September 08, 2011 Vantage Point - Functionality in focus as HDL hypothesis awaits confirmation Amy Brown When it comes to HDL, the consensus of opinion is that higher is better. Yet efforts over many years to prove that agents which boost “good cholesterol” and can prevent heart attacks and strokes have failed to translate this widely accepted wisdom into firm clinical evidence (AHA 2010 – Debate over benefits of raising HDL set to continue,November 18, 2010). The answer to this quandary is thought to lie in HDL functionality – how efficiently these particles are working to clear cholesterol from the body rather than simply how much of it is present in the blood stream. Much work is going on to gain a greater understanding of the mechanisms these complex lipoproteins employ in an attempt to characterise functional HDL; the existence of dysfunctional particles has been proposed to explain the failure of HDL boosting therapies tested to date. What is still sorely needed, however, is a positive study to confirm that researchers are on the right track; huge ongoing studies with three drugs – dalcetrapib, anacetrapib and niacin – might hold the key. New era One of the major groups of blood lipoproteins, HDL or high density lipoproteins are a diverse family of particles that play several roles, some of which are only now being understood. Most famously these particles transport cholesterol from cells throughout the body, including the walls of arteries, to the liver for excretion. This process is known as reverse cholesterol transport. However, it seems that not all HDL particles are very good at this vital function. What is less clear is why HDL loses its functionality, what functional and dysfunctional HDL particles look like, and how the effectiveness of an individual patient’s HDL at clearing cholesterol can be measured. “In epidemiology there is very good proof that the higher the HDL the better the prognosis, and the other way round, when there is a lower defensive capacity of the arteries against bad cholesterol. That that is true, is not in question,” says Dr Heinz Drexel, a cardiologist at the Vorarlberg Institute in Austria. “But which HDL particle is the important one – it is an open question. Now everyone wants to look in more detail into HDL function; this really is the beginning of a new era in HDL research,” he says. Old view Compared to low density lipids (LDL), which transport cholesterol into the artery wall, HDL particles are much more complex and varied. They include small lipid-poor particles, which remove cholesterol from atherosclerotic plaque, while bigger particles transport cholesterol to the liver. Over time HDL particles gradually remodel, and indeed are in a constant state of flux – a young or nascent particle is thought to be a more efficient transporter of cholesterol. As a particle matures and becomes larger, it eventually becomes the HDL particle that is measured in standard laboratory tests, along with LDL and triglycerides, to paint a picture of a patient’s lipid profile. Another vital component of the system is the cholesterylester transfer protein, or CETP, which shuttles free cholesterol between lipoproteins like LDL and HDL, in both directions – so it is involved in both the build up and removal of artery plaque. “The traditional view of HDL was that it is involved in reverse cholesterol transport - picking up free cholesterol from cells known as macrophages and delivering it to the liver – and that there is an equilibration step that involves CETP. "When people in the past talked about HDL functionality it was that aspect they concentrated on,” says Jane Armitage, Professor of Clinical Trials and Epidemiology at Oxford University. Broader view That functionality is now believed to encompass many more pathways and processes that are central to HDL’s cardioprotective attributes. The particles carry a multitude of surface proteins thought to have anti- inflammatory, anti-coagulant and anti-oxidant properties, while HDL could also be involved in immunological responses. For example, certain proteins associated with small HDL particles seem to be closely correlated with the rate of LDL oxidation. This is important because for many years it has been believed that LDL particles only deposit cholesterol on the artery wall when they have been oxidised. “HDL could be playing an important role in protecting against LDL oxidation,” Professor Armitage says. Meanwhile, it is suspected that certain states of inflammation, which are known to contribute to vascular disease, might cause HDL to lose its functionality. “There are certain conditions – such as high oxidative stress or response to surgery – when the body tries to protect itself. These effects can cause the HDL to become less functional, it stops being able to remove cholesterol and becomes pro-inflammatory,” says Roger Newton, one of the developers of Lipitor and founder of Esperion, a company that has been researching therapeutic HDL agents for almost 15 years. How to measure? Advances in proteomics and lipidomics are helping researchers to better understand the HDL particle and the processes it influences. Taking advantage of this knowledge from a therapeutic perspective is another matter, because although HDL functionality is now accepted as important, ways to measure it are lacking. “The major challenge has been to find the right tools, but we are getting there,” says Stephen Nicholls, a cardiovascular specialist with the Cleveland Clinic. He points to a study published in the New England Journal of Medicine earlier this year conducted in patients coming into hospital for an angiogram. From blood samples, they measured a patient’s ability to promote cholesterol efflux – the removal of cholesterol from macrophages and a major function of HDL. “They were able to show a relationship whereby those patients who were the most effective at promoting cholesterol efflux were least likely to have coronary disease on their angiogram. That relationship was stronger than HDL levels. “That lends some support to the concept that we can be measuring functionality,” Dr Nicholls says. Other techniques are being developed, but these are far from being used in daily practise. But it is becoming increasingly clear that a simple measure of the quantity of HDL in the blood is a poor indicator of a patient’s cardiovascular risk. “A static measurement of a dynamic process is exactly that,” says Esperion’s Mr Newton. “Just because you have HDL that’s increased by 31% it doesn’t necessarily mean you are increasing reverse cholesterol transport. There are some conditions where you will have only smaller changes in HDL but the flux through the system is faster.” Spurring research How HDL functions, what its functions are and how they are relevant to a person's health are questions still not understood. However, faith that these cardioprotective particles can be harnessed to prevent heart attacks and strokes continues to spur research. Esperion for example is trying to develop a version of apo-A1, the HDL precursor protein, that is resistant to oxidative stress and will not lose its function. Resverlogix meanwhile is pushing on with RVX-208, a drug designed to promote production of apo-A1, despite disappointing results from a phase II study last year (AHA 2010 – Resverlogix falls back to earth, November 18, 2010). The most high profile products in development are the CETP inhibitors, Roche’s dalcetrapib and Merck & Co’s anacetrapib, both in large phase III trials. They are following in the wake of Pfizer’s CETP inhibitor torcetrapib, a potent HDL booster that was scrapped after a phase III study surprisingly revealed the drug increased a patient’s risk of dying. Modulating function Data generated so far suggest neither of the new contenders raise blood pressure – an unexpected side effect of torcetrapib that might explain the deaths (ESC - Jury remains out on dalcetrapib despite 'safe so far' verdict, August 30, 2011). However, exactly why the drug was dangerous remains unclear. “It may have been off target effects, but another possibility is that some of the anti-inflammatory function of HDL was being impaired by torcetrapib. It is a bit of a puzzle that they did see increases in deaths from other causes as well,” says Professor Armitage. It is notable that Roche no longer refers to dalcetrapib as a CETP inhibitor, now choosing the word modulator to describe its flagship cardiovascular agent. “A CETP inhibitor will block all the transfer of cholesterol between lipoproteins, effectively freezing the system, which is how we believed torcetrapib acted,” David Kallend, leader for Roche’s dalcetrapib development programme, said on a conference call held after this year’s European Society of Cardiology conference. “With CETP modulation you stop the transfer of cholesterol to the atherogenic lipoproteins such as LDL and VLDL but you leave open the remodelling of HDL. It is important to remodel the HDL in order to have function and reverse cholesterol transport, and to really keep this dynamic process cycling within the body,” he says. Focus on function While the failure of torcetrapib was a disappointment, it did prompt much of the research into functionality going on today. “In many ways the torcetrapib story stimulated interest in the functionality concept and the question of 'is it less important about how much you raise it and is it more important the kind of HDL you promote?', could you have lower elevations of HDL but find the right kind?,” says Dr Nicholls. Mid-stage trials have demonstrated that dalcetrapib and anacetrapib certainly boost HDL – the former by around 30% and the latter by more than double. Aware that this might not be enough to affect outcomes, both companies have been searching for signs that the HDL particles promoted are functional. Roche has conducted studies that suggest dalcetrapib is a more potent promoter of reverse cholesterol transport than torcetrapib and anacetrapib, while trials measuring efflux found dalcetrapib substantially boosts the transfer of cholesterol.