FormalPara Key Summary Points

Atherosclerotic cardiovascular events continue to occur despite maximal lipid-lowering therapy (LLT).

High-sensitivity C-reactive protein (hsCRP) has been shown to predict atherosclerotic cardiovascular disease (ASCVD) risk.

hsCRP remains a useful tool to identify an inflammatory residual risk phenotype and may be used to tailor therapy in patients with low-grade chronic inflammation.

To date, repurposed drugs such as colchicine and canakinumab have been shown to significantly reduce the relative risk of major cardiovascular events.

Future trials may indicate that drugs that target interleukin-6, such as ziltivekimab and tocilizumab, could be useful therapies for reducing ASCVD risk.

In the future, combined LLT and anti-inflammatory targeted drug therapies may be used to reduce ASCVD.

Introduction

It is now well understood that cardiovascular events can still repeatedly occur in high-risk and even low-risk patients despite achieving effective lowering of low-density lipoprotein cholesterol (LDL-C) using statins and other drugs [1]. This residual risk of atherosclerotic cardiovascular disease (ASCVD) can be affected by a wide variety of pathophysiological processes, but inflammation is likely to play a key role.

Uncontrolled persistent inflammation underlies all phases of atherogenesis. Early stages of atherosclerosis are marked by endothelial dysfunction and increased vascular permeability, which facilitate infiltration of plasma LDL and triglyceride-rich lipoprotein (TRL) remnants into the subendothelial intimal matrix. LDL and TRL particles in the intimal layer are exposed to oxidative or other types of conformational modifications that may increase their atherogenic potential. These modified lipoproteins initiate subsequent stages of atherosclerosis, including triggering macrophages and other cells to express adhesion molecules and to secrete inflammatory cytokines. The balance between inflammatory and anti-inflammatory activity, in part, modulates the progression of atherosclerosis. Numerous studies suggest that the inflammatory burden is a key factor in plaque stability [2]. Indeed, stable plaques are characterized by a chronic inflammatory infiltrate [3]. Therefore, early detection of atherosclerosis using inflammatory biomarkers could play an important role in treatment decision-making.

Traditional biomarkers for ASCVD include LDL-C, TRL, lipoprotein(a), and glucose. However, approximately 20% of all cases of myocardial infarction (MI) and stroke occur in patients with no evident common risk factors [4]. Hence, there is great interest in identifying additional pathophysiological biomarkers of inflammation as well as an inflammatory coronary plaque imaging phenotype. The clinical utility of these other biomarkers and the need for their further clinical evaluation will be highlighted in the current review. We will also discuss new lipid-lowering therapies as well as alternative therapeutic approaches that predominantly target inflammation underlying cardiovascular disease (CVD).

Cardiovascular Residual Risk Biomarkers

High-Sensitivity C-Reactive Protein (hsCRP)

The majority of studies have focused on well-established hsCRP as an ASCVD inflammation marker. The current US 2018 multi-society guidelines in ASCVD risk assessment use hsCRP as a risk enhancer [5]. For example, patients with an intermediate 10-year risk score (7.5–20%) and hsCRP > 2 mg/L are considered to be at increased risk. This recommendation comes from several primary prevention trials, including the Physicians Health Study [6], where the highest quartile of hsCRP was associated with a 2.9-fold greater risk for MI and 1.9-fold greater risk for stroke (p < 0.001, p = 0.02, respectively). Subsequently, the Emerging Risk Factors Collaboration [7] meta-analysis of 54 prospective cohorts of primary prevention showed that log-transformed hsCRP was associated with 1.23-fold increased risk for coronary heart disease. Similar outcomes for ischemic stroke and vascular-associated risk were found (relative risk [RR] of 1.32; 95% confidence interval [CI], 1.18–1.49 and 1.34; 95% CI 1.20–1.50). These associations were further strengthened by a meta-analysis of > 160,000 individuals in a primary prevention study that identified an adjusted RR for coronary heart disease of 1.37 (95% CI 1.27–1.48), which is comparable to traditional risk factors, such as non-high-density lipoprotein cholesterol (HDL-C) and systolic blood pressure. These associations were also strongly predictive of ischemic stroke (RR 1.27, 95% CI 1.15–1.40) and ASCVD mortality (RR 1.55, 95% CI 1.37–1.76), even after adjustment for traditional risk factors [8]. These findings demonstrate that hsCRP, as a surrogate marker of the interleukin (IL)-1β-to-IL-6-to-hsCRP pathway, is a reliable and important risk marker, but not a risk factor, for ischemic heart disease.

Triglyceride-Rich Lipoproteins

For decades, triglycerides (TGs) have been considered a risk factor for ASCVD, but the relationship between TG and ASCVD events remains controversial. Two key questions arise: Is it the TG itself or the lipoprotein which transports it (TRL or TRL remnants) that is the causal factor? Secondly, is this risk association with TRL due to its cholesterol or its TG content, or both? To date, epidemiological and Mendelian randomization studies have demonstrated a causal relationship of TRL to ASCVD risk [9, 10].

TRLs are considered to be more atherogenic per particle than LDL [11]. Due to their size (< 70 nm), they can only be transported through the artery wall in one direction (Fig. 1) [12]. TRL biochemical composition is also critical for arterial retention and subsequent events, since larger TRL particles can contain up to four times the cholesterol content per particle as LDL particles, even though TGs are their main component [13]. Additionally, TRLs are enriched in apoC-III and apoE, which are both implicated in TRL binding and retention in the arterial wall. TRL uptake by macrophages activates the NLRP3 inflammasome [14,15,16]. Recent genetic research supports the causality of TRLs in cardiovascular (CV) risk, implicating various genes related to TG metabolism such as APOA5, APOC3, ANGPTL3, ANGPTL4, and lipoprotein lipase (LPL) [17]. Additional studies have shown that an increase in TG levels is accompanied by increased levels of hsCRP [18].

Fig. 1
figure 1

Inflammatory signaling pathways and targeted therapeutic interventions. LDL low-density lipoprotein, TRL triglyceride-rich lipoprotein, hsCRP high-sensitivity C-reactive protein, PCSK9i proprotein convertase subtilisin/kexin type 9 inhibitor, BA bempedoic acid, IL-6 interleukin 6, IL-1β interleukin 1 beta, IL-18 interleukin-18, TNF-α tumor necrosis factor alpha, NLRP3 nucleotide-binding oligomerization domain-like receptor 3, Lp-PLA2 lipoprotein phospholipase A2. This original figure was created using BioRender

Full size image

The uptake of unmodified TRL in the arterial wall can induce macrophage foam cell formation [19]. Free fatty acids and other bioactive lysolipids generated during TRL-TG lipolysis can exert proinflammatory effects in macrophages and smooth muscle (SM) cells [20]. Lipid-laden foam cells and SM cells in lesions are abundant in LPL and promote endothelial dysfunction [21]. TG-laden macrophages also stimulate proinflammatory cytokine release via the polarization of macrophages to the M1 phenotype, thereby increasing the inflammatory burden of arterial lesions [22]. Cholesterol crystal formation may also occur upon very-low-density lipoprotein (VLDL) or remnant uptake by macrophages, with activation of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome and an inflammatory response [16, 23, 24].

Lipoprotein(a)

Similar to LDL, lipoprotein(a) [Lp(a)] is an apoB-containing lipoprotein with apolipoprotein(a) [apo(a)] covalently bound. Lp(a) has been identified as a causal factor for coronary heart disease based on epidemiological and genetic studies [25,26,27]. Although the circulating levels of Lp(a) are much lower than other lipoproteins, it has been shown that the per-particle ASCVD risk of Lp(a) is six times that of LDL (point estimate of 6.6; 95% CI 5.1–8.8) [28]. It is estimated that up to 1% of the population has extreme levels of Lp(a) above 430 nmol/L (180 mg/dL), which is associated with a more than threefold increased risk of ASCVD—the same lifetime risk for ASCVD as untreated heterozygous familial hypercholesterolemia [26].

The relationship between Lp(a) and inflammation is complex. The presence of oxidized phospholipids (OxPLs) in Lp(a) increases arterial wall inflammation (and also initiates aortic valve calcification) by activating monocytes and endothelial cells. Furthermore, Lp(a) has a direct link to IL-6, since the LPA gene contains a class II IL-6 response element (CTGGGA) which upregulates apo(a) expression [29]. In one study, serological testing of 1153 patients revealed that Lp(a) levels were higher in patients with elevated IL-6 (> 6 pg/mL) [30]. These authors also found that LPA gene expression in the liver was influenced by IL-6 response genes. In other in vitro experiments, human peripheral blood monocytes stimulated with Lp(a) had significantly increased IL-6 secretion [31]. This is supported by the finding that IL-6 SNP–174G/C was associated with increased odds of having elevated Lp(a) [32]. This is also true for IL-1, as individuals with a proinflammatory interleukin-1 genotype have higher CV risk associated with Lp(a) [33, 34].

Several clinical studies have reported that hsCRP also affects Lp(a) levels. A large study conducted in a Danish population showed that Lp(a) levels increased with increasing CRP levels (e.g., median Lp(a) was 18.0 mg/dL with CRP < 1 mg/L and 21.1 mg/dL with CRP > 10 mg/L) [35]. This is also supported by the ACCELERATE and MESA trials, where the former showed that when hsCRP was above 2 mg/dL, each unit increase in log of Lp(a) was associated with 13% greater risk of CV death, nonfatal MI, or stroke [36], with the latter study also showing that only individuals with hsCRP > 2 mg/dL and Lp(a) > 50 mg/dL demonstrated a risk associated with ASCVD. This association was not shown in an analysis of the JUPITER trial, where Lp(a) was a significant determinant of residual risk in both treatment groups where LDL-C < 130 mg/dL and hsCRP was either less than or greater than 2 mg/dL. Interestingly, in this study, when patients were stratified by the median of CRP values (4 mg/L), a significant association was observed between Lp(a) levels and the incident primary endpoint in the group with hsCRP below the median, but not in the group with hsCRP above the median [37]. Conversely, it has also been shown that rheumatoid arthritis (RA) treatment with IL-6 inhibitors (tocilizumab) results in a 50% reduction in Lp(a) [37]. Overall, treatments that block IL-6 reduced Lp(a) by 16–41% [38]. However, more recent data from the Copenhagen General Population study showed that high levels of Lp(a) were associated with ASCVD and aortic valve stenosis irrespective of whether hsCRP levels were above or below 2 mg/L. Taken together, these studies indicate that hsCRP has limited ability for identifying a higher CV risk [39].

Interleukin-1 (IL-1)

The IL-1 family is composed of 11 proteins which include IL-1α, IL-1β, IL-1Ra, IL-18, IL-33, IL-36α, IL-36β, IL-36γ, IL-36Ra IL-37, and IL-38. Both IL-1α and IL-1β bind to the IL-1 receptor and are the most potent proinflammatory cytokines implicated in the development of chronic inflammatory diseases such as RA. However, they have also been described to play a role in other diseases, such as type 2 diabetes (T2D). IL-1 signaling also leads to upregulation of other inflammatory markers, including IL-6, hsCRP, fibrinogen, and plasminogen activator inhibitor [40].

There is strong evidence to support the involvement of IL-1 in SM cell mitogenesis and extracellular matrix metabolism. This mechanism has been corroborated by studies of gene polymorphism of IL-1 receptor agonists associated with lower incidence of stent restenosis [41]. Moreover, extended direct exposure of IL-1β in porcine coronary arteries was found to induce intimal thickening and vasospasm, which was attenuated by an IL-1β-neutralizing antibody [42].

Interleukin 18 (IL-18) is a proinflammatory cytokine that has been implicated in several disease states. Canonical NLRP3 inflammasome activation leads to IL-1β and pro-IL-18 cleavage [43]. In animal models of acute MI, pressure overload, and lipopolysaccharide (LPS)-induced dysfunction, IL-18 regulates cardiomyocyte hypertrophy and induces cardiac contractile dysfunction and extracellular matrix remodeling. In humans, high IL-18 levels correlate with increased risk of developing ASCVD [44].

Interleukin-6

Interleukin-6 (IL-6) is a type 2 cytokine (i.e., activates humoral immune response) that generates diffusible molecules with autocrine, paracrine, or endocrine functions. IL-6 and IL-1 receptors trigger downstream transduction pathways, such as JAK/STAT, mitogen-activated protein (MAP) kinases, and Pi3k-Akt. IL-6 originates from different sources in the human body, but is mainly produced by macrophages, T helper cells, B cells, vascular endothelial cells, SM cells, and fibroblasts [45]. Mendelian randomization studies have demonstrated a link between genetic variation associated with plasma IL-6 levels and coronary events, suggesting a causal relationship for the IL-6 pathway [45].

IL-6 was found to be strongly associated with incident MI in the Physicians Health Study. Those in the highest quartile had a 2.3-fold greater risk of MI than those in the lower quartile (p = 0.03). This was also explored in the Women’s Health study, which found the same risk association, where the highest quartiles of IL-6 had 2.2-fold greater risk of CV events than the lowest quartile [46]. Furthermore, a meta-analysis of 29 trials found that for each standard deviation increase in IL-6, there was a 25% increase in nonfatal MI or coronary heart disease death [47].

Despite its association with ASCVD, however, there have not been a sufficient number of studies to confirm IL-6 as a risk enhancer, most likely due to the lack of access to reliable IL-6 assays.

Lp-PLA2

Lipoprotein-associated phospholipase-A2 (Lp-PLA2), a plasma biomarker of inflammation, is primarily produced by macrophages and macrophage-derived foam cells within atherosclerotic plaques. This enzyme plays a key role in the breakdown of oxidized phospholipids, releasing pro-atherogenic and proinflammatory oxidized fatty acids and lysophosphatidylcholine, thereby inducing oxidative stress and promoting atherosclerotic plaque progression [48]. Lp-PLA2 is primarily associated with LDL, and studies have shown that higher plasma levels are independently associated with the risk of developing coronary artery disease (CAD) events, regardless of non-HDL-C levels. Despite promising results in reducing plaque volume using darapladib, a selective Lp-PLA2 inhibitor, the Integrated Biomarker and Imaging Study 2 failed to demonstrate clear CV benefits [49]. As a result, the practical utility of measuring Lp-LPA2 remains uncertain in the absence of an effective targeted therapeutic with proven ASCVD benefits.

NLRP3 Inflammasome

The inflammasome is a macromolecular complex that is triggered by injury which amplifies the inflammatory response [50]. The pyrin-containing NLRP is a family of intracellular receptors that sense patterns of pathogens of injury. The NLRP3 inflammasome is the most extensively analyzed and has been shown to play a role in multiple autoimmune diseases. Its activation leads to production of caspases 1, 4, 5, and 11, and promotes the maturation and secretion of proinflammatory cytokines such as IL-1β and IL-18. This complex biochemical cascade activation culminates in cell death, called “pyroptosis” [51].

Atherosclerotic lesions contain deposits of cholesterol crystals in the necrotic core that induce inflammation by stimulation of the NLRP3 inflammasome. It was found that in ApoE-deficient mice fed a high-cholesterol diet, the formation of cholesterol crystals induced acute inflammation, whereas in mouse models deficient in NLRP3 inflammasome components, no inflammation was observed [16]. Moreover, deletion of the Nlrp3, Asc, and Il-1 alpha (IL1A) and beta (IL1B) genes in mice with hypercholesterolemia prevented the development of atherosclerosis [16, 52, 53]. An association between the NLRP3 inflammasome and other CVDs such as myocarditis, sarcoidosis, septic cardiomyopathy, chronic non-ischemic cardiomyopathy, and pericarditis has also been described [54].

A number of potential surrogate markers for inflammation in ASCVD currently under investigation, including fibrinogen, uric acid, midkine, pentraxins, cystatin C, microRNA (miRNA), and matrix metalloproteinases (MMPs), are beyond the scope of this review, and have been described in other recent reviews [55].

Imaging and Inflammation

Recent advances in imaging modalities, in particular cardiac computed tomography angiography (CCTA), have made this visualization technique a sensitive tool for monitoring vascular inflammation and atherosclerotic coronary plaque characterization. During atherosclerotic plaque buildup, the arterial wall microenvironment includes metabolically active cells which release proinflammatory mediators. The latter induce alterations in both luminal and perivascular morphological adipose tissue. This complex inter-tissue relationship can be evaluated by CCTA as perivascular adipose tissue (PVAT) attenuation, also known as the perivascular fat attenuation index (FAI) [56]. FAI around the proximal segment of the left anterior descending (LAD) artery or right coronary artery (RCA) was a strong predictor of all-cause and cardiac mortality in the CRISP-CT study, involving 4000 individuals undergoing diagnostic CCTA [57]. The PVAT concept has been effectively applied in a recently developed algorithm, namely CaRi-HEART (Caristo Diagnostics, Oxford, UK), that enables a quantitative analysis of vascular inflammation [58]. Moreover, FAI is differentially associated with the coronary plaque phenotype, which can be determined by CCTA. Indeed, detecting high-risk or vulnerable plaque features not only improves clinical decision-making but also aids in validating novel biomarkers and CAD drug development [59]. It has been shown that other sources of fat depots besides perivascular fat might contribute to cardio-metabolic disease progression [60]. For example, a CCTA-based approach for assessing epicardial and thoracic (pericardial) adipose tissue was found to be longitudinally associated with coronary and cardiac characteristics in psoriasis [60]. Moreover, by applying the same imaging modality for detecting visceral adipose tissue (VAT), it was revealed that a reduction in psoriatic inflammation was associated with lower VAT and improved longitudinal coronary plaque burden [61].

Although CCTA-derived imaging markers have proved its sensitivity for ASCVD risk prediction, a spatial resolution similar to that of invasive coronary angiography or intravascular ultrasound is still a matter of concern. As an alternative imaging technique, positron emission tomography/computed tomography (PET/CT) has been used less extensively due to its high cost and limited availability [62]. One of the most commonly utilized tracers for assessing inflammation is 18F-fluorodeoxyglucose (FDG) [63]. In atherosclerotic plaque, macrophages have higher metabolic activity due to hypoxia-induced anaerobic glycolysis as compared to the residing cell milieu. Thus, levels of FDG uptake by plaque macrophages represent its activity related to the degree of inflammation, whereas 18F-FDG uptake by other metabolically active organs, such as bone marrow, spleen, and liver, was associated with systemic inflammation and subclinical atherosclerosis in patients with psoriasis and RA. Despite its diagnostic utility, 18F-FDG PET-CT lacks cell specificity and needs to be further technically advanced for wider routine use [64].

Lipid-Lowering Therapies and Inflammation

Statins, Ezetimibe, and PCSK9 Inhibitors

Several clinical trials have shown the potential for statins to reduce markers of inflammation. For example, in the PRINCE study [65] and in a subset of patients from the CARE study, it was found that treatment with pravastatin 40 mg daily reduced hsCRP levels longitudinally by 16.9% and 21.6%, respectively [66].

Statin therapy intensity has also been shown to play a role in the hsCRP reduction. For example, in the JUPITER trial, patients on 20 mg of rosuvastatin lowered their levels of hsCRP by 37% [67]. The magnitude of reduction in hsCRP achieved in the treatment group of the JUPITER trial was proportional to the reduction in CV risk. Furthermore, those that achieved hsCRP levels of < 2 mg/L had a 55% reduction in the primary endpoint compared with those that did not [67]. In line with these findings, treatment with atorvastatin 80 mg was shown to achieve hsCRP of < 2 mg/L in 57.5% of patients in the PROVE-IT TIMI 22 study [68]. It is also important to note that those patients who achieved hsCRP levels of < 2 mg/L had fewer CV events independently of LDL-C.

There are limited data in the assessment of inflammation markers with ezetimibe or PCSK9 inhibitor treatment. For example, in the IMPROVE-IT study, a 14% reduction in hsCRP was demonstrated in the combined simvastatin and ezetimibe treatment group [69]. In this study, patients treated with a statin alone or in combination with ezetimibe who achieved both LDL-C < 70 mg/dL and hsCRP < 2 mg/L had better CV outcomes (CV death, major coronary event, or stroke) than patients with higher LDL-C and hsCRP levels (adjusted hazard ratio [HR] of 0.73 [95% CI 0.66–0.81]) [70]. Despite the considerable reduction in LDL-C, PCSK9 inhibitor trials have provided scant evidence for an improvement in hsCRP. In the FOURIER trial, only 30% of patients had hsCRP < 3 mg/L, and in the follow-up period, the median reduction in hsCRP was 0.2 mg/L [71]. Despite this nonsignificant improvement in hsCRP levels, however, the absolute relative risk reduction (RRR) in those with high levels of hsCRP was 2.6% for the primary endpoint.

PCSK9i also lowers Lp(a) to some extent. Several studies have shown that Lp(a) can be lowered by 24–29% with the PCSK9 inhibitor evolocumab [72, 73]. However, a study by Stiekema et al. found that while evolocumab effectively lowered LDL-C levels and achieved a slight decrease in Lp(a) levels, there was no significant change in arterial wall inflammation among patients who underwent 18F-FDG PET/CT to assess carotid or thoracic aortic artery wall inflammation before and after treatment with evolocumab [74].

Bempedoic Acid

Bempedoic acid (BA) has consistently been shown to lower hsCRP independently of statins. A pooled analysis of the four phase 3 studies of BA (CLEAR Harmony, CLEAR Wisdom, CLEAR Serenity, and CLEAR Tranquility) showed that in those with baseline hsCRP of > 2 mg/L, the percentage changes were essentially the same for those on statins and those with low or no statins (41.9 vs. 43.3%, respectively) [75]. It was also shown that those who at baseline had hsCRP > 2 mg/L, 38.7% and 40.7% treated with ASCVD/HeFH or with low-dose or no statin, respectively, achieved levels of hsCRP < 2 mg/L at 12 weeks of treatment. Overall, among all of those treated with BA with baseline high hsCRP > 2 mg/L, 68.6% achieved low levels at 12 weeks. In these studies, a total of 20.8% in the ASCVD and/or HeFH pool receiving both statins and BA attained the dual goal of LDL-C < 70 mg/dL and hsCRP < 2 mg/L, compared to 4.3% of those receiving statins and placebo, regardless of baseline hsCRP levels. Additionally, more patients in the low-dose or no statin pool achieved both LDL-C < 100 mg/dL and hsCRP < 2 mg/dL than those receiving placebo (32% vs. 5.3%, respectively). In the CLEAR Outcomes trial, at 6 months of BA treatment, hsCRP was reduced by 21.6% and LDL-C by 21.1%. A sub-study within this trial found that BA had similar efficacy in reducing CV events across all levels of hsCRP and LDL-C. Further studies are needed to determine whether the reduction in hsCRP levels with BA contributed to the observed benefits [76].

Lp(a)-Lowering Medications

Several Lp(a)-lowering therapies are currently in phase II/III studies. Antisense oligonucleotides (pelacarsen), inhibitors of apo(a) binding to LDL (muvalaplin), and small interfering RNA (olpasiran, SLN360 and lepodisiran)-based therapies have shown an impressive 65–98% reduction in plasma Lp(a) levels. However, the effect of these drugs on inflammation has only been partially explored with pelacarsen. One study, based on transcriptome analysis in circulating monocytes of healthy individuals, showed that pelacarsen lowered Lp(a) by 47% and concomitantly reduced the proinflammatory gene expression in monocytes of patients with ASCVD with elevated Lp(a) and demonstrated a functional reduction in the transendothelial migration capacity of monocytes. [74].

New Anti-inflammatory Therapies

IL-1β-Targeted Therapy

Canakinumab, a monoclonal antibody (mAb) targeting IL-1β, demonstrated its efficacy in the CANTOS phase III randomized placebo-controlled clinical trial involving 10,061 participants with a history of MI and hsCRP levels exceeding 2.0 mg/L. Canakinumab at a dose of 150 mg led to a 15% reduction in the primary composite endpoint of MI, stroke, and CV death, with an HR of 0.85 (95% CI 0.74–0.98) when compared to placebo [77]. Key secondary endpoints, including MI, unstable angina necessitating urgent revascularization, and any coronary revascularization, were also diminished. However, no significant reduction was observed in CV mortality or stroke. Notably, CANTOS marked a groundbreaking moment by demonstrating the clinical benefits of targeting inflammation. Despite an elevated risk of fatal infection associated with canakinumab therapy, there was a reduction in cancer-related deaths.

Moreover, individuals achieving hsCRP < 2 mg/L with canakinumab treatment exhibited a 25% reduction in CV events and a 31% decrease in all-cause mortality, with only a 5% reduction in those who did not achieve hsCRP < 2 mg/L [78]. Similarly, another post hoc analysis of this trial showed that with treatment with canakinumab, those who achieved on-treatment IL-6 levels below the study median value of 1.65 ng/L experienced a 32% reduction in major adverse cardiovascular events (MACE) and a 52% reduction in CV mortality. In contrast, for those who did not achieve this, there was no significant benefit for any endpoint.

However, the true effect on lipids in this trial still needs to be studied in detail. In the CANTOS trial, no difference in LDL-C levels was observed, as 90% of the population were on statins, and the median LDL-C between groups was ≈ 80 mg/dL [40]. Because of different IL-1 phenotypes, and furthermore because IL-6-modulating therapies can lower Lp(a), canakinumab could have potentially affected Lp(a) levels in the CANTOS trial.

To our knowledge, only a single study has explored the effects of canakinumab treatment on Lp(a) levels. In this study, 189 patients with ASCVD and T2D were evaluated and randomized to receive 150 mg of canakinumab for 12 months. After 3 months, the treatment group had a significant 27% reduction in Lp(a), which persisted for the 12-month duration of the trial (a reduction of 4.30 mg/dl [range −8.5 to −0.55 mg/dl]; p = 0.025) [79]. Consistent with these findings, an analysis of the CANTOS trial presented at the 2021 European Atherosclerosis Society found that treatment with canakinumab lowered Lp(a) by 23%. This analysis suggests that the Lp(a) lowering by canakinumab accounted for over 40% of the favorable results in CANTOS [80]. Further studies are needed to confirm these findings. In summary, the CANTOS trial demonstrated, for the first time, the proof of principle that therapeutic targeting of the immune system can be beneficial for CV outcomes.

Translating these findings to clinical practice can be challenging, as this post hoc analysis often raises more questions than it answers. It is currently not known whether patients should be treated to target IL-1β, or whether hsCRP and IL-6 should be monitored in order to stop treatment in those that cannot achieve a significant reduction in inflammation, as determined by hsCRP levels.

Anakinra, a recombinant interleukin-1 receptor antagonist, has also been tested in acute myocardial infarction (AMI). The VCU-ART [81,82,83,84,85] phase II clinical trials program included three sequential studies (n = 139) which analyzed hsCRP levels in patients with acute ST-segment elevation MI (STEMI). In each study, patients were randomized to receive anakinra or placebo. Treatment with anakinra was well tolerated and led to a decrease in the hsCRP area under the curve (AUC) in the 14-day period after the AMI. It also showed a significantly lower incidence of new-onset heart failure (HF) and HF-related hospitalization in the 1-year follow-up. The MRC-ILA-Heart study [86] (n = 182) studied patients with AMI, but without STEMI. In this study, anakinra also reduced hsCRP levels at 7 days but failed to improve clinical outcomes. These studies show that NLRP3 inflammasome activation with enhanced IL-1 activity during and after AMI is also a contributor to HF risk. Additional IL-1 modulators have been approved for indications other than ASCVD. Rilonacept (IL-1 trap) and gevokizumab (IL-1β binding protein) have been tested for inflammatory diseases, but to date not for ASCVD.

IL-6-Targeted Therapy

Tocilizumab, a humanized recombinant mAb targeting the IL-6 receptor, has demonstrated its efficacy and safety in several clinical trials. It has been approved for the treatment of several autoimmune diseases such as giant cell arteritis and RA, and has been considered for secondary prevention of ASCVD and MI. Recently, the randomized, double-blind, placebo-controlled phase III ASSAIL-MI trial [87] showed a larger myocardial salvage index in the tocilizumab group than under placebo. Also, microvascular obstruction was less extensive in the tocilizumab arm, but there was no significant difference in the final infarct size.

In a recent double-blind placebo-controlled trial in 117 patients with non-ST segment elevation myocardial infarction (NSTEMI), 2.1-fold lower hsCRP levels were reported in the tocilizumab group compared to placebo (2.0 vs. 4.2 mg/L/h, p < 0.001). Tocilizumab also reduced troponin levels (234 vs. 159 ng/L/h, placebo versus tocilizumab, respectively, p = 0.007), suggesting that treated patients sustained less myocardial damage than those on placebo [88].

A newer drug, ziltivekimab, a fully human mAb directed against the IL-6 ligand, has shown safety and efficacy in reducing biomarkers of inflammation and thrombosis in patients with high ASCVD risk. Recently, the RESCUE trial [89] confirmed that the use of ziltivekimab reduced hsCRP in a dose-dependent manner, from 77% to 93%, and other markers including fibrinogen, serum amyloid A (SAA), haptoglobin sPLA2, and Lp(a) in patients with ASCVD and chronic kidney disease (CKD) [90]. Currently, the ZEUS trial will compare ziltivekimab to placebo in 6200 randomized patients with stage 3–4 CKD or ASCVD and with elevated hsCRP, to formally test whether reducing circulating IL-6 reduces CV event rates. An important secondary aim to address is whether chronic IL-6 inhibition slows the progression of renal disease. This ambitious trial represents the next step toward understanding the benefits of targeting residual inflammatory risk (NCT05021835).

Sarilumab, a recently approved human mAb directed against both soluble and membrane-bound interleukin 6 receptor-α, shown to be effective in RA, could be also promising for managing ASCVD [91]. Currently there are only trials for the interaction with simvastatin registered in ClinicalTrials.gov (NCT02017639).

IL-18-Targeted Therapy

Currently, IL-18 blockers are under investigation. The role of IL-18 in inflammasome activation suggests inhibitors may target ASCVD. Recently, a novel IgG mAb that neutralizes IL-18 (GSK1070806) [92] has been reported to be well tolerated in healthy individuals, those with T2D, and obese subjects [93].

Methotrexate

Methotrexate (MTX) is a Food and Drug Administration (FDA)-approved folic acid antagonist which reduces T-cell proliferation and secretion of cytokines and MMPs, and thus is a potential treatment for atherosclerotic disease. MTX is indicated for the treatment of RA, juvenile idiopathic arthritis, and certain types of cancer. Its use for atherosclerosis has been shown in mouse models. In mice fed a high-cholesterol diet, MTX reduced both the size of the lesion areas of by 75% and the intima–media ratio by twofold. The drug inhibited macrophage migration into the intima by 50% and the presence of apoptotic cells by 84%, but did not inhibit the intimal proliferation of SM cells [94]. In observational studies of patients with RA, those treated with MTX had a 15% reduction in CV events [95], and in patients with psoriasis, a 50% reduction in the composite of ASCVD-related death, stroke, or MI was reported. A meta-analysis of 10 observational studies also showed that in patients with RA, psoriasis, or polyarthritis, treatment with MTX was associated with 21% lower risk for total ASCVD and 18% lower risk for MI. These studies prompted the Cardiovascular Reduction Trial (CIRT), a randomized double-blind, placebo-controlled trial looking to determine whether low doses of MTX could reduce CV events. This study was stopped early for futility, as MTX neither decreased MACE over 5 years (HR 1.01; 95% CI [0.82–1.25]; p = 0.91 versus placebo) nor lowered the concentration of inflammatory markers, particularly hsCRP [96]. Although patients included in the trial did not have residual inflammatory risk at baseline, this could not fully account for the trial results. Thus, only patients with CVD with rheumatological disease appear to benefit from low-dose MTX treatment.

Colchicine

Colchicine inhibits cytoskeletal reorganization, phagocytosis, mitosis, and other intracellular activity and is commonly prescribed for gout, familial Mediterranean fever, and off-label for pericarditis. Within its myriad effects, it also inhibits NLRP3 inflammasome-driven inflammation. Small studies have shown that in patients with stable CAD under statin treatment, the introduction of colchicine reduced hsCRP levels by 60%. Other studies have shown that in patients with acute coronary syndrome (ACS) and stable CAD randomized to receive either colchicine (1 mg, followed by 0.5 mg 1 h later) or no colchicine therapy prior to coronary angiography, those treated had significantly reduced transcoronary gradients of IL-1β, IL-18, and IL-6 cytokines by 40–88% (p = 0.028, 0.032, and 0.032, respectively) [97].

More recently, the LODOCO2 [98] and the COLCOT [99] studies demonstrated the utility of low-dose colchicine to reduce the risk of recurrent ischemic CV events. The COLCOT study involving 4745 post-MI patients demonstrated a remarkable 23% reduction in MACE, including CV death, cardiac arrest, MI, stroke, or urgent revascularization, over 22.6 months. This trial establishes the role of an anti-inflammatory agent in post-MI risk reduction. Subsequently, the LODOCO2 trial, which included 2762 patients with chronic coronary disease to receive low doses of colchicine, demonstrated a 31% reduction in MACE compared to placebo. Neither of the trials used hsCRP as an inclusion criterion. Given these results, in 2023 the FDA granted colchicine a broad label to reduce the risk of CV events in secondary prevention, or with multiple CV risk factors (https://us.agephapharma.com/blog/2023/06/20/us-fda-approves-first-anti-inflammatory-drug-for-cardiovascular-disease/).

Not all colchicine clinical trials, however, have been positive. In the COPS trial, the addition of colchicine to standard medical therapy in patients hospitalized with ACS did not affect CV outcomes at 12 months (HR 0.65, 95% CI 0.38–1.09, p = 0.10) [100]. Post hoc analysis demonstrated a significant reduction in the primary outcome in favor of colchicine. Unfortunately, this study was underpowered, thereby limiting interpretation of the mortality data.

Clinical trials are underway to investigate the use of colchicine in several different clinical settings. For example, the use of colchicine in ACS (COACS) (NCT01906749) trial examines vascular inflammation assessed with PET (COLPET) (NCT02162303). The CONVINCE trial, a large trial in phase 3, is planned to study the role of low-dose colchicine in adults who have suffered a stroke. This trial will evaluate patients (n = 2623) with ischemic stroke or transient ischemic attack, who will be followed though 60 months, and will assess the recurrence of stroke or nonfatal major cardiac events (NCT02898610). The most anticipated trial is CLEAR SYNERGY, the largest ongoing trial (n = 7000), which will be performed over 2 years in patients with MI. This study contains four arms (colchicine 0.5 mg twice daily and/or spironolactone 25 mg daily and/or SYNERGY stent and/or colchicine/spironolactone-placebo), providing the best approach for assessing the utility of colchicine in secondary ASCVD prevention (NCT03048825). This last trial is expected to be completed by 2025.

NLRP3 Inflammasome Inhibition Targeted Therapy

To date, the interest in NLRP3 inflammasome inhibition has led to the development of both direct and indirect inhibitors. Related to CVD, two direct inhibitors, MCC950 and VX-765, have been reported to prevent atherosclerosis in animal models [101,102,103]. Unfortunately, after the promising results of MCC950 in apoE-knockout (KO) mice in preventing atherosclerosis, its development was stopped due to its hepatotoxicity [104]. VX-740 and its analog VX-765, which are peptidomimetic inhibitors of caspase-1, have been reported to reduce oxidized LDL-induced vascular SM cell apoptosis in vitro and to prevent the development of atherosclerosis in apoE-KO mice [103]. Indirect inhibition of NLRP3 inflammasome can also be achieved by arglabin treatment, which has been shown to reduce both inflammation and plasma lipids by increasing autophagy and by directing macrophages to an anti-inflammatory phenotype in ApoE2-knock-in (KI) mice [105].

New glucose-lowering therapies have also been shown to indirectly target the inflammasome. In particular, sodium–glucose cotransporter-2 inhibitors and GLP-1 receptor agonists directly reduce CV risk [106, 107]. Several studies which evaluated the effects of these dugs on dysfunctional PVAT and diabetic mouse models have indicated that they may indirectly inhibit the NLRP3 inflammasome [108, 109]. Currently, there is growing interest in the evaluation of the ability of these treatments to reduce inflammation in clinical trials.

Other direct or indirect inhibitors of the NLRP3 inflammasome are currently under development for non-CVD inflammatory diseases, and their potential benefit in ASCVD risk reduction will be explored in the near future.

Corticosteroids

Corticosteroids are widely known as powerful anti-inflammatory agents and have been used to treat several autoimmune diseases. However, long-term use, especially at high doses, is associated with an increase in other ASCVD risk factors (hypertension, T2D, obesity, insulin resistance, and dyslipidemia) and a significant increase in ASCVD risk [110]. More recently, meta-analyses have shown that even low-dose corticosteroid treatment can increase CV events in RA. Therefore, corticosteroid appears to be of little use in treating low-grade inflammation for the prevention of ASCVD [111].

The mechanism of action and trials for all drugs discussed herein are summarized in Table 1.

Table 1 Summary table of current drugs in trials or development for treatment of cardiovascular inflammation
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Current Challenges and Future Direction

The individualized approach in immunotherapy has a promising future in ASCVD treatment. A major challenge is in tailoring therapy based on distinct characteristics of patients, such as immune profile or genetic background, and by correctly identifying the clinical ASCVD phenotype.

Identifying individuals with high residual cholesterol continues to be the main focus in ASCVD risk assessment, following the “lower-the-better” concept for LDL-C. This has resulted in the development of several lipid-lowering therapies that further reduce ASCVD risk when added to treatment with a statin. The detection of an inflammatory phenotype (measured mainly by hsCRP) and data collected over the past 30 years on the fundamental role inflammation plays in atherosclerosis provide the basis for therapies targeting residual inflammatory risk. Moreover, the impact of inflammation on the progression of ASCVD events no longer remains a theory insofar as the CANTOS trial showed proof of the clinical impact of residual inflammatory risk.

While defining residual inflammatory risk continues to be based solely on hsCRP levels (as a surrogate marker of the IL-1β-to-IL-6-to-hsCRP pathway), it is now clear that treatment that achieves lowering of hsCRP < 2 mg/L results in RRR for MACE (as shown in Fig. 2). Nevertheless, there are discrepancies in the degree of hsCRP-lowering potential and RRR between different drugs as shown in Fig. 3. It appears that a residual inflammatory phenotype, irrespective of LDL-C, confers a higher risk for MACE [112]. The hypothesis that other biomarkers may further aid in clinical decision-making is based on the evidence that incorporating IL-6 as a measurement of response to therapy may play a role in tailoring immunotherapy to reduce residual risk. The variable disease settings and inclusion criteria among clinical trials targeting inflammation in ASCVD challenges the implementation in a clinical setting. The small number of ongoing trials in this setting is surprising, given the success of immunotherapy in atherosclerosis. Currently, inflammation can be detected via imaging modalities, but its application in drug development is still limited.

Fig. 2
figure 2

Percentage reduction in hsCRP with different therapies. hsCRP high-sensitivity C-reactive protein, PCSK9i proprotein convertase subtilisin/kexin type 9 inhibitor. This original figure was created using GraphPad Prism

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Fig. 3
figure 3

Relative risk reduction for major cardiovascular events in trials in a population who achieved high-sensitivity C-reactive protein (hsCRP) < 2 mg/L. This original figure was created using GraphPad Prism

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One essential need is the early identification of patients who would most benefit from a specific treatment. This was clearly demonstrated in the CANTOS trial, as those who did not show a decrease in IL-6 did not have ASCVD benefits, compared to those who showed a decrease. Perhaps those who do not show a decrease in IL-6 may benefit more from other therapies, such as tocilizumab. Figure 4 presents a potential guideline for clinical decision-making and precision medicine when considering treatment for inflammatory states that may increase ASCVD risk.

Fig. 4
figure 4

Potential assessment of secondary prevention phenotype. Dotted lines indicate unanswered questions. LDL-C low-density lipoprotein cholesterol, hsCRP high-sensitivity C-reactive protein, PCSK9i proprotein convertase subtilisin/kexin type 9 inhibitor, BA bempedoic acid, IL-6 interleukin 6. This original figure was created using BioRender

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As we gain further information and insights, the clinical picture may become clearer. Nevertheless, several questions arise from the clinical trials. Even though colchicine has been approved for secondary prevention, we still do not know exactly when to initiate or how long to maintain treatment. It is yet to be determined whether hsCRP is a useful tool for decisions regarding add-on treatment or use as a therapeutic goal, and if so, what target level. Would adding other biomarkers help to make better decisions regarding the benefit of the therapy? Ridker et al. [112] showed that identifying inflammatory residual risk (hsCRP > 2 mg/dL), independently of LDL-C, was associated with an 18–20% greater risk of MACE, 83–91% greater risk of CV mortality, and 75–83% greater risk of all-cause mortality. This post hoc analysis implies that further lowering of LDL-C to < 70 mg/dL may be beneficial. Although the American College of Cardiology/American Heart Association/multi-society cholesterol guidelines [113] still endorse an LDL-C threshold of ≥ 70 mg/dL for adding non-statin treatment in secondary prevention, other guidelines such as the 2019 European Society of Cardiology/European Atherosclerosis Society Guidelines for Management of Dyslipidemia [114] go further and recommend < 55 mg/dL and even < 40 mg/dL for patients with multiple CV events within 2 years despite optimal statin therapy. However, post hoc analysis of the FOURIER trial showed that there is continued effectiveness, even with LDL-C < 40 mg/dL, for patients with high ASCVD risk, further validating the European guidelines [115]. Thus, as shown in Fig. 4, both lipid residual risk and inflammatory risk should be addressed in all high-risk patients with ASCVD.

Additional challenges that should be carefully considered in implementing anti-inflammatory therapies include the secondary effects of such therapies. For example, in the CANTOS trial, even though the use of canakinumab was associated with a lower incidence of lung cancer compared to placebo [116], it was associated with a small but statistically significant increase in non-opportunistic fatal infection. Another example is the CIRT trial, where the use of methotrexate was linked to a small increase in the incidence of skin cancer [96]. Finally, all of these trials are focused on secondary and not primary prevention. Identifying an inflammatory phenotype in primary prevention could help target immunotherapy in those at higher risk for ASCVD events.

Conclusion

Despite achieving low levels of LDL-C through guideline-proven therapies, residual risk from low chronic inflammation continues to drive cardiovascular events and therefore should potentially be targeted. The abovementioned studies provide proof of principle for reducing ASCVD risk by treating inflammation. New drugs targeting either interleukins or the NLRP3 inflammasome may be the most promising in managing recurrent ASCVD events. The future of these clinical trials and the implementation of new biomarkers may improve the scope of cardiovascular drug development and precision medicine approaches. Currently, however, there are many more questions than answers on how to implement these new drugs in cardiovascular medicine.

Although the role of inflammation is clear and several biomarkers are available, hsCRP is the most commonly studied and used clinically. Based on the reviewed literature, identifying a low-grade inflammation phenotype using hsCRP may be a reasonable tool for reclassifying ASCVD risk. Growing evidence supports its use to tailor therapy, but research is ongoing to determine whether an alternative anti-inflammatory biomarker may be superior. The use of imaging in inflammation detection and response to therapy, though promising, still requires further investigation. Development of new lipid- and inflammation-targeted therapies should rely on the concept of an individualized, tailored approach for efficiently addressing all forms of residual risk in ASCVD.

This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.