A Best Evidence Interview With Aroon D. Hingorani, MD, PhD
The Best Evidence Study
Sofat R, Hingorani AD, Smeeth L, et al. Separating the mechanism-based and off-target actions of cholesteryl ester transfer protein inhibitors with CETP gene polymorphisms. Circulation. 2010;121:52-62.
This study was selected as the subject of this interview because of its high ranking in Medscape Best Evidence, which uses the McMaster Online Rating of Evidence System. Of a possible top score of 7, clinicians who used this system ranked this study as 4 for relevance and 5 for newsworthiness
About the Interviewee
Aroon D. Hingorani, MD, PhD, the principal investigator on the study is British Heart Foundation Senior Research Fellow and Professor of Genetic Epidemiology in the Department of Epidemiology and Public Health in the Division of Population Health at University College London. He leads the Genetic Epidemiology Group, which investigates the implications (and applications) of new genetic advances for personal and public health. Reecha Sofat, MD, MRCP, the lead author, and Juan P. Casas, MD, PhD, the senior co-author of the study were also present. Dr. Sofat is a British Heart Foundation Schillingford Training Fellow, and Dr. Casas is a Senior Lecturer at the London School of Hygiene and Tropical Medicine and Senior Genetic Epidemiologist, underpinned by British Heart Foundation funding at University College London.
Introduction
Although lowering low-density lipoprotein cholesterol (LDL-C) levels, primarily with statins, has been shown to decrease cardiovascular morbidity and mortality, residual risk is known to remain even in patients on an aggressive statin regimen. Much of the interest in additional pharmacologic intervention to reduce this risk has focused on high-density lipoprotein cholesterol (HDL-C), higher concentrations of which have been shown to be associated with a lower risk for coronary heart disease (CHD) independent of LDL-C. A novel pharmacologic approach to increase levels of HDL-C is with inhibitors of cholesteryl ester transfer protein (CETP), a plasma protein that is an important mediator in the exchange of lipids between HDL particles and other lipoproteins. The ability of CETP inhibition to increase circulating levels of HDL-C was demonstrated in preclinical and clinical studies. As is well known, however, the first large-scale clinical outcomes trial with a CETP inhibitor, torcetrapib, was terminated prematurely and development of the drug was discontinued. The Investigation of Lipid Level management to Understand its iMpact IN ATherosclerotic Events (ILLUMINATE) trial showed a significant increase in cardiovascular events and deaths in patients at high cardiovascular risk taking torcetrapib in combination with atorvastatin vs those taking atorvastatin alone. The reason(s) for the increase in cardiovascular events and deaths, whether it was a mechanism-based (on-target) effect of CETP inhibition or an idiosyncratic (off-target) effect, has been uncertain. In the ILLUMINATE trial, as well as previous studies, torcetrapib administration in humans was associated with an increase in blood pressure, as well as an increase in serum sodium and aldosterone and a reduction in potassium. It was suggested that these effects may have contributed to the increased cardiovascular mortality associated with torcetrapib. Other CETP inhibitors are in clinical development, notably anacetrapib and dalcetrapib, both of which are in advanced stages. It is of fundamental importance therefore to answer the question of whether the blood pressure-increasing effect of torcetrapib was related to the mechanism of CETP inhibition or whether it was an off-target effect specific to the torcetrapib molecule. Both anacetrapib and dalcetrapib differ in molecular structure from torcetrapib and in studies to date neither drug has been associated with any blood pressure elevation.
Sr. Hingorani and his colleagues hypothesized that common single-nucleotide polymorphisms (SNPs) in the CETP gene could help distinguish mechanism-based from off-target actions of CETP inhibitors to inform on the validity of CETP as a therapeutic target. They coordinated a large international collaboration of studies to compare the effect of 3 CETP SNPs -- Taq1B (rs708272), -629C>A (rs1800775), and 1405V (rs5882) -- with torcetrapib treatment on lipid fractions, blood pressure, and electrolytes in almost 70,000 subjects from genetic studies and 18,000 from randomized trials. They found that CETP SNPs and torcetrapib treatment were both associated with reduced CETP activity and had a directionally concordant effect on 8 lipid and lipoprotein traits: total cholesterol, LDL-C, and HDL-C; HDL2; HDL3; apolipoproteins A-I and B; and triglycerides. The genetic effect on HDL-C (0.13 mmol/L, 95% confidence interval [CI] 0.11-0.14 mmol/L) was consistent with that expected of a 10-mg dose of torcetrapib (0.13 mmol/L, 95% confidence interval 0.10-0.15). In clinical trials, torcetrapib at a dose of 60 mg increased systolic blood pressure (SBP) and diastolic blood pressure (DBP) by an average of 4.47 mm Hg and 2.08 mm Hg, respectively. However, the effect of CETP SNPs on SBP (0.16 mm Hg) and DBP (-0.04 mm Hg) was null and significantly different from that expected with 10 mg of torcetrapib. This led the investigators to conclude that the blood pressure-elevating effect of torcetrapib is an off-target action that is unlikely to be shared by chemically dissimilar CETP inhibitors. The investigators suggested in their paper that genetic studies could be a new source of randomized evidence for drug-target validation in humans that could be used in drug development programs.
Sitting with lead author Reecha Sofat, MD, MRCP, and senior co-author Juan P. Casas, MD, PhD, Aroon D. Hingorani, MD, PhD, spoke with Linda Brookes, MSc, for Medscape Cardiology, to discuss some of the implications of this report for Medscape's readers.
The Interview
Medscape: Is this the first time that a study like this has been done to look at the target action of a drug?
Dr. Hingorani: We believe that this analysis is the first of its kind to use variants in the gene encoding a drug target to profile the mechanism-based effects of pharmacological modifications at the same target. It is an extension of a principle that is becomingly increasing used in genetic epidemiology called Mendelian randomization. The initial idea for the analysis came from our team, Dr. Sofat, Dr. Casas, and me, together with colleagues based at the University of Bristol and other schools in the United Kingdom.
Medscape: What are the advantages of doing this type of study?
Dr. Hingorani: From the perspective of drug development, essentially in cardiovascular disease, it has become difficult to reliably identify new targets for disease prevention, and, following the notable success of statins, a relatively large number of drugs have been tested and failed in that respect. One of the reasons for this is that there are many plausible drug targets that derive from cell culture studies, animal models, and observational epidemiology, but none of them provides completely conclusive evidence on the causal relationship between the target or marker and disease outcome. So pharmaceutical companies are left with having to use the evidence that is available before deciding whether to commit to designing the drug that modifies the target and taking it into clinical development. In the end the final arbiter of the validity of a target is the randomized controlled trial, in which the drug is randomly allocated to 2 equal groups. If the drug is effective, the group that receives the drug will benefit and all other sources of error and biases, confounding factors, etc, will have been overcome by virtue of the randomized trial design. The randomized trial is not only a test of the efficacy and safety of a particular drug molecule, however. If it is a new drug in its class it is also the most comprehensive assessment of the validity of the target, because it will be the largest experiment done in humans during the development of the drug. However, one problem with the randomized controlled trial in this role is, of course, that you have to design the drug and make and test it to get to the stage of the trial. So it is a sort of "Catch-22": you have to develop the drug to test it in the trial, which is the experiment that gives you the best evidence on the target validity. Ideally you would like evidence on target validity to come before the commitment to make the new drug.
We hypothesized that genetic information could provide randomized evidence in human beings without the necessity of making the drug. Genetic variants are allocated at random at conception according to Mendel's law; hence, Mendelian randomization. A genetic study is akin to a natural randomized controlled trial, if you use the right variant that has that effect on the target of interest. So a study of alleles of the CETP gene that reduce CETP activity is akin to a very long-term randomized intervention trial of a "clean" CETP inhibitor, free from the off-target effects of individual drug molecules. Genetic studies of this type may be an alternative way of obtaining randomized evidence on the efficacy and safety of CETP inhibition in humans without the recruitment of new trial participants, prospective follow-up, or exposure to a drug. We used common SNPs in the CETP gene to distinguish whether the hypertensive action of torcetrapib was mechanism based on or off target. We found that the genetic variant did exactly the same thing as the drug in relationship to lipid variables, but it had no effect on blood pressure. So the concordance of the effect of the genotype and the drug on the lipids suggests that those are clearly mechanism-based, whereas the fact that the drug affected blood pressure but the genetic variant did not suggests an idiosyncratic effect for torcetrapib, unlike other CETP inhibitors that are chemically dissimilar.
Medscape: Can the effects of these genetic variants really be regarded as equivalent to administering a pharmacologic agent acutely in a person at risk for cardiovascular disease?
Dr. Hingorani: In coronary heart disease prevention, people are given medicines for prevention over a long period of time, not simply an acute treatment, but you are correct in pointing out that preventive treatment is usually given later in life, in mid life, whereas the genotype you are allocated is present from birth. That is one of the potential differences between the two situations. However, in favor of the idea that genetic studies can profile the effects of drugs is that in our study we saw concordant effects of the genotype and the drug treatment in clinical trials on several blood lipid fractions. It is important to remember that we did not just do a genetic study, we did a meta-analysis of torcetrapib clinical trials, and we saw matching effects of the gene and of the drug on 8 different lipid variables, suggesting that genotype is providing good profiles of what a drug might do to the same target.
Juan P. Casas, MD, PhD: It is very important to understand this is not pharmacogenetics in the sense that it is usually used about personalizing medicine,doing genetic tests to use a drug optimally to reduce adverse effects or to use it in people who are likely to respond better. This is using genetic studies to identify drug targets that would be helpful for the population as a whole, irrespective of their genotype. The genetic element is simply a tool to mimic the effects of a randomized controlled trial. So we sometimes we refer to this approach as developing "impersonal" rather than "personalized" medicine,
Medscape: Randomized trials have strict inclusion and exclusion criteria, for example, according to what additional drugs patients are permitted to take.
Dr. Hingorani: We are studying drugs that are used for primary prevention. They are given to people who are at risk for heart disease but who do not have any overt manifestations of the disease at the outset. So in that context, doing genetic studies in individuals in the population in mid-life, for example, is a similar situation. We are not applying these genetic studies, at least currently, to test the effects of acute treatment -- for myocardial infarction, for example.
Medscape: The role of CETP appears complex and is still under debate. Was this a particular difficult target to study?
Dr. Hingorani: Withthe process we applied you only need to know that the variants that you are dealing with have an effect on the enzyme activity or mass to show that they are a marker for the functional effect on the target of interest. The reason we focused on these particular variants, Taq1B (rs708272), 629C>A (rs1800775), and 1405V (rs5882), was because they are the ones that have been most widely typed by these studies, and, of course, we needed to amass as much information as possible to give us adequate power. We did not address in this paper whether these variants had any effect on coronary heart disease risk. The approach that we have used could clearly be applied in the same way, and a study that examined the same panel of variants in relationship to coronary heart disease risk was reported in 2008.We focused on whether the blood pressure elevation of torcetrapib was likely to be on- or off-target, and our conclusion was that it is likely to be off-target. That conclusion is supported by other lines of evidence. In animal studies and in short term trials in humans, neither of the other CETP inhibitors, anacetrapib or dalcetrapib, led to blood pressure elevations. Those are separate, independent lines of evidence that are concordant with our conclusion.
Medscape:In your analysis, you used data from 31 genetic studies and 7 randomized trials from around the world, including Europe and the United States. How did you acquire all these data?
Dr. Hingorani: We had to generate a data set that to be adequately powered needed to combine data from a large number of studies. So these meta-analyses could not have been done without many collaborators who were very generous in sharing data, published and unpublished, to facilitate this analysis. All the analyses were done within the individual studies, summarized, and then sent to the coordinating center in London. So it was a collective and combined effort involving all the centers and we are very grateful for all their efforts.
Medscape: In the ILLUMINATE trial, torcetrapib was used at a dose of 60 mg in trial, whereas the effects of the genetic variant that you studied were approximately equivalent to a 5- or 10-mg dose. How were these 2 effects compared directly?
Dr. Hingorani: That was an estimate based on information from dose-ranging studies for torcetrapib and its effect on HDL-C. We looked at the effect of different doses of torcetrapib on HDL-C and used that information to express the gene effect as a torcetrapib equivalent to help clinicians interpret the size of the genetic effect. With this approach, you have to bear in mind that the genetic variants by and large will have a smaller effect on the target than a drug that you might eventually develop.
Medscape: There is evidence that the blood pressure effect may not have accounted for the adverse effects of torcetrapib. A recent study suggested that very low levels of CETP could be associated with an increased risk for mortality.
Dr. Hingorani: Of course we do not know the answer to that. That could be addressed in 2 ways. You could analyze the association between the same CETP variants and the risk for coronary heart disease, which was reported in 2008, as I mentioned previously. However, the final arbiter is a randomized controlled trial of potential dissimilar CETP inhibitors, anacetrapib or dalcetrapib.
Medscape: People who took torcetrapib would have also been taking a statin (which also inhibits CETP to a certain extent) and they may have been taking antihypertensive medication. Would this have affected your analyses?
Dr. Hingorani: In the trials all the patients were already on statins and some of the patients in the population studies were on statins. In the genetic analysis people with variant CETP alleles were no more likely to receive statin medication. Moreover, in analysis stratified by LDL-C level there was still no association of CETP genotype with blood pressure.
Medscape: Could this sort of analysis be used for another first-in-class molecule to help avoid the sorts of problems that arose with torcetrapib?
Dr. Hingorani: You could think of studies of the right SNP or SNPs for the gene encoding the target of interest, as a natural randomized trial but without exposure to a drug. That information if conducted in the right way in a large enough study could, we think, help you decide how high you would prioritize the development of that drug.
Medscape: At what stage of drug development would genetic analysis be applied?
Dr. Hingorani: It could be applied at a number of stages in drug development. In this case, where blood pressure emerged as a consequence of exposure to this first-in-class drug there was uncertainty as to whether that was an on or off target action, it was a relatively late stage in drug development. Much earlier on, of course, you could use genetic studies in populations to help gain additional evidence on the validity of a drug target.
Medscape: Would you need the same amount of data as you had for this analysis to study the target of another drug?
Dr. Hingorani: It would partly depend on the effect size of the genetic variant, but we think that, on average, we will need to pool data on a large scale. This is now becoming routine in genetic studies. Many genome-wide association studies (GWAS) have been done. These are a different type of study and they are done for different reasons, but the principle of pooling information from many studies to generate an adequately powered data set is becoming routine in genetic analysis. Many consortia have been established to do GWAS, and these data could be harnessed and redeployed for studies like this analysis. We think that that could be an additional utility and value of studies that have genome-wide data. Of course, there will be GWAS data on many different potential drug targets.
Medscape: Which area of drug therapy do you think would be most interesting to study in this way?
Dr. Hingorani: We think that there is interest both in identifying better targets for cardiovascular disease prevention and also for testing mechanism-based cardiovascular toxicity of drugs. We are not suggesting that these studies will replace randomized controlled trials. Randomized trials will always be required, partly because we do not know whether modifying the target later in life will reverse the process, and secondly, for every drug you test you have to quantify the benefits compared with current treatmentand you can only do this in adequately in a randomized trial setting. That said, however, we think that this approach, which is a totally new sort of information for drug development, can provide randomized evidence on a large scale in human populations without exposing them to a new molecule. With that information we think we can help in the prioritization of what drugs will be taken forward to Phase 3 and beyond.
Medscape: So you are continuing to study this approach to investigating drug targets?
Dr. Hingorani: We see the value in the approach and we are taking it forward together with our collaborators. Coming back to the broader utility of this approach, if you think about it from the perspective of the drug developer, in cardiovascular disease there are literally hundreds of plausible targets for which to design medications. Given that it costs $500 million or more to develop a drug and 10-20 years to get it from initial development to clinical trials, we think that this approach might help prioritization of targets. In other words, if you have genetic variants in a target that exert a favorable profile on a range of risk factors and biomarkers, then they might emerge as higher priority targets for drug development. That clearly might be helpful and it would help mitigate this problem of late-stage failure.