Lipoprotein(a) plasma levels are not associated with survival after acute coronary syndromes: An observational cohort study

Platelets, Biomarkers of Coagulation and Fibrinolysis, and Early Coronary Stent Thrombosis

Background/Objectives: Acute stent thrombosis (ST) is a rare yet severe complication following percutaneous coronary intervention (PCI). Herein, we investigated the possible association between routinely available coagulation and fibrinolysis markers with early ST. Methods: Within a single-center registry, we investigated the association between the preprocedural platelet count, plasma levels of fibrinogen and D-Dimer, and the incidence of early ST in the first 30 days after PCI. Results: Out of 10,714 consecutive patients who underwent PCI using drug-eluting stents (DESs), the preprocedural platelet count, fibrinogen, and D-Dimer measurements were available in 6337, 6155, and 956 patients, respectively. Fifty-eight patients (0.92%) experienced an early ST within 30 days after PCI. Compared with those without ST, patients with early ST showed significantly elevated preprocedural platelet counts (p < 0.05) and fibrinogen levels (p < 0.05). D-Dimer levels were not associated with early ST. Patients in the fifth quintile of platelet count had a significantly increased risk for early ST (HR 2.43; 95% CI 1.43-4.14; p = 0.001) compared with patients in the lower four quintiles. In addition, patients in the fifth quintile of fibrinogen also had a significantly increased risk for early ST (HR 1.86; 95% CI 1.07-3.26; p < 0.05) compared with patients in the lower four quintiles. These associations were independent of clinical risk factors, the number of stents, the presence of acute coronary syndromes, and white blood cell count. Conclusions: Preprocedural platelet counts and fibrinogen plasma levels can identify patients at elevated risk of early ST after implantation of DESs in addition to procedure-level and device-related risk factors.

Prognostic value of elevated lipoprotein (a) in patients with acute coronary syndromes: a systematic review and meta-analysis

Background

Elevated lipoprotein (a) level was recognized as an independent risk factor for significant adverse cardiovascular events in acute coronary syndrome (ACS) patients. Despite this recognition, the consensus in the literature regarding the prognostic significance of elevated lipoprotein (a) in ACS was also limited. Consequently, we conducted a thorough systematic review and meta-analysis to evaluate the prognostic relevance of elevated lipoprotein (a) level in individuals diagnosed with ACS.

Methods and results

A thorough literature review was conducted by systematically searching PubMed, Embase, and Cochrane databases until September 2023. This review specifically examined cohort studies exploring the prognostic implications of elevated lipoprotein (a) level in relation to major adverse cardiovascular events (MACE), including death, stroke, non-fatal myocardial infarction (MI), and coronary revascularization, in patients with ACS. The meta-analysis utilized aggregated multivariable hazard ratios (HR) and their respective 95% confidence intervals (CI) to evaluate prognostic implications between high and low lipoprotein (a) levels [the cut-off of high lipoprotein (a) level varies from 12.5 to 60 mg/dl]. Among 18,168 patients in the identified studies, elevated lipoprotein (a) was independently associated with increased MACE risk (HR 1.26; 95% CI: 1.17-1.35, P < 0.00001) and all-cause mortality (HR 1.36; 95% CI: 1.05-1.76, P = 0.02) in ACS patients. In summary, elevated lipoprotein (a) levels independently forecast MACE and all-cause mortality in ACS patients. Assessing lipoprotein (a) levels appears promising for risk stratification in ACS, offering valuable insights for tailoring secondary prevention strategies.

Systematic Review Registration

PROSPERO (CRD42023476543).

Association of Periprocedural Inflammatory Activation With Increased Risk for Early Coronary Stent Thrombosis

BACKGROUND

Stent thrombosis is a rare but deleterious event. Routine coronary angiography with percutaneous coronary intervention (PCI) is often deferred in the presence of laboratory markers of acute inflammation to prevent complications. The aim of this study was to investigate whether an acute inflammatory state is associated with an increased risk of early stent thrombosis.

METHODS AND RESULTS

Within a prospective single-center registry, the association between preprocedural acute inflammatory activation, defined as C-reactive protein plasma levels >50 mg/L or a leukocyte count >12 g/L, and occurrence of early (≤30 days) stent thrombosis was evaluated. In total, 11 327 patients underwent PCI and of those, 6880 patients had laboratory results available. 49.6% of the study population received PCI for an acute coronary syndrome and 50.4% for stable ischemic heart disease. In patients with signs of acute inflammatory activation (24.9%), PCI was associated with a significantly increased risk for stent thrombosis (hazard ratio, 2.89; P<0.00001), independent of age, sex, kidney function, number and type of stents, presence of multivessel disease, choice of P2Y12 inhibitor, and clinical presentation. Elevated laboratory markers of acute inflammation were associated with the occurrence of stent thrombosis in both patients with acute coronary syndrome (hazard ratio, 2.63; P<0.001) and in patients with stable ischemic heart disease (hazard ratio, 3.57; P<0.001).

CONCLUSIONS

An acute inflammatory state at the time of PCI was associated with a significantly increased risk of early stent thrombosis. Evidence of acute inflammation should result in deferred PCI in elective patients, while future studies are needed for patients with acute coronary syndrome.

Confounding Factors Responsible for Elevated Lp(a) Levels in Patients with Coronary Artery Disease

BACKGROUND

Cardiovascular diseases (CVDs) are a leading cause of global mortality, motivating research into novel approaches for their management. Lipoprotein(a) (Lp(a)), a unique lipoprotein particle, has been implicated in atherosclerosis and thrombosis, suggesting its potential as a therapeutic target for CVDs.

AIM

This study aimed to investigate the association of Lp(a) levels with various cardiovascular parameters and events among patients with confirmed cardiovascular disease.

METHODOLOGY

A prospective study was conducted, enrolling 600 participants, predominantly comprising males (79%), with a mean age of 52.78 ± 0.412 years diagnosed with cardiovascular disease. The follow-up was done for 18 months. Patient demographics, blood investigations, and occurrence of major adverse cardiac events (MACE) were collected. SPSS version 21 was used to statistically analyze the relationships between elevated Lp(a) levels and factors such as age, glycated hemoglobin, mortality, MACE, cardiac death, target vessel revascularization, and stroke.

RESULTS

The study revealed significant (P < 0.05) associations between elevated Lp(a) levels and advanced age, increased glycated hemoglobin levels, as well as occurrences of all-cause mortality, MACE, cardiac death, target vessel revascularization, and stroke. Notably, a significant (P < 0.05), association between high Lp(a) levels and acute coronary syndrome (ACS) emerged, suggesting Lp(a)'s role in advanced cardiac events.

CONCLUSION

The findings highlight the potential significance of Lp(a) as a notable risk factor in cardiovascular health. The observed associations between elevated Lp(a) and adverse cardiovascular events, including ACS, underscore its pathogenic role. Consequently, this study supports the rationale for further research into Lp(a)-specific therapeutic interventions, offering substantial promise in refining the management strategies for cardiovascular diseases.

Clinical characteristics associated with elevated levels of lipoprotein(a) in patients with vascular risk

Objectives

Lipoprotein(a) (Lp(a)) is increasingly used in the evaluation of patients with vascular risk due to its association with cardiovascular events. The purpose of this study was to identify the clinical characteristics of patients with elevated levels of Lp(a) attended in an outpatient vascular risk unit.

Methods

An observational, retrospective study was conducted to assess the clinical characteristics of patients with elevated levels of Lp(a) (≥50 mg/dL), as compared to patients with normal values (<50 mg/dL). The sample was composed of 878 patients identified as having a high vascular risk due to a diagnosis of vascular disease, attended in a vascular risk unit between 2021 and 2022.

Results

The highest levels of Lp(a) were independently associated with a higher probability of having a history of peripheral arterial disease (p=0.024), polygenic familial hypercholesterolemia (PH, p=0.030) and combined familial hypercholesterolemia (CFH, p=0.015); and using PCSK9 inhibitor treatment (p=0.029) and combination therapy with statins and ezetimibe (p=0.018). In contrast, there were no significant differences in relation to familial history of early cardiovascular disease (p=0.143) or personal history of cardiovascular disease (p=0.063), which contrasts with other series.

Conclusions

Elevated levels of Lp(a) were associated with a history of peripheral arterial disease, diagnosis of FHP and CFH, and need for more intense lipid-lowering treatments.

Características clínicas asociadas a niveles elevados de lipoproteína(a) en pacientes atendidos por riesgo vascular

Objectivos

La lipoproteína(a) (Lp(a)) es cada vez más relevante en la evaluación de pacientes con riesgo vascular debido a su asociación con una mayor incidencia de eventos cardiovasculares. Este estudio tiene como objetivo identificar las características clínicas de los pacientes con niveles elevados de Lp(a) atendidos en consultas externas por riesgo vascular.

Métodos

Estudio observacional retrospectivo en donde se compararon las características clínicas de los pacientes con niveles elevados de Lp(a) (≥50 mg/dL) con la de los pacientes con valores normales (<50 mg/dL), en un total de 878 pacientes atendidos por riesgo o enfermedad vascular durante los años 2021 y 2022.

Resultados

Los valores más elevados de Lp(a) se asociaron de forma independiente con una mayor probabilidad de antecedentes de enfermedad arterial periférica (p=0,024), hipercolesterolemia familiar poligénica (HFP, p=0,030) e hipercolesterolemia familiar combinada (HFC, p=0,015), el tratamiento de inhibidores de PCSK9 (p=0,029) y la combinación de estatinas y ezetimiba (p=0,018). Sin embargo, no se obtuvieron diferencias significativas para las variables antecedentes familiares de enfermedad cardiovascular precoz (p=0,143) ni para antecedentes de enfermedad cardiovascular previa (p=0,063) a diferencia de lo identificado en otras series.

Conclusiones

Los niveles elevados de Lp(a) se asociaron con antecedentes de enfermedad arterial periférica, diagnóstico de HFP y HFC, así como con la necesidad de utilizar tratamientos hipolipemiantes más intensos.

Circulating lipoprotein (a) and all-cause and cause-specific mortality: a systematic review and dose-response meta-analysis

AIMS

To investigate the association between circulating lipoprotein(a) (Lp(a)) and risk of all-cause and cause-specific mortality in the general population and in patients with chronic diseases, and to elucidate the dose-response relations.

METHODS AND RESULTS

We searched literature to find prospective studies reporting adjusted risk estimates on the association of Lp(a) and mortality outcomes. Forty-three publications, reporting on 75 studies (957,253 participants), were included. The hazard ratios (HRs) and 95% confidence intervals (95%CI ) for the top versus bottom tertile of Lp(a) levels and risk of all-cause mortality were 1.09 (95%CI: 1.01-1.18, I2: 75.34%, n = 19) in the general population and 1.18 (95%CI: 1.04-1.34, I2: 52.5%, n = 12) in patients with cardiovascular diseases (CVD). The HRs for CVD mortality were 1.33 (95%CI: 1.11-1.58, I2: 82.8%, n = 31) in the general population, 1.25 (95%CI: 1.10-1.43, I2: 54.3%, n = 17) in patients with CVD and 2.53 (95%CI: 1.13-5.64, I2: 66%, n = 4) in patients with diabetes mellitus. Linear dose-response analyses revealed that each 50 mg/dL increase in Lp(a) levels was associated with 31% and 15% greater risk of CVD death in the general population and in patients with CVD. No non-linear dose-response association was observed between Lp(a) levels and risk of all-cause or CVD mortality in the general population or in patients with CVD (Pnonlinearity > 0.05).

CONCLUSION

This study provides further evidence that higher Lp(a) levels are associated with higher risk of all-cause mortality and CVD-death in the general population and in patients with CVD. These findings support the ESC/EAS Guidelines that recommend Lp(a) should be measured at least once in each adult person's lifetime, since our study suggests those with higher Lp(a) might also have higher risk of mortality.

Baseline Lipoprotein(a) Levels and Long-Term Cardiovascular Outcomes After Acute Myocardial Infarction

BACKGROUND

Lipoprotein(a) is a known independent risk factor for atherosclerotic cardiovascular disease. However, the prognostic impact of the baseline lipoprotein(a) levels on long-term clinical outcomes among patients with acute myocardial infarction remain unclear.

METHODS

We analyzed 1,908 patients with acute myocardial infarction from November 2011 to October 2015 from a single center in Korea. They were divided into 3 groups according to their baseline lipoprotein(a) levels: groups I (< 30 mg/dL, n = 1,388), II (30-49 mg/dL, n = 263), and III (≥50 mg/dL, n = 257). Three-point major adverse cardiovascular events (a composite of nonfatal myocardial infarction, nonfatal stroke, and cardiac death) at 3 years were compared among the 3 groups.

RESULTS

The patients were followed for 1094.0 (interquartile range, 1,033.8-1,095.0) days, during which a total of 326 (17.1%) three-point major adverse cardiovascular events occurred. Group III had higher rates of three-point major adverse cardiovascular events compared with Group I (23.0% vs. 15.7%; log-rank P = 0.009). In the subgroup analysis, group III had higher rates of three-point major adverse cardiovascular events compared with group I in patients with non-ST-segment elevation myocardial infarction (27.0% vs. 17.1%; log-rank P = 0.006), but not in patients with ST-segment elevation myocardial infarction (14.4% vs. 13.3%; log-rank P = 0.597). However, in multivariable Cox time-to-event models, baseline lipoprotein(a) levels were not associated with an increased incidence of three-point major adverse cardiovascular events, regardless of the type of acute myocardial infarction. Sensitivity analyses in diverse subgroups showed similar findings to those of the main analysis.

CONCLUSION

Baseline lipoprotein(a) levels in Korean patients with acute myocardial infarction were not independently associated with increased major adverse cardiovascular events at 3 years.

Therapeutic RNA-silencing oligonucleotides in metabolic diseases

Recent years have seen unprecedented activity in the development of RNA-silencing oligonucleotide therapeutics for metabolic diseases. Improved oligonucleotide design and optimization of synthetic nucleic acid chemistry, in combination with the development of highly selective and efficient conjugate delivery technology platforms, have established and validated oligonucleotides as a new class of drugs. To date, there are five marketed oligonucleotide therapies, with many more in clinical studies, for both rare and common liver-driven metabolic diseases. Here, we provide an overview of recent developments in the field of oligonucleotide therapeutics in metabolism, review past and current clinical trials, and discuss ongoing challenges and possible future developments.

Lipoprotein(a) as predictor of coronary artery disease and myocardial infarction in a multi-ethnic Asian population

BACKGROUND AND AIMS

The role of Lp(a) in multi-ethnic Asian populations with coronary artery disease (CAD) has not been well established. The aims of this study were (i) to investigate whether Lp(a) is a predictor of CAD, and (ii) amongst patients with CAD, to ascertain whether Lp(a) is a predictor of acute myocardial infarction (AMI) and severity of CAD.

METHODS

We compared three cardiovascular phenotypes from patients recruited at coronary angiography. CAD was defined as ≥50% coronary artery stenosis and subdivided into a group with AMI history (CAD+AMI+) and a group without (CAD+AMI-). Minimal CAD group (CAD-) was defined as normal or <30% coronary artery stenosis and no AMI. The severity of CAD was defined using the modified Gensini score.

RESULTS

We studied 2025 patients comprising 94.5% men and 61.4% of Chinese ethnicity. The median Lp(a) level was highest in CAD+AMI+, followed by CAD+AMI- and CAD- (26.2, 20.1, and 15.8 nmol/L respectively). Similarly, the frequency of patients with Lp(a) ≥120 nmol/L were in the same order (11.8%, 9.1% and 2.4%). Lp(a) levels were highest among Asian Indians, followed by Malays and Chinese patients (p < 0.001). Lp(a) levels and Lp(a) ≥120 nmol/L were significant predictors of CAD (Odds ratio (OR) = 1.12 per 10 nmol/L increment, p < 0.001, and OR = 5.41 p = 0.004 respectively). Among patients with CAD, higher Lp(a) levels were associated with increased AMI risk (OR = 1.02 per 10 nmol/L increment, p = 0.024). Lp(a) ≥120 nmol/L was positively associated with CAD severity (p = 0.020).

CONCLUSIONS

Plasma Lp(a) concentration is a positive predictor of CAD and AMI in a mostly male South East Asian population.

Associations of Lipoprotein(a) With Coronary Atherosclerotic Burden and All-Cause Mortality in Patients With ST-Segment Elevation Myocardial Infarction Treated With Primary Percutaneous Coronary Intervention

Background: The coronary atherosclerotic burden in patients with ST-segment elevation myocardial infarction (STEMI) has been identified as the main predictor of prognosis. However, the association of lipoprotein(a) [Lp(a)], a well-established proatherogenic factor, with atherosclerotic burden in patients with STEMI is unclear.

Methods: In total, 1,359 patients who underwent percutaneous coronary intervention (PCI) for STEMI were included in analyses. Three prespecified models with adjustment for demographic parameters and risk factors were evaluated. Generalized additive models and restricted cubic spline analyses were used to assess the relationships of Lp(a) with Gensini scores and the no-reflow phenomenon. Kaplan–Meier curves were generated to explore the predictive value of Lp(a) for long-term all-cause mortality. Furthermore, mRNA expression levels of LPA in different groups were compared using the GEO database.

Results: Patients in the highest tertile according to Lp(a) levels had an increased incidence of heart failure during hospitalization. Furthermore, patients with high levels of Lp(a) (>19.1 mg/dL) had sharply increased risks for a higher Gensini score (P_{for trend} = 0.03) and no-reflow (P_{for trend} = 0.002) after adjustment for demographic parameters and risk factors. During a median follow-up of 930 days, 132 deaths (9.95%) were registered. Patients with high levels of Lp(a) (>19.1 mg/dL) had the worst long-term prognosis (P_{for trend} < 0.0001). In a subgroup analysis, patients with higher Lp(a) still had the highest all-cause mortality. Additionally, the mRNA expression levels of LPA in patients with STEMI with lower cardiac function were higher than those in other groups (P = 0.003). A higher coronary atherosclerotic burden was correlated with higher LPA expression (P = 0.01).

Conclusion: This study provides the first evidence that Lp(a) (at both the protein and mRNA levels) is independently associated with coronary atherosclerotic lesions and prognosis in patients with STEMI treated with PCI.

Clinical Trial Registration:http://www.chictr.org.cn/index.aspx, identifier: ChiCTR1900028516.

Precision Treatment in ACS-Role of Assessing Fibrinolysis

Despite advancements in pharmacotherapy and interventional strategies, patients with acute coronary syndrome (ACS) remain at risk of recurrent thrombotic events. In addition to an enhanced tendency to thrombus formation, impairment in the ability to naturally dissolve or lyse a developing thrombus, namely impaired endogenous fibrinolysis, is responsible for a major part of this residual risk regardless of optimal antiplatelet medication. Global assessment of endogenous fibrinolysis, including a point-of-care assay, can identify patients with ACS at persistent high cardiovascular risk and might play an important role in allowing the personalisation of potent antithrombotic therapy to enhance fibrinolytic status, providing precision treatment of ACS to improve long-term outcome.

Lipoprotein(a): a genetic marker for cardiovascular disease and target for emerging therapies

Lipoprotein(a) [Lp(a)] is an established cardiovascular risk factor, and growing evidence indicates its causal association with atherosclerotic disease because of the proatherogenic low-density lipoprotein (LDL)-like properties and the prothrombotic plasminogen-like activity of apolipoprotein(a) [apo(a)]. As genetics significantly influences its plasma concentration, Lp(a) is considered an inherited risk factor of atherosclerotic cardiovascular disease (ASCVD), especially in young individuals. Moreover, it has been suggested that elevated Lp(a) may significantly contribute to residual cardiovascular risk in patients with coronary artery disease and optimal LDL-C levels. Nonetheless, the fascinating hypothesis that lowering Lp(a) could reduce the risk of cardiovascular events - in primary or secondary prevention - still needs to be demonstrated by randomized clinical trials. To date, no specific Lp(a)-lowering agent has been approved for reducing the lipoprotein levels, and current lipid-lowering drugs have limited effects. In the future, emerging therapies targeting Lp(a) may offer the possibility to further investigate the relation between Lp(a) levels and cardiovascular outcomes in randomized controlled trials, ultimately leading to a new era in cardiovascular prevention. In this review, we aim to provide an updated overview of current evidence on Lp(a) as well as currently investigated therapeutic strategies that specifically address the reduction of the lipoprotein.

Lipoprotein(a): A Concealed Precursor of Increased Cardiovascular Risk? A Real-World Regional Lipid Clinic Experience

OBJECTIVE

Lipoprotein(a) [Lp(a)] is an independent cardiovascular risk factor. We present real-life characteristics of patients with increased Lp(a) levels attending a University Lipid Clinic.

METHODS

We retrospectively studied patients attending the University of Ioannina Hospital Lipid Clinic with Lp(a) levels ≥30 mg/dL who were followed-up for a median of 22 months.

RESULTS

One hundred eight patients (median age 59 years, 49% females) were included with median Lp(a) levels 67 mg/dL (30-320). Of patients, 25.1% had established atherosclerotic cardiovascular disease (ASCVD): 11.1 and 5.6% positive personal history of myocardial infarction (MI) and stroke, respectively, 6.5% carotid artery disease and 1.9% lower extremities arterial disease (LEAD). In addition, 35.2% of participants had heterozygous familial hypercholesterolemia (heFH), 37.9% positive family history of premature ASCVD, 29.6% hypertension, 12.0% diabetes and 5.5% chronic kidney disease (CKD). Of patients, 67.6% were receiving statin therapy and 16.6% additional ezetimibe at baseline visit, and 83 and 35% were receiving statin treatment and additional ezetimibe, respectively, during follow-up. Low-density cholesterol (LDL-C) levels and LDL-C_{corrected for Lp(a)} levels were significantly reduced in lipid-lowering therapy naive patients by 37 and 40% (p <0.05), in lipid-lowering therapy intensified patients by 31 and 36% (p <0.05), and in patients on stable lipid-lowering treatment by 15% (p <0.05) and 10% (p >0.05), respectively, during follow-up. Lp(a) levels increased by 9% (p <0.05).

CONCLUSION

Our data confirm the high prevalence of established ASCVD, hFH and positive familial history of premature ASCVD in patients with elevated Lp(a) levels. Lp(a) levels slightly increased during follow-up.

Gender difference in lipoprotein(a) concentration as a predictor of coronary revascularization in patients with known coronary artery disease

BACKGROUND AND AIMS

Whether there is a gender difference in the impact of elevated plasma Lp(a) levels on recurrent coronary events remains unclear. We, therefore, evaluated the association between Lp(a) levels and the occurrence of major adverse coronary events in a large series of coronary patients (32% women).

METHODS

This single-center prospective cohort study investigated 3034 consecutive patients admitted to the Coronary Care Unit with a diagnosis of coronary ischemia. According to the inclusion criteria, 2374 patients completed the follow-up (mean of 2 years). The end-points were non-fatal myocardial infarction (MI), revascularization and coronary deaths.

RESULTS

Elevated Lp(a) levels were significantly associated with rate of revascularization, but not with non-fatal MI and cardiac death. According to Lp(a) stratification (≤30 mg/dl, >30-50 mg/dl and ≥50 mg/dl), there was a significant rise of revascularization events in the whole sample of participants, with a trend in hazard ratio (HR) of 1.23 (95% CI 1.04-1.46) and a 6% rise for every 10 mg/dl increment in Lp(a) levels. This effect was mainly driven by women (HR 2.04, 95%CI 1.33-3.12) who showed a 14% incremental risk for every 10 mg/dl rise in Lp(a) levels.

CONCLUSIONS

In patients with coronary artery disease, elevated plasma Lp(a) levels were found to be a potentially useful predictor of the need for coronary revascularizations, especially in women.

Lipoprotein(a) Lowering-From Lipoprotein Apheresis to Antisense Oligonucleotide Approach

It is well-known that elevated lipoprotein(a)—Lp(a)—levels are associated with a higher risk of cardiovascular (CV) mortality and all-cause mortality, although a standard pharmacotherapeutic approach is still undefined for patients with high CV risk dependent on hyperlipoproteinemia(a). Combined with high Lp(a) levels, familial hypercholesterolemia (FH) leads to a greater CVD risk. In suspected FH patients, the proportion of cases explained by a rise of Lp(a) levels ranges between 5% and 20%. In the absence of a specific pharmacological approach able to lower Lp(a) to the extent required to achieve CV benefits, the most effective strategy today is lipoprotein apheresis (LA). Although limited, a clear effect on Lp(a) is exerted by PCSK9 antagonists, with apparently different mechanisms when given with statins (raised catabolism) or as monotherapy (reduced production). In the era of RNA-based therapies, a new dawn is represented by the use of antisense oligonucleotides APO(a)L_rx, able to reduce Lp(a) from 35% to over 80%, with generally modest injection site reactions. The improved knowledge of Lp(a) atherogenicity and possible prevention will be of benefit for patients with residual CV risk remaining after the most effective available lipid-lowering agents.