Increasing evidence points to the central role inflammation plays in cardiovascular disease. The complex interplay between the innate and adaptive immune system plays a critical role in the development and progression of atherosclerosis and highlights critical regulators of cardiovascular disease.1 Interleukin 1 (IL-1) has long been known to be a cornerstone in the inflammatory response, which mediates multiple disease states.2 IL-1α, IL-1β, and IL-1 receptor antagonist are all proteins produced by the IL-1 gene family but with very different functions. Although IL-1β and IL-1α are potent mediators of inflammation, IL-1 receptor antagonist is an anti-inflammatory endogenous protein that competitively blocks the type I IL-1 receptor and prevents binding by IL-1β and IL-1α. After binding of IL-1α or IL-1β to the type I IL-1 receptor, IL-1 receptor accessory protein is recruited to initiate signal transduction leading to activation of nuclear factor kappa-light-chain-enhancer of activated B cells, c-Jun N-terminal kinase, p38 mitogen-activated protein kinases, and other intracellular transcription factors critical in generating a robust inflammatory response. The promise of IL-1 inhibition is so great in the field of atherosclerosis that the pharmaceutical industry has invested billions of dollars in the hopes to that novel inhibitors of IL-1-mediated inflammation will lead to significant reductions in major adverse cardiovascular events.3
Rather than attempting to suppress IL-1β after its production, an alternative strategy may be to prevent its formation in the first place. The inflammasome has emerged as a critical mediator of IL-1β production, and development of drugs to suppress the inflammasome may result in improved outcomes for a variety of disease states. Among many pattern recognition receptors that play a critical role in the innate immune response, the intracellular nucleotide-binding oligomerization domain-like receptors (NLR) are a family of proteins characterized by the presence of a nucleotide-binding domain and a C-terminal leucine-rich repeat, which seem to be critical in their ability to oligomerize and form the inflammasome. The NLR with a Pyrin domain 3 (NLRP3) inflammasome is also characterized by oligomerization of 7 NLRP3 molecules, making it the largest inflammasome, and by the presence of pyrin domain, which recruits the adapter protein apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) that can then enzymatically cleave procaspase-1 into caspase-1.4 Once active, caspase-1 interacts with p20 and p10 to form a complex that can then go on to proteolytically cleaved pro-IL-1β and pro-IL-18 into their active forms.
Acute onset of myocardial ischemia and infarction is well known to lead to a robust inflammatory response. After acute myocardial infarction (AMI), neutrophils and subsequently monocytes, both major producers of IL-1β, infiltrate the tissue and help generate a robust inflammatory response, which can lead to further cell death and eventual “repair” by scar formation. However, this process can be associated with adverse remodeling of the left ventricle and subsequent development of heart failure. The ability to inhibit IL-1β production in the heart after AMI is very attractive because blunting the inflammatory response may lead to improved tissue repair and a sustained improvement in left ventricular function as previously reviewed.5 The inflammasome has been shown to play a significant role in ischemia/reperfusion injury through a series of elegant experiments involving knockout of ASC.6 These authors show that the inflammasome plays a role in IL-1β production by bone marrow–derived inflammatory cells and that hypoxia can help induce inflammasome-mediated production of caspase-1 in cardiac fibroblasts but not in the cardiomyocytes.6 Furthermore, mice lacking NLRP3 have significantly smaller infarcts and better preservation of left ventricular function after ischemic injury.7 Thus, drugs targeting the NLRP3 inflammasome may help significantly improve morbidity and mortality after AMI in patients through suppression of inflammation and prevent the development of heart failure.
In this issue of the journal, Marchetti et al8 present a series of in vitro and in vivo experiments characterizing the impact of a new molecule 16673-34-0 on the NLRP3 inflammasome and its impact on a model of murine ischemia/reperfusion injury and acute peritonitis. Despite it being an intermediate substrate in the synthesis of glyburide, this molecule importantly had no significant impact on glucose levels likely because of its lack of the cyclohexylurea moiety. Molecule 16673-34-0 resulted in a 90% inhibition of inflammasome activity as measured by caspase-1 activity within the heart of mice after ischemia/reperfusion injury, which corresponded with a reduction in infarct size measured by pathology and troponin levels. Although the data seem promising, several important issues remain, which need further investigation. Although the authors demonstrate that the molecule specifically inhibits NLRP3, the mechanism of action needs further clarification. Although caspase-1 activity was assessed, additional studies assessing IL-1β and IL-18 production after treatment will also be helpful. Furthermore, the drug seems to have questionable solubility, which is likely due to the 3 aromatic benzene rings, as evidenced by the use of dimethyl sulfoxide as a solvent. Although the solubility is improved through the introduction of a sulfonamide in the final synthetic step seen in figure 1 of the article, further improvement in solubility could be achieved through removal of the methyl group attached to oxygen in position 2 of the aromatic ring. However, given the uncertainty regarding the mechanism of action of the molecule, such a modification may significantly impact its ability to inhibit the NLRP3 inflammasome. Finally, pharmacokinetic studies are necessary to determine serum drug levels after administration because it is unclear how the authors determined the schedule for drug administration based on the available data from this article.
We look forward to further studies from this group clarifying these issues and believe this drug may not only have promise for management of patients after AMI, but also may serve as a potential therapy for attenuating the progression of atherosclerosis. Thus, treatment with patients after percutaneous coronary intervention for AMI may not only improve left ventricular dimensions and prevent heart failure, but also reduce future atherothrombotic events and potentially reduce the risk of restenosis. Thus, further mechanistic and animal studies are warranted to investigate the therapeutic potential of molecule 16673-34-0.
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