Ieukemia is a serious white blood cell cancer that more than 44,000 Americans develop every year. Currently there are approximately 218,000 people in the U.S. living with the disease, and each year 21,000 people die from leukemia. While advancements have been made in the diagnosis and treatment of leukemia, the 5-year survival rates for many forms of the disease remain relatively low and have changed little over the last 20 years. Thus, there is a great need to identify newer therapeutics with greater activity and fewer side effects. Bacterial toxins have been used as anti-cancer agents and represent a new class of targeted therapeutics. The bacterium A. actinomycetemcomitans produces a leukotoxin that has specificity for certain white blood cells. In vitro and in vivo studies with leukotoxin indicate that it could be an effective anti-leukemia therapeutic with high specificity. To further develop leukotoxin as a novel therapeutic agent, Actinobac Biomed Inc. was recently founded. FDA-required studies are currently underway.
It all started as a protein band on a gel. I was a graduate student at Columbia University, in the Department of Microbiology, studying an oral bacterium, Aggregatibacter (that was formerly Actinobacillus) actinomycetemcomitans that causes a juvenile form of periodontal disease. I was examining proteins that are released by the bacterium and found a secreted protein that was not supposed to be there. The protein is known as leukotoxin. Up until this time, it was believed that leukotoxin was not a secreted protein, and so this discovery was both novel and surprising. (I have learned that in science the most powerful tool is observation.) This result could have been ignored, but instead I chose to let the experiment dictate how to proceed. We subsequently worked out techniques to isolate large amounts of purified leukotoxin and began studies to understand how the toxin is expressed and secreted by the bacterium.
Leukotoxin is a protein that kills only white blood cells, and only those from humans and primates. Why would a bacterium want to produce such an agent? For any pathogen to cause disease, it must be able to evade the host immune response, which is composed predominantly of white blood cells. Thus, leukotoxin allows A. actinomycetemcomitans to avoid being killed by our white blood cells. Indeed, bacteria are exquisitely adapted to survive within their hosts as they provoke disease.
When I started as an assistant professor at UMDNJ in 2003, I began to ponder the idea of using leukotoxin as a therapeutic agent. The use of a bacterial toxin as therapy is not completely new. BOTOX is the neurotoxin from Clostridium botulinum used to treat neuromuscular disorders, and the drug ONTAK, used for the treatment of T-cell lymphoma, is composed of diphtheria toxin from Corynebacterium diphtheriae. As a microbiologist, I knew that bacteria were smarter and more efficient (evolutionarily-speaking) than we are. If A. actinomycetemcomitans figured out how to target only white blood cells, perhaps it would also kill preferentially the cancerous white blood cells (such as in leukemia and lymphoma) in the body, in contrast to conventional chemotherapy, which affects many different types of cells, hence causing a multitude of side-effects. But while leukotoxin has been shown to kill cancerous white blood cells under laboratory conditions, there was nothing known about how it might work in a living animal, such as mice or primates.
I acknowledged early on that in order to continue our efforts with animals and leukemia models, it would be necessary to work with a hematologist/oncologist who studies blood cancers. My search brought me to Roger Strair, MD/ PhD, director of hematological malignancies at The Cancer Institute of New Jersey (CINJ) and professor of medicine at UMDNJ-Robert Wood Johnson Medical School (RWJMS). He was very enthusiastic about our initial proposal and indicated that there is a great need to identify novel therapies for the treatment of hematological malignancies. And so, we began meeting to discuss projects. An important question we had to answer was whether leukotoxin is an effective anti-cancer agent in an animal. However, because it is specific for human and primate cells, we had to use a human xenograft model. As Dr. Strair described to me, in this model human leukemia cells are injected into immunodeficient mice. After several weeks, these mice develop disease and get sick.
In collaboration with Dr. Strair, we were fortunate to receive a grant from the Foundation of UMDNJ and the Office of Patents and Licensing/NJCST to perform these pilot studies with leukotoxin and mice. The results we obtained were striking (see Figure 1). The mice that received the leukemia cells, but no other treatment, got sick and died after several weeks. However, almost all of the mice that were treated with leukotoxin remained healthy and never developed disease. The leukotoxin-treated mice lived to be at least a year old, which is about the normal life-span for these mice. To further understand the effects of the toxin in vivo, we administered leukotoxin intravenously to nonhuman primates. We found that leukotoxin was specific for only white blood cells, was very well-tolerated by the animals at effective doses, and no toxicity was observed. While these studies will be followed up, the results thus far look positive.
Encouraged by our data, Dr. Strair asked very important questions that we still had to address. For example, does leukotoxin target cancer cells better than normal cells? One drawback to many current chemotherapeutic agents is the lack of specificity for the cancerous cells. We proceeded by testing leukotoxin against normal human white blood cells and leukemia cells, and noted a significant difference. The normal white blood cells were relatively unaffected by leukotoxin while leukemic white blood cells were very effectively killed. We discovered that the reason for this difference is due to the levels and type of receptor that leukotoxin uses to recognize cells.
Given these very exciting results, I was encouraged to start a biopharmaceutical company to further develop leukotoxin as a therapeutic agent. With an initial investment commitment of $500,000 from the Foundation Venture Capital Group, I founded Actinobac Biomed, Inc. in January of 2009. Actinobac Biomed has licensed the patents for the clinical use of leukotoxin from UMDNJ. To date, I act as an advisor to the company and chair the Scientific Advisory Board. The goal of the company is to bring leukotoxin and derived agents into clinical trials for testing. The types of studies required by the FDA for approval of a drug are often quite different from the experiments we carry out in an academic laboratory. Learning about the drug approval process and necessary studies has been a very satisfying challenge for me. It is not often that a basic scientist in academia has the chance to see his work in a truly translational setting and I feel very fortunate to have been given this opportunity. I look forward to the day when a patient may actually be treated with that obscure protein band that was not supposed to be there.
Scott Kachlany received his BS degree in microbiology from Cornell University in 1997 and PhD in 2001 from the Department of Microbiology at Columbia University. In 2003, he joined the faculty of UMDNJ-New Jersey Dental School as an assistant professor in the Department of Oral Biology and jointly in the Department of Microbiology and Molecular Genetics at UMDNJ-New Medical School. In 2008, he was promoted to associate professor.
Roger Strair received his BS in biology from SUNY Stonybrook and his MD and PhD degrees from Albert Einstein College of Medicine. He completed a residency and fellowship at Harvard Medical School — Brigham Women’s Hospital. Dr. Strair is director of hematologic malignancies at The Cancer Institute of New Jersey and
professor of medicine at UMDNJ-Robert Wood Johnson Medical School.