CETP INHIBITION: WHERE NOW?

A position statement by Philip Barter and Kerry-Anne Rye The Heart Research Institute, Sydney, Australia

This position paper is written in the light of the recent failure to demonstrate benefits of the cholesteryl ester transfer protein (CETP) inhibitor, torcetrapib, in human atherosclerosis imaging trials1-3 and of the adverse outcomes in those treated with torcetrapib in a human clinical end-point trial4. The reason for the failure of torcetrapib is not known but may have been the consequence of an off-target adverse pharmacological effect of the drug unrelated to CETP inhibition.

After examining the evidence, we recommend further investigation of the potential benefits of CETP inhibition in humans using inhibitors that do not share the adverse pharmacological effects of torcetrapib.

Rationale for targeting CETP as an anti-atherogenic strategy in humans

The CETP in human plasma promotes transfers of cholesterol from high density lipoproteins (HDLs) to low density lipoproteins (LDLs) and triglyceride-rich lipoproteins such as very low density lipoproteins (VLDLs) and chylomicrons5 (Figure 1). Inhibition of CETP has the potential to shift the balance of plasma cholesterol in favour of the protective HDL fraction (Figure 2).

Thus, inhibition of CETP has the potential to be anti-atherogenic by a number of mechanisms:

  • an increase in concentration of the protective HDL fraction;
  • a reduction in concentration of the (pro-atherogenic) LDL fraction;
  • a reduction in the cholesterol content of chylomicrons and VLDLs.

Evidence that inhibiting CETP is anti-atherogenic

CETP is not present in all animal species. Activity is prominent in humans, non-human primates and rabbits but is absent in the plasma of most other species, including mice, rats, dogs and pigs, to name but a few13.

Species such as rabbits and humans that express CETP activity in plasma tend to be highly susceptible to the development of atherosclerosis, while those such as rodents that lack the protein are naturally resistant to atherosclerosis (Figure 3).

Introduction of the human CETP gene into mice results in a reduction in HDL levels and a small increase in VLDL and LDL cholesterol and apoB (the major LDL protein) levels 14,15 and, in most (but not all) studies, increases susceptibility to development of atherosclerosis 5.

In contrast to mice, rabbits are highly susceptible to the development of diet-induced atherosclerosis. Rabbits also have a naturally high level of CETP 13. Furthermore, it has been demonstrated in several rabbit models of atherosclerosis that inhibiting CETP by a number of different mechanisms results in a marked reduction in susceptibility to development of atherosclerosis 16-19.

Small molecule chemical inhibitors of CETP have been used to study the inhibition of atherosclerosis in of cholesterol-fed rabbits. In one study using JTT-705, CETP activity was reduced by more than 90%, HDL cholesterol was almost doubled and non-HDL cholesterol was decreased by about 50% 18. These changes were accompanied by a 70% reduction in atherosclerotic lesions in the aortas of these animals. It was not possible to determine the relative importance of the increased HDLs versus the decreased LDLs in the reduction of atherosclerosis observed in these rabbit studies. This issue was addressed in a second rabbit study using another CETP inhibitor, torcetrapib 19.

When torcetrapib was used to inhibit CETP in cholesterol-fed rabbits there was a substantial increase in HDL cholesterol levels but little change in the concentration of non-HDL cholesterol. In this setting the major inhibition of atherosclerosis was most likely the consequence of the increase in HDL in the CETP inhibited animals 19.

In humans several mutations of the CETP gene have been identified as causing CETP deficiency and elevated levels of HDL cholesterol 5.

HDLs isolated from CETP-deficient subjects in which the deficiency is either genetic or a consequence of treatment with torcetrapib have been shown to have either a normal or an enhanced ability to promote the efflux of cholesterol from macrophages 20, 21.

In general, people with CETP deficiency have a reduced risk of coronary heart disease. It was found in two large studies that when CETP deficiency coincides with an HDL cholesterol level > 60 mg/dl (1.5 mmol/L) there is a low rate of coronary heart disease, comparable to that observed in subjects in whom an elevated HDL cholesterol level is not associated with a deficiency of CETP 22-24. These results are consistent with a proposition that a deficiency of CETP is protective so long as it induces a substantial increase in HDL cholesterol.

There are several reported polymorphisms of the human CETP gene. The relationship between these polymorphisms and susceptibility to atherosclerosis is variable, with no consistent picture emerging from numerous studies in which the relationship of the polymorphisms to atherosclerotic cardiovascular disease was investigated 5.

All of currently available evidence from animal and human studies supports the proposition that inhibiting CETP in humans has the potential to be profoundly anti-atherogenic.

Testing the anti-atherogenic potential of inhibiting CETP in humans

Small molecule inhibitors of CETP have been tested in human subjects and shown to have predictable effects on plasma lipoproteins. They substantially increase the concentration of HDL cholesterol and decrease that of LDL cholesterol and apoB 25-30.

To date the only human studies investigating the effects of CETP inhibition on atherosclerosis and cardiovascular events have been conducted with torcetrapib. However, this program was terminated following the discovery of an excess number of deaths in people taking the drug in the ILLUMINATE trial, a very large study designed to investigate the effects of CETP inhibition on cardiovascular outcomes 4. Three vascular imaging studies of the effects of torcetrapib on atherosclerosis were completed before the program was terminated and have now been reported 1-3. To date, there have been no human cardiovascular outcome trials reported using JTT-705.

The ILLUMINATE trial included 15,000 people with manifest cardiovascular disease or type-2 diabetes. All were treated with atorvastatin at a dose necessary to reduce the LDL cholesterol level to less than 100 mg/dL (2.6 mmol/L) before being randomized in a double-blind fashion to receive torcetrapib 60 mg per day or matching placebo. The follow-up was estimated to be 4.5 years in order to achieve enough events to test the hypothesis that treatment with torcetrapib was cardioprotective. This trial was terminated in December 2006 after a median follow up of only 18 months because of a statistically significant excess of deaths in the group treated with torcetrapib 4. The explanation for the excess mortality is currently not known but may have related to a torcetrapib-mediated increase in aldosterone.

Evidence that torcetrapib induces aldosterone secretion

A small blood pressure raising effect of torcetrapib was known before commencement of the human intervention trials but hazards were considered to be minor relative to the potential benefits of raising the concentration of HDLs. It was found in the ILLUMINATE trial that the torcetrapib-induced increase in systolic blood pressure was associated with an increase in plasma levels of aldosterone, a reduction in serum potassium and increases in serum sodium and bicarbonate levels 4 (Figure 4). In subsequent basic studies presented in two late breaking basic science posters at the America Heart Association scientific meetings held in Orlando, Florida, USA in November 2007 (DePasquale M, Knight D, Loging W, et al Mechanistic studies of hemodynamics with a series of cholesteryl ester transfer protein inhibitors. Late Breaking Science Session 12. American Heart Association Scientific Sessions, 5 Nov 2007, Orlando, FL, USA and Forrest MJ, Bloomfield D, Briscoe RJ, et al. Torcetrapib-induced blood pressure elevation is independent of CETP inhibition and is accompanied by an increase in circulating aldosterone levels. Late Breaking Science Session 11. American Heart Association Scientific Sessions, 5 Nov 2007, Orlando, FL, USA) it was found that blood pressure elevating effects of torcetrapib are apparent in animals that lack CETP as well as in those possessing CETP. Torcetrapib (and also structural analogs of torcetrapib that lack CETP inhibitory activity) not only raised blood pressure in vivo in animals but also induced aldosterone secretion in vitro from adrenal cortical cells by a mechanism that does not involve direct interactions with mineralocorticoid or glucocorticoid receptors. The increase in blood pressure following administration of torcetrapib to rats (a species lacking CETP) was abolished in animals that had been subjected to prior adrenalectomy. Finally, it was reported in these posters that there exist other potent CETP inhibitors that have no effect on blood pressure or on the secretion of alsdosterone by adrenal cortical cells. It has since been reported that the potent CETP inhibitor, anaceptrapib, does not raise blood pressure in humans 31.

The blood pressure, electrolyte, and clinical profile of increased cardiovascular mortality and morbidity in the ILLUMINATE trial are consistent with known hazards of aldosterone excess. The observed higher CHD mortality in the ILLUMIATE trial in those with greater reductions in potassium and greater increases in bicarbonate 4 (Figure 5) is consistent with this proposition. Use of torcetrapib in this trial was also associated with an excess of deaths from cancer and infections, although the incidence of non-fatal cancers and infections was not increased . It is possible that patients with pre-existing cancer and those with infections had their survival rate compromised by a worsening cardiovascular status.

While harm caused by an increase in aldosterone associated with the use of torcetrapib is one possible explanation for the observed adverse outcomes in the ILLUMINATE trial, it does not rule out the possibility that CETP inhibition per se may have been the reason. It has been suggested (but not proven) that inhibiting CETP may compromise HDL functionality, even to the extent of generating HDLs that are pro-atherogenic. The ILLUMINATE study did not address the issue of how torcetrapib impacts HDL functionality, although it was interesting to note that, within the group treated with torcetrapib, cardiovascular event rates tended to be lower in those whose increase in HDL-cholesterol or apolipoprotein A-I was greater than the median compared with those whose increases were below the median level of change 4. In fact, the greater the concentration of HDL-C achieved during treatment with torcetrapib in the ILLUMINATE trial, the lower the cardiovascular event rate (Barter, presented at the American Heart Association Scientific Sessions, 5 Nov 2007, Orlando, FL, USA) (Figure 6). This observation is consistent with the original rationale for considering CETP as a therapeutic target and inconsistent with a hypothesis of dysfunctional HDLs as the cause of the excess cardiovascular disease observed in ILLUMINATE.

In all three of the vascular imaging trials that were completed prior to the termination of the ILLUMINATE trial, patients were treated with atorvastatin to achieve optimal levels of LDL cholesterol before being randomised to receive torcetrapib at a daily dose of 60 mg or matching placebo. All subjects continued on atorvastatin throughout the trial. The treatments were continued for two years.

ILLUSTRATE 1 involved the use of intravascular ultrasound (IVUS) to assess the effect of torcetrapib on coronary atheroma burden while RADIANCE-1 2 and RADIANCE-2 3 used ultrasound to assess the effects of torcetrapib on carotid intima-media thickness. The ILLUSTRATE trial involved people with demonstrable coronary atheroma, while RADIANCE 1 and 2 involved patients with familial hypercholesterolemia and mixed hyperlipidemia, respectively.

Treatment with torcetrapib at a dose of 60 mg per day increased the concentration of HDL cholesterol by approximately 60% in all of these imaging trials. The LDL cholesterol level was reduced by approximately 20% over and above that achieved by atorvastatin. As in previous studies with torcetrapib, the blood pressure was increased by the treatment. Systolic blood pressure increased by 3-5 mm Hg in the three trials.

The effects of torcetrapib on atheroma were the same in all three trials, with no evidence that addition of torcetrapib to atorvastatin provided any benefits over and above those of atorvastatin alone. The extent to which this increase in systolic blood pressure impacted on these outcomes is not known.

It is known that aldosterone has adverse effects on the vasculature that can be explained only partly by an associated increase in blood pressure 32. Aldosterone induces arterial stiffening through collagen deposition in the extracellular matrix and subjects; for a given level of blood pressure, patients with hyperaldosteronism have a thicker common carotid IMT compared to subjects with essential hypertension 33, 34. Thus, to the extent that torcetrapib does increase aldosterone, then the changes observed in the carotid and coronary arteries may have little to do with atherosclerosis and much to do with other changes to the artery wall. This makes it extremely difficult to interpret the effects of torcetrapib in the imaging studies.

Overall, it must be concluded that the results of the ILLUMINATE, ILLUSTRATE and RADIANCE studies neither validate nor invalidate the hypothesis that raising HDL-cholesterol by inhibiting CETP may be cardioprotective. The possibility that inhibiting CETP may be beneficial will remain hypothetical until it is put to the test in a trial with a CETP inhibitor that does not share the off-target adverse pharmacological effects of torcetrapib

Recommendation

Given that demonstrable off-target pharmacological effects of torcetrapib (unrelated to CETP inhibition) may have been responsible both for the adverse outcome in the ILLUMINATE trial and the negative results of the imaging trials, there remains a compelling case for further testing of the hypothesis that CETP inhibition will be anti-atherogenic by conducting both imaging and large-scale outcome trials with CETP inhibitors that do not manifest such off-target pharmacology.

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