Along with Christopher Leamon, you’ve created the Trojan horse process to attack diseased cells. What exactly is this process?
One of the problems with medicine today is that good drugs often distribute equally into diseased cells and healthy cells. And when then accumulate in healthy cells they often cause toxicity. For example, with chemotherapy for cancer, a patient’s hair may fall out, the gut can bleed, the bone marrow is suppressed leading to a compromised immune system. All of these are consequences of good drugs going to the wrong cells.
We have identified very specific receptors or markers [that are over expressed] on cancer cells and other diseased cells that distinguish them from healthy cells. We use these different markers to deliver the drug very specifically to the diseased cell, thereby avoiding the collateral toxicity to the healthy cells. Our technology is based on two simple principles: find a homing molecule that will home in on the diseased cell but not the healthy cell, and then link that to a very effective drug that will treat the disease. So the drug piggy backs on the homing molecule and accumulates in the diseased cell and not in the healthy cell. Sometimes we take advantage of the fact, for example cancer cells have an enormous appetite for folic acid. They need a lot of this vitamin to divide; folic acid is needed for the synthesis of DNA.
Because cancer cells need to divide fast, they need a lot of DNA and therefore need a lot of folic acid. We exploit this hunger for folic acid by attaching a really nasty drug to the vitamin. The cancer cell eats the folic acid – which is the Trojan horse that delivers the killing agent specifically into the cancer cell.
We started with cancer but now have developed these targeted therapies for all sorts of autoimmune, inflammatory and infectious diseases, even some Central Nervous System diseases.
Once one has developed the homing molecule (targeting ligand) one can use it to deliver almost any cargo to the diseased cell. We started off delivering imaging agents initially. That allowed us to image where exactly the diseased cells are located in the body.
We can deliver a fluorescent dye to the cancer cell using this method. When the surgeon turns on the fluorescent lamp he/she can see exactly where the cancer cells lie, and then she can take out the cancer tissue without harming the healthy tissue. This is remarkable because the cancer tissue just stands out very brightly; it is very easy to distinguish it from normal tissue. This turns out to be very important because approximately 40% of all cancers tend to recur after surgery at the site of the original cancer because the surgeon has failed to remove all of the malignant diseased tissue when the surgery was performed. If we can help the surgeon see the residual malignant tissue left behind when she or he thinks all of it has been removed, we can save a lot of lives.
What is the story of the first time you discovered this process?
It’s a really interesting story. It was discovered by accident.
I assigned one of my graduate students t demonstrate that endocytosis – a process by which a cell takes in things from the outside by pinching in on the membrane and forming a compartment inside the cell – occurred in plants. The dogmas was that it does not take place, and I knew that was just hogwash – wasn’t correct. He said he would do it and I described the experiment. Plants recognize bacteria, so I said, take a piece of bacteria and radiolabeled it so that the radioactivity goes into the bacteria. He [did not do the experiment] as he was afraid of radioactivity. He came back and said to me “Dr Low, I am scared to use radioactivity.” I have never had a student like that before! So we had to put something else to attach to that bacteria that will go into the plant cell so we tried biotin, which is a vitamin. There are really easy ways to see where biotin is. He went and added this piece of the bacteria linked [attached covalently] to the vitamin to the plant cells. The plant cells all took it in. Then I said - go run a control and link biotin to something we know plant cells don’t need. He tried it and came to me and said,”Doctor, our experiment failed, I am so disappointed.” And I said – what do you mean it failed? He said, “Well the biotin linked insulin went into the cell, too.” I told him “Mark, it didn’t fail. You’ve just demonstrated that plants have an endocytosis pathway for the vitamin biotin.” That happens to be present through all of the plant kingdom! We had discovered two things, that there was a vitamin mediated uptake of an attached particle, secondly, we had demonstrated that endocytosis occurs in plants. Then I wondered if Vitamins were taken in that way in animal cells. To cut a long story short, biotin went into all sorts of animal cells, but folate went into only cancer cells. And when we found out this, we said “Yes!” This was great for it gave us our own private doorway into cancer cells that would not be shared by other cells.
You mentioned Folic acid as the molecule which is needed by cancer cells. When you attack other diseases what molecule do you use as the Trojan horse?
That is a good question. We use different homing molecules, or targeting ligands as they are called, for different cell types. Even within cancer, it turns out there are some cancers which are not so hungry for folic acid. That’s part of the challenge. Right now we are doing our very best to find a cocktail of targeting ligands that will cover 100% of human cancers. We’re pretty close to that. The goal is eventually to make cancer treatable without having side effects.
What animals do you use to test the drug?
[We use different animals] One of the applications of this targeting concept has been to develop a method to accelerate bone fracture repair. In the U.S. each year there are 6.3 million bone fractures. Many of these people miss a lot of work as an ankle can be 12 weeks in a cast. We figured there’s be a great relief if we could reduce the 12 weeks to six weeks! So we developed a homing molecule that targets just the fracture. We linked that to a cargo which is a bone-growth stimulator. This gives very rapid repair of the broken bone, without causing bone growth everywhere else. These same drugs cannot be administered systemically but if you can target it… In this experiment, we used sheep to test the drug.
Are there diseases that cannot be tackled with your method?
Theoretically, probably not. But we’re having a tough time getting our drugs across the blood-brain barrier. For that reason, we have not been able to treat the Central Nervous system diseases, like Alzheimer’s, Parkinson’s disease, Multiple Sclerosis… We have been able to treat brain cancers – when there is a cancer in the brain, it breaks down the blood-brain barrier, so you can get the drugs in. But, in general, the blood-brain barrier remains intact except for rare conditions with Alzheimer’s, Parkinson’s and Multiple Sclerosis. So most of the time you cannot get the drug in unless it naturally crosses the blood-brain barrier.
There are a number of ways to disrupt the blood-brain barrier – ultrasound, osmotic shock… they’re all very damaging. There are other ways to piggyback drugs on other transporters that can get past the barrier, but their capacities are very low. No one has been able to deliver enough drug to make that a viable option.
What are the side-effects of your method?
Side effects can arise from two problems that can vary from almost negligible to very prominent, depending on the targeting ligand (the homing molecule) and the drug we’re using. One is that we inject these, they circulate and they gradually accumulate at the site of the tumour or rheumatoid arthritis or Crohn’s disease, whatever it is. If you don’t make the bridge – the link between the targeting molecule and the therapeutic warhead – stable enough, it can break before it reaches the diseased tissue. If that happens then you just release a free drug – no better than a non-targeted drug. And we do see that sometimes. These are things you can’t always predict from animal studies.
Second is, nature did not design receptors to be only on cancer cells. The receptors we target are also invariably expressed in normal cells. We try to find receptors whose expression in normal cells is greatly reduced. We don’t go after receptors that are present even in 10% of the abundance they are on a cancer or some other disease tissue. This gives us a window and reduces the toxicity.
In some cases for example folic acid – its major expression is on cancer cells. It’s also expressed in inflammatory macrophages – which are involved in autoimmune diseases. This is good, because we can use folic acid to target anything from Oediopathic pulmonary fibrosis to rheumatoid arthritis, Crohn’s disease and much more. The third place where folic acid is expressed is on the proximal tubules of the kidneys, and that is a healthy cell. The purpose of the receptor on the proximal tubules in the kidneys is to capture folic acid from the urine and dump it back into the bloodstream. You know, if we didn’t have that salvage receptor in the kidneys, we would be folic deficient in fifteen minutes.
It turns out that we don’t see any toxicity in the kidneys because they don’t keep it. They just capture it from the urine and put it back in the blood. So we’re lucky there.
