Interleukin-1 Beta as a Target for Atherosclerosis Therapy - Biological Basis of CANTOS and BeyondLiterature - Peter Libby - J Am Coll Cardiol. 2017: vol. 70. DOI: 10.1016/j.jacc.2017.09.028
Inflammatory and immune mechanisms participate in atherothrombosis
Both experimental and clinical evidence have pointed to a role for inflammation in atherogenesis and its complications. Anti-inflammatory therapies can reduce CV events, and also part of the event reduction seen with statin therapy may be attributed to anti-inflammatory effects. Since statins reduce both LDL-c levels and inflammation simultaneously, studies on statin therapy cannot unequivocally demonstrate that targeting inflammation per se can reduce risk of atherosclerotic events. Moreover, considerable residual risk remains despite statin treatment, thus there is an unmet need to further reduce CV risk.
Cytokines serve as key messengers of inflammatory signaling
Inflammation transduces the atherogenic effects of classic risk factors, more so than primarily triggering atherogenesis. For instance, cytokines are proteins that mediate inflammation and modulate immunity. Cytokines, for instance interleukins, contribute to atherosclerosis.
IL-1: A primordial proinflammatory cytokine
Two related, but functionally distinct isoforms of IL-1 exist: IL-1α and IL-1β. IL-1 has a wide variety of effects in host defenses and in the pathogenesis of multiple diseases, affecting a diverse range of cell types. IL-1 induces expression of many cytokines, and it can also enhance its own gene expression. It can increase expression of leukocyte adhesion molecules and thrombogenic markers, as well as activate of cells involved in innate immunity, among many other effects.
IL-1 Family and its regulation
IL-1 action is regulated at multiple levels. The IL-1 receptor antagonist (IL-1ra) is structurally related and has an inhibitory effect, as compared with IL-1α and IL-1β. The balance between the proinflammatory IL-1α and IL-1β isoforms and IL-1ra yields one important level of control.
The functional difference between IL-1α and IL-1β lies in that IL-1α typically resides on the cell surface and signals only in the proximity by direct contact, while IL-1β can act at a distance. The IL-1 receptor 1 (IL-1R1) transduces IL-1β signaling. The IL-1 receptor II (IL-1RII) lacks a cytoplasmic domain, and therefore, although it can bind the ligands, it cannot transfer a signal. IL-1RII thus acts as a ‘decoy’, and consequently as a level of negative regulation of IL-1 signaling.
IL-1α and IL-1β can generate a positive feedback loop when they induce their own gene expression, but they can also increase IL-1ra expression, resulting in negative feedback.
The inflammasome activates IL-1β
Unlike IL-1α, IL-1β requires processing before it is biologically active. The enzyme caspase-1 cleaves a 33 kDa precursor to the 17 kDa mature active form. Macrophages in human atherosclerotic plaques contain the proteolytic enzyme caspase-1. Activation of caspase-1 results from inflammatory signals transduced by the inflammasome. The inflammasome is a supramolecular assembly that generally requires two signals to assemble and act. Coactivators or triggers include components of infectious agents, or crystals such as urate crystals (in the pathogenesis of gout) and cholesterol crystals. Disturbed flow, moderate hypoxia and acidosis can also augment inflammasome action, which are stimuli relevant to atherosclerosis. Indeed, human atheromata harbor activated inflammasomes.
IL-1 has manifold effects on the cardiovascular system
Il-1 induces inflammatory functions of human endothelial cells, it stimulates adhesion molecules that recruit leukocytes (e.g. ICAM-1 and VCAM-1), and it induces chemokines such as MCP-1 and CCL-2.
When exposed to inflammatory stimuli, atheroma produce IL-1. These observations led to the hypothesis that Il-1 participates in atherogenesis.
Moreover, Il-1 has multiple effects on human vascular smooth muscle cells (SMCs), a cell type known to be involved in atherogenesis. IL-1 can induce autocrine production of platelet-derived growth factor, which can stimulate SMC proliferation. Treatment with IL-1 strongly induces SMCs and other cells and results in Il-6 expression. IL-6 elicits the acute phase response, such that hepatocytes increase synthesis of acute phase reactants like fibrinogen and plasminogen activator inhibitor and CRP. Thus, IL-1 mediates an amplification loop, since one Il-1 molecule can activate multiple IL-6 molecules, which drive overexpression of atherothrombotic mediators.
IL-1 has been shown to affect function of cardiac myocytes and blood vessel wall cells, it impairs contractile function. It can also aggravate ischemia-reperfusion injury and expansive cardiac remodeling in an experimental MI-model. Experimental and small human studies have suggested that IL-1 antagonism can improve expansive remodeling after MI and limit CRP-release after ACS. Various experiments with genetically induced loss-of-function or gain-of-function of IL-1, manipulation of IL-1ra and pharmacologic inhibition of Il-1 strongly point to a role for this cytokine in atherogenesis. Moreover, activated platelets can express IL-1α and produce microparticles containing functional IL-1β, representing another link between IL-1 and atherothrombosis.
Therapeutic targeting of IL-1β
Direct anti-inflammatory therapy to improve outcomes in survivors of ACS is of urgent interest when considering the residual burden of recurrent CV events, despite standard medical care consisting of high-potency statins, potent anti-platelet combinations and late generation stenting. Evidence has pointed at IL-1β as a potential therapeutic target.
IL-1 action can be inhibited with several strategies. The IL-1ra anakinra has been shown to confer clinical benefit in some cases of rheumatoid arthritis, acute gouty arthritis and diabetes mellitus. It blocks both IL-1 isoforms, meaning it may impair host defenses and thus increase susceptibility to infection. The human monoclonal antibody canakinumab selectively neutralizes IL-1β, and has been found to benefit several IL-1β-mediated inflammatory conditions. An antibody directed against IL-1α is now also being evaluated clinically. While anakinra has to be administered once daily, canakinumab has a prolonged biological half-life and can be administered every 3 months.
CANTOS: affirmation of the role of inflammation in atherothrombosis
The CANTOS trial was a large trial that tested the hypothesis that administration of canakinumab to neutralize IL-1β activity could improve outcomes in individuals who have had an acute MI. Over 10000 individuals with an acute MI at least one month ago, aggressively treated with all standard secondary prevention therapies for MI, with residual inflammation (hsCRP >2.0 mg/dL) were included. Patients were randomized to placebo or one of three doses of canakinumab, injected four times a year. The primary endpoint was a composition of non-fatal MI, non-fatal stroke or CV death.
Canakinumab 150 mg and 300 mg lowered the primary endpoint by 15%, while 50 mg did not significantly lower events. The higher doses lowered hsCRP by about 60%, leaving LDL-c unaffected. Thus, CANTOS affirmed the inflammatory hypothesis of atherosclerosis and set the stage for a new era of CV therapeutics. Moreover, the CANTOS results highlight the utility of assessing both LDL and hsCRP post ACS, as targeting either biomarker can provide clinical benefit. This fits in the current era of precision medicine, in which therapies are allocated rationally based on readily measured biomarkers. Patients with LDL-c levels above targets can benefit from additional lipid-lowering on top of statin treatment, to lower their residual LDL risk. Those who have persistently elevated CRP despite standard of care, could receive canakinumab to lower their residual inflammatory risk. Of note, the FOURIER trial evaluating a PCSK9 inhibitor on top of statin therapy and CANTOS showed event reductions of a similar magnitude, despite targeting different aspects of residual risk.
Beyond CANTOS: other targets, other indications
IL-1β is not the only potential target to inhibit inflammation in atherosclerosis. And considering the redundancy of inflammatory signaling pathways, targeting one mediator may not block all inflammatory processes implicated in atherogenesis. Other studies should now examine the role of other strategies to modify inflammation and immunity, in various clinical scenarios.
Some confusion may need to be clarified about targeting inflammation versus targeting oxidative stress. The enzyme lipoprotein-associated phospholipase A2 (LP-PLA2) arises primarily from inflammatory cells, and generates pro-oxidant lipid species. Targeting LP-PLA2 with darapladib has failed to reduce CV events in two large clinical trials. Thus, these trials did not address the inflammation hypothesis, as they did not reduce inflammation itself, but rather oxidative stimuli.
Ongoing clinical trials evaluate other means of targeting inflammation. Colchicine has been shown to reduce CV events in the small LoDoCo trial. Both the COLCOT and LoDoCo2 studies are now following up on this preliminary finding in a larger setting. Low-dose methotrexate is being evaluated for potential CV benefit in the ongoing CIRT trial.
Experimental studies have identified more inflammatory targets with well-characterized mechanisms of action that merit consideration for future investigation. Antibodies directed at IL-1α may be interesting as it shares many proinflammatory actions with IL-1β, including induction of IL-6. Clinical trials evaluating these antibodies have started. Tocilizumab is an antibody that selectively neutralizes IL-6, can treat several inflammatory disease, but adversely perturbs the lipid profile.
IL-18 is a member of the IL-1 family is also activated by the inflammasome, and it may be implicated in atherosclerosis. Inhibition of the inflammasome NLRP3 may simultaneously impair action of this pair of proinflammatory cytokines, although this strategy may come with a higher risk of interfering with host defenses.
CD40 and its ligand CD154 also appear to be involved in CV and metabolic disease. Antibody strategies encountered thrombotic complications, but a late-generation antisense oligonucleotide strategy may be more promising, as well as a small molecule inhibitor of CD40 interaction with its signal transducing partner TRAF.
TNF-inhibiting strategies are successful in certain rheumatologic and inflammatory diseases, but they can also adversely affect lipid profiles. They receive little interest for treatment of CV patients.
MCP-1 and its receptor CCR-2 are also involved in atherogenesis. It has been revealed that monocyte heterogeneity is relevant for CV disease, and a proinflammatory subset of monocytes recruited through the CCR-2 receptor seems relevant in several CV conditions. Antibodies that neutralize CCR-2 merit clinical evaluation.
All interventions that interfere with innate immunity may increase susceptibility to infections or impair tumor surveillance. In CANTOS, a small but significant increase in fatal infections was seen. Analysis of risk factors for infection may help mitigate this risk. Interestingly, CANTOS showed a substantial and canakinumab dose-dependent reduction of fatal malignancies; 67% lower incidence of lung cancer and 77% lower fatal lung cancer rate. It is proposed that IL-1 blockade may impede the invasion and metastasis of existing cancers.
Since inflammation also contributes to other clinical scenarios, and since cytokines modulate the healing of acute ischemic myocardial injury experimentally, some of the anti-cytokine strategies may exert beneficial healing effects after an ischemic insult in patients with ACS. Data from over 1400 primary events in CANTOS will provide safety data on the use of canakinumab during ACS.
Conclusions: entering an era of targeting inflammation in cardiovascular disease
Decades of research on the participation of immune and inflammatory pathways in CV disease have culminated in clinical benefit. Extrapolation of preclinical study results to humans requires caution, but the burden of residual risk mandates careful consideration of novel approaches to target inflammatory pathways.