits in ACS.
PRECLINICAL STUDIES AND PHASE I CLINICAL STUDIES OF LOSMAPIMOD
Losmapimod is a novel, selective, reversible, competitive inhibitor of the α and β isoforms of p38 MAPK.19 It inhibits p38α MAPK and p38β MAPK to a similar degree with a pKi of 8.1 and 7.6, respectively. In a preclinical study of hypertensive rats that were prone to stroke, losmapimod improved survival and endothelial function, in addition to reducing the production of IL-1.27 In early phase I studies involving losmapimod, there were no serious adverse events associated with the drug use.28 A pharmacokinetic/pharmacodynamic meta-analysis from six phase I studies demonstrated that losmapimod plasma concentration had no significant effect on QT-interval prolongation.29 In a study comparing the safety, tolerability, and pharmacokinetic and pharmacodynamic parameters of losmapimod following an intravenous administration versus an oral route, there were no deaths, nonfatal serious adverse events, or adverse events leading to withdrawal. The only adverse event reported more than once was headache, which occurred in the oral group.30 Overall, these early studies provided the pharmacokinetic and pharmacodynamic groundwork for the use of losmapimod in humans and positive data on the safety and tolerability of losmapimod.
PHASE II CLINICAL STUDIES OF LOSMAPIMOD
In a phase II randomized controlled trial of 56 hypercholesterolemic patients treated with oral losmapimod or placebo, p38 MAPK inhibition improved nitric oxide-mediated vasomotor function in patients with hypercholesterolemia. In addition to an improvement in endothelium-dependent vasodilatory response to acetylcholine, treatment with losmapimod also improved endothelium-independent vasodilatory response to intraarterial sodium nitroprusside.26 The study demonstrated that losmapimod inhibited p38 MAPK at the cellular level by showing a reduction in the phosphorylation of heat shock protein, a known downstream bioassay of p38 MAPK activity. In addition, there was a significant reduction in the level of high-sensitivity CRP (hs-CRP) in the losmapimod group, suggesting that the drug plays a role in the reduction of systemic inflammation. Furthermore, there were no serious adverse events in patients from the losmapimod group compared with those from the placebo group.26
One of the largest phase II trials involving losmapimod was the SOLSTICE trial (Study Of LoSmapimod Treatment on Inflammation and InfarCt sizE). The goal of the trial was to assess the safety of losmapimod and explore potential clinical benefits with use in non-ST elevation MI patients.31 The study was a randomized, double-blinded, and placebo-controlled trial involving 535 patients. Early suppression of inflammatory markers (hs-CRP and IL-6) was seen in patients treated with losmapimod, but the levels were similar by 12 weeks. The mean troponin I area under the curve values also did not differ between the two groups. Although infarct size, as measured by circulating markers of myonecrosis, was similar in the two groups, delayed enhancement cardiac magnetic resonance imaging in a substudy indicated a trend toward reduced infarct sizes at 3–5 days and at 12 weeks in the losmapimod group.31 There were also significant improvements in predefined secondary measures including left ventricular ejection fraction, left ventricular end-diastolic and end-systolic volumes, and B-type natriuretic peptide concentrations at 12 weeks. Similar to previous studies, losmapimod was well tolerated and was not associated with any major safety concerns.31 These promising effects on biomarkers of inflammation, infarct size, and cardiac function showed that p38 MAPK inhibition with losmapimod can have positive cardiovascular outcomes in patients with ACS.
LATITUDE-TIMI 60 TRIAL
On the basis of promising clinical data, the Losmapimod To Inhibit p38 MAP Kinase as a Therapeutic Target and Modify Outcomes After an Acute Coronary Syndrome (LATITUDE)-TIMI 60 trial was initiated in 2014.19 The primary goal of the trial was to assess whether losmapimod can reduce the risk of a subsequent cardiovascular event when added to standard of care in patients hospitalized with MI, including ST-segment elevation MI and non-ST–segment elevation MI. The trial was randomized, placebo-controlled and double-blinded, and planned to occur in three stages. The first stage of the trial (part A) involved 3,500 patients and assessed the safety and efficacy of the drug. After completion of part A, there was a review to assess whether to proceed to part B, which would have been event driven with approximately 22,000 patients to provide the primary assessment of the efficacy of losmapimod. Study patients were older than age 35, had been hospitalized with a MI, and had at least one indicator of higher cardiovascular risk.19
The primary end point was major adverse cardiovascular events, defined as the composite of cardiovascular death, MI, or severe recurrent ischemia requiring urgent coronary artery revascularization. The principal secondary end point was the composite of cardiovascular death or MI.32 Through week 12, the primary end point occurred in 8.1% of patients in the losmapimod group compared with 7.0% in the placebo group (hazard ratio [HR], 1.16; 95% confidence interval [CI], 0.91–1.47; P = 0.24; Fig. 2).32 Additionally, there were no differences in the rate of the principal secondary end point of cardiovascular death or MI (HR, 1.13; 95% CI, 0.88–1.47). Results were similar at week 24, the primary end point occurred in 10.3% of patients in the losmapimod group compared with 9.7% in the placebo group (HR, 1.11; 95% CI, 0.90–1.38). Interestingly, in patients who suffered ST elevation MI, the primary end point was 6.5% in the losmapimod group compared with 7.6% in the placebo group (HR, 0.84; 95% CI, 0.51–1.40).32 Because the finding was within a small subgroup, the results were not statistically significant, and thus need to be validated in a larger study. As repeatedly reported in previous studies, there were no significant differences between the incidences of any serious adverse events between the two groups. Losmapimod did reduce the levels of the inflammatory biomarker hs-CRP and N-terminal pro-BNP concentration at 4 weeks and at the end of the treatment period at 12 weeks; however, this did not translate to clinical benefits. After the completion of part A, a selected group of individuals involved in the trial leadership from the TIMI Study Group and the sponsor reviewed the data from part A and made the decision to not proceed with part B.32 Overall, the findings do not support a strategy of p38 MAPK inhibition with losmapimod in patients hospitalized with MI.
One limitation of the study was that the treatment course was limited in duration, and as a result one cannot preclude the possibility that losmapimod may have efficacy as part of a longer anti-inflammatory therapy.32 Additionally, a possible hypothesis for the inefficacy of losmapimod is that the p38 MAPK isoforms play different and perhaps opposite roles in inflammation and cardiovascular disease. Thus, it is difficult to predict what may happen in a clinical trial with an inhibitor of both the α and β isoforms of p38 MAPK. For instance, p38α activation in cardiac myocytes causes apoptosis and cell death, whereas p38β activation is responsible for hypertrophy and survival.33 Furthermore, carbon monoxide, a product of heme oxygenase activity, exerts its anti-apoptotic and anti-inflammatory activities partly through activating p38β MAPK.34 Thus, inhibiting p38β MAPK may result in loss of anti-apoptotic and anti-inflammatory properties. This may explain why the use of losmapimod did not produce clinical benefits in the setting of post-MI patients.
OTHER ANTI-INFLAMMATORY MEDICATIONS AND THEIR ROLE IN CAD
In addition to losmapimod, there has been great interest in studying the role of novel anti-inflammatory medications in CAD. Many large-scale phase III trials are now underway with agents that impact the classical IL-6 signaling cascade, such as canakinumab and methotrexate, and with agents that impact the non-IL-6-dependent pathways, such as varespladib and darapladib.20
ILs are important mediators of the systemic anti-inflammatory response. IL-1, for example, plays a critical role in atherothrombosis and sits proximal to the classical IL-6 signaling cascade.20 Additionally, activated macrophages within atheromatous plaque express IL-1 that leads to recruitment of additional inflammatory cell lines into the plaque and endothelial wall.20 Canakinumab is a human monoclonal anti-human IL-1β antibody that is indicated for treating several rare IL-1β over-expression disorders such as cryopyrin-associated periodic syndrome, and systemic juvenile idiopathic arthritis.35 Canakinumab has been shown to reduce the plasma levels of fibrinogen, IL-6 and CRP in high vascular risk patients.36 Based on these data, a large-scale Canakinumab Anti-Inflammatory Thrombosis Outcomes Study (CANTOS) was launched in 2011 to investigate whether IL-1β inhibition with canakinumab can reduce recurrent cardiovascular event rates in stable CAD patients who remain at high inflammatory risk due to a persistent elevation of CRP. With over 10,000 participating post-MI patients, the study would provide a novel cytokine-based therapy for the secondary prevention of cardiovascular disease, if proven positive.37 , 38 The results of CANTOS, supporting the inflammatory hypothesis of atherothrombosis, were recently reported on, demonstrating that canakinumab injected at a dose of 150 mg every 3 months led to a significantly lower rate of recurrent cardiovascular events compared with placebo, which was independent of lipid lowering. However, there was no effect of the drug on total mortality when compared with placebo.39
Methotrexate is an agent that has received interest for its broad range of anti-inflammatory properties, including its ability to reduce the levels of tumor necrosis factor α, IL-6, and CRP.40 One meta-analysis reported that patients with rheumatoid arthritis or psoriatic arthritis taking low-dose methotrexate have a 21% lower risk of future cardiovascular events.41 In 2013, the Cardiovascular Inflammation Reduction Trial was initiated in which 7,000 patients with prior MI and either type 2 diabetes or metabolic syndrome are given either low-dose methotrexate or placebo. The goal of the trial is to determine whether low-dose methotrexate can reduce the rate of recurrent cardiovascular events. Additionally, if positive, the trial would also support the inflammatory hypothesis of atherothrombosis.42
Lipoprotein-associated phospholipase A2 (Lp-PLA2) and secretory PLA2 (sPLA2) are members of the phospholipase A2 superfamily that hydrolyze phospholipids, leading to the production of atherogenic lipid fractions and increased oxidant stress.20 Lp-PLA2 is highly expressed in atherosclerotic lesions and secretes cytokines that contribute to plaque vulnerability and susceptibility to rupture.20 Darapladib is a potent and reversible oral inhibitor of Lp-PLA2 and has been shown to reduce the enzyme’s activity in human carotid plaque.43 In the Integrated Biomarker and Imaging Study 2 involving patients with CAD, darapladib arrested the progression of the necrotic core of coronary artery plaques.44 Based on these data, the Stabilization of Atherosclerotic Plaque by Initiation of Darapladib Therapy (STABILITY) trial was initiated in 2010 to evaluate the agent’s efficacy in preventing ischemic events in patients with stable CAD. Approximately 15,000 patients with chronic coronary heart disease receiving the current standard of care were enrolled with a median treatment duration of 2.75 years. The primary end point, which was the composite of major adverse cardiovascular events, MI, or stroke, was similar between the two groups, occurring in 9.7% of patients in the darapladib group versus 10.4% in the placebo group (HR, 0.94; CI, 0.85–1.03; P = 0.20).45 Another trial, the Stabilization of Plaques using Darapladib-Thrombolysis in MI-52(SOLID-TIMI52), was initiated in 2011 to determine the efficacy of darapladib in patients after an ACS. Approximately 13,000 patients were enrolled with a median treatment duration of 2.5 years.46 The trial demonstrated that darapladib did not reduce the risk of major coronary events. The primary end point, which was the composite of coronary heart disease death, MI, or urgent revascularization for myocardial ischemia, occurred in 16.3% of the patients in the darapladib group versus 15.6% with placebo at 3 years (HR, 1.00; CI, 0.91–1.09; P = 0.93).47
In addition to its role in atherogenesis, sPLA2 is implicated in ischemic myocardial tissue damage and inflammation.48 Varespladib is a nonspecific pan-sPLA2 inhibitor with favorable effects on atherosclerotic lesions in animal studies.49 In 2012, the Vascular Inflammation Suppression to Treat Acute Coronary Syndrome for 16 weeks (VISTA-16) study was initiated to determine the effects of varespladib on cardiovascular risk in patients with ACS. Approximately, 5000 patients were enrolled and were subjected to either varespladib or placebo daily for 16 weeks in addition to the standard therapy. The primary outcome measures were a composite of cardiovascular mortality, nonfatal MI, nonfatal stroke, or unstable angina with evidence of ischemia.50 The trial was terminated early due to futility and possible harm. Although the drug lowered levels of low-density lipoprotein cholesterol and CRP, there was no evidence of a reduction in primary cardiovascular outcome. In fact, treatment with the drug led to an increase in the risk of MI (3.4%) when compared with 2.2% in the placebo group (HR, 1.66; CI, 1.16–2.39; P = 0.05). Additionally, the composite secondary end point of cardiovascular mortality, MI, and stroke were greater in the varespladib group than in the placebo group.51
In addition to the above anti-inflammatory medications, colchicine has also been investigated for its role in CAD. In 2008, the Low-Dose Colchicine for Secondary Prevention of Cardiovascular Disease (LoDoCO) study was initiated to determine the effectiveness of low-dose colchicine in reducing the risk of cardiovascular events in patients with clinically stable CAD. The primary outcome was the composite of ACS, out-of-hospital cardiac arrest, or noncardioembolic ischemic stroke. The trial demonstrated that the addition of colchicine to standard therapy reduced the primary trial end point (HR, 0.33; 95% CI, 0.18–0.59; P < 0.001).52 The positive benefits appeared early after trial initiation, were sustained throughout the median 3-year follow-up period, and were largely driven by a reduction in ACS unrelated to stent disease. However, 11% of patients withdrew from therapy early on due to intestinal tolerance and a further 5% later withdrew due to a range of side effects.52 Additionally, a retrospective, cross-sectional study of patients with a diagnosis of gout was done in 2012 to investigate the effect of colchicine on the prevalence of MI. The results indicated that the prevalence was lower in the colchicine group compared with the noncolchicine group. There were also fewer deaths and lower levels of CRP within the colchicine group.53 However, a meta-analysis of 39 randomized parallel-group trials demonstrated that colchicine had no effect on all-cause mortality (RR, 0.94; 95% CI, 0.82–1.09; participants = 4174; studies = 30; I^2 = 27%; moderate quality of evidence).54
Many anti-inflammatory medications have been extensively studied for their roles in CAD. One novel drug, losmapimod, a p38 MAPK inhibitor, was investigated to assess its efficacy when added to standard of care for patients with acute MI. The LATITUDE-TIMI 60 trial, a phase III trial, demonstrated that losmapimod did not reduce the risk of adverse cardiovascular outcomes in patients with acute MI. Other anti-inflammatory agents such as darapladib and varespladib have also not shown any benefits in reducing adverse cardiovascular events in patients with ACS. Thus, although these medications have been shown to decrease the levels of biomarkers of inflammation, pharmacological inhibition to achieve such levels does not necessarily lead to clinical benefits. One anti-inflammatory drug, colchicine, did show reductions in the prevalence of MI, but had no effect on all-cause mortality.
The results of CANTOS provide evidence to support the inflammatory hypothesis of atherothrombosis, demonstrating a benefit of canakinumab in reducing recurrent cardiovascular events in patients with MI, independent of lipid lowering. However, there was no benefit from the drug on total mortality.
The results of the ongoing Cardiovascular Inflammation Reduction Trial (CIRT) with methotrexate should provide additional data regarding the potential benefit for anti-inflammatory therapy for cardiovascular risk reduction and safety in a clinical setting.
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