The Role of Downstream Kynurenine Pathway Metabolites in the Modulation of Cardiovascular Disease Development in Chronic Kidney Disease

Abstract

Cardiovascular diseases (CVDs) represent a significant and escalating health challenge in patients with chronic kidney disease (CKD). In this population, the cardiovascular incidents are markedly elevated, and CVD represents the leading cause of mortality. The pathogenesis of CVD in the course of CKD is multifactorial, and some evidence indicates that disturbances in the kynurenine pathway (KP), the major route of tryptophan metabolism, can also play a significant role in this process. The enhanced activation of the KP and reduced clearance of its metabolites contribute to their accumulation during CKD progression, potentially exacerbating cardiovascular risk. Few data suggest that certain downstream KP metabolites, including 3-hydroxykynurenine (3-HKYN), quinolinic acid (QUIN), and anthranilic acid (AA), are associated with established CVD risk factors, while others, such as 3-hydroxyanthranilic acid (3-HAA) and kynurenic acid (KYNA), exhibit more complex and ambiguous effects, with potential protective actions on the cardiovascular system. The aim of this review is to summarize current knowledge of the roles of individual downstream kynurenine (KYN) metabolites in the development of CVD in the CKD population. Since for some kynurenines there are only isolated or ambiguous reports in this field, the review has been completed with data on the contribution of downstream KYN metabolites in the development of CVD in the general population and in experimental models.

Introduction

Cardiovascular diseases (CVDs) represent the leading cause of death worldwide, and approximately 85% of cases result from atherosclerotic disorders like coronary artery disease, peripheral artery disease, and stroke. ¹ Recognized CVD risk factors include traditional factors, such as dyslipidemia, hypertension, diabetes mellitus, and physical inactivity, as well as non-traditional ones, which include inflammation, oxidative stress (OX), and endothelial dysfunction.^2,3 In addition, the presence of CKD is recognized as a significant risk factor for CVD. ⁴ In this patient population, the leading cause of mortality is not end-stage renal disease (ESRD) but CVD. ⁵ The high risk of CVD affects patients with CKD even in the early stages of the disease, and the risk increases progressively with declining kidney function, reaching over 75% in the advanced stage, which is a serious problem due to the increasing prevalence of CKD.^5-7 It is estimated that over 800 million individuals worldwide are affected by CKD, and the projections indicate that by 2040, CKD will become the fifth leading cause of death globally. ⁸ The development of CVD in the course of CKD is a complex process involving multiple mechanisms (Figure 1), and the impact of additional factors continues to be examined. ⁷

Figure 1.

Mechanisms leading to the development of CVD in the course of CKD.

Source: Figure was created using Microsoft PowerPoint.

Abbreviations: Ca, calcium; CCL-2, monocyte chemoattractant protein-1; ICAM-1, intercellular adhesion molecule-1; IL-1, interleukin-1; IL-8, interleukin-8; NO, nitric oxide; Pi, phosphorus; ROS, reactive oxygen species; TF, tissue factor; VCAM-1, vascular cell adhesion molecule-1; vWF, von Willebrand factor.

Endothelial Cell Dysfunction

The functional endothelium regulates vascular permeability, vascular tone, and maintains an antithrombotic and anti-inflammatory state. Patients with CKD develop endothelial cell dysfunction, which plays a key role in the pathogenesis of CVD. Endothelial dysfunction initiates the atherosclerotic process, in which the chronic inflammation associated with CKD activates endothelial cells to produce pro-inflammatory cytokines, such as interleukin-1 (IL-1) and IL-8, chemokines such as monocyte chemoattractant protein-1 (MCP-1/CCL2), and endothelial–leukocyte adhesion molecules such as VCAM-1 (vascular cell adhesion molecule-1), ICAM-1 (intercellular adhesion molecule-1), and P-selectin. These changes promote leukocyte adhesion and transmigration into the vessel wall, where, together with accumulated lipids, they contribute to the formation of atherosclerotic plaque. Moreover, reduced production of nitric oxide (NO) and prostacyclin is observed, which are crucial for inhibiting platelet activation and promoting vascular relaxation.^9,10

Inflammation and Oxidative Stress

CKD is characterized by a chronic low-grade inflammatory state. ¹¹ The adhesion of macrophages to vascular endothelial cells causes their damage, which may contribute to the development of atherosclerosis. Furthermore, even a slight increase in serum cytokine concentrations and other pro-inflammatory factor levels can predict the risk of CVD. ¹² OX in uremia may result from increased reactive oxygen species (ROS) production, as evidenced by enhanced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and expression observed in patients and experimental models with renal insufficiency, even at early stages of CKD. In addition to an increased generation of ROS, the pathogenesis of CKD is associated with insufficient efficiency of antioxidant systems. ¹¹ ROS reduces NO bioavailability, oxidizes LDL particles, and activates transcription factors such as nuclear factor-κB (NF-κB), thereby promoting the expression of proinflammatory cytokines. This creates a vicious cycle in which OX amplifies inflammation, while inflammation further enhances ROS generation, contributing to endothelial dysfunction and disease progression. Excessive ROS production in the course of CKD leads to enhanced OX, which contributes to the pathogenesis of CVDs such as atherosclerosis, hypertension, and aortic aneurysms. ¹³

Atherosclerosis

Dysfunction of vascular endothelial cells, OX, and inflammation represent key factors leading to the development and progression of atherosclerosis, which is very prevalent in CKD patients and is the main cause of CVD. ¹⁴ The higher cardiovascular risk in the CKD population is also driven by the increased prevalence of both traditional and uremia-related atherosclerotic risk factors, including dyslipidemia, anemia, hemostatic abnormalities, and hyperhomocysteinemia. ¹⁵ The development of atherosclerosis initiates with the deposition of low-density lipoproteins (LDL) in medium- and large-sized arteries inducing a chronic inflammatory response. ¹⁶ Within the progression of an atherosclerotic plaque, macrophages differentiate into a foam cells phenotype as a result of the accumulation of oxidized LDL (oxLDL). ¹⁷

Disturbances in the Hemostasis System

The chronic inflammatory state, endothelial dysfunction, and impaired clearance of prothrombotic factors contribute to hemostatic disturbances. Endothelial injury in CKD leads to an increased expression of tissue factor (TF), a key initiator of coagulation, and elevated secretion of von Willebrand factor (vWF) involved in platelet adhesion and aggregation. The expression of endothelial thrombomodulin (TM) decreases, resulting in reduced activity of protein C and thereby impairing anticoagulant mechanisms.^9,10 Additionally, enhanced hydrostatic forces associated with CKD contribute to the degradation of the endothelial glycocalyx, which further promotes activation of the coagulation cascade and impairs the anticoagulant properties of the endothelium. ¹⁸ The predominance of a prothrombotic state in CKD consequently results in an increased risk of atherothrombosis. ¹⁹

Alterations of Bone Mineral Metabolism

Disturbances in the homeostasis of mineral metabolism and high concentration of inorganic phosphorus (Pi) and hypercalcemia are considered major determinants in the progression of vascular calcification (VC) in CKD patients.^20,21 Intimal calcification is associated with atherosclerotic plaque burden and may contribute to plaque destabilization and rupture. It is recognized as a robust predictor of cardiovascular events and mortality. Medial arterial calcification leads to increased arterial stiffness, elevated pulse wave velocity, and the development of left ventricular hypertrophy, potentially resulting in heart failure. Both intimal and medial calcification are significant contributors to elevated cardiovascular mortality in patients with CKD. ²² Elevated serum calcium (Ca) and Pi levels promote mineralization in vascular smooth muscle cells (VSMCs), whereas Pi complexes first activate signaling pathways that precede calcification.^23,24 Disturbed Ca and Pi metabolism in uremia is often accompanied by dysregulation of calciotropic hormones, like parathyroid hormone (PTH) and vitamin D. High PTH levels implicate in rapid bone turnover, which is associated with Ca and Pi release into circulation. In contrast, suppressed PTH can produce an adynamic/low turnover-bone disease, which is often associated with the presence of cardiovascular calcifications. ²⁵

Kynurenine Pathway

The kynurenine pathway (KP) is the main route of tryptophan (TRP) metabolism, leading to the generation of kynurenine (KYN) and its metabolic intermediaries, terminating in the generation of nicotinamide adenine dinucleotide (NAD+), an important cellular energy source. ²⁶ KP starts with the conversion of TRP to KYN by the enzymes indoleamine 2,3-dioxygenase 1 (IDO-1), indoleamine 2,3-dioxygenase 2 (IDO-2), and tryptophan 2,3-dioxygenase (TDO). ²⁷ Subsequently, KYN is metabolized through 3 different branches, which depend on the expression and activity of enzymes in the particular organ.^28,29 Kynurenine 3-monooxygenase (KMO), catalyzes the formation of 3-hydroxykynurenine (3-HKYN). The further metabolism of 3-HKYN involves kynureninase (KYNU), which catalyzes the hydrolysis reaction to 3-hydroxyanthranilic acid (3-HAA). The 3-HAA is metabolized mainly through the action of 3-hydroxyanthranilate-3,4-dioxygenase (HAO), an enzyme with the highest metabolic activity in this pathway. HAO catalyzes the conversion of 3-HAA to quinolinic acid (QUIN) and ultimately to NAD+. Alternatively, KYN can be metabolized by KYNU and kynurenine aminotransferase (KAT). KYNU catalyzes the metabolism of KYN to anthranilic acid (AA). KAT catalyzes the transamination reaction of KYN to kynurenic acid (KYNA). Metabolites produced in the KP are involved in several physiological processes, like immune regulation, inflammation, and metabolism. ³⁰ The toxic effects of kynurenines are associated with their increased concentrations and ability to promote proinflammatory conditions, OX, and apoptosis. ³¹ Moreover, kynurenines can modulate the activity of various cellular signaling pathways through activation of the aryl hydrocarbon receptor (AhR), leading to the interruption of homeostasis of different organs. ³²

The AhR is a ligand-dependent transcription factor with a basic helix-loop-helix (bHLH) structure. Activation of AhR induces or represses the expression of many genes and leads to the production of a diverse spectrum of biological and toxic effects across a wide range of species and tissues. ³³ The inactive form of the AhR is cytoplasmic, and it forms a complex with several chaperone proteins, including Heat Shock Protein 90 (HSP90) and an X-associated protein 2 (XAP2). These proteins maintain the proper folding of AhR, allow an accurate recognition of the ligand by the receptor, and indirectly ensure an effective transcriptional effect. ³⁴ In the classic (canonical) mechanism, after cytosolic AhR binds to a ligand, AhR changes its conformation and moves to the nucleus, where it dimerizes with AhR nuclear transporter (ARNT). The AhR–ARNT complex binds to specific DNA sequences (termed DRE or XRE for dioxin- or xenobiotic-responsive element) and induces transcription of AhR-responsive genes, such as cytochrome (CYP) 1A1, CYP1A2, CYP1B1, and AhR repressor (AhRR), which is the inhibitor of AhR.^35,36 AhR activation is linked to a wide range of toxic responses, including carcinogenic and teratogenic effects, immunosuppression, inflammation, impaired fertility, and hepatotoxicity. For many years, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and chlorinated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), were considered the only agonists of the AhR, mediating its toxic effect. However, AhR can also be activated by several endogenous ligands, among which some kynurenines, such as KYN, KYNA, and QUIN, are also mentioned. ³⁷ A graphical presentation of the interactions between KP metabolites and AhR activation is shown in Figure 2.

Figure 2.

The activation of the kynurenine pathway and its interaction with AhR system.

Abbreviations: AA: anthranilic acid; AhR: aryl hydrocarbon receptor; AHRR: aryl hydrocarbon receptor repressor; ARNT: aryl hydrocarbon receptor nuclear transporter; CYP1A1: cytochrome P450; family 1; member 1A; CYP1A2: cytochrome P450; family 1; member 2A; CYP1B1: cytochrome P450; family 1; sub family B; 3-HAA: 3-hydroxyanthranilic acid; HAO: 3-hydroxyanthranilate-3,4-dioxygenase; 3-HKYN: 3-hydroxykynurenine; HSP90: Heat Shock Protein 90; IDO: indoleamine 2,3-dioxygenase; QUIN: quinolinic acid; KAT: kynurenine aminotransferase; KMO: kynurenine 3-monooxygenase; KYN: kynurenine; KYNA: kynurenic acid; KYNU: kynureninase; TDO: tryptophan 2,3-dioxygenase; TRP: tryptophan; XAP2: X-associated protein 2. Figure was created using Microsoft PowerPoint.

On the other hand, AhR is postulated to play a critical role in maintaining physiological homeostasis. Currently, a wide range of exogenous compounds are known to bind to this receptor, including flavonoids such as resveratrol, quercetin, curcumin, and microbial metabolites, which can activate AhR. ³⁸ The ligand-AhR complex is involved in modulating inflammatory responses, cell proliferation, and immune system activity.^39,40 Studies have shown that activation of the AhR by KYN can lead to the differentiation of T lymphocytes into regulatory T cells (Tregs), ⁴¹ and contribute to brain damage after stroke. ⁴⁰

Disturbances of the KP in the course of CKD

During CKD, disturbances in TRP metabolism via KP and the accumulation of its toxic metabolites occur in the body. ³² Debnath et al ⁴² showed that plasma TRP levels were nearly 60% lower at stage 5 CKD compared with stage 1 CKD. The main mechanism of decreased TRP concentration springs from the chronic inflammatory state associated with CKD. Pro-inflammatory factors, essentially the cytokine interferon-γ (IFN-γ), stimulate the activity of the IDO, leading to increased conversion of TRP into KYN, which is reflected by the KYN/TRP ratio. ⁴³ Additionally, IDO activity increases with CKD severity and is correlated with markers of kidney function, such as creatinine and creatinine clearance.^42,44 Moreover, studies conducted by Saito et al ⁴⁵ and Pawlak et al ²⁸ showed that TDO activity is elevated in the liver of rats with experimental renal insufficiency compared to the control group. Due to enhanced TRP metabolism in the course of CKD, a significant increase in KP metabolites concentration has been observed. The increase may also be caused by impaired vitamin B6 absorption in this population. Vitamin B6 serves as a coenzyme for KMO and KYNU, the enzymes that catalyze the conversion of KYN into its downstream metabolites. The activity of these KYN metabolizing enzymes will be significantly reduced, leading to higher KYN levels.^45,46 TRP and metabolites of the KP are excreted via the kidneys, and their impaired function can cause accumulation of KP metabolites. ⁴⁴ A recently conducted large prospective cohort study of patients with CKD demonstrated that elevated KYN concentrations are associated with an increased incidence of both fatal and non-fatal cardiovascular events. Moreover, KYN was identified as an independent factor strongly linked to non-atherosclerotic cardiovascular events in this patient population. Interestingly, higher serum-free KYN levels were also significantly associated with an increased risk of cardiovascular mortality, whereas no similar association was observed for non-cardiovascular or all-cause mortality. ⁴⁷ Moreover, KYN is considered to potentially serve as a marker of CVD. ²⁹

Understanding the influence of KP seems crucial for explaining the mechanisms leading to the main cause of mortality among CKD patients, namely CVD. Currently, there is a lack of data that summarizes the contribution of individual KP metabolites in the pathogenesis of CVD in the course of CKD. This paper aims to analyze and compare the involvement of specific downstream metabolites of KYN in the pathogenesis of CVD in the CKD population. Since for some metabolites of the KP there are only isolated or ambiguous reports in this regard, this review also considers the role of the kynurenines in the development of CVD in the general population and in experimental models.

A literature search was performed in the PubMed database for articles published between 1995 and 2025 using combinations of keywords including: “kynurenine pathway”, “3-hydroxykynurenine,” “3-hydroxyanthranilic acid,” “quinolinic acid,” “kynurenic acid,” “anthranilic acid,” “cardiovascular disease,” and “chronic kidney disease.”

The Role of Downstream Metabolites of KYN in CVD Development

3-HKYN

3-HKYN in Human Studies

A cohort study, conducted in the general population, showed that 3-HKYN is associated with CVD mortality. Moreover, CRP levels, a well-established predictor of CVD outcomes, were also positively correlated with this metabolite. ⁴⁸ Another prospective study demonstrated that elevated 3-HKYN concentrations in the elderly population are associated with an increased risk of first acute coronary events, such as unstable angina pectoris, acute myocardial infarction, or sudden death. ⁴³

3-HKYN in In Vitro Studies

The exposure of human aortic endothelial cells (HAECs) to 3-HKYN markedly increased the expression of key pro-apoptotic factors, including poly (ADP-ribose) polymerase (PARP), caspase-7, caspase-3, and intensified terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. In contrast to this, the application of an IDO or a KMO inhibitor suppressed the upregulation of these apoptotic markers. It has been shown that 3-HKYN promotes endothelial cell apoptosis by increasing NADPH oxidase activation, thereby inducing ROS production and accelerating the release of mitochondrial cytochrome c, which in turn drives apoptotic cell death of endothelial cells. ⁴⁹

Taken together, 3-HKYN is associated with an increased risk of acute coronary events and higher cardiovascular mortality. It is also associated with inflammation, enhances OX, and exerts pro-poptotic effects on endothelial cells. The above data support an adverse impact of 3-HKYN on the cardiovascular system.

3-HKYN in CKD

Studies on CKD patients conducted by our team^3,50-52 indicated that increased levels of 3-HKYN positively correlate with non-traditional cardiovascular risk factors in this population. OX is a key contributor to endothelial dysfunction and participates in all phases of plaque development and progression. It has been shown that in ESRD patients, plasma 3-HKYN concentrations were positively associated with OX markers, including Cu/Zn superoxide dismutase (Cu/Zn SOD) and malondialdehyde (MDA).^3,50 3-HKYN was also positively correlated with endothelial dysfunction markers, such as TM, sICAM-1, sVCAM-1, and inflammatory indicator – hsCRP (high sensitive C-reactive protein).^3,50,51 3-HKYN contributes to disorders in the hemostasis system. It has been demonstrated that plasma concentrations of TF were significantly elevated in patients with CKD compared to healthy controls. Moreover, the TF/tissue factor pathway inhibitor (TFPI) system was positively correlated with elevated 3-HKYN levels. ⁵² The increased levels of vWF, which mediates platelet adhesion and is involved in clot formation, have been linked to an elevated risk of cardiovascular events such as coronary artery disease and stroke.^53,54 In the study performed on patients with ESRD, the elevated vWF levels were positively correlated with 3-HKYN. ³ Another study demonstrated that intima-media thickness (IMT), an early marker of systemic atherosclerosis, was positively related to the concentration of 3-HKYN. ⁵¹ Furthermore, it has been shown that in CKD patients, the levels of 3-HKYN were associated with the increased risk of CVD occurrence. ⁵⁰

The above results indicate that 3-HKYN is associated with increased OX, endothelial dysfunction, and inflammation, and may contribute to the increased risk of thrombotic complications. To sum up, 3-HKYN is also linked to the development of atherosclerosis and an increased risk of CVD in CKD patients.

3-HAA

Literature data indicate that 3-HAA has antioxidant, immune regulatory, and NO inhibitory activities.^55,56 As a highly reactive compound, 3-HAA modified the local redox environment, which accounts for its toxic effects on some cell types. However, whether 3-HAA can be pro- or anti-oxidant in any one situation will depend on the local redox conditions. ⁵⁷

3-HAA in In Vitro Studies

Oh et al ⁵⁵ show for the first time that exogenous 3-HAA is able to suppress inducible nitric oxide synthase (iNOS) expression by enhancing the heme oxygenase-1 (HO-1) gene expression in murine macrophages stimulated with interferon (IFN)-γ and lipopolysaccharide (LPS). Pae et al ⁵⁸ confirmed this observation using human umbilical vein endothelial cells (HUVECs). They showed that exposure of HUVECs to 3-HAA exerts a protective effect on the vascular endothelium by activation of the NF-E2 related factor 2/antioxidant response element (Nrf2/ARE) signaling pathway, which subsequently leads to the transcription of the heme oxygenase-1 (HO-1) gene. The increase in HO-1 activity inhibits the activation of transcriptional NF-κB and the expression of pro-inflammatory genes, like MCP-1 and VCAM-1, both of which are abundantly expressed in atherosclerotic plaques, promoting monocyte recruitment and adhesion to the endothelium, and thereby initiating the atherogenic process. ⁵⁸ It has also been shown that 3-HAA possesses anti-inflammatory and immunomodulatory activities – it inhibited LPS-induced inflammatory mediators and NF-κB activity in macrophages ⁵⁹ and suppressed T cells response by inhibiting dendritic cells activation. ⁶⁰ Berg et al ⁶¹ confirmed these anti-inflammatory effects showing that macrophages treated with 3-HAA had decreased IL-1β and procaspase-1 levels. Additionally, in the human hepatoma cell line HepG2, 3-HAA treatment reduced sterol response element binding protein-2 (SREBP-2) expression and decreased apolipoprotein B (apoB) levels by approximately 47%. ⁶¹ Furthermore, Wang et al ⁶² showed that IDO knockdown in human aortic smooth muscle cells (HASMCs) treated with IFN-γ resulted in inhibition of MMP-2 expression. To identify which kynurenine metabolite was responsible for MMP-2 upregulation, the researchers incubated HASMCs with the main exogenous metabolites of the pathway. A significant increase in MMP-2 expression and activity was observed only in response to 3-HAA. ⁶²

3-HAA in Animal Models

Numerous in vivo studies demonstrate the anti-atherosclerotic effects of 3-HAA. Zhang et al ¹⁷ found that 3-HAA exerts its beneficial effects by modifying lipid metabolism and reducing inflammation. In Ldlr−/− mice fed a high-fat diet, treatment with 3-HAA significantly decreased oxidized low-density lipoprotein (oxLDL) uptake by macrophages compared with control mice. Administration of 3-HAA also lowered plasma triglycerides and chylomicron/very low-density lipoprotein (VLDL) fraction, and significantly increased HDL levels. Moreover, Thomas et al ⁵⁶ demonstrated that 3-HAA is a highly efficient co-antioxidant for plasma lipid peroxidation by its ability to interact with α-tocopherol in lipoproteins. This mechanism may represent a local extracellular antioxidant defense against LDL oxidation and atherosclerosis progression.

Zhang et al ¹⁷ observed an 80% reduction of atherosclerotic lesion area in the aortic arch and a 40% reduction in the aortic root in mice treated with 3-HAA compared to the control group. The authors further reported a significant decrease in CD4+ T cell infiltration, which plays a role in macrophage activation and the promotion of inflammation, as well as reduced expression of MHC class II protein I-A, which is induced by IFN-γ, indicating reduced vascular inflammation in these mice. ¹⁷ However, according to Metghalchi et al, ⁶³ the administered dose of 3-HAA in the above experiment was supraphysiological, and the protective effects were attributed to the impact of 3-HAA on lowering cholesterol and triglycerides rather than to its immunomodulatory role.

Polyzos et al ¹⁶ conducted a study in which Apoe⁻/⁻ mice placed on a high-fat diet were given drinking water supplemented with an IDO-1 inhibitor. The authors observed that IDO inhibition led to an increase in atherosclerotic lesion size of approximately 58% in the aortic arch and 54% in the aortic root. In addition, aortic mRNA levels of pro-inflammatory genes, such as VCAM-1 and CCL2, were significantly higher, together with an increase in CD68⁺ macrophage accumulation in the plaque. Subsequently, the intraperitoneal administration of 3-HAA to these mice was able to reverse the acceleration of atherosclerosis. This effect involved reduced VCAM-1 expression in the atherosclerotic plaque and even in the smooth muscle cells (SMCs) of the tunica media and a reduction in CD68⁺ macrophages in the plaque. ¹⁶ Despite the comments of Metghalchi et al ⁶³ that pharmacological inhibition of IDO was incomplete, likely nonselective, and that the exacerbation of atherosclerosis may be due to an unfavorable change in the lipid profile induced by the IDO inhibitor, the results of Polyzos et al ¹⁶ suggested the protective role of 3-HAA against atherosclerosis progression.

In line with the above results, Berg et al ⁶¹ demonstrated the anti-atherosclerotic effects of 3-HAA. The researchers observed that Ldlr−/− mice fed a western diet and receiving an HAO inhibitor, which increases endogenous 3-HAA levels, showed approximately a 50% reduction in the atherosclerotic lesion area in the aortic arch compared to the control animals. Simultaneously, the reductions in total plasma cholesterol and triglycerides, as well as a decrease in VLDL production, have been shown. This effect was comparable to that obtained after administration of exogenous 3-HAA. ⁶¹

Experimental evidence indicates that activation of the inflammasome and secretion of interleukin-1 beta (IL-1β) are key drivers of atherosclerosis. The increase in 3-HAA concentration induced by HAO inhibition in mice led to a significant reduction in procaspase-1 levels and a trend decrease in caspase-1 levels in aortic protein extracts, indicating suppression of NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome activity. ⁶¹ The authors additionally noted that elevated 3-HAA levels were associated with decreased hepatic SREBP-2 expression, which is the key regulator of cholesterol synthesis in the liver. ⁶¹

Whereas, Wang et al ⁶² observed that 3-HAA plays an important role in the formation of abdominal aortic aneurysm (AAA). The incidence of angiotensin II (AngII) induced AAA was 73% in Apoe−/− mice, whereas in Apoe−/−/IDO−/− mice it developed in only 15% of cases, indicating a significant contribution of KP activation in VSMCs to AAA formation. The authors reported that Apoe−/− mice infused with AngII exhibited a marked increase in MMP-2 expression and activity, which is one of the key factors driving AAA development and progression, while this increase was not observed in Apoe−/−/IDO−/− mice. ⁶² Moreover, injection of 3-HAA into Apoe−/− or Apoe−/−/IDO−/− mice resulted in increased MMP-2 expression and activity. Moreover, reduction of endogenous 3-HAA levels through KMO inhibition in Apoe−/− mice diminished AAA formation as well as MMP-2 expression and activity.

3-HAA in Human Studies

Finally, analysis of human AAA samples revealed significantly elevated levels of MMP-2, IDO1, and KMO, and stronger anti-3-HAA staining compared to nonaneurysmal aortic samples. ⁶²

In sum, the majority of the presented studies indicated that 3-HAA exhibits anti-atherosclerotic effects in vitro, and administration of 3-HAA or an HAO inhibitor in animal models of atherosclerosis had beneficial effects on the lipid profile, inflammation, and reduced the atherosclerotic lesion area. However, in vitro and in vivo studies in mice and patients indicate a potential involvement of 3-HAA in the formation of AAA.

3-HAA in CKD

There are only isolated reports linking 3-HAA with CVD in this population. Chemotactic cytokines (CC-chemokines) induce the directed migration of leukocytes into tissues undergoing inflammation, and their elevated levels are observed in inflammatory diseases. In patients with CKD, CCL2 and macrophage inflammatory protein-1β (CCL4) were found to be associated with carotid atherosclerosis and CVD prevalence.^64-66 A previous study of our team ⁶⁷ showed that increased 3-HAA levels were independently associated with increased concentrations of CCL2 and CCL4 in the plasma of CKD patients. Additionally, 3-HAA was positively correlated with Cu/Zn SOD. ⁶⁷ The association between 3-HAA and CC chemokines, which mediate leukocyte migration into atherosclerotic lesions, may constitute a mechanistic contributor to plaque formation in the course of CKD. However, the mechanism by which 3-HAA might increase chemokine levels was unknown at that time. ⁶⁷

Therefore, in the setting of CKD, there is no clear answer as to how an increase in 3-HAA concentration may translate into cardiovascular health

In summary, the conflicting findings regarding the role of 3-HAA in CVD may be attributed to disease specific mechanisms, the surrounding redox conditions, and methodological variability across experimental studies. In atherosclerosis, 3-HAA appears to exert mainly protective effects through modulation of the hepatic SREBP-2/lipoprotein pathway, reduction of circulating lipid levels, and suppression of NLRP3 inflammasome activation in macrophages. ⁶¹ Moreover, 3-HAA functions as an effective co-antioxidant for α-tocopherol, thereby limiting LDL oxidation. ⁵⁶ In contrast, in AAA, 3-HAA may promote vascular injury by increasing MMP-2 expression and activity in vascular smooth muscle cells via NF-κB signaling, contributing to degradation of the elastic lamina and aneurysm progression. ⁶² In addition, some beneficial effects observed in animal studies may be related to supraphophysiological doses influencing lipid metabolism rather than direct immunomodulatory activity. In the uremic environment of CKD, the potential antioxidant effects of 3-HAA, reflected by its association with Cu/Zn SOD, may be limited by its association with the pro-inflammatory chemokines CCL2 and CCL4. This suggests that 3-HAA may also promote leukocyte recruitment and migration into the vascular wall, thereby contributing to plaque progression and reducing its overall beneficial effect in this population. ⁶⁷

QUIN

It is known that QUIN is a generator of ROS ⁶⁸ and plays a key role in the induction of atherosclerotic processes. ⁶⁹ QUIN enhances monocyte adhesion to endothelial cells and their translocation into the subendothelial region, where they differentiate into macrophages and accumulate lipids. This leads to the formation of foam cells, and their accumulation serves as an early marker of atherosclerotic lesion formation. ⁶⁹

QUIN in In Vitro Studies

Evidence from a study conducted on isolated rat thoracic aorta suggests that QUIN may exert a relaxant effect on blood vessels. In the study by Watts et al ⁷⁰ aortic rings with intact endogenous fat were contracted with phenylephrine and then incubated with downstream KYN metabolites. None of the investigated compounds induced vasorelaxation, whereas QUIN reduced contraction by nearly 80%. However, in the cleaned of fat thoracic aorta, QUIN did not reduce angiotensin II–induced contraction, which reveals that QUIN is not the cause of the reduction in maximum contraction. Furthermore, the 1 mM concentration used is likely to significantly exceed endogenous levels. ⁷⁰ Additionally, in vitro results may not fully reflect the effects of QUIN on VSMCs in vivo, and these metabolites may only be effective when acting synergistically.

QUIN in Human Studies

A study on atherosclerotic changes in the walls of aortic aneurysms indicates the involvement of QUIN in the development of advanced CVD. QUIN has been identified as one of the key metabolites, with its relative levels being more than 1.5-fold higher in aneurysmal lesions compared to early atherosclerotic lesions, where QUIN was almost undetectable. ⁷¹ Another study demonstrated significantly increased levels of the quinolinic branch enzymes -KMO and KYNU in atherosclerotic arteries compared with control arteries. Moreover, this branch shows a pro-inflammatory profile, as evidenced by the positive correlation of these enzymes with pro-inflammatory and metalloprotease genes. Additionally, KMO expression in atherosclerotic arterial tissue was found mainly in macrophages, whereas in control arteries, it was primarily expressed in SMCs, suggesting that inflammation plays a crucial role in the enhanced synthesis of QUIN. Importantly, KMO and KYNU expression levels were markedly upregulated in plaques derived from symptomatic patients than in those from asymptomatic individuals. Furthermore, a higher KP “deviation index,” which measures the activity of the quinolinic branch relative to the protective KYNA branch, significantly increased the risk of developing carotid disease-related symptoms. This suggests that enhanced activity of the quinolinic branch of the KP is strongly associated with atherosclerosis progression. ⁷²

Overall, the alteration of the KP toward the branch leading to QUIN production contributes to the development and instability of atherosclerotic plaques.

QUIN in CKD

The previous studies of our team^73,74 demonstrated that elevated plasma concentrations of QUIN in ESRD patients were positively correlated with the inflammatory marker – hsCRP and with OX markers – Cu/ZnSOD and MDA.^3,73 Furthermore, QUIN proved to be an independent factor significantly correlated with increased IMT, a recognized early marker of systemic atherosclerosis.^51,73,75 Additionally, it was observed that QUIN was positively associated with elevated concentrations of endothelial function biomarkers: TM, vWF, sICAM-1, sVCAM-1, and it was identified as an independent variable of increased TM levels in patients with ESRD.^5,51

The patients with ESRD exhibit a procoagulant state associated with elevated plasma concentrations of QUIN. In these patients, plasma levels of tissue factor (TF), a coagulation-initiating factor expressed or exposed at sites of vascular injury, and its inhibitor, TFPI, were significantly higher compared to healthy controls. The level of the hypercoagulability marker – prothrombin fragment 1 + 2 (F1 + 2) was also markedly increased and directly related to elevated QUIN concentrations. A positive association was observed between TF/TFPI system and QUIN, QUIN which was identified as an independent variable associated with the hypercoagulable state in ESRD patients. ⁷⁴ On the other hand, QUIN level was one of the independent variables significantly associated with the hyperfibrinolytic state, which is also causally related to the development of cardiovascular disorders in patients with CKD. ⁷⁶

QUIN appears to be also associated with vascular wall remodeling, regulated by the system of matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs). It has been shown that in patients with CKD undergoing chronic ambulatory peritoneal dialysis (CAPD), who also suffer from CVD, distinctly elevated concentrations of MMP-2 and its tissue inhibitor TIMP-2 are observed compared to patients without CVD. Both the level of MMP-2 and TIMP-2 were positively correlated with the QUIN concentration, and moreover, QUIN turned out to be a variable independently associated with the MMP-2 level. Additionally, OX marker – Cu/Zn SOD correlated positively with MMP-2/TIMP-2 system and QUIN in this group of patients. These results indicate that elevated QUIN concentration in CKD may be related to the intensified OX and unfavorable vascular wall remodeling that subsequently may increase the risk of CVD development. ⁷⁷

Benitez et al ⁷⁵ demonstrated the association of QUIN with increased aortic and coronary calcification in CKD patients. It was shown that QUIN is related to a shorter time to the occurrence of the first cardiovascular event, with a 1 µmol increase of QUIN concentration associated with a 28% higher risk. The authors also noticed that CKD patients with a history of CVD exhibited significantly higher QUIN levels compared to those without earlier cardiovascular events. ⁷⁵

Overall, current evidence indicates a link between increased QUIN levels and oxidative stress, inflammation, disturbations of the hemostatic system, endothelial dysfunction, aortic and coronary calcification, supporting the role of this KYN metabolite in atherosclerotic plaque formation and new CVD events in CKD patients. QUIN emerges as a promising candidate biomarker due to its established prognostic value. Prospective data indicate that each 1 µmol increase in its concentration is associated with a 28% higher risk of the first cardiovascular event. ⁷⁵ Furthermore, QUIN has been identified as an independent factor significantly correlated with increased IMT,^51,73,75 hypercoagulability, ⁷⁴ and MMP-2 ⁷⁷ levels, suggesting its pathological role persists independently of traditional risk factors. The feasibility of QUIN measurement in clinical research is demonstrated by its precise quantification in human plasma using targeted mass spectrometry (LC-MS). ⁷⁵ Finally, the reproducibility of these findings is evidenced by consistent results across independent cohorts, where elevated QUIN levels significantly predict both atherosclerosis and new cardiovascular events.^51,73,75

KYNA

It is well known that inflammation, immune activation, and OX play important roles in the pathogenesis of CVD.^78,79 So far, the connection of KYNA with inflammation, antioxidant activity, and the immune system modulation has been identified.^80-82 The immunomodulating and anti-inflammatory functions of KYNA enable its action in the formation of an immunosuppressive milieu via GPR35 and AhR-dependent signaling and reduction of inflammatory state.^83,84

KYNA in In Vitro Studies

Mechanistic studies at the cellular level predominantly highlight the anti-inflammatory and protective potential of KYNA. The release of TNF-α from leukocytes in response to the administration of LPS is attenuated in human peripheral blood mononuclear cells ⁸⁵ treated with KYNA. Moreover, KYNA reduces the secretion of the high mobility group box 1 (HMGB1) protein by human peripheral blood monocytes and the secretion of α-defensin by neutrophils. ⁸⁵ KYNA reduces the release of IL-4 by human natural killer T cells ⁸⁶ and suppresses the production of IL-1β and caspase-1 activation in macrophages by suppressing the activation of the NLRP3 inflammasome. ⁸⁷ These effects were mediated by the GPR35 receptor on these cells. AhR activation by KYNA can also lead to suppression of inflammatory responses and reduction in the amounts of proinflammatory cytokines, like IFN-γ, IL-6, TNF, and IL-17. ⁸⁸ KYNA has been found to counteract the harmful effects of homocysteine, which is associated with the development of atherosclerotic lesions and increases the risk of coronary heart disease and stroke, on endothelial cells in vitro. ⁸⁹

Lima et al ⁹⁰ demonstrated that incubation of KYNA with rat aortas undergoing I/R, normalized contractile and relaxation responses that did not differ significantly from vessels not exposed to ischemia. KYNA treatment altered the protein expression profile in ischemic vessels, with enrichment in proteins associated with tissue repair. ⁹⁰

Another study related to I/R, conducted on cardiomyocytes, showed that KYNA treatment attenuates cell death by reducing OX. Moreover, KYNA exhibited a pronounced anti-apoptotic effect, demonstrated by a decreased ratio of cells presenting morphological features characteristic of irreversible apoptosis, a reduced incidence of cells with extensive DNA damage, suppression of apoptotic enzymes such as caspase-3 and -7, a decrease in the expression of the pro-apoptotic protein Bcl-2 associated X (BAX), and an increase in the expression of the anti-apoptotic protein B-cell lymphoma extra-large (Bcl-XL). ⁹¹

Bigelman et al ⁹² also demonstrated that KYNA may exert cardioprotective effects. The experiments on myoblasts exposed to ischemic conditions in vitro showed that KYNA improved cell viability, increased mitochondrial fraction and activity, and reduced OX.

In vitro studies on human macrophages demonstrated that KYNA exerts anti-inflammatory effects by reducing the production of cytokines, such as TNF and IL-6, as well as pro-inflammatory markers including CCR7, CXCL10, and CCL5. This mechanism proved to be dependent on the AhR, since its activation mimicked the effects of KYNA in suppressing pro-inflammatory markers, whereas receptor blockade abolished KYNA activity. ⁷²

However, Metghalchi et al ⁶³ observed that during incubation of murine macrophages with KYNA, there was a selective inhibition of extracellular signal-regulated kinase ½ (Erk1/2) phosphorylation, resulting in reduced IL-10 production, through activation of the cyclic adenosine monophosphate (cAMP) signaling pathway, which may contribute to the progression of atherosclerosis.

KYNA in Animal Models

In animal models, the elevated KYNA in the blood has been experimentally induced by various inflammatory stimuli.^83,84 Thus, in the conditions of chronic, low-grade inflammation, the production of KYNA can be induced. However, in many cases, it is unclear whether the overproduction of KYNA is a primary disorder or a compensatory response to inflammatory signals. ⁸¹

Olenchock et al ⁹³ demonstrated that KYNA is a key metabolite necessary and sufficient for remote cardioprotection in I/R injury. Exogenous administration of KYNA or its mimetic reduced infarct size in I/R models, whereas pharmacological inhibition of the KP in mice suppressed this cardioprotective effect. Ischemia results in the inactivation of Egln1, an oxygen sensor, leading to α-ketoglutarate (αKG) accumulation. Elevated αKG levels stimulate KAT activity, thereby promoting the generation of KYNA. ⁹³

A study conducted by Kamel et al ⁹⁴ confirmed that KYNA reduced cardiomyocyte mortality under I/R conditions. Moreover, in rats injected with KYNA before the induction of myocardial ischemia followed by reperfusion, the infarct size was significantly reduced compared to control animals. The authors showed that KYNA exerts cardioprotective effects by decreasing phosphorylation of the transcription factor FOXO3α via ERK1/2 and Akt, which is associated with antioxidant defense and mitophagy, thereby preventing its degradation. These effects were associated with enhanced expression of the antioxidant marker–superoxide dismutase 2 (SOD2), and a tendency toward increased PARK2 and decreased p62 levels, indicating activation of mitophagy. ⁹⁴

In the line with in vitro findings, administration of KYNA in an experimental murine model of MI, enhanced cardiac recovery, leading to increased ejection fraction, stroke volume, and cardiac output, and additionally reducing the amount of collagen deposition in the myocardium. ⁹²

In addition, in vivo experiments confirmed a protective, anti-inflammatory effect of KYNA. Intraperitoneal administration of KYNA to mice with peritoneal inflammation reduced neutrophil migration and accelerated the resolution of inflammation. ⁷²

In another experiment on mice fed a high-fat diet, the administration of KYNA (5 mg/kg per day) slowed the growth of body weight, daily energy intake, and reduced the coronary artery risk index and atherosclerosis index. In the serum of these animals, KYNA diminished triglyceride levels, slowed the increase in low-density lipoprotein cholesterol levels, and raised high-density lipoprotein cholesterol levels. ⁹⁵ Moreover, administration of KYNA with drinking water has been shown to have immunomodulatory ⁹⁶ and antioxidant ⁹⁷ effects in animal models, and decrease heart rate in spontaneously hypertensive rats. ⁹⁸ In animal models of stroke, the different effects of various metabolites of the KP have been demonstrated,^99,100 however, the KYNA analog displayed a neuroprotective effect. ¹⁰¹

However, not all animal studies demonstrated beneficial effects of KYNA. Metghalchi et al ⁶³ showed in vivo that KYNA after resupplementation in Ldlr−/−/Ido1−/− mice induced a significant increase in atherosclerotic plaque size without affecting cholesterol levels. The authors further showed that KYNA concentration in the plasma of mice correlated positively with plaque size and negatively with IL-10 expression. ⁶³

KYNA in Human Studies

Data regarding the role of KYNA in the development of CVD in humans are also inconsistent. The authors of a cohort study conducted on a group of patients at high risk of developing CVD described that higher concentrations of KYNA were associated with an increased risk of composite CVD, defined as stroke, myocardial infarction, or death from cardiovascular causes. Moreover, an even stronger association was observed for non-stroke events. ¹⁰² Both homocysteine and KYNA serum levels were positively associated with aortic stiffness in hypertensive or normotensive patients suffering atrial fibrillation.^103,104 The high KYNA levels in human unstable atheromatous plaques were observed, which were not detected in stable fibrous plaques. Among patients with acute MI, elevated KYNA levels were independently associated with increased risk of recurrent MI or death. ⁶³ Moreover, the higher concentrations of several metabolites of the KP, including KYNA, were described as factors correlating with higher mortality and post-stroke cognitive impairments.^105,106

On the other hand, the elevated levels of KYNA in patients with hyperhomocysteinemia protect them from homocysteine-mediated atherosclerotic complications. ¹⁵ Baumgartner et al ⁷² observed a decreased activity of the enzymes leading to the production of KYNA–kynurenine oxoglutarate aminotransferases 1 and 2 (KYAT1, KYAT2) in carotid plaques of patients with carotid stenosis compared with macroscopically disease-free control arteries and with other enzymes of the pathway. The expression of these enzymes was inversely correlated with pro-inflammatory and metalloprotease genes, whereas it was positively correlated with genes that play a protective role in atherosclerotic disease, including anti-inflammatory and plaque-stabilizing genes. Moreover, the activity of KYAT1 and KYAT2 was significantly lower in patients with cerebrovascular symptomatic disease and unstable plaques. The authors of this study concluded that patients with higher expression of enzymes leading to KYNA formation had a lower probability of developing symptoms related to carotid artery disease and a reduced risk of requiring endarterectomy surgery. ⁷²

In conclusion, in the majority of in vitro studies and in animal experiments, KYNA is considered to be a potential cardioprotective metabolite. In I/R models, KYNA contributes to reduced infarct size, enhanced cardiac recovery, reduced collagen deposition in myocardium, and enhances tissue repair. At the cellular level, KYNA attenuates cell death and exerts anti-apoptotic, anti-inflammatory, and anti-oxidative effects. These KYNA effects improve cardiac performance parameters, including ejection fraction, stroke volume, and cardiac output, and decrease the probability of developing symptoms related to carotid artery disease. However, there were suggestions that elevated concentrations of KYNA may contribute to the progression of atherosclerosis by inhibiting IL-10 production and are associated with unstable plaques and an increased risk of recurrent MI.

On the other hand, human studies have not clearly demonstrated a beneficial effect of increased KYNA levels on the cardiovascular system, which may, however, reflect a compensatory mechanism, through which an increase in the concentration of this KP metabolite plays a protective role in relation to cardiovascular complications.

KYNA in CKD in Human Studies

KYNA is one of the end products of the KP, which is not subject to further metabolism. In the CVD system, KYNA is synthesized by several cell types: endothelial cells, human peripheral blood mononuclear cells, red blood cells, and skeletal muscle cells.^107-109 To date, there are few studies showing the association of KYNA with the atherosclerotic process and the occurrence of CVD in the population of patients with CKD.^51,73,74,110

Disturbances in the hemostatic system may contribute to a higher risk of CVD. Several studies of our group showed that in ESRD patients, KYNA was positively correlated with coagulation initiator – TF, and its inhibitor – TFPI, the marker of oxidative stress – Cu/Zn SOD, and with endothelial dysfunction markers: TM, vWF, sVCAM-1, and sICAM-1, which are independently associated with atherosclerosis.^51,74,110 On the other hand, KYNA was positively associated with TFPI, PAI-1, and inversely associated with F1 + 2 levels in peritoneally dialyzed (PD) patients, which could suggest its role in attenuating hypercoagulation. Moreover, KYNA concentrations were significantly lower in PD patients who developed CVD compared to PD patients without CVD, and decreased KYNA levels were found to be one of the independent factors significantly associated with the prevalence of CVD in these patients. ¹¹¹ This study showed a relationship between KYNA deficiency, coagulation activation, and CVD prevalence in peritoneally dialyzed patients.

Another study ¹⁵ revealed that increased concentrations of KYNA observed in ESRD patients with hyperhomocysteinemia may provide a protective effect against atherosclerosis induced by homocysteine. The concentration of KYNA was 3 times higher in ESRD patients on CAPD with hyperhomocysteinemia compared to those with normal homocysteine levels, and they were significantly lower in subjects with CVD than in those without CVD in these studied groups. A positive relationship existed between homocysteine and KYNA in all CAPD patients, and in patients with both hyperhomocysteinemia and CVD presence. Importantly, the marker of atherosclerosis-IMT value-was associated with homocysteine but not with KYNA. ¹⁵

What is worth emphasizing, some studies indicate the protective effect of KYNA against the progression of kidney function impairment. Liu et al ¹¹² showed that high levels of KYNA proved to be associated with a slower decline of the estimated glomerular filtration rate (eGFR) in ESRD patients with type 2 diabetes mellitus.

KYNA in Animal Models of CKD

A protective role of KYNA toward kidney function has also been demonstrated in an animal model of renal ischemia-reperfusion (I/R) injury ¹¹³ and in rats with spontaneous hypertension. ¹¹⁴ Uremic toxins, which accumulate in the body during CKD progression, negatively impact multiple components of the cardiovascular system, ultimately leading to an intensified atherosclerotic process. Therefore, the protective effect of KYNA on renal function may also be considered as an indirect mechanism, contributing to the prevention of cardiovascular events in this population.

The above results suggest a rather protective effect of KYNA in relation to the atherosclerotic process in the vascular wall and CVD prevalence. The observed link between KYNA levels and endothelial dysfunction markers, hemostatic disturbances, and OX probably reflects the compensatory mechanism by which increased KYNA synthesis could prevent the damage of blood vessels in CKD. Decreased KYNA levels have been identified as an independent factor associated with the prevalence of CVD, specifically in PD patients, highlighting the context-dependent nature of its role as a potential biomarker.

In summary, these contrasting effects of KYNA may be explained by the activation of different signaling pathways depending on the biological context. In experimental conditions, often involving pharmacological doses, KYNA appears to exert protective effects through AhR-mediated immunomodulation, reducing pro-inflammatory cytokine production and supporting tissue repair. In contrast, in the chronic uremic milieu of advanced CKD, persistently elevated endogenous KYNA may contribute to adverse responses, potentially by reducing IL-10 production via cAMP-dependent signaling and inhibition of Erk1/2 phosphorylation, thereby potentially accelerating atherosclerosis. Thus, elevated KYNA levels in humans may reflect an insufficient compensatory response, in which disruption of the IL-10 anti-inflammatory pathway may partially limit the beneficial effects associated with AhR activation.

AA

AA in Human Studies

In the general population, a cohort study showed that AA is associated with CVD-related mortality. ⁴⁸

AA Analogs in Animal Models

Interestingly, the synthetic AA derivative 3,4-dimethoxycinnamoyl anthranilic acid (3,4-DAA) was tested in experimental settings as a potential therapeutic compound for the treatment of CVD due to its anti-inflammatory properties. ¹¹⁵ Administration of 3,4-DAA to ApoE⁻/⁻ mice with accelerated atherosclerosis significantly reduced the lesion development measured by the intima-to-media ratio and limited macrophage infiltration in the atherosclerotic plaque. ¹¹⁵

AA Analogs in In Vitro Studies

Moreover, an incubation of atheroma cells from patients undergoing carotid endarterectomy with the 3,4-DAA effectively reduced the production of pro-inflammatory factors, such as IL-6, granulocyte–macrophage colony-stimulating factor (GM-CSF), TNF-α, C-X-C motif chemokine ligand 1 (CXCL1). Additionally, it is suggested that 3,4-DAA exerts an immunomodulatory effect, promoting the expression of IL-10, an anti-inflammatory cytokine, in murine splenic B cells. ¹¹⁵

However, these findings relate specifically to a pharmacological analog studied in vitro and in animal models and should not be directly interpreted as reflecting the biological effect of endogenous AA in humans.

AA in CKD

AA shows significant accumulation in the plasma of patients with CKD, ⁷⁵ particularly in those with ESRD and in individuals undergoing dialysis, compared with healthy controls.^{4,51,74,75,110} This elevated concentration seems to play an important role in the pathogenesis of atherosclerosis and CVD. It has been demonstrated that AA is strongly and positively associated with markers of endothelial dysfunction, such as vWF, TM, sICAM-1, and sVCAM-1,^51,110 with the OX marker Cu/Zn SOD, ¹¹⁰ as well as with markers of VC, including aortic and coronary calcium amounts quantified by the Agatston score, and with the left ventricular mass index in CKD patients. ⁷⁵ Moreover, AA was also associated with a shorter time to a cardiovascular event – every 1 nanomolar increase in AA levels resulted in a 2% higher risk of a new cardiovascular event. ⁷⁵

Our previous study ⁷⁴ showed that AA reflects a hypercoagulable state in uremic patients undergoing CAPD and HD treatment. The marker of thrombin generation – F1 + 2 levels – was independently and significantly associated with AA, and TF and TFPI were also linked to this metabolite. ⁷⁴ In patients with mild to moderate CKD, AA correlated with uPA and suPAR levels, suggesting stimulation of fibrinolysis, while in groups with severe to end-stage CKD, AA was inversely associated with PAI-1 and tPA levels. Additionally, AA was identified as an independent predictor of fibrinolytic activators - tPA concentrations in all CKD patients and of uPA in the mild to moderate CKD stage. ⁴

In summary, elevated AA levels in patients with CKD may pose an increased cardiovascular risk through their association with endothelial dysfunction, OX, VC, and disturbances in the balance of the coagulation and fibrinolytic systems. AA potentially serves as a clinical biomarker due to its significant prognostic utility and independence from traditional risk factors. Evidence indicates that each 1 nanomolar increase in baseline AA levels corresponds to a 2% higher risk of new cardiovascular events, and its link to fibrinolytic disturbances persists regardless of traditional cofounders like eGFR or CRP.^4,75 Additionally, AA may represent a candidate clinical biomarker due to the feasibility of its measurement using high-performance liquid chromatography (HPLC), a technique widely available in diagnostic laboratories. ⁴ The reproducibility of AA as a predictor is highlighted by its independent association with both cardiovascular events and fibrinolytic disturbances across diverse pre-dialysis and dialysis cohorts.^4,74,75,110

The characteristics of individual KP downstream metabolites, including their molecular mechanisms, associations with CKD, experimental evidence, and clinical relevance, are synthesized in Table 1.

Table 1.

Summary of downstream KP metabolites.

Metabolite	Mechanism	Association with CKD	Experimental evidence	Clinical relevance
3-HKYN	Increases NADPH oxidase activity, induces ROS generation and endothelial apoptosis. 49	Correlates with:	In vitro: Accelerates apoptosis in HAECs through cytochrome c release. 49	Associated with acute coronary events 43 and CVD mortality. 48
		- OX (SOD,MDA), 3 , 50
		- inflammation (hsCRP), 50 , 51
		- hemostatic disturbances (TF/TFPI, 52 vWF), 53 , 54
		- endothelial dysfunction (TM, VCAM-1, ICAM-1),3,50,51
		- atherosclerosis (IMT). 51
3-HAA	Supresses iNOS via HO-1 55 , 58 ; inhibits NLRP3 inflammasome 61 ; increases MMP-2 in AAA. 62	Associated with CC-chemokines (CCL2, CCL4) and Cu/Zn SOD. 63	In vitro: Increases MMP-2 in HASMCs. 62	Contribution to plaque progression 63 and pathogenesis of AAA. 62
			In vivo: Reduces atherosclerosis via SREBP-2 modulation (Ldlr−/− mice). 17 , 61
QUIN	Acts as a ROS generator 68 ; enhances monocyte adhesion 69 ; is independently associated with MMP-2 levels. 71	Correlates with	In vitro: Vasorelaxation in rat aorta (supraphysiological dose). 70	Linked to MMP-2/TIMP-2 and adverse remodeling in CAPD patients 77 ; 28% higher CVD risk per 1 µmol increase. 75
		- hsCRP, OX (SOD,MDA), 73
		- vascular remodeling (IMT, 51 , 73 VC, 75 MMP-2, TIMP-2) 77
		- endothelial dysfunction (TM, vWF, VCAM-1, ICAM-1), 5 , 51
		- hemostatic disturbances (TF/TFPI, F1 + 2). 74
KYNA	AhR-mediated immunomodulation; inhibits IL-10 via cAMP/Erk1/2 signaling. 63	Correlates with:	In vivo: Reduces infarct size in I/R models.,92-94 improves cardiac recovery. 92	Higher levels independently associated with recurrent MI or death in humans. 63
		- endothelial dysfunction (TM, vWF, VCAM-1, ICAM-1)
		- OX51,74,110;
		- hemostatic disturbances (TF, TFPI, PAI-1, F1 + 2) 111
		Deficiency linked to CVD in PD patients. 111
AA	Associated with hemostatic and fibrinolytic system disturbances (tPA, uPA, PAI-1). 4 , 74	Correlates with:	In vitro/in vivo: 3,4-DAA (synthetic analog) reduces lesions and macrophage infiltration. 115	Prognostic tool; 1 nM increase linked to a 2% higher risk of new CVD events. 75
		- endothelial dysfunction (vWF, TM, VCAM-1, ICAM-1) 51 , 110
		- hemostatic disturbances (F1 + 2, TF, TFPI, 74 tPA, uPA, suPAR, PAI-1 4 )
		- OX (Cu/Zn SOD) 110
		- VC. 75

Abbreviations: AA, anthranilic acid; AAA, abdominal aortic aneurysm; AhR, aryl hydrocarbon receptor; CAPD, continuous ambulatory peritoneal dialysis; cAMP, cyclic adenosine monophosphate; CKD, chronic kidney disease; CVD, cardiovascular disease; 3,4-DAA, N-(3,4-dimethoxycinnamoyl) anthranilic acid; Erk1/2, extracellular signal-regulated kinase 1/2; HAECs, human aortic endothelial cells; HASMCs, human aortic smooth muscle cells; F1 + 2, prothrombin fragment 1 + 2; 3-HAA, 3-hydroxyanthranilic acid; 3-HKYN, 3-hydroxykynurenine; HO-1, heme oxygenase-1; hsCRP, high-sensitivity C-reactive protein; ICAM-1, intercellular adhesion molecule-1; IL-10, interleukin-10; IMT, intima-media thickness; iNOS, inducible nitric oxide synthase; I/R, ischemia/reperfusion; KYNA, kynurenic acid; Ldlr, low-density lipoprotein receptor; MDA, malondialdehyde; MI, myocardial infarction; MMP-2, matrix metalloproteinase-2; OX, oxidative stress; PAI-1, plasminogen activator inhibitor-1; PD, peritoneal dialysis; QUIN, quinolinic acid; ROS, reactive oxygen species; SOD, superoxide dismutase; SREBP-2, sterol regulatory element-binding protein 2; suPAR, soluble urokinase-type plasminogen activator receptor; TIMP-2, tissue inhibitor of metalloproteinase-2; TF, tissue factor; TFPI, tissue factor pathway inhibitor; TM, thrombomodulin; tPA, tissue plasminogen activator; uPA, urokinase-type plasminogen activator; VC, vascular calcification; VCAM-1, vascular cell adhesion molecule-1; vWF, von Willebrand factor.

Impact of Standard CKD Therapies on KP Activity

In patients with CKD, the KP becomes markedly disturbed due to a chronic inflammatory state and impaired renal excretion of its metabolites. ¹¹⁶ Enhanced activation of this pathway promotes the accumulation of uremic toxins, which contribute to the pathophysiological link between declining kidney function and increased cardiovascular risk.^116,117 Interestingly, several classes of drugs commonly used in patients with CKD, such as renin-angiotensin system inhibitors (RASis) and cholesterol-lowering agents, have been reported to modulate the KP activity.^116,118

A clinical study in patients with CKD demonstrated that individuals receiving RASi therapy, including ACE inhibitors (ACEis) and angiotensin II receptor blockers (ARBs), showed significantly lower serum KYN concentrations compared to an untreated group. Moreover, in patients not receiving RASi, KYN levels were inversely correlated with the eGFR and directly associated with proteinuria and albuminuria, whereas IDO activity showed a negative association with eGFR. Additionally, CKD patients without RASi treatment who had a history of CVD demonstrated higher IDO activity and lower TRP concentrations compared to patients without cardiovascular events. ¹¹⁶ Similar effects on KP activity were observed for lipid-lowering therapy. Zinellu et al ¹¹⁸ demonstrated that patients with CKD receiving simvastatin or simvastatin in combination with ezetimibe exhibited a significant decrease in plasma KYN concentrations and the KYN/TRP ratio after 12 months of treatment. At baseline, these patients showed higher KYN levels and KYN/TRP ratio compared to healthy controls, and these parameters positively correlated with OX markers, including MDA and the allantoin/uric acid (All/UA) ratio. After treatment, the downregulation of KP activity was associated with a significant reduction in OX. ¹¹⁸ Moreover, further analysis of this patient group demonstrated that the decreased KP activity following cholesterol-lowering therapy was associated with decreased levels of asymmetric dimethyl-arginine (ADMA), a recognized marker of endothelial dysfunction. ¹¹⁹ Beyond the modulation of KYN concentration, experimental studies have also demonstrated that ACEis and ARBs, significantly reduce the production of KYNA, through the inhibition of KAT enzymes. However, these findings are limited as they were conducted in vitro on homogenates of healthy rat kidneys, and therefore may not reflect the alterations observed in human CKD.^117,120

Available findings suggest that the anti-inflammatory and antioxidant effects of standard CKD therapies, such as RASis and cholesterol-lowering agents, may contribute to the modulation of the KP through the downregulation of IDO activity, induced by inflammation. Reduced synthesis of KYN may therefore represent one of the possible mechanisms underlying the attenuation of CKD progression.^116,118 Furthermore, decreased KP activity is linked to lower ADMA levels and improved endothelial function, potentially contributing to a lower risk of CVD development in this population. ¹¹⁹ However, available data are limited, and further studies are required to clarify the precise roles and mechanisms through which standard CKD treatments modulate the KP, particularly regarding downstream metabolites, as kynurenine represents only an intermediate metabolite in this pathway. ¹¹⁶

Conclusions

The development of CVD in CKD can be explained by an integrative pathophysiological framework, as illustrated in Figure 3, in which the KP acts as a link between the uremic milieu and vascular injury. This model is initiated by the chronic low-grade inflammation characteristic of CKD, which stimulates IDO-1 activity and accelerates TRP metabolism, while progressive renal dysfunction impairs the renal clearance of kynurenines, leading to the systemic accumulation of toxic metabolites. Consequently, metabolites such as 3-HKYN, QUIN, and AA promote OX, endothelial dysfunction, vascular remodeling, and hemostatic disturbances, thereby contributing to accelerated atherosclerosis and increased cardiovascular risk in CKD. In contrast, KYNA and 3-HAA appear to exert context-dependent effects, showing cardioprotective and anti-inflammatory properties in experimental models, but potentially promoting adverse vascular responses in the chronic uremic environment, possibly through the suppression of anti-inflammatory pathways like IL-10 via cAMP/Erk1/2 signaling. Collectively, chronic KP activation together with impaired metabolite clearance may contribute to a vicious cycle linking inflammation, OX, and vascular injury, potentially participating in the increased cardiovascular risk in CKD patients.

Figure 3.

Framework of KP activation and CVD in CKD.

Source: Figure was created using Microsoft PowerPoint.

Abbreviations: AA, anthranilic acid; AAA, abdominal aortic aneurysm; AhR, aryl hydrocarbon receptor; CVD, cardiovascular disease; Erk1/2, extracellular signal-regulated kinase 1/2; 3-HAA, 3-hydroxyanthranilic acid; 3-HKYN, 3-hydroxykynurenine; IFN-γ, interferon-γ; IL-10, interleukin-10; KP, kynurenine pathway; KYN, kynurenine; KYNA, kynurenic acid; MMP-2, matrix metalloproteinase 2; NOS, nitric oxide synthase; QUIN, quinolinic acid; ROS, reactive oxygen species; SREBP-2, sterol response element binding protein-2; TDO, tryptophan 2,3-dioxygenase; TRP, tryptophan; VSMCs, vascular smooth muscle cells.

This review establishes a prognostic hierarchy, identifying QUIN and AA as the promising prognostic biomarkers due to their independent association with cardiovascular events and reproducibility across cohorts. The transition of these metabolites from research to clinical practice is facilitated by the feasibility of their measurement in peripheral blood using validated chromatographic and mass spectrometry. Their reproducibility across independent cohorts further supports their utility as independent prognostic tools that could enhance clinical prognostic evaluation in the CKD population. Among the studied metabolites, QUIN and AA demonstrate the highest clinical potential as prognostic biomarkers.

Furthermore, current evidence suggests that the pharmacological modulation of KMO and HAO enzymes represents a promising therapeutic strategy for CVD. Moreover, the development of synthetic analogs, such as 3,4-DAA, provides a novel direction for drug development aimed at reducing vascular inflammation. However, these experimental findings require further confirmation in clinical trials. Although manipulation of KP metabolism represents a promising therapeutic direction, further studies are needed not only to confirm its clinical utility in the CKD population but also to better elucidate the contribution of KYN derivatives to CVD pathogenesis. In particular, further clinical investigations focused on downstream KP metabolites are necessary to verify the potential clinical significance of current experimental findings. Improved understanding of these mechanisms may ultimately contribute to reducing mortality and improving quality of life in patients with CKD.