Why are we in need of newer treatments for atherosclerotic disease? Current treatment strategies have been effective in reducing deaths related to atherosclerotic disease of the heart as shown by the trend of age-adjusted deaths per 100 000 between 1999 and 2017 within the US population using CDC Wonder, a database that accumulates data on deaths per county in the USA using death certificate information, as shown in Fig. 1 (using ICD Codes 125.0 and 125.01 to delineate atherosclerotic cardiovascular disease) . Albeit, data from the National Inpatient Sample (a publicly available all-payer inpatient database containing data on hospital admissions) shows that the rate of discharges secondary to stent placement within the USA has increased markedly from baseline between 2003 and 2015 as seen in Fig. 2 .
Notwithstanding cardiovascular interventions and pharmacotherapy, cardiovascular event rates are still significant enough to pose a burden on the healthcare system within the USA, warranting investigational inquiries into the full spectrum of modalities to alleviate cardiovascular disease burden. The 4S trial investigated the effects of statin therapy in reducing the 10-year cardiovascular heart disease risk by following 4444 patients over a median of 5.4 years . It was shown that over the median follow-up period, major cardiovascular events occurred in 19% of the treatment arm compared to 28% of the control arm using the FRAMINGHAM curve provided for patient risk as shown in Fig. 3 . Despite statistically significant benefits seen in statin therapy, this 19% rate is still unacceptable. In the Heart Protection Study, the cardiovascular event rate at 5 years was 8.7% in the treatment arm versus 11.8% in the control arm (reported with a P < 0.0001) . The primary prevention study of AFCAPS/TEXCAPS featured a treatment arm that experienced a 6.8% rate of adverse cardiovascular events over our recurrent 5-year time interval versus 10.9% in placebo [RR 0.63, 95% confidence interval (CI) 0.50–0.79, P < 0.001)] .
Moreover, procedures from percutaneous coronary intervention (PCI) and surgery with the left internal mammary artery (LIMA) also did not reduce cardiovascular event rates profoundly. SYNTAX was one of the largest surgical PCI trials for patients with multi-vessel disease. It enrolled 1800 patients that were randomly assigned to coronary artery bypass grafting [coronary artery bypass graf (CABG)] (n = 897) or PCI (n = 903). A complex scoring system (SYNTAX score) was used to provide an evaluation of cardiovascular risk. At the end of 5 years, CABG saw improved results relative to most groups, however, the cardiovascular event rate for CABG with LIMA remained up to 25% with standard of medical care, seen in Fig. 4 . Another trial, the Future Revascularization Evaluation in Patients with Diabetes Mellitus: Optimal Management of Multivessel Disease trial showed that CABG was superior to PCI with drug-eluting stents (DES) in decreasing the rate of cardiovascular events (defined as a primary outcome of death, MI, and stroke, P < 0.001) to 18.7% (CABG) versus 26.6% (PCI) after a follow-up of 5 years [8,9]. In conclusion, newer treatments beyond surgery, DES, and drugs are needed to address this continued high cardiovascular event rate in patients with cardiovascular disease [9,10].
Patients with more complex disease receive the largest benefit with medications and revascularization, but within these high-risk groups, the absolute risk reduction with best treatment option continues to be marginal relative to control. This historical perspective on pursuits to mitigate atherosclerotic disease sets the stage for a discussion on emerging novel non-statin therapies in cardiovascular disease and the basic science surrounding disease abatement. In this review, we will analyze the basic science of PCSK9 inhibitors, their clinical benefits, and potential implications for diabetic management with cardiometabolic considerations.
Cholesterol is an organic molecule that is paramountin cellular homeostasis, serving many roles. These include, but are not limited to, contributing to the structural integrity of the cell membrane and promoting the synthesis of bioactive molecules such as corticosteroids, sex hormones, and Vitamin D. However, dysregulation of lipid metabolism and subsequent excessive deposition of cholesterol into the vasculature of humans is responsible for coronary heart disease . Of notable clinical interest is the prevalence and effects of dyslipidemia in patients with type 2 diabetes mellitus (T2DM). It is estimated that nearly 50% of all patients with T2DM have some form of dyslipidemia, and this dyslipidemia is still prominent in patients despite adequate glycemic control . Within the spectrum of dyslipidemias in patients with T2DM, an increased low-density lipoprotein cholesterol (LDL-C) is one of the most significant risk factors for the future development of adverse cardiovascular events . However, dysregulation of lipid metabolism as noted by pathologic deposits into the vasculature has other implications beyond cardiovascular disease in the patient with T2DM and cholesterol dysregulation such as mitochondrial dysfunction, exacerbation of diabetic sequelae, renal damage, changes in cardiovascular histology, and increased risk for stroke as shown in Fig. 5 [14–19].
While lifestyle recommendations such as increasing physical activity, increasing the amount of dietary viscous fiber, consumption of n-3 fatty acids, and avoiding trans fats is recommended by the American Diabetes Association (ADA) to lower LDL-C levels, pharmacotherapy is commonly indicated in patients refractory to these lifestyle modifications. Factors that necessitate such pharmacotherapy include severe or resistant disease, genetics, or socioeconomic limitations that mitigate LDL-C reduction [20–23]. Per the ADA, patients with T2DM who have ASCVD or 10-year ASCVD risk >20%, should be started on statins as a primary therapeutic option . However, in patients with T2DM and ASCVD, if LDL-C ≥70 mg/dl on maximum statin therapy, PCSK9 inhibitors are recommended to regulate dyslipidemia (grade A recommendation) . The American College of Cardiology and American Heart Association also recommend the use of PCSK9 inhibitors in patients with high ACSVD risk factors such as T2DM who have an elevated LDL-C ≥70 mg/dl if statin therapy is inadequate .
PCSK9 inhibitors are a class of monoclonal antibodies that were primarily developed to treat hyperlipidemia. However, these therapeutics have been shown to possibly have additional benefits of interest to patients with T2DM including changes in vascular tone, reduction in atheroma volumes, and reduced rates in adverse cardiovascular events. However, inhibition of PCSK9 also may have consequences related to glucose tolerance, insulin sensitivity, and endocrine function. Important adverse effects from the administration of PCSK9 inhibitors include injection site reactions such as rashes or swelling, fatigue, and flu-like symptoms [25–27]. PCSK9 is an enzyme primarily produced in the liver, and targeting the production of PCSK9 with new technologies may confer better health outcomes with an attenuated fiscal burden on patients and institutions. Using new small interfering RNA (siRNA) therapy, it is possible to lower LDL-C by inhibiting the translation of protein PCSK9. The ORION-1 randomized 501 patients (70–80% on statins) in a double-blind, placebo-controlled phase 2 trial using active treatment arm with Inclisiran. At 240 days, PCSK9 and LDL-C remained significantly lower relative to baseline (LDL-C approximately 130 mg/dl), and with treatment dropped about 50%. Moreover, a 70% reduction in PCSK9 levels with 25% drop in Lp(a) and a modest 14% drop in triglycerides was noted .
Another novel non-statin treatment for patient with elevated triglycerides who frequently have chronic hyperglycemia secondary to diabetes is found through modulation of angiopoietin-like 3 (ANGPTL3), a protein that acts as an inhibitor of lipoprotein lipase (LPL) and endothelial lipase . Genome-wide association studies found association between loss of function gene encoding for ANGPTL3 . Innovations in molecular and cell biology have paved the way for the development of antisense oligonucleotides (ASOs) targeting ANGPTL3 mRNA, which blocks the actions of ANGPTL3. Research using animal and human models have found that blockade with ANGPTL3 ASO reduced triglycerides by 63%, LDL by 32%, apoB by 36%, very low-density lipoprotein by 60%, non-HDL by 36%, and improved insulin resistance. Previous animal research reported loss-of-function variants in ANGPTL3 had increased levels of LPL, endothelial lipase activity, and insulin sensitivity with a fall in serum-free fatty acids . To date, neither Inclisiran nor ANGPTL3 ASO have completed cardiovascular outcome trials. Enrollment has gone underway, so our commentary on their clinical evidence will be limited in this review, with more focus placed on established therapies such as PCSK9 inhibitors. A Randomized Trial Assessing the Effects of Inclisiran on Clinical Outcomes Among People With Cardiovascular Disease (ORION-4) is underway .
An exciting emerging non-statin is a highly purified and concentrated EPA, icosapent ethyl, with a utility recently validated by a positive cardiovascular event trial. The REDUCE-IT trial enrolled 8179 patients receiving statin therapy with established cardiovascular disease or with diabetes in a multicenter, randomized, double-blind, placebo-controlled trial over a median duration of 4.9 years . Patients had a fasting triglyceride level of 135–499 mg/dl (1.52–5.63 mmol/L) and an LDL-C of 41-100 mg/dl (1.06–2.59 mmol/L). The primary end-point event (composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina) occurred in 17.2% of the patients in the icosapent ethyl group, as compared with 22.0% of the patients in the placebo group (hazard ratio 0.75, 95% CI 0.68–0.83, P < 0.001) . Potential mechanism of EPA has been addressed by Sheikh et al.  in a recent review of clinical benefits of EPA and atrial fibrillation. Icosapent ethyl is currently approved by the Food and Drug Administration (FDA) of the USA for hypertriglyceridemia refractory to behavioral and pharmacologic response, representing another therapeutic non-statin which has been shown to improve outcomes in dyslipidemia [35,36]. However, because it is a derivative of n-3 fatty acids, icosapent ethyl raises concern for potentially causing adverse bleeding in patients who are on anticoagulation therapy by increasing bleeding time. However, there has been a wave of research to show this phenomenon may not take place clinically [37–40]. The implications of PCSK9 inhibitors, ANGPTL3, concentrated EPA/icosapent ethyl, statins, and siRNA on the prevention and cardiovascular event reduction are visualized in Fig. 6 .
To understand how novel non-statin therapies may affect the physiology of the human body by affecting cholesterol regulation and metabolism, PCSK9 will be used as a model therapeutic. The rationale for this is designation is due to to the volume of literature surrounding PCSK9 as well as the interesting refractory clinical implications of altering cholesterol metabolism in spite of cholesterol presence via dietary consumption. A review of the function of PCSK9 inhibitors at the genetic and molecular level throughout the literature, to help explain how these mechanisms translate into clinically observable effects, was performed. Specifically, we examined their cardiometabolic properties.
Some of the first insight into the role of PCSK9 on cholesterol metabolism came from studies that showed differential expression of PCSK9 resulted in disparate LDL-C levels. In 2003, Abifadel et al.  showed that a gain-of-function mutation of PCSK9 was associated with increased LDL-C levels and played a role in the pathogenesis of autosomal dominant hypercholesteremia. Shortly thereafter in 2005, Cohen et al.  reported on a cohort of patients of African-American descent in the Dallas Heart Study who exhibited low LDL-C levels secondary to non-sense PCSK9 mutations. Subsequently, Cohen et al.  looked at 3363 African-American subjects with non-sense mutations in PCSK9 (2.6%). These patients had an 88% reduction in the risk of coronary heart disease (P < 0.008).
These findings spurred interest into the role that PCSK9 has on lipid metabolism. PCSK9, otherwise known as Proprotein convertase subtilisin/kexin type 9, is an enzyme encoded by the PCSK9 gene on chromosome 1 in humans . PCSK9 binds to LDL particle receptors (LDLR), a receptor primarily expressed in the liver but may also be found in other membranes located within the kidney, the central nervous system, pancreatic β-cells, and vasculature [46,47]. This receptor normally promotes LDL particle uptake from the extracellular fluid to the intracellular compartment, reducing serume LDL particle levels. When LDL binds to LDLR, an LDLR-LDL complex forms, and is then directed to the intracellular endosome. Within the endosome, LDLR-LDL undergoes structural changes due to the acidity of the endosome. These structural changes manifest as the retention of LDL within the cell and thus subsequently promotes the recycling of LDLR to the surface of the membrane [48,49]. This conformational change is prevented by PCSK9, sending the LDLR to the lysosome, leading to decreased LDL-C uptake [48,49]. Inhibition of PCSK9, therefore, allows more LDLR recycling, ncreased LDL-C uptake, and thus lower serum LDL-C levels. These peculiar findings prompted the development of PCSK9 inhibitors, with the first two drugs (alirocumab and evolocumab) approved for use by the US FDA in 2015.
Recent research has shown that PCSK9 exhibit effects that extend beyond LDL-C uptake that may be of interest to the diabetic patient with cardiometabolic comorbidities. Insulin plays a critical role in regulating LDL receptor expression by increasing hepatic LDLR levels, leading to decreased LDL-C levels. Conversely, PCSK9 controls LDLR levels in pancreatic β-cells, and may serve a role as placing a cap on the cholesterol influx within the pancreas to maintain physiological homeostasis . Therefore, inhibition of PCSK9 results in increased LDLR expression, leading to increased pancreatic cholesterol levels, followed by hypoinsulinemia and hyperglycemia secondary to possible β-cell dysfunction [50–52]. It is notable, however, that Da Dalt et al. , found no change in insulin resistance with PCSK9 deficiency . The effects of residual hyperglycemia on diabetic or pre-diabetic patients susceptible to new-onset diabetes, however, warrant concern. Hyperglycemia is thought to promote the autonomic neuropathy seen in patients with T2DM. One significant example is the resulting neurologic dysregulation of the myocardium, resulting in an increased QTc and compromised ventricular repolarization status, warranting concern for a possible arrhythmic exacerbation in diabetic patients . This is a consideration that needs to be taken into account when showing decreased adverse cardiac events associated with PCSK9 inhibitor use. Reassuring data for the concern of new-onset diabetes is provided in a pooled analysis from 10 ODYSSEY Phase 3 studies, which showed that alirocumab had no association with new-onset diabetes in trial participants . A meta-analysis of phase 2/3 randomized clinical trials assessed PCSK9 inhibitor use and the manifestation of diabetes and found a small, but albeit statistically significant increase in plasma glycemia and HgbA1c with short-term PCSK9 inhibitor use .
One aspect of extrahepatic systems in which cholesterol undergoes dynamical effects include the renal considerations of administering PSCK9 inhibitors. In terms of their metabolism, these therapies are monoclonal antibodies and thus do not undergo renal elimination, but rather elimination by the reticuloendothelial system, therefore metabolism in patients with T2DM and diabetic nephropathy is not expected for decreased eGFR [57,58]. PCSK9 is also produced by renal tissue, and reduces the expression of epithelial sodium channel in the renal tubules . This would potentially raise concern that PCSK9 inhibition would promote increased sodium retention and subsequent hypertension, increasing long-term cardiovascular outcomes in patients with T2DM by increasing left ventricular preload and subsequent remodeling as a consequence of prolonged left ventricular wall stress. However, this has not been observed in mouse models with PCSK9 deficiencies or during administration of monoclonal antibodies targeting PCSK9 [60,61]. Moreover, there is a discrepancy in the literature in elucidating the relationship between PCSK9 expression and renal function. Some studies have endorsed no correlation between serum PCSK9 and eGFR, while others have found that increased PCSK9 (possibly though proteasome-mediated channel degradation) was related to lower renal function [62,63]. The notion of eGFR in individuals with T2DM and dyslipidemia receiving PCSK9 is critical as reduced renal function may expedite one of the more feared consequences of prolonged T2DM, diabetic nephropathy. Further studies are needed to validate the long-term effects of PCSK9 inhibitors on renovascular and cardio-renal physiology.
The intestine is a major node of cholesterol and lipid flux throughout the human body, and has long been sought out as a target of modulating cholesterol transit from dietary intake to the vasculature . The mechanisms of cholesterol metabolism central to the intestine have been identified, and are currently being studied. Exploitation of the molecular biochemistry surrounding this organ system has been targeted, with Ezetimibe serving as a classical example as it blocks cholesterol absorption from the small bowel by inhibiting intracellular cholesterol transport molecules within the enterocyte, such as NPC1L1. This decreases the total load of cholesterol available for hepatic uptake, forcing the liver to absorb more cholesterol from the vasculature, and lowering cholesterol in circulation. Because of its role in serving as a junctional site of cholesterol metabolism, it should not be unreasonable to surmise whether or not PCSK9 has implications in intestinal cholesterol flow. The expression of PCSK9 in the intestine is speculated to play a role in cholesterol metabolism in this intestinal compartment. PCSK9 is expressed in human enterocytes, as evident in jejunal biopsies [65,66]. PCSK9 has been shown to negatively modulate transintestinal cholesterol excretion (TICE), a route of cholesterol metabolism that has garnered more interest into its effects in dyslipidemia pathogenesis [67,68]. Mice who exhibit loss of function in PCSK9 expression expressed increased TICE, leading to lower cholesterol levels. Moreover, injection of PCSK9 in mice led to decreased TICE and therefore, increased cholesterol absorption and loss of hepatic absorption of extra-intestinal cholesterol from the circulation, leading to elevated cholesterol [69,70]. Moreover, it was noted that PCSK9 deficient individuals exhibited an increase in chylomicron cholesterol clearance, representing an auxiliary function in the role PCSK9 inhibition has in cholesterol reduction with regards to the intestine . Studying the effects of PCSK9 inhibition in T2DM with respect to postprandial lipid flux represents an avenue in which LDL-C mitigation could confer mortality benefit secondary to a reduction in adverse cardiovascular events from a different angle.
PCSK9 inhibitors may also play a role in the hypothyroid patient, possibly exacerbating the dyslipidemia experienced in patients with T2DM . Moreover, hypothyroidism increases LDL-C levels by downregulating LDLR . PCSK9 levels are increased in hypothyroidism as well, which drive LDLR levels down and increase LDL-C levels further, exacerbating the dyslipidemia that patients with T2DM with subclinical hypothyroidism may experience [74,75]. PCSK9 inhibition, therefore, offers a promising secondary effect of ameliorating LDL burden in the diabetic patient with hypothyroidism, however, no formal FDA guidelines exist for their use, neither do robust widescale clinical trials investigating this relationship.
Much of the literature emphasizing the expression of PCSK9 expression is focused on the liver. The presence of PCSK9 in endothelial cells, vascular smooth cells, and macrophages suggests that PCSK9 has a role in the development of atheromas and thus, vascular tone. PCSK9 expression in these cells suggest that it may be a culprit of LDL retention in the arterial intima . LDL deposition in the arterial intima has been associated with arterial stiffness. PCSK9 inhibition may serve as an avenue for promoting vascular contractility, a relationship that has been extensively studied in elucidating the benefits of cardiovascular mortality seen in PCSK9 expression . The latter is promising to the patient with T2DM who already experiences vascular compromise secondary to hyperglycemic inflammation and the accumulation of advanced glycation end products, with the potential relief of vascular stiffness burden may confer mortality benefits in these patient populations by decreasing fixed afterload that the heart has to pump against.
Because of their multisystemic effects, PCSK9 inhibitor use in Random Clinical Trials (RCTs) has received interest to examine their efficacy in human populations. The Further Cardiovascular Outcomes Research with PCSK9 Inhibition in Subjects with Elevated Risk (FOURIER) trial was an RCT with the goal to evaluate the effects of a PCSK9 inhibitor, evolocumab, had on subjects with elevated cardiovascular risk on statin therapy . Study participants were randomized to evolocumab therapy (n = 13 784) versus placebo (n = 13 780). Of these patients, 11 031 had T2MD versus 16 533 without T2DM. The primary outcome of this trial was the incidence of cardiovascular death, MI, stroke, coronary revascularization, or hospitalization for angina. A diabetic subanalysis of FOURIER showed that evolocumab significantly reduced primary outcome incidence in both patients with T2DM and normoglycemic patients. Hazard ratios with respect to the primary outcome for patients with T2MD and normoglycemic patients were 0.83 (95% CI 0.75–0.93; P = 0.0008) and 0.87 (95% CI 0.79–0.96; P = 0.0052) . It is noteworthy that despite evolocumab showing similar efficacy in primary outcome reduction, patients with T2DM had a greater absolute risk reduction over time with PCSK9 inhibition compared to normoglycemic patients [2.7% (95% CI 0.7–4.8) versus 1.6% (95% CI 0.1–3.2) reduction in the primary endpoint over 3 years] . The latter is explained by the increased risk of cardiovascular events in diabetics.
In a trial by Nicholls et al. , patients with prior CAD on statin therapy were randomized and received either evolocumab or placebo in hopes of analyzing if PCSK9 therapy would confer differential coronary plaque composition. This trial, Global Assessment of Plaque Regression with a PCS9 antibody as measured by intravascular ultrasound, showed that evolocumab therapy promoted regression of plaque formation compared to statin (−1.2% vs. +0.17%; P < 0.0001). However, no statistically significant difference was noted in volumes of calcium, fibrofatty, fibrous, or necrotic components of the plaques studied. Coupled with the notion that PCSK9 inhibitors reduce mortality as shown by clinical trials, we can deduce that LDL-C lowering therapy and possibly other non-statin lipid-lowering medications confer their cardiovascular event outcome benefits in a mechanism that supersedes plaque composition and morphology.
The ODYSSEY trial (Evaluation of Cardiovascular Outcomes After an Acute Coronary Syndrome During Treatment with Alirocumab) was an RCT conducted to compare the effects of alirocumab relative to placebo with a composite endpoint of coronary heart disease death, non-fatal MI, fatal and non-fatal ischemic stroke, and unstable angina requiring hospitalization in study participants who had acute coronary syndrome 1–12 months prior to randomization . Study participants were randomly assigned in equal groups of those receiving alirocumab titrated to achieve LDL-C concentrations between 25 and 50 mg/dl. The study included 5444 patients with T2DM (28.8%), 8246 (43.6%) had prediabetes, and 5234 patients without diabetes (27.7%). The results of ODYSSEY showed similar relative reductions in the incidence of the primary endpoint for each category of glycemic patients. Notably, there was a greater absolute reduction in primary endpoint incidence in patients with T2DM (2.3%, 95% CI 0.4–4.2) relative to those with impaired glucose tolerance or prediabetes (1.2%, 95% CI 0.0–2.4) and patients without T2DM (1.2%, 95% CI −0.3 to 2.7).
The latter two trials indicate that there are significant benefits for T2DM patients in terms of cardiovascular outcomes. However, more extensive studies are needed to assess the efficacy of these therapeutics on concomitant cardiometabolic sequelae in patients with T2DM. These include outcomes in diabetic nephropathy, diabetic retinopathy, and and major adverse cardiovascular effects (MACE). These are attributed to the effects of mitigation of LDL-C deposition in arterial intima, vascular tone, and normotensive effects PCSK9 inhibitors confer as observed in the literature. If PCSK9 inhibitors are to become as part of a standard pharmacotherapy toolkit in patients with comorbid conditions, their effects also need to be serially studied with mainstay medications in T2DM patients with increased ASCVD risk factors, such as ACE inhibitors, Metformin, Insulin, GLP-1 agonists, SGLT-2 inhibitors, and anti-platelet therapeutics.
Through this work, we sought to not only report on novel non-statin drugs and their roles in cardiac diabetology, but to show how evaluations of their systemic effects may shape our perspective on what outcomes are truly clinically beneficial and what endeavors can be pursued to show the latter. Inadequate clinical outcomes with interventional procedures and conventional pharmacotherapy in those with atherosclerotic disease spurred innovation and inquiry to cardiometabolic therapeutics elsewhere. PCSK9 inhibitors, whose development was primarily galvanized through empiric observation of genetics differences, reinforces a valuable lesson that understanding the translational biology of therapeutics such as LDL-C and LDLR function may predict the challenges in implementation given possible side effects and notable metabolic consequences. These questions then propose clinical trials which validate, or in some cases, reject held dogmas in medicine. In this review, we showed that PCSK9 inhibitors offer a good model given the understanding of the biochemistry and molecular biology involved in their function. Inquiries in insulin resistance, renovascular implications, plaque and cardiovascular disease, as well as endocrine functions soon followed and our aim was to express the results of these inquiries. Studies involving the later mostly showed that these therapeutics are safe and reduce adverse cardiovascular events, if adequate clinical supervision and judgement is practiced on behalf of providers to appraise their patient’s total health parameters. These lessons can be extended to novel non-statin drugs, of which our understanding surrounding their function is everchanging. Moreover, as we gain insight into cholesterol metabolism and cardiometabolic regulation, retroactive insight into these therapeutics shape our understanding on reducing negative disease outcomes, such as in atherosclerotic disease.
Conflicts of interest
There are no conflicts of interest.
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