Cholesteryl ester transfer protein inhibition: a pathway to reducing risk of morbidity and promoting longevity

Abstract

Purpose of review

To review the evidence and describe the biological plausibility for the benefits of inhibiting cholesteryl ester transfer protein (CETP) on multiple organ systems through modification of lipoprotein metabolism.

Recent findings

Results from observational studies, Mendelian randomization analyses, and randomized clinical trials support the potential of CETP inhibition to reduce atherosclerotic cardiovascular disease (ASCVD) risk through a reduction of apolipoprotein B-containing lipoproteins. In contrast, raising high-density lipoprotein (HDL) particles, as previously hypothesized, did not contribute to ASCVD risk reduction. There is also an expanding body of evidence supporting the benefits of CETP inhibition for safeguarding against other conditions associated with aging, particularly new-onset type 2 diabetes mellitus and dementia, as well as age-related macular degeneration, septicemia, and possibly chronic kidney disease. The latter are likely mediated through improved functionality of the HDL particle, including its role on cholesterol efflux and antioxidative, anti-inflammatory, and antimicrobial activities.

Summary

At present, there is robust clinical evidence to support the benefits of reducing CETP activity for ASCVD risk reduction, and plausibility exists for the promotion of longevity by reducing risks of several other conditions. An ongoing large clinical trial program of the latest potent CETP inhibitor, obicetrapib, is expected to provide further insight into CETP inhibition as a therapeutic target for these various conditions.

INTRODUCTION

Cholesteryl ester transfer protein (CETP) is a hydrophobic glycoprotein that is a member of the lipid transfer protein family [1–3]. It facilitates the bidirectional exchange of cholesteryl esters and triglycerides among lipoprotein particles leading to a net mass transfer of cholesteryl esters from high-density lipoprotein (HDL) to low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL) particles. In addition, triglycerides are transferred in the opposite direction from LDL and VLDL to HDL [1,2]. Interest in CETP inhibition as a therapeutic target began with the discovery in observational studies that some CETP gene polymorphisms were associated with reduced coronary heart disease (CHD) incidence and CHD mortality, although these results have not been entirely consistent [2,4–6]. However, taking all evidence into consideration, observational studies, Mendelian randomization (MR) analyses, and randomized clinical trials of pharmaceutical agents indicate that CETP inhibition confers cardiovascular benefit and reduces risk of atherosclerotic cardiovascular disease (ASCVD) [6,7,8^▪▪,9^▪▪]. Additionally, emerging evidence suggests that CETP inhibition may promote longevity [10^▪,11], presumably by lowering the risk of several conditions associated with aging such as new-onset type 2 diabetes mellitus (T2D), dementia, chronic kidney disease (CKD), and age-related macular degeneration (AMD), as well as promoting survival in septicemia [12^▪▪,13^▪▪]. 

Box 1.

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Cholesteryl ester transfer protein inhibition and atherosclerotic cardiovascular disease

The somewhat inconsistent association between CETP levels or activity and CHD incidence and mortality in observational studies can be largely explained by including different CETP gene polymorphisms, other genes, and environmental factors [2,5,6,14]. A meta-analysis of the CETP C-629A polymorphism found no association with CHD risk in the overall analysis, and a significant increased risk of CHD in the subgroup analysis of Caucasian individuals [15]. In contrast, another study revealed that the CETP TaqIB variant was associated with a lower risk of composite ischemic cardiovascular disease risk in both Asians and Caucasians [16]. Results of much larger MR studies invariably support that genetically reduced CETP levels or activity are associated with lower CHD risk in both ancestries [17,18]. The results of a MR meta-analysis of the CETP rs708272 polymorphism indicated that carriers of one B1 allele as well as the B1B1 genotype had 17% and 25% reduced risk of CHD, respectively, for a 0.2 μg/ml reduction in circulating CETP [19]. Furthermore, results from a MR analysis that examined the association between changes in LDL cholesterol (LDL-C) and the risk of cardiovascular events related to CETP gene variants indicated that a higher CETP genetic score (reduced CETP activity) was associated with decreased LDL-C and apolipoprotein (Apo) B concentrations and reduced risk of major cardiovascular events [20]. These results point to lowering LDL-C and Apo B as the driving factor for reducing ASCVD risk with CETP inhibition, rather than increasing HDL-C, as was originally hypothesized [6].

Results from clinical trials of CETP inhibitors have demonstrated that, compared to placebo, torcetrapib, evacetrapib, anacetrapib, and, most recently obicetrapib, reduced LDL-C and Apo B levels, as well as increased HDL-C [21–24,25^▪▪]. Dalcetrapib 600 mg once daily did increase HDL-C modestly, but had a minimal effect on LDL-C [26]. In the Randomized Evaluation of the Effects of Anacetrapib through Lipid Modification cardiovascular outcomes trial, anacetrapib 100 mg once daily, as an adjunct to a high-intensity statin, reduced LDL-C by 17% (beta-quantification), non-HDL-C and Apo B by 18%, and major adverse cardiovascular events by 9% [rate ratio, 0.91; 95% confidence interval (CI), 0.85, 0.97; P = 0.004] [23]. The sponsor did not apply for regulatory approval for anacetrapib after a review of its clinical profile, which included an extended half-life and accumulation in adipose tissue [27,28]. Previous CETP inhibitors also failed to reach market approval for a variety of compound specific, not drug class, reasons [7,8^▪▪]. A recent systematic review and meta-analysis of nine randomized controlled trials of CETP inhibitors compared to placebo with at least 6 months follow-up between 2003 and 2023 revealed that the use of a CETP inhibitor was associated with significantly reduced cardiovascular disease-related mortality [risk ratio (RR), 0.89; 95% CI, 0.81–0.98; P = 0.02; I² = 0%) and risk of myocardial infarction (RR, 0.92; 95% CI, 0.86–0.98; P = 0.01; I² = 0%) [9^▪▪]. Intuitively, these results were primarily attributable to anacetrapib.

Obicetrapib is a next generation, highly selective CETP inhibitor that was developed as a tetrahydroquinoline derivative with a pyrimidine and an ethoxycarbonyl structure and two chiral centers for improved physical and biopharmaceutical properties [7,8^▪▪,29,30]. Unlike anacetrapib, obicetrapib does not have clinically relevant accumulation; 10 mg obicetrapib has a mean terminal half-life of 131 h [29,30]. It potently inhibits CETP activity by 97% at the 10 mg dose and in Phase I and II clinical trials produced significant reductions in the concentrations of LDL-C, non-HDL-C, Apo B, lipoprotein(a), and total and small LDL particles. Significant increases in HDL-C, Apo A1, prebeta HDL levels, as well as cholesterol efflux capacity were also observed [8^▪▪,24,25^▪▪,31–34]. Several Phase III trials of obicetrapib are in progress, including the Cardiovascular Outcome Study to Evaluate the Effect of Obicetrapib in Patients with Cardiovascular Disease (PREVAIL, NCT05202509).

Cholesteryl ester transfer protein and effects on high-density lipoprotein cholesterol concentration and high-density lipoprotein functionality

Even though the benefits of pharmacologic CETP inhibition for ASCVD risk reduction appear to be largely mediated through its effects on Apo B-containing particles, its effects on HDL might still have important clinical implications for other conditions. Genetically or pharmacologically inhibited CETP activity results in higher concentrations of HDL-C, HDL particles, Apo A1, prebeta-1 and prebeta 2 HDL, as well as increased HDL particle size and higher cholesterol efflux capacity [2,24,25^▪▪,32,33,35–38]. HDL particles have a key role in cholesterol efflux, which is the first step in reverse cholesterol transport, that is, the transfer of cholesterol from peripheral cells to HDL and from HDL to the liver for excretion through the bile synthesis pathway. HDL also has antioxidative, vasodilatory, anti-inflammatory, antimicrobial, and antithrombotic activities [13^▪▪,39]. Low levels of HDL-C are an established ASCVD risk factor, although recent studies suggest there is a U-shape to this relationship such that markedly elevated HDL-C may be associated with increased cardiovascular morbidity and mortality [40]. However, a more recent analysis with much longer follow-up has shown that this high HDL-C to mortality association is based on unreported abuse of alcohol and can therefore be considered a classical confounder [41]. MR analyses do not support HDL-C levels as a causal cardiovascular risk factor, and it is now believed that other HDL-related factors, such as the concentration of HDL particles and specific HDL sub-populations with different proteomic/lipidomic profiles, may be better indicators of disease risk [40,42,43,44^▪].

Apo A1, of which there are ∼2–6 molecules per HDL particle, is the most prevalent HDL apolipoprotein. It activates cholesterol efflux in a process mediated by the ATP binding cassette (ABC) transporters. ABCA1 is induced by liver X receptors (LXR) when macrophages accumulate cholesterol, thereby promoting the transport of free cholesterol and phospholipids to HDL [43]. HDL particles can exist as mature, spherical alpha HDL, but also as lipid-poor Apo A1-containing particles called prebeta-1 HDL [43,45,46]. Prebeta-1 HDL is the principal acceptor of cholesterol in the cholesterol efflux process. After cholesterol has been effluxed from cells to prebeta-1 HDL, it is esterified via lecithin cholesterol acyltransferase (LCAT), and the cholesteryl esters drive larger prebeta HDL species and, ultimately, alpha HDL. Then the cholesteryl esters are transferred via CETP to LDL and VLDL particles. There is some disagreement about whether prebeta-1 HDL levels should be considered anti or pro-atherogenic, because, somewhat counter-intuitively, some observational studies have reported that prebeta-1 HDL levels are positively associated with CHD and myocardial infarction risk [43,45,46]. There may be defects in the dynamic process of cholesterol efflux and HDL maturation when CHD is present which might explain the accumulation of prebeta-1 HDL [46]. Furthermore, quantification of HDL particles and subfractions by different methods have provided disparate results. Accordingly, relationships between HDL particle subfractions and ASCVD risk have not yet been fully elucidated.

In ROSE2, nuclear magnetic resonance lipoprotein particle analysis demonstrated that the rise in HDL-C with obicetrapib was next to an increase of prebeta HDL also related to an increase in the concentration of large HDL particles and, consequently, HDL particle size increased 19.3% in the 10 mg obicetrapib arm compared with 0.00% for placebo [25^▪▪]. Prior studies of evacetrapib and obicetrapib demonstrated that these CETP inhibitors increased prebeta-1 HDL particles in patients with dyslipidemia taking statins when assessed using nondenaturing two-dimensional gel electrophoresis and/or immunofixation [33,35]. However, in patients with ASCVD or diabetes on atorvastatin, evacetrapib, compared to placebo, increased HDL1 and HDL2 subclasses, and significantly reduced smaller, more dense HDL3 and prebeta-1 HDL levels, when assessed using a native polyacrylamide gel electrophoresis system with lipid prestaining [47]. Additional studies are needed to further understand the effects of CETP inhibition on the remodeling of HDL and flux of cholesterol through this lipid fraction.

There are at least 80 different proteins present on the HDL particle. Beyond Apo A1, other highly abundant HDL proteins include Apo A2, Apo C3, and Apo E [42]. Results from investigations examining HDL subspecies according to their contents of various proteins have reported different correlations between these subspecies and relative risk of CHD and cholesterol efflux capacity [48,49]. An investigation of HDL subspecies in the Study of Evacetrapib in Participants with High Cholesterol and the Investigation of Lipid Level Management to Understand its Impact on Atherosclerotic Events reported that treatment with evacetrapib or torcetrapib, respectively, on a background of atorvastatin increased Apo A1 in HDL subspecies that also contained Apo C3 and Apo E [21,50,51]. These results point to the importance of investigating effects of CETP inhibition on HDL particle concentration and composition, rather than focusing simply on HDL-C.

Diabetes

T2D, which is associated with 2- to 4-fold higher risk of ASCVD, is commonly accompanied by a dyslipidemic profile characterized by elevated triglycerides and non-HDL-C, increased small, dense LDL, and decreased HDL-C, as well as quantitative and qualitative changes in HDL particles that impair their ability to efflux cholesterol and that reduce their antioxidant, antiapoptotic, and anti-inflammatory properties [44^▪,52,53]. These HDL defects are associated with reduced secretory capacity of pancreatic beta-cells, increased apoptosis, and impaired glucose uptake in skeletal muscle. There is substantial evidence that CETP inhibition has a role in reducing risk of T2D through its effects on HDL particles [12^▪▪,53]. While the exact mechanisms have not yet been defined, results from cell and in vivo rodent studies indicated that dalcetrapib, anacetrapib, and torcetrapib stimulated reverse cholesterol transport in pancreatic beta cells, as well as increased insulin secretion and reduced apoptosis [12^▪▪]. Recently, a 2x2 factorial MR analysis of UK Biobank participants of European ancestry was conducted that examined previously constructed genetic scores for CETP and sodium-glucose co-transporter-2 (SGLT2) in relation to glycated hemoglobin (HbA1c) and incidence of T2D [54]. Results demonstrated that individuals with genetic inhibition of both CETP and SGLT2 had significantly lower HbA1c than controls, and that joint inhibition was associated with decreased incidence of diabetes compared to controls and to those with SGLT2 inhibition alone [54].

Clinical trials of CETP inhibitors also support a beneficial effect on diabetes risk. Indeed, two meta-analyses of randomized controlled trials, comparing CETP inhibitors to control conditions, found significant reductions in the risk of new-onset diabetes of 12% and 16%, respectively [55,56^▪]. Glycemic measures were also significantly improved in those with and without diabetes across most trials [56^▪]. Patients with diabetes in the Investigation of Lipid Level Management to Understand its Impact in Atherosclerotic Events trial who received torcetrapib plus atorvastatin had lower levels of plasma glucose and HbA1c than participants who received statin alone [57]. A posthoc analysis demonstrated that although the correlations between the change in HDL-C and changes in glucose, insulin, or HbA1c failed to achieve statistical significance, when the differences in the glycemic variable responses between the treatment groups were adjusted for the change in HDL-C, their statistical significance was attenuated [57]. The ongoing Phase III Randomized Study to Evaluate the Effect of Obicetrapib on Top of Maximum Tolerated Lipid-Modifying Therapies (BROADWAY; NCT05142722), which is investigating the lipid effects of obicetrapib in participants with a history of ASCVD and/or underlying heterozygous familial hypercholesterolemia, includes exploratory endpoints of percentage changes in HbA1c, homeostasis model assessment of insulin resistance (HOMA-IR), and fasting blood glucose [58].

Neurodegeneration

Alzheimer's disease (AD) is the most common cause of dementia. CETP variants leading to decreased CETP expression and/or activity are associated with good cognitive performance and with a reduced risk of developing AD, particularly among subjects carrying the APOE4 allele, the single greatest risk factor for AD [59,60,61^▪,62]. Apo E is a major cholesterol carrier in the brain. It transports cholesterol between astrocytes and neurons and is necessary for neuronal growth, membrane repair/remodeling, synaptogenesis, clearance and degradation of amyloid beta, and immune modulation [63]. In the brain, there is a unique Apo E-HDL particle. Apo E must be properly lipidated to perform its functions and this process of lipidation is influenced by structural differences associated with Apo E isoforms (E2, E3, and E4). Apo E2 is the most efficient isoform for promoting cholesterol efflux from cells, whereas Apo E4 is the least efficient [63].

Understanding the potential mechanisms whereby CETP inhibition may be a therapeutic target for AD is still in its early stages [12^▪▪,61^▪,62]. In CETP-expressing amyloid precursor protein (APP) mouse models, CETP inhibition with evacetrapib rescued memory deficit, and memory performance was correlated with HDL and LDL plasma levels, suggesting that high CETP activity may play an aggravating role in AD [64]. In mice expressing the human CETP gene, evacetrapib was able to cross the blood-brain barrier and was detectable in brain tissue [62]. In humans, brain CETP expression is quite low, but it is found in the cerebrospinal fluid at ∼12% of the concentration found in plasma [62]. As described by Mehta et al., inhibiting CETP is hypothesized to reduce the overall brain content of cholesterol (oxysterols) and Apo E4, and increase Apo A1 and prebeta HDL in the circulation and in the choroid plexus, which might restore dysfunctional neural cholesterol metabolism [12^▪▪,13^▪▪,62]. Initial results from a Phase IIa trial of 10 mg obicetrapib in 13 patients with early AD carrying an APOE4 variant demonstrated reduced 24- and 27-hydroxycholesterol levels in cerebrospinal fluid and increased amyloid beta 42/40 ratio in plasma [65]. A significant body of evidence indicates that these parameters are sensitive biomarkers for AD risk [66,67]. Further investigation of obicetrapib in the AD patient population is planned.

Other potential clinical applications for cholesteryl ester transfer protein inhibition

Beyond the therapeutic areas described above, there are numerous other plausible clinical applications for CETP inhibition, such as for the reduction of CKD, AMD, and septicemia [12^▪▪,13^▪▪,68,69]. In a drug target MR analysis conducted by Schmidt et al., lower CETP concentration was associated with lower risk of CKD (odds ratio, 0.94; 95% CI, 0.91–0.97) [70]. This is supported by indications that HDL composition and function are important predictors of CKD, suggesting a role for CETP inhibition [13^▪▪,68]. Evidence from MR analyses suggested that haplotypes in the CETP gene leading to lower CETP activity were associated with higher risk of AMD [70–72]. However, these results were obtained in epidemiological surveys designed to elucidate the pathogenesis of ASCVD and were not meant to evaluate AMD per se. In fact, they are fundamentally flawed for this use because of self-reported diagnosis of AMD, no correction for age, and, as explained by Ference, on-target MR cannot be used to unravel “side-effects” of a therapy that targets the protein of interest [73,74]. As such, results from MR cannot be used for estimating the potential adverse drug reactions or evidence for a causal relationship between a genetically instrumented exposure and an observed outcome. There was no association between CETP allele carrier status and any stage of AMD. A recent MR analysis targeted at AMD in all its clinical stages that utilized retinal photography-based diagnosis of AMD, revealed no relationship between low CETP activity haplotypes and the prevalence of AMD at any stage [75]. Another counter argument is that there have been no case reports of AMD in patients with homozygous or heterozygous CETP deficiency or reports of issues with the eye in clinical trials of CETP inhibitors [12^▪▪]. In contrast, there is evidence that raising HDL, which carries lipophilic antioxidants, and increasing prebeta HDL with CETP inhibition may, in fact, benefit eye health [76]. Further investigation of obicetrapib in this regard is planned. A potential role for CETP inhibition in sepsis is also promising [12^▪▪,13^▪▪,69]. CETP inhibition is suggested to increase HDL-mediated scavenging of endotoxins [77]. Genetic scores for decreased CETP function were associated with significantly decreased sepsis mortality in the UK Biobank and in the Identification of SNPs Predisposing to Altered Acute Lung Injury Risk sepsis cohorts (hazard ratios [95% CIs] were 0.77 [0.59–1.00] and 0.60 [0.37–0.98], respectively, per 1 mmol/L increased HDL-C) [69]. In an APOE∗3-Leiden CETP mouse cecal-ligation and puncture model of sepsis, CETP inhibition with anacetrapib preserved HDL-C and Apo A1 levels and increased survival, relative to placebo [69]. Ongoing mice model studies of sepsis using obicetrapib are also underway.

CONCLUSION

At present, there is robust evidence to support the potential benefits of reducing CETP for ASCVD risk reduction and increasing indications of its ability to promote longevity by reducing risks of several other conditions including T2D, dementia, CKD, AMD, and septicemia. Ongoing clinical trials of obicetrapib are expected to provide further insight into CETP inhibition as a therapeutic target.

Acknowledgements

The authors wish to thank Carol F. Kirkpatrick, PhD, MPH, RDN and Mary R. Dicklin, PhD, with Midwest Biomedical Research, for their assistance in the preparation of this manuscript.

Financial support and sponsorship

This manuscript was funded by NewAmsterdam Pharma B.V., Naarden, The Netherlands.

Conflicts of interest

Michael H. Davidson, Andrew Hsieh, and John J.P. Kastelein are employees of NewAmsterdam Pharma B.V. and they also report that they receive stock or stock options.

REFERENCES AND RECOMMENDED READING

Papers of particular interest, published within the annual period of review, have been highlighted as:

• ▪ of special interest

• ▪▪ of outstanding interest