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Home » Angiogenesis Center » Robert L. Engler, MD

Robert L. Engler, MD

Dr. Engler has been a pioneer in basic cardiovascular research and the development of genetic and other innovations in the treatment of cardiovascular disease. He has served as a Professor of Medicine at UC San Diego, directed the Coronary Care Unit at the San Diego VA Hospital, and was associate chief of staff and chief, research service at the VA hospital.

Dr. Engler is currently Chief Medical Advisor to Cardium Therapeutics. In this interview, he explains how angiogenesis might solve the problem of angina and ischemia in patients who have persistent chest pain even after angioplasty or bypass surgery. Dr. Engler also discusses the various approaches that have been taken and what direction the field of angiogenetic research and clinical trials is currently taking.

 

 

Robert L. Engler, MD
Robert L. Engler, MD

Q: Can you describe what angiogenesis is, as it is applied to the problem of angina from heart disease?
Dr. Engler: We have at our disposal now two main ways to approach patients with angina. One is with medications -- that either decrease the oxygen needs of the heart, or that dilate blood vessels that may be constricted, or that alter the heart's need for oxygen in other ways. The second approach we have is to perform bypass surgery of specific areas where the coronary arteries are narrowed and restrict the flow of blood -- and by bypassing those, we can deliver more blood downstream from the blockage. Another technique is to do angioplasty with or without stenting, and thus dilate the narrowing. Those two types of interventions, drugs and revascularization, are what we have available.

However, atherosclerosis of the coronary arteries is a diffuse disease. It's only in a few isolated areas that it causes narrowing. So if we go in and dilate or put a stent in one place that's narrowed, the patient can come back a few months or a few years later with another place that's become narrowed.

The heart's normal, natural response to lack of blood flow is to grow new blood vessels: the process of angiogenesis that leads to what we call collateral blood vessels that take blood from a well-perfused area of the heart to an area of the heart that needs more blood flow. Angiogenesis does occur in virtually all of our patients with angina. The problem is that it stops too soon. The angiogenesis is insufficient to relieve the patient's symptoms of angina, caused by myocardial ischemia.

If we could find a method to jump-start or restart that process, that would be a powerful new mechanism for treating patients with angina. And there's an awful lot of hope and excitement that we will be able to find a method to increase angiogenesis in patients with angina.

Q: What is the current status of angiogenesis therapy for the heart?
Dr. Engler: Basically two major approaches have been tried. The first one was protein or growth factor therapy. There are two main types of proteins or growth factors, VEGF and FGF, known respectively as Vascular Endothelial Growth Factor and Fibroblast Growth Factor, that have been evaluated in clinical trials. One from each family has been administered as a protein infusion, and the trial results were very disappointing. There was really no overall clinical effect that could be measured in the patients. That was actually not too surprising because proteins have a very short half-life in the blood. These protein growth factors are gone literally in hours, and in order to stimulate the growth of new blood vessels, the process requires several weeks.

The second approach was gene therapy. With gene therapy we're not really altering or changing the genes in the patient, as one would try to do with certain genetic diseases. What we do is take a normal human gene, and put it into some cells in the body in a particular location and try to get the intrinsic cells in the body to produce the growth factor and secrete it. And when we do that, the secretion of the growth factor usually goes on for several weeks or several months, which is sufficient time to get new blood vessels to grow.

This area was pioneered by the late Jeff Isner in Boston who made enormous contributions to the field. Jeff was using naked DNA or plasma DNA for transfection of cells; plasma DNA is not as efficient as using a vector. One other large trial used the adenovirus as a vector for putting the gene into the heart by direct injection into the heart muscle. The injection was done through a thoracotomy, that is, a surgery. You actually open the chest and do multiple injections in the surface of the heart.

The other method that's been tried is with a catheter to go inside the heart and have a small needle on the end of the catheter and inject the DNA into the inside wall of the heart. There was a large trial initiated but that trial was stopped because they had some problems with perforation of the heart by the needle that caused some bleeding into the sac around the heart, which can be a very dangerous situation.

Another method was discovered by Kirk Hammond at the VA Hospital in San Diego. Kirk was trying to find a way to treat his patients that had angina whose vessels were such that he couldn't do angioplasty on them, or who had already had all the angioplasty they could get, and had bypass surgery, and were still having angina.

So he tried, in a pig model, to see what would happen if he put an adenovirus into the coronary artery -- to see if he could get the adenovirus to put the gene into the heart, so the heart itself would become a source of growth factor. And he made the remarkable observation that if you infuse the adenovirus directly into the coronary artery one time, you get a remarkably highly efficient uptake of the gene in the heart and very little escapes the heart. What does escape the heart gets so diluted in the rest of the body that it has very little effect, virtually no effect in the rest of the body.

Q: So it's not going to go throughout the body and start growing blood vessels everywhere?
Dr. Engler: Exactly. And we also believe that in most cases these genes, at the level that we're giving them, actually won't cause the uncontrolled growth of blood vessels because we think you need to have some other growth factors that are produced during ischemia. That's not completely true with growth factors like VEGF, because if you just inject a large amount of it, it will stimulate the growth of blood vessels no matter where you put it.

In any event, Collateral Therapeutics was formed and we developed Fibroblast Growth Factor 4 in the Adenovector through Phase II clinical trials. Schering AG acquired the company and initiated two Phase 2b/3 clinical trials. Schering took an interim look at the results of one trial. The statistical analysis indicated it would not reach statistical significance, as the trial was designed, so they stopped the gene therapy trials for angiogenesis.

At Cardium Therapeutics we did a meta-analysis of the trial (which was presented by Dr. Tim Henry at a recent gene therapy meeting) and in the meta-analysis it appears that there are actually some very remarkable effects of the gene therapy. It was highly effective in relieving angina and improving exercise performance in women, but seemed to have very little effect that we could detect in men. So we have bought back the technology from Schering, and Cardium is going forward with the development of that product in a Phase 3 trial.

Q: Do you have any theories as to why would this have more of an effect in women than in men?
Dr. Engler: No one knows for sure the answer to your question. And it's been an important question in cardiology for several decades, because we've known for a long time that both the rate of progression and the manifestation of coronary artery disease are quite different in men than women.

One of the important differences is that women appear to have what we call microvascular disease -- that is, the very small blood vessels are diseased. Atherosclerosis in general affects only larger surface blood vessels, the main vessels to the heart. The blood flow to the heart is like a tree. There are the main trunks, and then there are smaller branches, and even smaller branches. And atherosclerosis generally affects mostly the main trunks and the largest branches. And that's true in men.

However, in women, we observe something different. Nearly 50% of women who get angiograms performed on their heart, to look for narrowings in the arteries because they have angina, 50% of those women do not have significant or serious narrowings. And the conclusion is that they must have microvascular disease -- that is, small blood vessel dysfunction. And angiogenesis starts in the microcirculation, it starts with the smallest of blood vessels, with budding and forming of new blood vessels. And so it could be that because women's disease tends to be microvascular and angiogenesis is a microvascular treatment, it is much more effective in women.

Q: So would it be correct to say that you can grow new blood vessels using angiogenesis?
Dr. Engler: Angiogenesis is not going to replace a large main coronary artery. The angiogenic process does not generate those large conduit vessels that take the blood from the aorta, down to the smaller vessels. The collateral vessels that grow from angiogenesis take blood from medium sized arteries over to other medium sized arteries. So if there's a medium to large-sized artery that's blocked, and one that's not so severely blocked, the increased collateral blood vessels will take blood through smaller channels from the unblocked vessel over to the territory of the blocked vessel.

As you know, there are three main coronary arteries: the circumflex, the left anterior descending, and the right coronary artery. If all three of those arteries are severely blocked, angiogenesis by itself is not likely to help that patient. The patients that are good candidates for angiogenesis are the ones that have at least one or two of their major coronary arteries that do not have blockages, or bypass grafts that are open. And that's probably another aspect of why this therapy appears to be effective in women but not in men -- because of the fact that a lot of the women that present with angina, their larger vessels are not as diseased as men.

Q: A lot of patients on our Forum write in with problems – patients who have been through stents and angioplasty and they're still not feeling well, still having angina. Is there a possibility that this is a patient population that might be helped by angiogenesis?
Dr. Engler: Oh absolutely. You hit the nail on the head. I think that's a very important population that angiogenesis would be very helpful for -- patients who continue to have persistent angina despite having had revascularization, either by angioplasty, stent, or by bypass surgery.

There are a number of patients who have persistent angina despite angioplasty or bypass surgery. The repeat catheterization shows the vessels and or bypass grafts are open and they may say, "But I'm still feeling this heaviness in my chest and I'm getting these pains still". It does appear that most of those patients are having persistent angina. And that is exactly the type of population, patients with persistent angina, that angiogenesis therapy would be expected to help.

Q: Can you describe more about the catheter-based system that Cardium is working with right now? There's no needle involved….
Dr. Engler: We do an ordinary diagnostic cardiac catheterization, with a standard cardiac catheter that every cardiologist who does angiography uses. We put the catheter into the main coronary arteries, into each one separately, and we do a single infusion of the adenovirus that has the growth factor gene. The heart does its job, because it takes up avidly a lot of that adenovirus on the first time through the heart.

That's why we have to infuse it directly into the coronary arteries because we want to get the growth factor concentrated and targeted to the heart, and not have the Fibroblast growth factor all over the body. Fibroblast growth factors are locally acting hormones. They are a type of hormone that is produced, secreted by cells, stays local, and has all of its activity in the environment in which it is secreted. The adenovirus with the gene that isn't taken up by the heart becomes so diluted in the rest of the system that there's not enough gene transfer there to produce detectable amounts of growth factor.

So basically the cardiologist delivers this in a matter of 15 minutes. Once the cardiac catheterization is done, once you get the catheters in the vessels and get all ready to go, it'll take about 15 or 20 minutes, to infuse the adenovirus into the coronary arteries and then you're done.

There's no balloon involved, there's no angioplasty involved and there's no stent involved. The guiding catheter is put in the main coronary artery, and then a small selective catheter is passed down through the guiding catheter and goes a short distance further into the coronary arteries. And the reason we do that is so we can be sure that virtually all of the product that we're infusing goes down the coronary arteries and doesn't spill back into the aorta.

You have to do that because when the heart contracts there's actually backward flow of blood in the large surface coronary arteries because the heart generates so much pressure when it contracts. So if you were to just infuse it through the guiding catheter, when the heart contracts during systole, a lot of that product would get pushed back into the aorta and go to the rest of the body and not be targeted to the heart.

Q: How many years are we away from this therapy being used?
Dr. Engler: I can just give you a rough guideline. Cardium is going forward with a phase 3 trial that typically would take 12-18 months to complete. The FDA might require an additional trial, even if the first trial showed the product to be effective, in order to ensure that enough patients had been tested for safety reasons.

Q: Are there other areas in the body that you see this affecting, for example peripheral vessels in the leg?
Dr. Engler: At Cardium we are not pursuing that application at the current time, but there are others that are. And generally those trials involve injecting genes for growth factors in multiple locations in muscles of the leg to help increase the growth of collateral blood vessels in the leg. And there are a significant number of patients in the world with what we call claudication. That is insufficient blood flow to the legs when they walk, quite analogous to the patient who gets angina because he gets insufficient blood flow to the heart when he exercises, or when the demands on the heart go up. And in theory, angiogenesis could well work in the peripheral vascular disease area and there are trials still ongoing.


This interview was conducted by Burt Cohen of Angioplasty.Org -- posted April 2007

** photo and graphics courtesy of Cardium Therapeutics, Inc.

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